11.3 Divisions of the Skeletal System
Skulls on Display
This somewhat macabre display (Figure 11.3.1) can be viewed at the Slovak National Museum in Bratislava, Slovakia. The are meant to represent normal human skeletal anatomy. The skull is part of the , which is one of the two major divisions of the human skeleton. The other division is the .
Axial Skeleton
The , shown in blue in Figure 11.3.2, consists of a total of 80 bones. Besides the skull, it includes the rib cage and vertebral column. It also includes the three tiny ossicles (hammer, anvil, and stirrup) in the middle ear and the hyoid bone in the throat, to which the tongue and some other soft tissues are attached.
Skull
The is the part of the human skeleton that provides a bony framework for the head. It consists of 22 different bones. There are eight bones in the cranium, which encloses the brain, and 14 bones in the face.
Cranium
The forms the entire upper portion of the skull. As shown in Figure 11.3.3, it consists of eight bones: one frontal bone, two parietal bones, two temporal bones, one occipital bone, one sphenoid bone, and one ethmoid bone. The ethmoid bone separates the nasal cavity from the brain. The sphenoid bone is one of several bones, including the frontal bone, that help form the eye sockets. The other bones of the cranium are large and plate-like. They cover and protect the brain. The bottom of the skull has openings for major blood vessels and nerves. A large opening, called the foramen, connects the spinal cord and brain.
Facial Bones
The 14 facial bones of the skull are located below the frontal bone of the cranium, and they are depicted in Figure 11.3.4. Large bones in the face include the upper jaw bones, or maxillae (singular, maxilla), which form the middle part of the face and the bottom of the two eye sockets. The maxillae are fused together, except for an opening between them for the nose. The lower edge of the maxillae contains sockets for the upper teeth. The lower jaw bone, or mandible, is also large. The top edge of the mandible contains sockets for the lower teeth. The mandible opens and closes to chew food and is controlled by strong muscles. There are two zygomatic (or cheek) bones and two nasal bones. The nasal region also contains seven smaller bones, as indicated in Figure 11.3.4.
Vertebral Column
The — also called the spine or backbone — is the flexible column of (singular, vertebra) that connects the trunk with the and encloses the spinal cord. It consists of 33 vertebrae that are divided into five regions, as shown in Figure 11.3.5: the , , , , and regions. From the neck down, the first 24 vertebrae (cervical, thoracic, and lumbar) are individual bones. The five sacral vertebrae are fused together, as are the four coccygeal vertebrae.
The vertebral column consists of 24 individual vertebrae that are separated by s of cartilage. An additional nine vertebrae are fused together at the base of the spine. Note the S-shaped curve of the vertebral column in the profile view in Figure 11.3.5 on the left.
The human vertebral column reflects adaptations for upright bipedal locomotion (walking upright on two legs). For example, the vertebral column is less like a rigid column than an S-shaped spring (see profile view in Figure 11.3.5). Although newborn infants have a relatively straight spine, the curves develop as the backbone starts taking on its support functions, such as keeping the trunk erect, holding up the head, and helping to anchor the limbs. The S shape of the vertebral column allows it to act like a shock absorber, absorbing much of the jarring of walking and running so the forces are not transmitted directly from the pelvis to the skull. The S shape also helps protect the spine from breaking, which would be more likely with a straight, more rigid vertebral column. In addition, the S shape helps to distribute the weight of the body — particularly of the internal organs, so the weight load is not all at the bottom, as would occur with a straight spine.
Rib Cage
The (also called thoracic cage) is aptly named, because it forms a sort of cage that holds within it the organs of the upper part of the trunk, including the and . It is shown in Figures 11.3.6–11.3.8. The rib cage includes the 12 thoracic vertebrae and the sternum, as well as 12 pairs of ribs, which are attached at joints to the vertebrae. The ribs are divided into three groups, called true ribs, false ribs, and floating ribs. The top seven pairs of ribs are true ribs. They are attached by cartilage directly to the sternum. The next three pairs of ribs are false ribs. They are attached by cartilage to the ribs above them, rather than directly to the sternum. The lowest two pairs of ribs are floating ribs. They are attached by cartilage to muscles in the abdominal wall. The attachments of false and floating ribs let the lower part of the rib cage expand to accommodate the internal movements of breathing.
Appendicular Skeleton
The , shown in red (Figure 11.3.9), consists of a total of 126 bones. It includes all the bones of the limbs (arms, legs, hands, and feet,) as well as the bones of the shoulder (shoulder girdle) and pelvis (pelvic girdle).
Upper Limbs
Each upper limb consists of 30 bones. As shown in Figure 11.3.10, there is one bone (called the humerus) in each of the upper arms, and there are two bones (called the ulna and radius) in each of the lower arms. The remaining bones of the upper limb are shown in Figure 11.3.11. Each wrist contains eight carpal bones — which are arranged in two rows of four bones each — and each hand contains five metacarpal bones. The bones in the fingers of each hand include 14 phalanges (three in each finger except the thumb, which has two phalanges). The thumb has the unique ability to move into opposition with the palm of the hand, and with each of the fingers when they are slightly bent. This allows the hand to handle and manipulate objects such as tools.
Lower Limbs
Each lower limb consists of 30 bones. As shown in Figure 11.3.12 to the left, there is one bone (called the femur) in each of the upper legs, and there are two bones (called the tibia and fibula) in each of the lower legs. The knee cap (or patella) is an additional leg bone at the front of each knee, which is the largest joint in the human body.
The remaining bones of the lower limbs are in Figure 11.3.13 to the right. Each ankle contains seven tarsal bones (including the talus and calcaneus), and each foot contains five metatarsal bones. The tarsals and metatarsals form the ankle, heel, and arch of the foot. They give the foot strength while allowing flexibility. The bones in the toes of each foot consist of 14 phalanges (three in each toe except the big toe, which has two phalanges)
Bones of the lower leg (fibula and tibia), ankle (talus), heel (calcaneus), foot (metatarsals), and toes (phalanges)
Shoulder Girdle
The (also called shoulder girdle) attaches the upper limbs to the trunk of the body. It is connected to the by muscles alone. This allows a considerable range of motion in the upper limbs. The shoulder girdle consists of just two pairs of bones, with one of each pair on opposite sides of the body (see Figure 11.3.14). There are a right and left clavicle (collarbone), and a right and left scapula (shoulder blade). The scapula is a pear-shaped flat bone that helps form the shoulder joint. The clavicle is a long bone that serves as a strut between the shoulder blade and the sternum.
Pelvic Girdle
The attaches the legs to the trunk of the body, and also provides a basin to contain and support the organs of the abdomen. It is connected to the vertebral column of the axial skeleton by ligaments. The pelvic girdle consists of two halves — one half for each leg — but the halves are fused with each other in adults at a joint called the pubic symphysis. Each half of the pelvic girdle includes three bones, as shown in Figure 11.3.15 to the right: the ilium (flaring upper part of the pelvic girdle), pubis (lower front), and ischium (lower back). Each of these bones helps form the acetabulum, which is a depression into which the top of the femur (thighbone) fits. When the body is in a seated position, it rests on protrusions (called tuberosities) of the two ischial bones.
11.3 Summary
- The consists of a total of 80 bones. It includes the skull, vertebral column, and rib cage. It also includes the three tiny ossicles in the middle ear and the hyoid bone in the throat.
- The provides a bony framework for the head. It consists of 22 different bones: eight in the (which encloses the brain) and 14 in the face (which includes the upper and lower jaw).
- The is a flexible, S-shaped column of 33 vertebrae that connects the trunk with the skull and encloses the spinal cord. The vertebrae are divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal regions. The S shape of the vertebral column allows it to absorb shocks and distribute the weight of the body.
- The rib cage holds and protects the organs of the upper part of the trunk, including the heart and lungs. It includes the 12 thoracic vertebrae, the sternum, and 12 pairs of ribs.
- The consists of a total of 126 bones. It includes the bones of the four limbs, shoulder girdle, and pelvic girdle.
- Each upper limb consists of 30 bones. There is one bone (called the humerus) in the upper arm, and two bones (called the ulna and radius) in the lower arm. The wrist contains eight carpal bones, the hand contains five metacarpals, and the fingers consist of 14 phalanges. The thumb is opposable to the palm and fingers of the same hand.
- Each lower limb also consists of 30 bones. There is one bone (called the femur) in the upper leg, and two bones (called the tibia and fibula) in the lower leg. The patella covers the knee joint. The ankle contains seven tarsal bones, and the foot contains five metatarsals. The tarsals and metatarsals form the heel and arch of the foot. The bones in the toes consist of 14 phalanges.
- The shoulder girdle attaches the upper limbs to the trunk of the body. It is connected to the axial skeleton only by muscles, allowing mobility of the upper limbs. Bones of the shoulder girdle include a right and left clavicle, as well as a right and left scapula.
- The pelvic girdle attaches the legs to the trunk of the body, and supports the organs of the abdomen. It is connected to the axial skeleton by ligaments. The pelvic girdle consists of two halves that are fused together in adults. Each half consists of three bones: the ilium, pubis, and ischium.
11.3 Review Questions
- What are the advantages of an S-shaped vertebral column?
- What is the rib cage? What is its function? What types of ribs are there?
- Explain the advantage of having some ribs that are not attached directly to the sternum.
- What is the shoulder girdle? Why does it allow considerable upper limb mobility?
- Describe some of the similarities between the upper limbs and the lower limbs.
- Describe the pelvic girdle and the bones it contains.
11.3 Explore More
Bones of the skull – Learn in 4 minutes! Neural Academy, 2018.
Craniosynostosis – Mayo Clinic, 2011.
Attributions
Figure 11.3.1
Human_skulls_on_display by Kiwiev on Wikimedia Commons is used under a CC0 1.0 Universal Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/deed.en) license.
Figure 11.3.2
Axial_skeleton_diagram_blank.svg by Quico/ Qllach on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain). (This is a derivative work from Axial skeleton diagram.svg, by Mariana Ruiz Villarreal [LadyofHats].)
Figure 11.3.3
822px-Cranial_bones_en_v2.svg by Was a bee (adapted/ reallocated text on original image File:Cranial bones en.svg. by Edoarado) on Wikimedia Commons is used under a CC0 1.0 Universal Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/deed.en) license.
Figure 11.3.4
Facial_skeleton_-_en.svg by Was a bee (adapted original image File:Es-Human skull front simplified (bones).svg. by Cristobal carrasco) on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 11.3.5
Spinal_column_curvature by vsion on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 11.3.6
True_ribs_animation from en:Anatomography on Wikimedia Commons is used under a CC BY-SA 2.1 JP (https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en) license. (Creator/ licensor: “BodyParts3D, © The Database Center for Life Science licensed under CC Attribution-Share Alike 2.1 Japan.”)
Figure 11.3.7
False_ribs_animation from en:Anatomography on Wikimedia Commons is used under a CC BY-SA 2.1 JP (https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en) license. (Creator/ licensor: “BodyParts3D, © The Database Center for Life Science licensed under CC Attribution-Share Alike 2.1 Japan.”)
Figure 11.3.8
Floating_ribs_animation from en:Anatomography on Wikimedia Commons is used under a CC BY-SA 2.1 JP (https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en) license. (Creator/ licensor: “BodyParts3D, © The Database Center for Life Science licensed under CC Attribution-Share Alike 2.1 Japan.”)
Figure 11.3.9
Appendicular_skeleton_diagram.svg by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 11.3.10
Humerus,_ulna_and_radius_(female) by Mikael Häggström on Wikimedia Commons is used and adapted by Christine Miller (addition of labels), as it has been released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 11.3.11
Human_left_hand_bones_with_metacarpal_numbers_and_carpal_letters.svg by Mariana Ruiz Villarreal [LadyofHats], Nyks, Bibi Saint-Pol. Bloubéri. and Whidou on Wikimedia Commons is used under a CC0 1.0 Universal Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/deed.en) license.
Figure 11.3.12
Human_leg_bones_labeled.svg by Jecowa (original uploader) at English Wikipediaon Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 11.3.13
Blausen_0411_FootAnatomy by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.3.14
Pectoral_girdle_front_diagram.svg by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 11.3.15
2048px-Blausen_0723_Pelvis by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
References
Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
Mayo Clinic. (2011, ). Craniosynostosis – Mayo Clinic. YouTube. https://www.youtube.com/watch?v=fcHB2pvH2uc
Neural Academy. (2018, ). Bones of the skull – Learn in 4 minutes! YouTube. https://www.youtube.com/watch?v=WRmNC_yPQZ8
Created by: CK-12/Adapted by: Christine Miller
What Is Pseudoscience?
is a claim, belief, or practice that is presented as scientific but does not adhere to the standards and methods of science. True science is based on repeated evidence-gathering and testing of falsifiable hypotheses. Pseudoscience does not adhere to these criteria. In addition to phrenology, some other examples of pseudoscience include astrology, extrasensory perception (ESP), reflexology, reincarnation, and Scientology,
Characteristics of Pseudoscience
Whether a field is actually science or just pseudoscience is not always clear. However, pseudoscience generally exhibits certain common characteristics. Indicators of pseudoscience include:
- The use of vague, exaggerated, or untestable claims: Many claims made by pseudoscience cannot be tested with evidence. As a result, they cannot be falsified, even if they are not true.
- An over-reliance on confirmation rather than refutation: Any incident that appears to justify a pseudoscience claim is treated as proof of the claim. Claims are assumed true until proven otherwise, and the burden of disproof is placed on skeptics of the claim.
- A lack of openness to testing by other experts: Practitioners of pseudoscience avoid subjecting their ideas to peer review. They may refuse to share their data and justify the need for secrecy with claims of proprietary or privacy.
- An absence of progress in advancing knowledge: In pseudoscience, ideas are not subjected to repeated testing followed by rejection or refinement, as hypotheses are in true science. Ideas in pseudoscience may remain unchanged for hundreds — or even thousands — of years. In fact, the older an idea is, the more it tends to be trusted in pseudoscience.
- Personalization of issues: Proponents of pseudoscience adopt beliefs that have little or no rational basis, so they may try to confirm their beliefs by treating critics as enemies. Instead of arguing to support their own beliefs, they attack the motives and character of their critics.
- The use of misleading language: Followers of pseudoscience may use scientific-sounding terms to make their ideas sound more convincing. For example, they may use the formal name dihydrogen monoxide to refer to plain old water.
Persistence of Pseudoscience
Despite failing to meet scientific standards, many pseudosciences survive. Some pseudosciences remain very popular with large numbers of believers. A good example is astrology.
Astrology is the study of the movements and relative positions of celestial objects as a means for divining information about human affairs and terrestrial events. Many ancient cultures attached importance to astronomical events, and some developed elaborate systems for predicting terrestrial events from celestial observations. Throughout most of its history in the West, astrology was considered a scholarly tradition and was common in academic circles. With the advent of modern Western science, astrology was called into question. It was challenged on both theoretical and experimental grounds, and it was eventually shown to have no scientific validity or explanatory power.
Today, astrology is considered a pseudoscience, yet it continues to have many devotees. Most people know their astrological sign, and many people are familiar with the personality traits supposedly associated with their sign. Astrological readings and horoscopes are readily available online and in print media, and a lot of people read them, even if only occasionally. About a third of all adult Americans actually believe that astrology is scientific. Studies suggest that the persistent popularity of pseudosciences such as astrology reflects a high level of scientific illiteracy. It seems that many Americans do not have an accurate understanding of scientific principles and methodology. They are not convinced by scientific arguments against their beliefs.
Dangers of Pseudoscience
Belief in astrology is unlikely to cause a person harm, but belief in some other pseudosciences might — especially in health care-related areas. Treatments that seem scientific but are not may be ineffective, expensive, and even dangerous to patients. Seeking out pseudoscientific treatments may also delay or preclude patients from seeking scientifically-based medical treatments that have been tested and found safe and effective. In short, irrational health care may not be harmless.
Scientific Hoaxes, Frauds, and Fallacies
Pseudoscience is not the only way that science may be misused. Scientific , frauds, and fallacies may misdirect the pursuit of science, put patients at risk, or mislead and confuse the public. An example of each of these misuses of science and its negative effects is described below.
The Piltdown Hoax
Piltdown Man (see picture left) was a paleontological hoax in which bone fragments were presented as the fossilized remains of a previously unknown early human. These fragments consisted of parts of a skull and jawbone, reported to have been found in 1908 in a gravel pit at Piltdown, East Sussex, England. The significance of the specimen remained the subject of controversy until it was exposed in 1953 as a hoax. It eventually came to light that the specimen consisted of the lower jawbone of an orangutan deliberately combined with skull bones of a modern human. The Piltdown hoax is perhaps the most infamous paleontological hoax ever perpetrated, both for its impact on the direction of research on human evolution and for the length of time between its "discovery" and its full exposure as a forgery.
In 1912, the head of the geological department at the British Museum proposed that Piltdown man represented an evolutionary missing link between apes and humans. With its human-like cranium and ape-like jaw, it seemed to support the idea then prevailing in England that human evolution began with the brain. The Piltdown specimen led scientists down a blind alley in the belief that the human brain increased in size before the jaw underwent size reductions to become more like the modern human jaw. This belief confused and misdirected the study of human evolution for decades, and actual fossils of early humans were ignored because they didn't support the accepted paradigm.
The Vaccine-Autism Fraud
You may have heard that certain vaccines put the health of young children at risk. This persistent idea is not supported by scientific evidence or accepted by the vast majority of experts in the field. It stems largely from an elaborate medical research fraud that was reported in a 1998 article published in the respected British medical journal, The Lancet. The main author of the article was a British physician named Andrew Wakefield. In the article, Wakefield and his colleagues described case histories of 12 children, most of whom were reported to have developed autism soon after the administration of the MMR (measles, mumps, rubella) vaccine.
Several subsequent peer-reviewed studies failed to show any association between the MMR vaccine and autism. It also later emerged that Wakefield had received research funding from a group of people who were suing vaccine manufacturers. In 2004, ten of Wakefield's 12 coauthors formally retracted the conclusions in their paper. In 2010, editors of The Lancetretracted the entire paper. That same year, Wakefield was charged with deliberate falsification of research and barred from practicing medicine in the United Kingdom. Unfortunately, by then, the damage had already been done. Parents afraid that their children would develop autism had refrained from having them vaccinated. British MMR vaccination rates fell from nearly 100 per cent to 80 per cent in the years following the study. The consensus of medical experts today is that Wakefield's fraud put hundreds of thousands of children at risk because of the lower vaccination rates and also diverted research efforts and funding away from finding the true cause of autism.
Correlation-Causation Fallacy
Many statistical tests used in scientific research calculate correlations between variables. Correlation refers to how closely related two data sets are, which may be a useful starting point for further investigation. Correlation, however, is also one of the most misused types of evidence, primarily because of the logical fallacy that correlation implies causation. In reality, just because two variables are correlated does not necessarily mean that either variable causes the other.
A few simple examples, illustrated by the graphs below, can be used to demonstrate the correlation-causation fallacy. Assume a study found that both per capita consumption of mozzarella cheese and the number of Civil Engineering doctorates awarded are correlated; that is, rates of both events increase together. If correlation really did imply causation, then you could conclude from the second example that the increase in age of Miss America causes an increase in murders of a specific type or vice versa.
An actual example of the correlation-causation fallacy occurred during the latter half of the 20th century. Numerous studies showed that women taking hormone replacement therapy (HRT) to treat menopausal symptoms also had a lower-than-average incidence of coronary heart disease (CHD). This correlation was misinterpreted as evidence that HRT protects women against CHD. Subsequent studies that controlled other factors related to CHD disproved this presumed causal connection. The studies found that women taking HRT were more likely to come from higher socio-economic groups, with better-than-average diets and exercise regimens. Rather than HRT causing lower CHD incidence, these studies concluded that HRT and lower CHD were both effects of higher socio-economic status and related lifestyle factors.
Check out this “Rough Guide to Spotting Bad Science” infographic from Compound Interest:
1.7 Summary
- Pseudoscience is a claim, belief, or practice that is presented as scientific, but does not adhere to scientific standards and methods.
- Indicators of pseudoscience include untestable claims, lack of openness to testing by experts, absence of progress in advancing knowledge, and attacks on the motives and character of critics.
- Some pseudosciences, including astrology, remain popular. This suggests that many people do not possess the scientific literacy needed to distinguish pseudoscience from true science, or to be convinced by scientific arguments against them.
- Belief in a pseudoscience such as astrology is unlikely to cause harm, but belief in pseudoscientific medical treatments may be harmful.
- In addition to pseudoscience, other examples of the misuse of science include scientific hoaxes (such as the Piltdown hoax), scientific frauds (such as the MMR vaccine-autism fraud), and scientific fallacies (such as the correlation-causation fallacy).
1.7 Review Questions
- Define pseudoscience. Give three examples.
- What are some indicators that a claim, belief, or practice might be pseudoscience rather than true science?
- Astrology was once considered a science, and it was common in academic circles. Why did its status change from a science to a pseudoscience?
- What are possible reasons that some pseudosciences remain popular even after they have been shown to have no scientific validity or explanatory power?
- List three other ways besides pseudoscience that science can be misused, and identify an example of each.
- Explain how misuses of science may waste money and effort. How can they potentially cause harm to the public?
- Many claims made by pseudoscience cannot be tested with evidence. From a scientific perspective, why is it important that claims be testable?
- What do you think is the difference between pseudoscience and belief?
- If you see a website that claims that an herbal supplement causes weight loss and they use a lot of scientific terms to explain how it works, can you be assured that the drug is scientifically proven to work? If not, what are some steps you can take to determine whether or not the drug does in fact work?
- Why do you think it was problematic that Andrew Wakefield received funding from a group of people who were suing vaccine manufacturers?
- What do you think it says about the 1998 Wakefield paper that ten of the 12 coauthors formally retracted their conclusions?
1.7 Explore More
https://www.youtube.com/watch?v=E91bGT9BjYk
How to spot a misleading graph - Lea Gaslowitz, TED-Ed, 2017.
https://www.youtube.com/watch?v=sxYrzzy3cq8
How statistics can be misleading - Mark Liddell, TED-Ed, 2016.
Attributions
Figure 1.7.1
Zodiac Signs Cancer Aquarius Aries Gemini Leo from Max Pixel, is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
Figure 1.7.2
Piltdown Man - McGregor model, by James Howard McGregor on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 1.7.3
Sterkfontein Piltdown man, by Anrie on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.
Figure 1.7.4
Spurious Correlations (Causation Fallacy) - Consumption of mozzarella cheese and awarded Doctorates by Tyler Vigen on Tylervigen.com is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 1.7.5
Spurious Correlations (Causation Fallacy) - Miss America and Murder, by Tyler Vigen, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 1.7.6
A rough guide to spotting bad science, by Compound Interest, is used under a CC BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/ca/) license
References
TED-Ed. (2017, July 6). How to spot a misleading graph - Lea Gaslowitz. YouTube. https://www.youtube.com/watch?v=E91bGT9BjYk&feature=youtu.be
Wakefield, A.J., Murch, S.H., Anthony, A., Linnell, J., Casson, D.M., Malik, M., et al. (1998). Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children. Lancet, 351: 637–41.
Wikipedia contributors. (2020, June 18). Andrew Wakefield. Wikipedia. https://en.wikipedia.org/w/index.php?title=Andrew_Wakefield&oldid=963243135
Created by: CK-12/Adapted by: Christine Miller
Ouch!
The person in Figure 1.2.1 is getting a flu vaccine. You probably know that getting a vaccine can hurt — but it's usually worth it. A vaccine contains dead or altered forms of germs that normally cause a disease, such as flu or measles. The germs in vaccines have been inactivated or weakened so they can no longer cause illness, but are still "noticed" by the immune system.
They stimulate the immune system to produce chemicals that can kill the actual germs if they enter the body, thus preventing future disease. How was such an ingenious way to prevent disease discovered? The short answer is more than two centuries of science.
Science as Process
You may think of as a large and detailed body of knowledge, but science is also the process by which this knowledge is obtained. Science uses experimentation, evidence, and logic to continuously test ideas. Over time and through repeated experimentation and testing, scientific knowledge advances.
We've been accumulating knowledge of vaccines for more than two centuries. The discovery of the first vaccine, as well as the process of vaccination, dates back to 1796. An English doctor named Edward Jenner observed that people who became infected with cowpox did not get sick from smallpox, a similar but much more severe disease (Figure 1.2.2). Jenner decided to transmit cowpox to a young boy to see if it would protect him from smallpox. He gave the boy cowpox by scratching liquid from cowpox sores into the boy's skin. Then, six weeks later, he scratched liquid from smallpox sores into the boy's skin. As Jenner predicted, the boy did not get sick from smallpox. Jenner had discovered the first vaccine, although additional testing was needed to show that it really was effective.
Almost a century passed before the next vaccine was discovered, a vaccine for cholera in 1879. Around the same time, French chemist Louis Pasteur found evidence that many human diseases are caused by germs, which earned him the title of "father of germ theory." Since Pasteur's time, vaccines have been discovered for scores of additional diseases caused by germs, and scientists are currently researching vaccines for many others.
Benefits of Science
Medical advances such as the discovery of vaccines are one of the most important benefits of science, but science and scientific knowledge are also crucial for most other human endeavors. Science is needed to design safe cars, predict storms, control global warming, develop new technologies of many kinds, help couples have children, and put humans on the moon. Clearly, the diversity of applications of scientific knowledge is vast!
1.2 Summary
- Science is a large and detailed body of knowledge. It is also the process by which this knowledge is obtained.
- Science uses experimentation, evidence, and logic to continuously test ideas. Over time and through repeated experimentation and testing, scientific knowledge advances.
- Medical advances such as the development of vaccines are one of the most important benefits of science, but science and scientific knowledge are also crucial for most other human endeavors.
1.2 Review Questions
- Explain why science is considered both a process and a body of knowledge.
- State three specific examples of human endeavors that are based on scientific knowledge.
- How does science influence your daily life?
- Jenner used a young boy as a research subject in his smallpox vaccine research. Today, scientists must follow strict guidelines when using human subjects in their research. What unique concerns do you think might arise when human beings are used as research subjects?
- What gave Jenner the idea to develop a vaccine for smallpox?
- Why do you think almost a century passed between the development of the first vaccine (for smallpox) and the development of the next vaccine (for cholera)
1.2 Explore More
https://www.youtube.com/watch?time_continue=1&v=yqUFy-t4MlQ&feature=emb_logohttps://www.ted.com/talks/uri_alon_why_science_demands_a_leap_into_the_unknown?language=en
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.
Attributions
Figure 1.2.1
Vacina, centro de vacinação, by Hyttalo Souza on Unsplash, used under the Unsplash License (https://unsplash.com/license).
Figure 1.2.2
Child with Smallpox/ID#3265, by CDC/ James Hicks, from the Centre for Disease Control and Prevention, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https://www.youtube.com/watch?v=yqUFy-t4MlQ&feature=youtu.be
Wikipedia contributors. (2020, August 9). Edward Jenner. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Edward_Jenner&oldid=971970576
Wikipedia contributors. (2020, August 5). Louis Pasteur. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Louis_Pasteur&oldid=971330056
Created by: CK-12/Adapted by: Christine Miller
Defining Science
is a distinctive way of gaining knowledge about the natural world that starts with a question and then tries to answer the question using evidence and logic. It is an exciting exploration of all the whys and hows that any curious person might ask about the world. You can be part of that exploration! Besides your curiosity, all you need is a basic understanding of how scientists think and how is done. In this concept, you'll learn how to think like a scientist.
Thinking Like a Scientist
Thinking like a scientist rests on certain underlying assumptions. Scientists assume that:
Nature Is Understandable
Scientists think of nature as a single system controlled by natural laws. By discovering natural laws, scientists strive to increase their understanding of the natural world. Laws of nature are expressed as scientific laws. A is a statement that describes what always happens under certain conditions in nature.
Scientific Ideas Are Open to Change
Science is both a process and body of knowledge. Scientific knowledge is generated through systematic processes, such as and experimentation. Scientists are always testing and revising their ideas, and as new observations are made, existing ideas may be challenged. Ideas may be replaced with new ideas that better fit the facts, but more often, existing ideas are simply revised. Through many new discoveries over time, scientists gradually build an increasingly accurate and detailed understanding of the natural world.
Scientific Knowledge May Be Long Lasting
Many scientific ideas have stood the test of time. About 200 years ago, the scientist John Dalton proposed atomic theory — the theory that all matter is made of tiny particles called atoms. This theory is still valid today. During the two centuries since the theory was first proposed, scientists have learned a lot more about atoms and the even smaller particles that compose them. Nonetheless, the idea that all matter consists of atoms remains valid. There are many other examples of basic scientific ideas that have been tested repeatedly and proven sound. You will learn about many of them as you study human biology.
Not All Questions Can be Answered by Science
rests on evidence and logic, and evidence comes from observations. Therefore, science deals only with things that can be observed. An is anything that is detected through human senses or with instruments or measuring devices that extend human senses. Things that cannot be observed or measured by current means — such as supernatural beings or events — are outside the bounds of science. Consider these two questions about life on Earth:
- Did life on Earth evolve over time?
- How did life on Earth originate?
The first question can be answered by science on the basis of scientific evidence (such as fossils and logical arguments). The second question could be a matter of belief, but no evidence can be gathered to support or refute it. Therefore, it is outside the realm of science.
1.3 Summary
- Science is a distinctive way of gaining knowledge about the natural world that tries to answer questions using evidence and logic.
- Scientists assume that nature can be understood through systematic study.
- Scientific ideas are open to revision.
- Sound scientific ideas withstand the test of time.
- Science cannot provide answers to all of our questions.
1.3 Review Questions
- Define science.
- What is the general goal of science?
- Identify four basic assumptions that scientists make when they study the natural world.
- Do observations in science have to be made by the naked eye? Can you think of a way in which scientists might be able to make observations about something they cannot directly see?
- If something cannot be observed, can it be tested scientifically? Explain your reasoning.
- Scientific knowledge builds upon itself. Give an example of a scientific idea from the reading where the initial idea developed further as science advanced.
- Discuss this statement: “Scientific ideas are always changing, so they can't be trusted.” Do you think this is true?
- Why do you think that scientific knowledge expands as technology becomes more advanced?
1.3 Explore More
https://youtu.be/3nAETHZTObk
Nature of Science with the Ameoba Sisters, 2019.
References
Amoeba Sisters. (2019, Jun 6). Nature of science with Ameoba Sisters. YouTube. https://www.youtube.com/watch?v=3nAETHZTObk
Wikipedia contributors. (2020, July 25). John Dalton. In Wikipedia. https://en.wikipedia.org/w/index.php?title=John_Dalton&oldid=969425891
Created by: CK-12/Adapted by Christine Miller
After reading this chapter, you should be able to see numerous connections between chemistry, human life, and health. In Joseph’s situation, chemistry is involved in the reasons why his father has diabetes, why his personal risk of getting diabetes is high, and why the dietary changes he is considering could be effective.
Type 2 diabetes affects populations worldwide and is caused primarily by a lack of response in the body to the hormone insulin, which causes problems in the regulation of blood sugar, or glucose. Insulin is a peptide hormone, and as you have learned, peptides are chains of amino acids. Therefore, insulin is in the class of biochemical compounds called proteins. Joseph is at increased risk of diabetes partly because there is a genetic component to the disease. DNA, which is a type of chemical compound called a nucleic acid, is passed down from parents to their offspring, and carries the instructions for the production of proteins in units called genes. If there is a problem in a gene (or genes) that contributes to the development of a disease, such as type 2 diabetes, this can get passed down to the offspring and may raise that child’s risk of getting the disease.
But genetics is only part of the reason why Joseph is at an increased risk of diabetes. Obesity itself is a risk factor, and one that can be shared in families due to shared lifestyle factors (such as poor diet and lack of exercise), as well as genetics. Consumption of too many refined carbohydrates (like white bread and soda) may also contribute to obesity and the development of diabetes. As you probably now know, these simple carbohydrates are more easily and quickly broken down in the digestive system into glucose than larger complex carbohydrate molecules, such as those found in vegetables and whole grains. This can lead to dramatic spikes in blood sugar levels, which is particularly problematic for people with diabetes because they have trouble maintaining their blood sugar at a safe level. You can understand why Joseph’s father limits his consumption of refined carbohydrates, and in fact, some scientific studies have shown that avoiding refined carbohydrates may actually help reduce the risk of getting diabetes in the first place.
Joseph’s friend recommended eating a low fat, high carbohydrate diet to lose weight, but you can see that the type of carbohydrate — simple or complex — is an important consideration. Eating a large amount of white bread and rice may not help Joseph reduce his risk of diabetes, but a healthy diet that helps him lose weight may lower his risk of diabetes, since obesity itself is a factor. Which specific diet will work best to help him lose weight probably depends on a variety of factors, including his biology, lifestyle, and food preferences. Joseph should consult with his doctor about his diet and exercise plan, so that his specific situation can be taken into account and monitored by a medical professional.
Drinking enough water is usually good advice for everyone, especially if it replaces sugary drinks like soda. You now know that water is important for many of the chemical reactions that take place in the body. But you can have too much of a good thing — as in the case of marathon runners who can make themselves sick from drinking too much water! As you can see, proper balance, or homeostasis, is very important to the health of living organisms.
Finally, you probably now realize that “chemicals” do not have to be scary, toxic substances. All matter consists of chemicals, including water, your body, and healthy fresh fruits and vegetables, like the ones pictured in Figure 3.12.2. When people advocate “clean eating” and avoiding “chemicals” in food, they are usually referring to avoiding synthetic — or man-made — chemical additives, such as preservatives. This can be a healthy way to eat because it involves eating a variety of whole, fresh, unprocessed foods. But there is no reason to be scared of chemicals in general — they are simply molecules and how they react depends on what they are, what other molecules are present, and the environmental conditions surrounding them.
Chapter 3 Summary
By now, you should have a good understanding of the basics of the chemistry of life. Specifically, you have learned:
- All consists of chemical substances. A chemical substance has a definite and consistent composition and may be either an or a .
- An is a pure substance that cannot be broken down into other types of substances.
- An is the smallest particle of an element that still has the properties of that element. Atoms, in turn, are composed of subatomic particles, including negative , positive , and neutral . The number of protons in an atom determines the element it represents.
- Atoms have equal numbers of electrons and protons, so they have no charge. Ions are atoms that have lost or gained electrons, so they have either a positive or negative charge. Atoms with the same number of protons but different numbers of neutrons are called isotopes.
- There are almost 120 known elements. The majority of elements are metals. A smaller number are nonmetals, including carbon, hydrogen, and oxygen.
- A is a substance that consists of two or more elements in a unique composition. The smallest particle of a compound is called a molecule. Chemical bonds hold together the atoms of molecules. Compounds can form only in chemical reactions, and they can break down only in other chemical reactions.
- Biochemical compounds are carbon-based compounds found in living things. They make up and other structures of organisms and carry out life processes. Most biochemical compounds are large molecules called that consist of many repeating units of smaller molecules called .
- There are millions of different biochemical compounds, but all of them fall into four major classes: , , , and .
- Carbohydrates are the most common class of biochemical compounds. They provide cells with energy, store energy, and make up organic structures, such as the cell walls of plants. The basic building block of carbohydrates is the monosaccharide.
- Sugars are short-chain carbohydrates that supply us with energy. Simple sugars, such as glucose, consist of just one monosaccharide. Some sugars, such as sucrose (or table sugar) consist of two monosaccharides and are called disaccharides.
- Complex carbohydrates, or polysaccharides, consist of hundreds or even thousands of monosaccharides. They include starch, glycogen, cellulose, and chitin.
- is made by plants to store energy and is readily broken down into its component sugars during digestion.
- Glycogen is made by animals and fungi to store energy and plays a critical part in the homeostasis of blood glucose levels in humans.
- is the most common biochemical compound in living things. It forms the cell walls of plants and certain algae. Humans cannot digest cellulose, but it makes up most of the crucial dietary fibre in the human diet.
- makes up organic structures, such as the cell walls of fungi and the exoskeletons of insects and other arthropods.
- Lipids include fats and oils. They store energy, form cell membranes, and carry messages.
- Lipid molecules consist mainly of repeating units called . Fatty acids may be or , depending on the proportion of hydrogen atoms they contain. Animals store fat as saturated fatty acids, while plants store fat as unsaturated fatty acids.
- Types of lipids include triglycerides, phospholipids, and steroids.
- contain glycerol (an alcohol) in addition to fatty acids. Humans and other animals store fat as triglycerides in fat cells.
- contain phosphate and glycerol in addition to fatty acids. They are the main component of cell membranes in all living things.
- are lipids with a four-ring structure. Some steroids, such as cholesterol, are important components of cell membranes. Many other steroids are hormones.
- In living things, proteins include , antibodies, and numerous other important compounds. They help cells keep their shape, make up muscles, speed up chemical reactions, and carry messages and materials (among other functions).
- are made up of small monomer molecules called .
- Long chains of amino acids form polypeptides. The sequence of amino acids in polypeptides makes up the primary structure of proteins. Secondary structure refers to configurations such as helices and sheets within polypeptide chains. Tertiary structure is a protein's overall three-dimensional shape, which controls the molecule's basic function. A quaternary structure forms if multiple protein molecules join together and function as a complex.
- The chief characteristic that allows proteins' diverse functions is their ability to bind specifically and tightly with other molecules.
- include and . They encode instructions for making proteins, helping make proteins, and passing the encoded instructions from parents to offspring.
- Nucleic acids are built of monomers called , which bind together in long chains to form polynucleotides. DNA consists of two polynucleotides, and RNA consists of one polynucleotide.
- Each nucleotide consists of a sugar molecule, phosphate group, and nitrogen base. Sugars and phosphate groups of adjacent nucleotides bind together to form the "backbone" of the polynucleotide. Bonds between complementary bases hold together the two polynucleotide chains of DNA and cause it to take on its characteristic double helix shape.
- DNA makes up genes, and the sequence of nitrogen bases in DNA makes up the genetic code for the synthesis of proteins. RNA helps synthesize proteins in cells. The genetic code in DNA is also passed from parents to offspring during reproduction, explaining how inherited characteristics are passed from one generation to the next.
- A is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a , and a substance that forms in a chemical reaction is called a . During the chemical reaction, bonds break in reactants and new bonds form in products.
- Chemical reactions can be represented by . According to the , mass is always conserved in a chemical reaction, so a chemical equation must be balanced, with the same number of atoms of each type of element in the products as in the reactants.
- Many chemical reactions occur all around us each day, such as iron rusting and organic matter rotting, but not all changes are chemical processes. Some changes, such as ice melting or paper being torn into smaller pieces, are physical processes that do not involve chemical reactions and the formation of new substances.
- All chemical reactions involve energy, and they require to begin. release energy. absorb energy.
- are chemical reactions that take place inside living things. The sum of all the biochemical reactions in an organism is called . Metabolism includes catabolic reactions (exothermic reactions) and anabolic reactions (endothermic reactions).
- Most biochemical reactions require a biological called an to speed up the reaction by reducing the amount of activation energy needed for the reaction to begin. Most enzymes are proteins that affect just one specific substance, called the enzyme's substrate.
- Virtually all living things on Earth require liquid water. Only a tiny per cent of Earth's water is fresh liquid water. Water exists as a liquid over a wide range of temperatures, and it dissolves many substances. These properties depend on water's polarity, which causes water molecules to "stick" together through weak bonds called hydrogen bonds.
- The human body is about 70 per cent water (outside of fat). Organisms need water to dissolve many substances and for most biochemical processes, including photosynthesis and cellular respiration.
- A solution is a mixture of two or more substances that has the same composition throughout. Many solutions consist of water and one or more dissolved substances.
- is a measure of the hydronium ion concentration in a solution. Pure water has a very low concentration and a pH of 7, which is the point of neutrality on the . Acids have a higher hydronium ion concentration than pure water and a pH lower than 7. Bases have a lower hydronium ion concentration than pure water and a pH higher than 7.
- Many acids and bases in living things are secreted to provide the proper pH for enzymes to work properly.
Now you understand the chemistry of the molecules that make up living things. In the next chapter, you will learn how these molecules make up the basic unit of structure and function in living organisms — cells — and you will be able to understand some of the crucial chemical reactions that occur within cells.
Chapter 3 Review
- The chemical formula for the complex carbohydrate glycogen is C24H42O21.
- What are the elements in glycogen?
- How many atoms are in one molecule of glycogen?
- Is glycogen an ion? Why or why not?
- Is glycogen a monosaccharide or a polysaccharide? Besides memorizing this fact, how would you know this based on the information in the question?
- What is the function of glycogen in the human body?
- What is the difference between an ion and a polar molecule? Give an example of each in your explanation.
- Define monomer and polymer.
- What is the difference between a protein and a polypeptide?
- People with diabetes have trouble controlling the level of glucose in their bloodstream. Knowing this, why do you think it is often recommended that people with diabetes limit their consumption of carbohydrates?
- Identify each of the following reactions as endothermic or exothermic.
- cellular respiration
- photosynthesis
- catabolic reactions
- anabolic reactions
- Pepsin is an enzyme in the stomach that helps us digest protein. Answer the following questions about pepsin:
- What is the substrate for pepsin?
- How does pepsin work to speed up protein digestion?
- Given what you know about the structure of proteins, what do you think are some of the products of the reaction that pepsin catalyzes?
- The stomach is normally acidic. What do you think would happen to the activity of pepsin and protein digestion if the pH is raised significantly?
Attributions
Figure 3.13.1
Prevalence_of_Diabetes_by_Percent_of_Country_Population_(2014)_Gradient_Map by Walter Scott Wilkens [Wwilken2], University of Illinois - Urbana Champaign Department of Geography and GIScience, on Wikimedia Commons, is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.
Figure 3.13.2
Healthy plate by Melinda Young Stuart on Flickr is used under a CC BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/) license.
Created by CK-12/Adapted by Christine Miller
Case Study: Our Invisible Inhabitants
Lanying is suffering from a fever, body aches, and a painful sore throat that feels worse when she swallows. She visits her doctor, who examines her and performs a throat culture. When the results come back, he tells her that she has strep throat, which is caused by the bacteria Streptococcus pyogenes. He prescribes an antibiotic that will either kill the bacteria or stop it from reproducing, and advises her to take the full course of the treatment even if she is feeling better earlier. Stopping early can cause an increase in bacteria that are resistant to antibiotics.
Lanying takes the antibiotic as prescribed. Toward the end of the course, her throat is feeling much better — but she can’t say the same for other parts of her body! She has developed diarrhea and an itchy vaginal yeast infection. She calls her doctor, who suspects that the antibiotic treatment has caused both the digestive distress and the yeast infection. He explains that our bodies are home to many different kinds of microorganisms, some of which are actually beneficial to us because they help us digest our food and minimize the population of harmful microorganisms. When we take an antibiotic, many of these “good” bacteria are killed along with the “bad,” disease-causing bacteria, which can result in diarrhea and yeast infections.
Lanying's doctor prescribes an antifungal medication for her yeast infection. He also recommends that she eat yogurt with live cultures, which will help replace the beneficial bacteria in her gut. Our bodies contain a delicate balance of inhabitants that are invisible without a microscope, and changes in that balance can cause unpleasant health effects.
What Is Human Biology?
As you read the rest of this book, you'll learn more amazing facts about the human organism, and you'll get a better sense of how biology relates to your health. Human biology is the scientific study of the human species, which includes the fascinating story of human evolution and a detailed account of our genetics, anatomy, physiology, and ecology. In short, the study focuses on how we got here, how we function, and the role we play in the natural world. This helps us to better understand human health, because we can learn how to stay healthy and how diseases and injuries can be treated. Human biology should be of personal interest to you to the extent that it can benefit your own health, as well as the health of your friends and family. This branch of science also has broader implications for society and the human species as a whole.
Chapter Overview: Living Organisms and Human Biology
In the rest of this chapter, you'll learn about the traits shared by all living things, the basic principles that underlie all of biology, the vast diversity of living organisms, what it means to be human, and our place in the animal kingdom. Specifically, you'll learn:
- The seven traits shared by all living things: or the maintenance of a more-or-less constant internal environment; multiple levels of organization consisting of one or more ; the use of energy and ; the ability to grow and develop; the ability to evolve adaptations to the environment; the ability to detect and respond to environmental stimuli; and the ability to .
- The basic principles that unify all fields of biology, including gene theory, homeostasis, and evolutionary theory.
- The diversity of life (including the different kinds of biodiversity), the definition of a species, the classification and naming systems for living organisms, and how evolutionary relationships can be represented through diagrams, such as phylogenetic trees.
- How the human species is classified and how we've evolved from our close relatives and ancestors.
- The physical traits and social behaviors that humans share with other primates.
As you read this chapter, consider the following questions about Lanying's situation:
- What do single-celled organisms (such as the bacteria and yeast living in and on Lanying) have in common with humans?
- How are bacteria, yeast, and humans classified?
- How do the concepts of and apply to Lanying’s situation?
- Why can stopping antibiotics early cause the development of antibiotic-resistant bacteria?
Attribution
Figure 2.1.1
Photo (face mask) by Michael Amadeus, on Unsplash is used under the Unsplash license (https://unsplash.com/license).
Reference
Mayo Clinic Staff (n.d.). Strep throat [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/strep-throat/symptoms-causes/syc-20350338
Created by: CK-12/Adapted by Christine Miller
What Are You Made of?
Your entire body is made of cells and cells are made of molecules.If you look at your hand, what do you see? Of course, you see skin, which consists of . But what are skin cells made of? Like all living cells, they are made of matter. In fact, all things are made of matter. is anything that takes up space and has mass. Matter, in turn, is made up of chemical substances. A is matter that has a definite composition that is consistent throughout. A chemical substance may be either an element or a compound.
Elements and Atoms
An is a pure substance. It cannot be broken down into other types of substances. Each element is made up of just one type of .
Structure of an Atom
An is the smallest particle of an element that still has the properties of that element. Every substance is composed of atoms. Atoms are extremely small, typically about a ten-billionth of a metre in diametre. However, atoms do not have well-defined boundaries, as suggested by the atomic model shown below.
Every is composed of a central area — called the — and one or more subatomic particles called , which move around the nucleus. The nucleus also consists of subatomic particles. It contains one or more s and typically a similar number of . The number of protons in the nucleus determines the type of element an atom represents. An atom of hydrogen, for example, contains just one proton. Atoms of the same element may have different numbers of neutrons in the nucleus. Atoms of the same element with the same number of protons — but different numbers of neutrons — are called .
Protons have a positive electric charge and neutrons have no electric charge. Virtually all of an atom's mass is in the protons and neutrons in the nucleus. Electrons surrounding the nucleus have almost no mass, as well as a negative electric charge. If the number of protons and electrons in an atom are equal, then an atom is electrically neutral, because the positive and negative charges cancel each other out. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an .
The negatively-charged electrons of an atom are attracted to the positively-charged protons in the nucleus by a force called , for which opposite charges attract. Electromagnetic force between protons in the nucleus causes these subatomic particles to repel each other, because they have the same charge. However, the protons and neutrons in the nucleus are attracted to each other by a different force, called , which is usually stronger than the electromagnetic force. Nuclear force repels the positively-charged protons from each other.
Periodic Table of the Elements
There are almost 120 known elements. As you can see in the Periodic Table of the Elements shown below, the majority of elements are metals. Examples of metals are iron (Fe) and copper (Cu). Metals are shiny and good conductors of electricity and heat. Nonmetal elements are far fewer in number. They include hydrogen (H) and oxygen (O). They lack the properties of metals.
The periodic table of the elements arranges elements in groups based on their properties. The element most important to life is carbon (C). Find carbon in the table. What type of element is it: metal or nonmetal?
Compounds and Molecules
A is a unique substance that consists of two or more elements combined in fixed proportions. This means that the composition of a compound is always the same. The smallest particle of most compounds in living things is called a .
Consider water as an example. A molecule of water always contains one atom of oxygen and two atoms of hydrogen. The composition of water is expressed by the chemical formula H2O. A model of a water molecule is shown in Figure 3.2.4.
What causes the atoms of a water molecule to “stick” together? The answer is chemical bonds. A is a force that holds together the atoms of molecules. Bonds in molecules involve the sharing of electrons among atoms. New chemical bonds form when substances react with one another. A is a process that changes some chemical substances into others. A chemical reaction is needed to form a compound, and another chemical reaction is needed to separate the substances in that compound.
3.2 Summary
- All consists of chemical substances. A has a definite composition which is consistent throughout. A chemical substance may be either an element or a compound.
- An is a pure substance that cannot be broken down into other types of substances.
- An is the smallest particle of an element that still has the properties of that element. Atoms, in turn, are composed of subatomic particles, including negative , positive , and neutral . The number of protons in an atom determines the element it represents.
- Atoms have equal numbers of electrons and protons, so they have no charge. Ions are atoms that have lost or gained electrons, and as a result have either a positive or negative charge. Atoms with the same number of protons — but different numbers of neutrons — are called .
- There are almost 120 known elements. The majority of elements are metals. A smaller number are nonmetals. The latter include carbon, hydrogen, and oxygen.
- A compound is a substance that consists of two or more elements in a unique composition. The smallest particle of a compound is called a . Chemical bonds hold together the atoms of molecules. Compounds can form only in chemical reactions, and they can break down only in other chemical reactions.
3.2 Review Questions
- What is an element? Give three examples.
- Define compound. Explain how compounds form.
- Compare and contrast atoms and molecules.
- The compound called water can be broken down into its constituent elements by applying an electric current to it. What ratio of elements is produced in this process?
- Relate ions and isotopes to elements and atoms.
- What is the most important element to life?
- Iron oxide is often known as rust — the reddish substance you might find on corroded metal. The chemical formula for this type of iron oxide is Fe2O3. Answer the following questions about iron oxide and briefly explain each answer.
- Is iron oxide an element or a compound?
- Would one particle of iron oxide be considered a molecule or an atom?
- Describe the relative proportion of atoms in iron oxide.
- What causes the Fe and O to stick together in iron oxide?
- Is iron oxide made of metal atoms, metalloid atoms, nonmetal atoms, or a combination of any of these?
- 14C is an isotope of carbon used in the radiocarbon dating of organic material. The most common isotope of carbon is 12C. Do you think 14C and 12C have different numbers of neutrons or protons? Explain your answer.
- Explain why ions have a positive or negative charge.
- Name the three subatomic particles described in this section.
3.2 Explore More
https://www.youtube.com/watch?v=yQP4UJhNn0I&feature=emb_logo
Just how small is an atom? TED-Ed, 2012
Attributions
Figure 3.2.1
Man Sitting, by Gregory Culmer, on Unsplash, is used under the Unsplash license (https://unsplash.com/license).
Figure 3.2.2
Lithium Atom diagram, by AG Caesar, is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en)
Figure 3.2.3
Periodic Table Armtuk3, by Armtuk, is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/) license.
Figure 3.2.4
Water molecule, by Sakurambo, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
TED-Ed. (2012, April 16). Just how small is an atom. YouTube. https://www.youtube.com/watch?v=yQP4UJhNn0I&feature=youtu.be
Created by: CK-12/Adapted by Christine Miller
A Bag Full of Jell-O
The simple cut-away model of an animal (Figure 4.4.1) shows that a cell resembles a plastic bag full of Jell-O. Its basic structure is a filled with . Like Jell-O containing mixed fruit (Figure 4.4.2), the cytoplasm of the cell also contains various structures, including a and other . Your body is composed of trillions of cells, but all of them perform the same basic life functions. They all obtain and use , respond to the environment, and . How do your cells carry out these basic functions and keep themselves — and you — alive? To answer these questions, you need to know more about the structures that make up cells, starting with the .
What is the Plasma Membrane?
The is a structure that forms a barrier between the inside the and the environment outside the cell. Without the plasma membrane, there would be no cell. Although it is very thin and flexible, the plasma membrane protects and supports the cell by controlling everything that enters and leaves it. It allows only certain substances to pass through, while keeping others in or out. To understand how the plasma membrane controls what passes into or out of the cell, you need to know its basic structure.
Phospholipid Bilayer
The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a . As shown in the simplified diagram in Figure 4.4.3, each individual phospholipid molecule has a phosphate group head (in red) and two fatty acid tails (in yellow). The head “loves” water () and the tails “hate” water (). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outward, toward either the cytoplasm (intracellular) or the fluid that surrounds the cell (extracellular).
molecules can easily pass through the if they are small enough, because they are water-hating like the interior of the membrane. molecules, on the other hand, cannot pass through the plasma membrane — at least not without help — because they are water-loving like the exterior of the membrane.
Other Molecules in the Plasma Membrane
The plasma membrane also contains other molecules, primarily other and . The yellow molecules in the diagram here, for example, are the cholesterol. Molecules of the steroid lipid cholesterol help the plasma membrane keep its shape. Proteins in the plasma membrane (shown blue in Figure 4.4.4) include: transport that assist other substances in crossing the cell membrane, receptors that allow the cell to respond to chemical signals in its environment, and cell-identity markers that indicate what type of cell it is and whether it belongs in the body.
Additional Functions of the Plasma Membrane
The may have extensions, such as whip-like (singular flagellum) or brush-like (singular cilium), shown below (Figure 4.4.5), that give it other functions. In single-celled organisms, these membrane extensions may help the organisms move. In multicellular organisms, the extensions have different functions. For example, the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose, while the flagellum on a human sperm cell allows it to swim.
Feature: My Human Body
If you smoke or use e-cigarettes (vaping) and need another reason to quit, here's a good one. We usually think of lung as the major disease caused by smoking. But smoking and vaping can have devastating effects on the body's ability to protect itself from repeated, serious respiratory infections, such as bronchitis and pneumonia.
are microscopic, hair-like projects on cells that line the respiratory, reproductive, and digestive systems. Cilia in the respiratory system line most of your airways, where they have the job of trapping and removing dust, germs, and other foreign particles before they can make you sick. Cilia secrete mucus that traps particles, and they move in a continuous wave-like motion that sweeps the mucus and particles upward toward the throat, where they can be expelled from the body. When you are sick and cough up phlegm, that's what you are doing.
Smoking prevents cilia from performing these important functions. Chemicals in tobacco smoke paralyze the cilia so they can't sweep mucus out of the airways. Those chemicals also inhibit the cilia from producing mucus. Fortunately, these effects start to wear off soon after the most recent exposure to tobacco smoke. If you stop smoking, your cilia will return to normal. Even if prolonged smoking has destroyed cilia, they will regrow and resume functioning in a matter of months after you stop smoking.
4.4 Summary
- The is a structure that forms a barrier between the inside the cell and the environment outside the cell. It allows only certain substances to pass in or out of the cell.
- The is composed mainly of a molecules. It also contains other molecules, such as the steroid cholesterol, which helps the membrane keep its shape, and transport proteins, which help substances pass through the membrane.
- The plasma membranes of some cells have extensions that have other functions, like flagella to help sperm move, or cilia to help keep our airways clear.
4.4 Review Questions
- What are the general functions of the plasma membrane?
- Describe the phospholipid bilayer of the plasma membrane.
- Identify other molecules in the plasma membrane. State their functions.
- Why do some cells have plasma membrane extensions, like flagella and cilia?
- Explain why hydrophilic molecules cannot easily pass through the cell membrane. What type of molecule in the cell membrane might help hydrophilic molecules pass through it?
- Which part of a phospholipid molecule in the plasma membrane is made of fatty acid chains? Is this part hydrophobic or hydrophilic?
- The two layers of phospholipids in the plasma membrane are called a phospholipid ____________.
- Steroid hormones can pass directly through cell membranes. Why do you think this is the case?
- Some antibiotics work by making holes in the plasma membrane of bacterial cells. How do you think this kills the cells?
- What is the name of the long, whip-like extensions of the plasma membrane that helps some single-celled organisms move?
4.4 Explore More
https://www.youtube.com/watch?v=yAXnYcUjn5k&feature=emb_logo
Insights into cell membranes via dish detergent - Ethan Perlstein, TED-Ed, 2013.
https://www.youtube.com/watch?v=qBCVVszQQNs
Inside the cell membrane, by The Amoeba Sisters, 2018.
Attributions
Figure 4.4.1
Animal Cell Unannotated, by Kelvin Song on Wikimedia Commons is used under a CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/deed.en) public domain dedication license.
Figure 4.4.2
Jello mold at the mexican bakery photo by Aimée Knight on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Figure 4.4.3
Phospholipid_Bilayer by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.
Figure 4.4.4
Lipid bilayer by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.
Figure 4.4.5
Spermatozoa-human-3140x by No specific author on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.4.6
Cilia/ Bronchiolar epithelium 3 - SEM by Charles Daghlian on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.4.7
Adverse effects of vaping (raster) by Mikael Häggström on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Amoeba Sisters. (2018, February 27). Inside the cell membrane. YouTube. https://www.youtube.com/watch?v=qBCVVszQQNs&feature=youtu.be
Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.. Womble, M., DeSaix. P. (2013, April 25). Figure 3.3 Phospolipid Bilayer [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane
Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.. Womble, M., DeSaix. P. (2013, April 25). Figure 3.4 Cell Membrane [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane
Ghosh, A., Coakley, R. C., Mascenik, T., Rowell, T. R., Davis, E. S., Rogers, K., Webster, M. J., Dang, H., Herring, L. E., Sassano, M. F., Livraghi-Butrico, A., Van Buren, S. K., Graves, L. M., Herman, M. A., Randell, S. H., Alexis, N. E., & Tarran, R. (n.d.). Chronic E-Cigarette Exposure Alters the Human Bronchial Epithelial Proteome. American Journal of Respiratory and Critical /Care Medicine, 198(1), 67–76. https://doi-org.ezproxy.tru.ca/10.1164/rccm.201710-2033OC
TED-Ed. (2013, February 26). Insights into cell membranes via dish detergent - Ethan Perlstein. YouTube. https://www.youtube.com/watch?v=yAXnYcUjn5k&feature=youtu.be
each of the twelve bones of the backbone to which the ribs are attached.
Created by: CK-12/Adapted by Christine Miller
Of Peas and People
These purple-flowered plants are not just pretty to look at. Plants like these led to a huge leap forward in biology. They're common garden peas, and they were studied in the mid-1800s by an Austrian monk named Gregor Mendel. Through careful experimentation, Mendel uncovered the secrets of heredity, or how parents pass characteristics to their offspring. You may not care much about heredity in pea plants, but you probably care about your own heredity. Mendel's discoveries apply to people, as well as to peas — and to all other living things that reproduce sexually. In this concept, you will read about Mendel's experiments and the secrets of heredity that he discovered.
Mendel and His Pea Plants
Gregor Mendel (Figure 5.10.2) was born in 1822. He grew up on his parents’ farm in Austria. He did well in school and became a friar (and later an abbot) at St. Thomas' Abbey. Through sponsorship from the monastery, he went on to the University of Vienna, where he studied science and math. His professors encouraged him to learn science through experimentation, and to use math to make sense of his results. Mendel is best known for his experiments with pea plants (like the purple flower pictured in Figure 5.10.1).
Blending Theory of Inheritance
During Mendel's time, the blending theory of inheritance was popular. According to this theory, offspring have a blend (or mix) of their parents' characteristics. Mendel, however, noticed plants in his own garden that weren’t a blend of the parents. For example, a tall plant and a short plant had offspring that were either tall or short — not medium in height. Observations such as these led Mendel to question the blending theory. He wondered if there was a different underlying principle that could explain how characteristics are inherited. He decided to experiment with pea plants to find out. In fact, Mendel experimented with almost 30 thousand pea plants over the next several years!
Why Study Pea Plants?
Why did Mendel choose common, garden-variety pea plants for his experiments? Pea plants are a good choice because they are fast-growing and easy to raise. They also have several visible characteristics that can vary. These characteristics — some of which are illustrated in Figure 5.10.4 — include seed form and colour, flower colour, pod form and colour, placement of pods and flowers on stems, and stem length. Each of these characteristics has two common values. For example, seed form may be round or wrinkled, and flower colour may be white or purple (violet).
Controlling Pollination
To research how characteristics are passed from parents to offspring, Mendel needed to control , which is the fertilization step in the sexual reproduction of plants. Pollen consists of tiny grains that are the male sex cells (or gametes) of plants. They are produced by a male flower part called the anther. Pollination occurs when pollen is transferred from the anther to the stigma of the same or another flower. The stigma is a female part of a flower, and it passes pollen grains to female gametes in the ovary.
Pea plants are naturally self-pollinating. In , pollen grains from anthers on one plant are transferred to stigmas of flowers on the same plant. Mendel was interested in the offspring of two different parent plants, so he had to prevent self-pollination. He removed the anthers from the flowers of some of the plants in his experiments. Then he pollinated them by hand using a small paintbrush with pollen from other parent plants of his choice.
When pollen from one plant fertilizes another plant of the same species, it is called . The offspring that result from such a cross are called . When the term hybrid is used in this context, it refers to any offspring resulting from the breeding of two genetically distinct individuals.
Mendel's First Set of Experiments
At first, Mendel experimented with just one characteristic at a time. He began with flower colour. As shown in Figure 5.10.5, Mendel cross-pollinated purple- and white-flowered parent plants. The parent plants in the experiments are referred to as the P (for parent) generation.
Figure 5.10.5 shows Mendel's first experiment with pea plants. The F1 generation results from the cross-pollination of two parent (P) plants, and it contains all purple flowers. The F2 generation results from the self-pollination of F1 plants, and contains 75% purple flowers and 25% white flowers.
F1 and F2 Generations
The offspring of the P generation are called the F1 (for filial, or “offspring”) generation. As shown in Figure 5.10.5, all of the plants in the F1 generation had purple flowers — none of them had white flowers. Mendel wondered what had happened to the white-flower characteristic. He assumed that some type of inherited factor produces white flowers and some other inherited factor produces purple flowers. Did the white-flower factor just disappear in the F1 generation? If so, then the offspring of the F1 generation — called the F2 generation — should all have purple flowers like their parents.
To test this prediction, Mendel allowed the F1 generation plants to self-pollinate. He was surprised by the results. Some of the F2 generation plants had white flowers. He studied hundreds of F2 generation plants, and for every three purple-flowered plants, there was an average of one white-flowered plant.
Law of Segregation
Mendel did the same experiment for all seven characteristics. In each case, one value of the characteristic disappeared in the F1 plants, later showing up again in the F2 plants. In each case, 75 per cent of F2 plants had one value of the characteristic, while 25 per cent had the other value. Based on these observations, Mendel formulated his first law of inheritance. This law is called the . It states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces.
Mendel's Second Set of Experiments
Mendel wondered whether different characteristics are inherited together. For example, are purple flowers and tall stems always inherited together, or do these two characteristics show up in different combinations in offspring? To answer these questions, Mendel next investigated two characteristics at a time. For example, he crossed plants with yellow round seeds and plants with green wrinkled seeds. The results of this cross are shown in Figure 5.10.6.
Figure 5.10.6 shows the outcome of a cross between plants that differ in seed colour (yellow or green) and seed form (shown here with a smooth round appearance or wrinkled appearance). The letters R, r, Y, and y represent genes for the characteristics Mendel was studying. Mendel didn’t know about genes, however, because genes would not be discovered until several decades later. This experiment demonstrates that, in the F2 generation, nine out of 16 were round yellow seeds, three out of 16 were wrinkled yellow seeds, three out of 16 were round green seeds, and one out of 16 was wrinkled green seeds.
F1 and F2 Generations
In this set of experiments, Mendel observed that plants in the F1 generation were all alike. All of them had yellow round seeds like one of the two parents. When the F1 generation plants self-pollinated, however, their offspring — the F2 generation — showed all possible combinations of the two characteristics. Some had green round seeds, for example, and some had yellow wrinkled seeds. These combinations of characteristics were not present in the F1 or P generations.
Law of Independent Assortment
Mendel repeated this experiment with other combinations of characteristics, such as flower colour and stem length. Each time, the results were the same as those shown in Figure 5.10.6. The results of Mendel's second set of experiments led to his second law. This is the . It states that factors controlling different characteristics are inherited independently of each other.
Mendel's Legacy
You might think that Mendel's discoveries would have made a big impact on science as soon as he made them, but you would be wrong. Why? Because Mendel's work was largely ignored. Mendel was far ahead of his time, and he was working from a remote monastery. He had no reputation in the scientific community and had only published sparingly in the past. Additionally, he published this research in an obscure scientific journal. As a result, when Charles Darwin published his landmark book on evolution in 1869, although Mendel's work had been published just a few years earlier, Darwin was unaware of it. Consequently, Darwin knew nothing about Mendel's laws, and didn’t understand heredity. This made Darwin's arguments about evolution less convincing to many.
Then, in 1900, three different European scientists — Hugo de DeVries, Carl Correns, and Erich von Tschermak — arrived independently at Mendel's laws. All three had done experiments similar to Mendel's and come to the same conclusions that he had drawn several decades earlier. Only then was Mendel's work rediscovered, so that Mendel himself could be given the credit he was due. Although Mendel knew nothing about genes, which were discovered after his death, he is now considered the father of genetics.
5.10 Cultural Connection
Corn is the world's most produced crop. Canada produces 13,000-14,000 metric Kilo tonnes of corn annually, mostly in fields in Ontario, Quebec and Manitoba. Approximately 1.5 million hectares are devoted to this crop which is critically important for both humans and livestock as a food source. Despite these high numbers of output, Canada is still only 11th on the list of world corn producers, with USA, China and Brazil claiming the top three places. How did corn become such an important part of modern agriculture?
We didn't always have corn as we know it. Modern corn is descended from a type of grass called teosinte (Figure 5.10.7) native to Mesoamerica (southern part of North America). It is estimated that Indigenous people have been harvesting corn and corn ancestors for over 9000 years. Excavations of the Xihuatoxtla Shelter in southwestern Mexico revealed our earliest evidence of domesticated corn: maize remains on tools dating back 8,700 years.
Ancient Indigenous peoples of southern Mexico developed corn from grass plants using a process we now call , also known as . Teosinte doesn't resemble the corn we have today- it had only a few kernels individually encased on very hard shells, and yet today we have multiple varieties of corn with row upon row of bare kernels. This means that ancient agriculturalists among the Indigenous people of Mexico were intentionally cross-breeding strains of teosinte, and later, early maize to create plants which had more kernels, and reduced seed casings. Watch the TED Ed video in the Explore More section to see what other changes agriculturalists have made to modern-day corn.
5.10 Summary
- Mendel experimented with the inheritance of traits in pea plants at a time when the blending theory of inheritance was popular. This is the theory that offspring have a blend of the characteristics of their parents.
- Pea plants were good choices for this research, largely because they have several visible characteristics that exist in two different forms. By controlling pollination, Mendel was able to cross pea plants with different forms of the traits.
- In Mendel's first set of experiments, he experimented with just one characteristic at a time. The results of this set of experiments led to Mendel's first law of inheritance, called the . This law states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces.
- In Mendel's second set of experiments, he experimented with two characteristics at a time. The results of this set of experiments led to Mendel's second law of inheritance, called the . This law states that the factors controlling different characteristics are inherited independently of each other.
- Mendel's work was largely ignored during his own lifetime. However, when other researchers arrived at the same laws in 1900, Mendel's work was rediscovered, and he was given the credit he was due. He is now considered the father of genetics.
5.10 Review Questions
- Why were pea plants a good choice for Mendel's experiments?
- How did the outcome of Mendel's second set of experiments lead to his second law?
- Discuss the development of Mendel's legacy.
- If Mendel’s law of independent assortment was not correct, and characteristics were always inherited together, what types of offspring do you think would have been produced by crossing plants with yellow round seeds and green wrinkled seeds? Explain your answer.
5.10 Explore More
https://www.youtube.com/watch?v=Mehz7tCxjSE&feature=emb_logo
How Mendel's pea plants helped us understand genetics - Hortensia Jiménez Díaz, TED-Ed, 2013.
https://www.youtube.com/watch?v=ogc367xyzfk&feature=emb_logo
10 Strange Hybrid Fruits, Junkyboss, 2016.
https://www.youtube.com/watch?v=i6teBcfKpik
The history of the world according to corn - Chris A. Kniesly, TED-Ed, 2019.
Attributions
Figure 5.10.1
Purple sweet pea flower by unknown on Yana Ray on publicdomainpictures.net is used under a CC0 1.0 public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
Figure 5.10.2
Gregor_Mendel by unknown from National Institutes of Health, Health & Human Services on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 5.10.3
Gregor Mendel in Lego by Alan on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
Figure 5.10.4
Mendels_peas by Mariana Ruiz [LadyofHats] on Wikimedia Commons is used under a CC0 1.0 public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
Figure 5.10.5
Mendel's first experiment with pea plants by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Figure 5.10.6
Mendel's Second Experiment by by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Figure 5.10.7
Maize-teosinte by John Doebley on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
References
Brainard, J/ CK-12 Foundation. (2016). Figure 5 Mendel's first experiment [digital image]. In CK-12 College Human Biology (Section 5.9) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-human-biology/section/5.9/
Brainard, J/ CK-12 Foundation. (2016). Figure 6 Mendel's second experiment [digital image]. In CK-12 College Human Biology (Section 5.9) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-human-biology/section/5.9/
Junkyboss. (2016, March 31). 10 Strange hybrid fruits. YouTube. https://www.youtube.com/watch?v=ogc367xyzfk&feature=youtu.be
TED-Ed. (2013, March 12). How Mendel's pea plants helped us understand genetics - Hortensia Jiménez Díaz. YouTube. https://www.youtube.com/watch?v=Mehz7tCxjSE&feature=youtu.be
TED-Ed. (2019, November 26). The history of the world according to corn - Chris A. Kniesly. YouTube. https://www.youtube.com/watch?v=i6teBcfKpik&feature=youtu.be
Wikipedia contributors. (2020, June 1). Carl Correns. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Carl_Correns&oldid=960172546
Wikipedia contributors. (2020, July 8). Charles Darwin. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Charles_Darwin&oldid=966652322
Wikipedia contributors. (2020, March 9). Erich von Tschermak. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Erich_von_Tschermak&oldid=944695823
Wikipedia contributors. (2020, July 7). Hugo de Vries. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Hugo_de_Vries&oldid=966513671
Created by: CK-12/Adapted by Christine Miller
Like Father, Like Son
This father-son duo share some similarities. The shape of their faces and their facial features look very similar. If you saw them together, you might well guess that they are father and son. People have long known that the characteristics of living things are similar between parents and their offspring. However, it wasn’t until the experiments of Gregor Mendel that scientists understood how those traits are inherited.
The Father of Genetics
Mendel did experiments with pea plants to show how traits such as seed shape and flower colour are inherited. Based on his research, he developed his two well known laws of inheritance: the and the . When Mendel died in 1884, his work was still virtually unknown. In 1900, three other researchers working independently came to the same conclusions that Mendel had drawn almost half a century earlier. Only then was Mendel's work rediscovered.
Mendel knew nothing about genes, because they were discovered after his death. He did think, however, that some type of "factors" controlled traits, and that those "factors" were passed from parents to offspring. We now call these "factors" genes. , now expressed in terms of genes, form the basis of , the science of heredity. For this reason, Mendel is often called the father of genetics.
The Language of Genetics
Today, we know that traits of organisms are controlled by on . To talk about inheritance in terms of genes and chromosomes, you need to know the language of genetics. The terms below serve as a good starting point. They are illustrated in the figure that follows.
- A is the part of a that contains the genetic code for a given . For example, in pea plants, a given gene might code for flower colour.
- The position of a given gene on a chromosome is called its (plural, loci). A gene might be located near the center, or at one end or the other of a .
- A given gene may have different normal versions, which are called . For example, in pea plants, there is a purple-flower allele (B) and a white-flower allele (b) for the flower-colour gene. Different alleles account for much of the variation in the traits of organisms, including people.
- In sexually reproducing organisms, each individual has two copies of each type of chromosome. Paired chromosomes of the same type are called . They are about the same size and shape, and they have all the same genes at the same loci.
Genotype
When occurs, sex cells (called ) unite during to form a single cell called a . The zygote inherits two of each type of chromosome, with one chromosome of each type coming from the father, and the other coming from the mother. Because have the same genes at the same loci, each individual also inherits two copies of each gene. The two copies may be the same allele or different . The alleles an individual inherits for a given gene make up the individual’s . As shown in Table 5.11.1, an organism with two of the same allele (for example, BB or bb) is called a . An organism with two different alleles (in this example, Bb) is called a .
Table 5.11.1
Allele Combinations Associated With the Terms Homozygous and Heterozygous
Phenotype
The expression of an organism’s genotype is referred to as its , and it refers to the organism’s traits, such as purple or white flowers in pea plants. As you can see from Table 5.11.1, different genotypes may produce the same phenotype. In this example, both BB and Bb genotypes produce plants with the same phenotype, purple flowers. Why does this happen? In a Bb heterozygote, only the B allele is expressed, so the b allele doesn’t influence the phenotype. In general, when only one of two alleles is expressed in the phenotype, the expressed allele is called , and the allele that isn’t expressed is called .
The terms dominant and recessive may also be used to refer to phenotypic traits. For example, purple flower colour in pea plants is a dominant trait. It shows up in the phenotype whenever a plant inherits even one dominant allele for the trait. Similarly, white flower colour is a recessive trait. Like other recessive traits, it shows up in the phenotype only when a plant inherits two recessive alleles for the trait.
5.11 Summary
- Mendel's laws of inheritance, now expressed in terms of , form the basis of genetics, which is the science of heredity. This is why Mendel is often called the father of genetics.
- A gene is the part of a that codes for a given . The position of a gene on a chromosome is its . A given gene may have different versions, called . Paired chromosomes of the same type are called . They have the same size and shape, and they have the same genes at the same .
- The alleles an individual inherits for a given gene make up the individual's . An organism with two of the same allele is called a homozygote, and an individual with two different alleles is called a heterozygote.
- The expression of an organism's genotype is referred to as its . A dominant allele is always expressed in the phenotype, even when just one dominant allele has been inherited. A recessive allele is expressed in the phenotype only when two recessive alleles have been inherited.
5.11 Review Questions
- Define genetics.
- Why is Gregor Mendel called the father of genetics if genes were not discovered until after his death?
- Imagine that there are two alleles, R and r, for a given gene. R is dominant to r. Answer the following questions about this gene:
- What are the possible homozygous and heterozygous genotypes?
- Which genotype or genotypes express the dominant R phenotype? Explain your answer.
- Are R and r on different loci? Why or why not?
- Can R and r be on the same exact chromosome? Why or why not? If not, where are they located?
5.11 Explore More
https://www.youtube.com/watch?v=pv3Kj0UjiLE
Alleles and Genes, Amoeba Sisters, 2018.
https://www.youtube.com/watch?v=OaovnS7BAoc
Genotypes and Phenotypes, Bozeman Science, 2011.
Attributions
Figure 5.11.1
Father holding his baby boy with matching haircut [photo] by Kelly Sikkema on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 5.11.2
Chromosome, Gene, Locus, and Allele by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Table 5.11.1
Allele Combinations Associated With the Terms Homozygous and Heterozygous by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Amoeba Sisters. (2018, February 1). Alleles and genes. YouTube. https://www.youtube.com/watch?v=pv3Kj0UjiLE&feature=youtu.be
Bozeman Science. (2011, August 4). Genotypes and phenotypes. YouTube. https://www.youtube.com/watch?v=OaovnS7BAoc&feature=youtu.be
Brainard, J/ CK-12 Foundation. (2016). Figure 2 Chromosome, gene, locus, and allele [digital image]. In CK-12 College Human Biology (Section 5.10) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-human-biology/section/5.9/
Goose Bumps
No doubt you’ve experienced the tiny, hair-raising skin bumps called goose bumps, like those you see in Figure 10.4.1. They happen when you feel chilly. Do you know what causes goose bumps, or why they pop up when you are cold? The answers to these questions involve the layer of skin known as the dermis.
What is the Dermis?
The is the inner of the two major layers that make up the skin, the outer layer being the . The dermis consists mainly of . It also contains most skin structures, such as and blood vessels. The dermis is anchored to the tissues below it by flexible collagen bundles that permit most areas of the skin to move freely over subcutaneous (“below the skin”) tissues. Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes.
Anatomy of the Dermis
The basic anatomy of the dermis is a matrix, or sort of scaffolding, composed of connective tissues. These tissues include collagen fibres — which provide toughness — and elastin fibres, which provide elasticity. Surrounding these fibres, the matrix also includes a gel-like substance made of proteins. The tissues of the matrix give the dermis both strength and flexibility.
The dermis is divided into two layers: the papillary layer and the reticular layer. Both layers are shown in Figure 10.4.2 below and described in the text that follows.
Papillary Layer
The is the upper layer of the dermis, just below the that connects the dermis to the epidermis above it. The papillary layer is the thinner of the two dermal layers. It is composed mainly of loosely arranged collagen fibres. The papillary layer is named for its fingerlike projections — or papillae — that extend upward into the epidermis. The papillae contain capillaries and sensory touch receptors.
The papillae give the dermis a bumpy surface that interlocks with the epidermis above it, strengthening the connection between the two layers of skin. On the palms and soles, the papillae create epidermal ridges. Epidermal ridges on the fingers are commonly called fingerprints (see Figure 10.4.3). Fingerprints are genetically determined, so no two people (other than identical twins) have exactly the same fingerprint pattern. Therefore, fingerprints can be used as a means of identification, for example, at crime scenes. Fingerprints were much more commonly used forensically before DNA analysis was introduced for this purpose.
Reticular Layer
The is the lower layer of the dermis, located below the papillary layer. It is the thicker of the two dermal layers. It is composed of densely woven collagen and elastin fibres. These protein fibres give the dermis its properties of strength and elasticity. This layer of the dermis cushions subcutaneous tissues of the body from stress and strain. The reticular layer of the dermis also contains most of the structures in the dermis, such as and hair .
Structures in the Dermis
Both papillary and reticular layers of the dermis contain numerous , which make the skin the body’s primary sensory organ for the sense of . Both dermal layers also contain blood vessels. They provide nutrients to remove wastes from dermal cells, as well as cells in the lowest layer of the epidermis, the . The circulatory components of the dermis are shown in Figure 10.4.4 below.
Glands
Glands in the reticular layer of the dermis include sweat glands and sebaceous (oil) glands. Both are exocrine glands, which are glands that release their secretions through ducts to nearby body surfaces. The diagram in Figure 10.4.5 shows these glands, as well as several other structures in the dermis.
Sweat Glands
produce the fluid called sweat, which contains mainly water and salts. The glands have ducts that carry the sweat to hair follicles, or to the surface of the skin. There are two different types of sweat glands: eccrine glands and apocrine glands.
- occur in skin all over the body. Their ducts empty through tiny openings called pores onto the skin surface. These sweat glands are involved in temperature regulation.
- are larger than eccrine glands, and occur only in the skin of the armpits and groin. The ducts of apocrine glands empty into hair follicles, and then the sweat travels along hairs to reach the surface. Apocrine glands are inactive until puberty, at which point they start producing an oily sweat that is consumed by bacteria living on the skin. The digestion of apocrine sweat by bacteria causes body odor.
Sebaceous Glands
s are exocrine glands that produce a thick, fatty substance called sebum. is secreted into hair follicles and makes its way to the skin surface along hairs. It waterproofs the hair and skin, and helps prevent them from drying out. Sebum also has antibacterial properties, so it inhibits the growth of microorganisms on the skin. Sebaceous glands are found in every part of the skin — except for the palms of the hands and soles of the feet, where hair does not grow.
Hair Follicles
s are the structures where hairs originate (see the diagram above). Hairs grow out of follicles, pass through the epidermis, and exit at the surface of the skin. Associated with each hair follicle is a sebaceous gland, which secretes sebum that coats and waterproofs the hair. Each follicle also has a bed of capillaries, a nerve ending, and a tiny muscle called an .
Functions of the Dermis
The main functions of the dermis are regulating body temperature, enabling the sense of touch, and eliminating wastes from the body.
Temperature Regulation
Several structures in the reticular layer of the dermis are involved in regulating body temperature. For example, when body temperature rises, the of the sends nerve signals to sweat glands, causing them to release sweat. An adult can sweat up to four litres an hour. As the sweat evaporates from the surface of the body, it uses energy in the form of body heat, thus cooling the body. The hypothalamus also causes dilation of blood vessels in the dermis when body temperature rises. This allows more blood to flow through the skin, bringing body heat to the surface, where it can radiate into the environment.
When the body is too cool, sweat glands stop producing sweat, and blood vessels in the skin constrict, thus conserving body heat. The arrector pili muscles also contract, moving hair follicles and lifting hair shafts. This results in more air being trapped under the hairs to insulate the surface of the skin. These contractions of arrector pili muscles are the cause of goose bumps.
Sensing the Environment
Sensory receptors in the dermis are mainly responsible for the body’s tactile senses. The receptors detect such tactile stimuli as warm or cold temperature, shape, texture, pressure, vibration, and pain. They send nerve impulses to the brain, which interprets and responds to the sensory information. Sensory receptors in the dermis can be classified on the basis of the type of touch stimulus they sense. s sense mechanical forces such as pressure, roughness, vibration, and stretching. s sense variations in temperature that are above or below body temperature. s sense painful stimuli. Figure 10.4.6 shows several specific kinds of tactile receptors in the dermis. Each kind of receptor senses one or more types of touch stimuli.
- Free nerve endings sense pain and temperature variations.
- Merkel cells sense light touch, shapes, and textures.
- Meissner’s corpuscles sense light touch.
- Pacinian corpuscles sense pressure and vibration.
- Ruffini corpuscles sense stretching and sustained pressure.
Excreting Wastes
The sweat released by is one way the body excretes waste products. Sweat contains excess water, salts (electrolytes), and other waste products that the body must get rid of to maintain . The most common electrolytes in sweat are sodium and chloride. Potassium, calcium, and magnesium electrolytes may be excreted in sweat, as well. When these electrolytes reach high levels in the blood, more are excreted in sweat. This helps to bring their blood levels back into balance. Besides electrolytes, sweat contains small amounts of waste products from , including ammonia and urea. Sweat may also contain alcohol in someone who has been drinking alcoholic beverages.
Feature: My Human Body
is the most common skin disorder in the Canada. At least 20% of Canadians have acne at any given time and it affects approximately 90% of adolescents (as in Figure 10.4.7). Although acne occurs most commonly in teens and young adults, but it can occur at any age. Even newborn babies can get acne.
The main sign of acne is the appearance of pimples (pustules) on the skin, like those in the photo above. Other signs of acne may include whiteheads, blackheads, nodules, and other lesions. Besides the face, acne can appear on the back, chest, neck, shoulders, upper arms, and buttocks. Acne can permanently scar the skin, especially if it isn’t treated appropriately. Besides its physical effects on the skin, acne can also lead to low self-esteem and depression.
Acne is caused by clogged, sebum-filled pores that provide a perfect environment for the growth of bacteria. The bacteria cause infection, and the immune system responds with inflammation. Inflammation, in turn, causes swelling and redness, and may be associated with the formation of pus. If the inflammation goes deep into the skin, it may form an acne nodule.
Mild acne often responds well to treatment with over-the-counter (OTC) products containing benzoyl peroxide or salicylic acid. Treatment with these products may take a month or two to clear up the acne. Once the skin clears, treatment generally needs to continue for some time to prevent future breakouts.
If acne fails to respond to OTC products, nodules develop, or acne is affecting self-esteem, a visit to a dermatologist is in order. A dermatologist can determine which treatment is best for a given patient. A dermatologist can also prescribe prescription medications (which are likely to be more effective than OTC products) and provide other medical treatments, such as laser light therapies or chemical peels.
What can you do to maintain healthy skin and prevent or reduce acne? Dermatologists recommend the following tips:
- Wash affected or acne-prone skin (such as the face) twice a day, and after sweating.
- Use your fingertips to apply a gentle, non-abrasive cleanser. Avoid scrubbing, which can make acne worse.
- Use only alcohol-free products and avoid any products that irritate the skin, such as harsh astringents or exfoliants.
- Rinse with lukewarm water, and avoid using very hot or cold water.
- Shampoo your hair regularly.
- Do not pick, pop, or squeeze acne. If you do, it will take longer to heal and is more likely to scar.
- Keep your hands off your face. Avoid touching your skin throughout the day.
- Stay out of the sun and tanning beds. Some acne medications make your skin very sensitive to UV light.
10.4 Summary
- The is the inner and thicker of the two major layers that make up the skin. It consists mainly of a matrix of s that provide strength and stretch. It also contains almost all skin structures, including and s.
- The dermis has two layers. The upper has papillae extending upward into the epidermis and loose connective tissues. The lower has denser connective tissues and structures, such as glands and hair follicles. Glands in the dermis include eccrine and apocrine sweat glands and sebaceous glands. Hair are structures where hairs originate.
- Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes. The dense connective tissues of the dermis provide cushioning. The dermis regulates body temperature mainly by sweating and by vasodilation or vasoconstriction. The many tactile sensory receptors in the dermis make it the main organ for the sense of touch. Wastes excreted in sweat include excess water, electrolytes, and certain metabolic wastes.
10.4 Review Questions
- What is the dermis?
- Describe the basic anatomy of the dermis.
- Compare and contrast the papillary and reticular layers of the dermis.
- What causes epidermal ridges, and why can they be used to identify individuals?
- Name the two types of sweat glands in the dermis, and explain how they differ.
- What is the function of sebaceous glands?
- Describe the structures associated with hair follicles.
- Explain how the dermis helps regulate body temperature.
- Identify three specific kinds of tactile receptors in the dermis, along with the type of stimuli they sense.
- How does the dermis excrete wastes? What waste products does it excrete?
- What are subcutaneous tissues? Which layer of the dermis provides cushioning for subcutaneous tissues? Why does this layer provide most of the cushioning, instead of the other layer?
- For each of the functions listed below, describe which structure within the dermis carries it out.
- Brings nutrients to and removes wastes from dermal and lower epidermal cells
- Causes hairs to move
- Detects painful stimuli on the skin
10.4 Explore More
https://www.youtube.com/watch?v=FX-FwK0IIrE
How do you get rid of acne? SciShow, 2016.
https://www.youtube.com/watch?v=VcHQWMAClhQ&feature=emb_logo
When You Can't Scratch Away An Itch, Seeker, 2013.
Attributions
Figure 10.4.1
Goose_bumps by EverJean on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 10.4.2
Layers_of_the_Dermis by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.3
Fingerprint_detail_on_male_finger_in_Třebíč,_Třebíč_District by Frettie on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.4
Blausen_0802_Skin_Dermal Circulation by BruceBlaus on Wikimedia commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.5
Anatomy_The_Skin_-_NCI_Visuals_Online by Don Bliss (artist) / National Cancer Institute (National Institutes of Health, with the ID 4604) is in the public domain (https://en.wikipedia.org/wiki/public_domain).
Figure 10.4.6
Blausen_0809_Skin_TactileReceptors by BruceBlaus on Wikimedia commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.7
Akne-jugend by Ellywa on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/public_domain). (No machine-readable author provided. Ellywa assumed, based on copyright claims).
References
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 5.7 Layers of the dermis [digital image]. In Anatomy and Physiology (Section 5.1 Layers of the skin). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/5-1-layers-of-the-skin
Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
SciShow. (2016, October 26). How do you get rid of acne? YouTube. https://www.youtube.com/watch?v=FX-FwK0IIrE
Seeker. (2013, October 26). When you can't scratch away an itch. YouTube. https://www.youtube.com/watch?v=VcHQWMAClhQ&feature=emb_logo
Created by: CK-12/Adapted by Christine Miller
Albinism
This child has much lighter skin and hair than his parents. He has a condition called albinism, which results from a lack of the pigment melanin in the skin, hair, and eyes. Although he looks different than his parents, albinism is actually a genetic trait. Genetic traits are characteristics that are encoded in DNA. Most forms of albinism are recessive, which is why the child's parents were able to pass the trait to him without exhibiting the condition themselves. You will learn more about this type of inheritance in this concept. Albinism is one of the few human traits that actually has a simple inheritance pattern, similar to the traits that Gregor Mendel studied in pea plants. The way these traits are inherited by offspring from their parents is called .
What Is Mendelian Inheritance?
Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two , one of which may be completely to the other. The pattern of inheritance of Mendelian traits depends on whether the traits are controlled by genes on s, or by genes on .
- Autosomal traits are controlled by genes on one of the 22 pairs of human . Autosomes are all the chromosomes except the X or Y chromosome, and they do not differ between males and females, so autosomal traits are inherited in the same way, regardless of the sex of the parent or offspring.
- Traits controlled by genes on the sex chromosomes are called . Because of the small size of the Y chromosome, most sex-linked traits are controlled by genes on the X chromosome. These traits are called . Single-gene X-linked traits have a different pattern of inheritance than single-gene autosomal traits, because males have just one X chromosome. In addition, males always inherit their X chromosome from their mother, and they pass on their X chromosome to all of their daughters, but none of their sons.
Studying Inheritance Patterns
There are two very useful tools for studying how traits are passed from one generation to the next. One tool is a pedigree, and the other is a Punnett square.
Pedigree
The chart below (Figure 5.13.2) is called a . A pedigree shows how a trait is passed from generation to generation within a family. A pedigree can show, for example, whether a Mendelian trait is an or trait. It can also be used to infer the genotype of different members of the family.
The trait represented by this chart is a hypothetical autosomal trait controlled by a dominant allele. At the top of the pedigree, you can see symbols representing a married couple. The husband has the trait (affected male), but the wife does not (unaffected female). The next row of the pedigree shows the couple's children, as well as the spouses of three of the children. For example, the first child on the left is an affected male married to an unaffected female. The third row of the pedigree shows the next generation (the grandchildren of the couple at the top of the pedigree). One child in this generation — the affected female on the left — is the sibling of an unaffected male.
Punnett Square
A is a chart that allows you to easily determine the expected ratios of possible genotypes in the offspring of two parents. You can see a hypothetical example in Figure 5.13.3. In this case, the gene is autosomal, and both parents are heterozygotes (Aa) for the gene. Half of the gametes produced by each parent will have the A allele, and half will have the a allele. That's because the two alleles are on homologous chromosomes, which always separate and go to separate gametes during meiosis. The alleles in the gametes from each parent are written down the side and across the top of the Punnett square. Filling in the cells of the Punnett square gives the possible genotypes of their children. It also shows the most likely ratios of the genotypes, which in this case is 25 per cent AA, 50 per cent Aa, and 25 per cent aa.
A Punnett square can also be used to show how the X and Y chromosomes are passed from parents to their children. This is illustrated in the Punnett square in Figure 5.13.4. It may help you understand the inheritance pattern of sex-linked traits.
Here's how to fill out a Punnett Square:
https://www.youtube.com/watch?v=prkHKjfUmMs&t=
Learn Biology: How to Draw a Punnett Square, mahalodotcom, 2011.
Try out this Punnett Square:
Autosomal Mendelian Traits in Humans
Not many human autosomal traits are controlled by a single gene with two alleles, but they are a good starting point for understanding human heredity. As discussed in the beginning of this concept, most forms of albinism in humans have a Mendelian inheritance pattern. Albinism is usually controlled by a single autosomal gene with two alleles. The allele for normal pigmentation (let's call it R) is dominant to the allele for albinism (r). Individuals with either an RR or Rr genotype will not have albinism, because the R allele is dominant over the recessive r allele.
However, consider what happens if two individuals with the Rr genotype reproduce with each other. The outcome would be similar to the example shown in the Punnett square above for two hypothetical Aa individuals. Their possible offspring could be RR (normal pigmentation), Rr (normal pigmentation), or rr (albinism). This explains why a child with albinism (rr), can have two parents that do not have albinism. Both unaffected parents must be a carrier of the recessive r allele, but they also have a dominant R allele that prevents them from having the condition themselves.
Some other human traits that have a Mendelian inheritance pattern are Huntington's disease and wet versus dry ear wax. You may have heard about other human traits that were previously thought to be Mendelian, such as dimples, a widow's peak hairline, hitchiker's thumb, and the ability to roll your tongue. As science has progressed, it is now understood that these are not actually Mendelian traits. In fact, most human traits are actually controlled by multiple genes, or otherwise have more than two alleles, which means they do not have a simple Mendelian inheritance pattern.
X-Linked Mendelian Traits in Humans
Because males have just one X chromosome, they have only one allele for any X-linked trait. Therefore, a recessive X-linked allele is always expressed in males. Because females have two X chromosomes, they have two alleles for any X-linked trait. Therefore, they must inherit two copies of the recessive allele to express an X-linked recessive trait. This explains why X-linked recessive traits are less common in females than males, and why they show a different pattern of inheritance than autosomal traits.
An example of a recessive X-linked trait is red-green colour blindness. People with this trait cannot distinguish between the colours red and green. More than one recessive gene on the X chromosome codes for this trait, which is fairly common in males, but relatively rare in females. Figure 5.13.6 shows a simple pedigree for this trait. A female with one of the recessive alleles for the trait does not have the trait herself, but can pass it on to her children. In this case, she is called a carrier of the trait. Half of any sons she has can be expected to have the trait, because there is a 50 per cent chance that they will inherit the X chromosome with the colour-blindness allele. Having only one X chromosome, the recessive allele will be expressed in the sons who inherit it. However, as long as the father is not affected, none of the woman's daughters will have the trait. The daughters have a 50 per cent chance of inheriting the X chromosome with the colour-blindness allele, but it won't be expressed because it is recessive to the normal allele they inherit from their father.
Another example of a recessive X-linked Mendelian trait is hemophilia, which is a disorder characterized by the blood's inability to clot normally. England's Queen Victoria (pictured in Figure 5.13.7) carried the disorder. Two of her five daughters inherited the hemophilia allele from their mother and were carriers. When they married royalty in other European countries, they spread the allele across Europe, including to the royal families of Spain, Germany, and Russia. Victoria's son Prince Leopold also inherited the hemophilia allele from his mother, and he actually suffered from the disease. Understandably, hemophilia was once popularly called "the royal disease."
Feature: My Human Body.
Are you colour blind, or do you think you might be? If you inherited this X-linked recessive disorder, a world without clear differences between certain colours seems normal to you. It's all you have ever known! Some people who are colour blind are not even aware of it. Simple tests have been devised to determine whether a person is colour blind, and to what degree. An example of such a test is pictured in Figure 5.13.5. What do you see when you look at this circle? Can you clearly perceive the number 74? If so, you probably have normal red-green colour vision. If you cannot see the number, you may have red-green colour blindness.
Being colour blind can cause a number of problems. These may range from minor frustrations to outright dangers. Here are a few examples:
- If you are colour blind, it may be difficult to colour-coordinate clothing and furnishings. You may end up wearing colour combinations that people with normal colour vision think are odd or clashing.
- Many LED indicator lights are red or green. Power strips and electronic devices, for example, may have indicator lights to show whether they are on (green) or off (red).
- Test strips for pH, hard water, swimming pool chemicals, and other common tests are also often colour coded. Litmus paper for testing pH, for example, turns red in the presence of an acid, but if you are colour blind, you may not be able to read the test result.
- Do you like your steak well done? If you are colour blind, you may not be able to tell if the meat is still undercooked (red) or grilled just right. You also may not be able to distinguish ripe (red) from unripe (green) fruits and vegetables, such as tomatoes. And some foods, such as dark green spinach, may look more like mud than food, making them totally unappetizing.
- Weather maps often are colour coded. Is that rain (green) in your forecast, or a wintry mix of sleet and freezing rain (pink or red)? If you can't tell the difference, you may go out on the roads when you shouldn't, putting yourself in danger.
- Being able to distinguish red from green traffic lights may be a matter of life or death. This can be very difficult for someone with red-green colour blindness. In some countries, people with this vision defect are not allowed to drive.
Figure 5.13.9 Examples of challenges faced by those who are colour blind
5.13 Summary
- refers to the inheritance of traits controlled by a single with two , one of which may be completely dominant to the other. The pattern of inheritance of Mendelian traits depends on whether the traits are controlled by genes on , or by genes on .
- Two tools for studying inheritance are and . A pedigree is a chart that shows how a trait is passed from generation to generation within a family. A Punnett square is a chart that shows the expected ratios of possible in the offspring of two parents.
- Examples of human autosomal Mendelian traits include albinism and Huntington's disease. Examples of human X-linked traits include red-green colour blindness and hemophilia.
5.13 Review Questions
- Define genetic traits and Mendelian inheritance.
- Explain why autosomal and X-linked Mendelian traits have different patterns of inheritance.
- Identify examples of human autosomal and X-linked Mendelian traits.
- Imagine a hypothetical human gene that has two alleles,Qand q. Q is dominant to q and the inheritance of this gene is Mendelian. Answer the following questions about this gene.
- If a woman has the genotype Q q and her husband has the genotype QQ, list each of their possible gametes. What proportion of their gametes will have each allele?
- What are the likely proportions of their offspring beingQQ,Qq, or qq?
- Is this an autosomal trait or an X-linked trait? How do you know?
- What are the chances of their offspring exhibiting the dominant Q trait? Explain your answer.
- Explain why fathers always pass their X chromosome down to their daughters.
5.13 Explore More
https://www.youtube.com/watch?v=h2xufrHWG3E&feature=emb_logo
Punnett Squares and Sex-Linked Traits, Amoeba Sisters, 2015.
https://www.youtube.com/watch?v=Gd09V2AkZv4
Pedigrees, Amoeba Sisters, 2017.
https://www.youtube.com/watch?v=veB31XmUQm8
Secrets of the X chromosome - Robin Ball, TED-Ed, 2017.
Attributions
Figure 5.13.1
Albino_baby_by_Felipe_Fernandes by Felipe Fernandes on Wikimedia Commons is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0) license.
Figure 5.13.2
Pedigree_Chart2.svg by Jerome Walker on Wikimedia Commons is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license. (Derivative work of original image created by GAThrawn22)
Figure 5.13.3
Punnett square by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 5.13.4
Inheritance of Sex Chromosomes by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Figure 5.13.5
Red-Green Colour Blindness by Unknown author on Wikimedia is released into the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original believed to be by Shinobu Ishihara)
Figure 5.13.6
XlinkRecessive by US National Institutes of Health on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 5.13.7
The_Young_Queen_Victoria (painted portrait by Franz Xaver Winterhalter (photograph from the Osborne House, Isle of Wight) on Wikimedia Commoons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 5.13.8
Deuteranopia/ Figure 9.10 by Web Style Guide on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
Figure 5.13.9
- LED traffic light by Bidgee on Wikimedia is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
- Universal indicator paper by Bordercolliez on Wikimedia is used under a CC0 1.0
Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en) - Photo tags: Fruit tomatoes food fruits and vegetables fresh by InspiredImages on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).
- Simple Control with Indicator LIghts by Sam D. Wilbur on Wikimedia is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
- Photo tags: Feet socks checkered striped pants colorful color by Suppenkasper on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).
References
Amoeba Sisters. (2015, January 23). Punnett squares and sex-linked traits. YouTube. https://www.youtube.com/watch?v=h2xufrHWG3E&feature=youtu.be
Amoeba Sisters. (2017, February 8). Pedigrees. YouTube. https://www.youtube.com/watch?v=Gd09V2AkZv4&feature=youtu.be
Brainard, J/ CK-12 Foundation. (2016). Figure 3 Inherited traits of sex chromosomes [digital image]. In CK-12 College Human Biology (Section 5.12) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/5.12/
mahalodotcom. (2011, January 14). Learn biology: How to draw a punnett square. YouTube. https://www.youtube.com/watch?v=prkHKjfUmMs&feature=youtu.be
TED-Ed. (2017, April 18). Secrets of the X chromosome - Robin Ball. YouTube. https://www.youtube.com/watch?v=veB31XmUQm8&feature=youtu.be
Wikipedia contributors. (2020, June 26). Albinism in humans. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Albinism_in_humans&oldid=964622728
Wikipedia contributors. (2020, June 29). Gregor Mendel. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Gregor_Mendel&oldid=965090119
Wikipedia contributors. (2020, June 17). Ishihara test. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Ishihara_test&oldid=963014774
Created by: CK-12/Adapted by Christine Miller
Figure 5.14.1 Collage of Diverse Faces.
This collage shows some of the variation in human skin colour, which can range from very light to very dark, with every possible gradation in between. As you might expect, the skin color trait has a more complex genetic basis than just one gene with two alleles, which is the type of simple trait that Mendel studied in pea plants. Like skin color, many other human traits have more complicated modes of inheritance than Mendelian traits. Such modes of inheritance are called , and they include inheritance of multiple allele traits, traits with codominance or incomplete dominance, and polygenic traits, among others. All of these modes are described below.
Multiple Allele Traits
The majority of human genes are thought to have more than two normal versions, or alleles. Traits controlled by a single gene with more than two alleles are called . An example is ABO blood type. Your blood type refers to which of certain proteins called antigens are found on your red blood cells. There are three common alleles for this trait, which are represented by the letters A, B, and O.
As shown in the table there are six possible ABO genotypes, because the three alleles, taken two at a time, result in six possible combinations. The A and B alleles are dominant to the O allele. As a result, both AA and AO genotypes have the same phenotype, with the A antigen in their blood (type A blood). Similarly, both BB and BO genotypes have the same phenotype, with the B antigen in their blood (type B blood). No antigen is associated with the O allele, so people with the OO genotype have no antigens for ABO blood type in their blood (type O blood).
Codominance
Look at the genotype AB in the ABO blood group table. Alleles A and B for ABO blood type are neither nor to one another. Instead, they are codominant. occurs when two alleles for a gene are expressed equally in the phenotype of . In the case of ABO blood type, AB heterozygotes have a unique phenotype, with both A and B antigens in their blood (type AB blood).
Incomplete Dominance
Another relationship that may occur between alleles for the same gene is . This occurs when the dominant allele is not completely dominant. In this case, an intermediate phenotype results in heterozygotes who inherit both alleles. Generally, this happens when the two alleles for a given gene both produce proteins, but one protein is not functional. As a result, the heterozygote individual produces only half the amount of normal protein as is produced by an individual who is homozygous for the normal allele.
An example of incomplete dominance in humans is Tay Sachs disease. The normal allele for the gene in this case produces an that is responsible for breaking down . A defective allele for the gene results in the production of a nonfunctional enzyme. Heterozygotes who have one normal and one defective allele produce half as much functional enzyme as the normal homozygote, and this is enough for normal development. Homozygotes who have only defective allele, however, produce only nonfunctional enzyme. This leads to the accumulation of lipids in the brain starting in utero, which causes significant brain damage. Most individuals with Tay Sachs disease die at a young age, typically by the age of five years.
Another good example of incomplete dominance in humans is hair type. There are genes for straight and curly hair, and if an individual is heterozygous, they will typically have the phenotype of wavy hair.
Polygenic Traits
Many human traits are controlled by more than one gene. These traits are called . The alleles of each gene have a minor additive effect on the phenotype. There are many possible combinations of alleles, especially if each gene has multiple alleles. Therefore, a whole continuum of phenotypes is possible.
An example of a human polygenic trait is adult height. Several genes, each with more than one allele, contribute to this trait, so there are many possible adult heights. One adult’s height might be 1.655 m (5.430 feet), and another adult’s height might be 1.656 m (5.433 feet). Adult height ranges from less than 5 feet to more than 6 feet, with males, on average, being somewhat taller than females. The majority of people fall near the middle of the range of heights for their sex, as shown in Figure 5.14.4.
Environmental Effects on Phenotype
Many traits are affected by the environment, as well as by genes. This may be especially true for polygenic traits. Adult height, for example, might be negatively impacted by poor diet or childhood illness. Skin color is another polygenic trait. There is a wide range of skin colors in people worldwide. In addition to differences in genes, differences in exposure to ultraviolet (UV) light cause some variation. As shown in Figure 5.14.5, exposure to UV light darkens the skin.
Pleiotropy
Some genes affect more than one phenotypic trait. This is called . There are numerous examples of pleiotropy in humans. They generally involve important proteins that are needed for the normal development or functioning of more than one organ system. An example of pleiotropy in humans occurs with the gene that codes for the main protein in collagen, a substance that helps form bones. This protein is also important in the ears and eyes. Mutations in the gene result in problems not only in bones, but also in these sensory organs, which is how the gene's pleiotropic effects were discovered.
Another example of pleiotropy occurs with sickle cell anemia. This recessive genetic disorder occurs when there is a mutation in the gene that normally encodes the red blood cell called hemoglobin. People with the disorder have two alleles for sickle cell hemoglobin, so named for the sickle shape (pictured in Figure 5.14.6) that their red blood cells take on under certain conditions (like physical exertion). The sickle-shaped red blood cells clog small blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain bone deformities, kidney failure, and strokes.
Epistasis
Some genes affect the expression of other genes. This is called . Epistasis is similar to dominance, except that it occurs between different genes, rather than between different alleles for the same gene.
Albinism is an example of epistasis. A person with albinism has virtually no pigment in the skin. The condition occurs due to an entirely different gene than the genes that encode skin color. Albinism occurs because a protein called tyrosinase, which is needed for the production of normal skin pigment, is not produced, due to a gene . If an individual has the albinism mutation, he or she will not have any skin pigment, regardless of the skin color genes that were inherited.
Feature: My Human Body
Do you know your ABO blood type? In an emergency, knowing this valuable piece of information could possibly save your life. If you ever need a blood transfusion, it is vital that you receive blood that matches your own blood type. Why? If the blood transfused into your body contains an antigen that your own blood does not contain, antibodies in your blood plasma (the liquid part of your blood) will recognize the antigen as foreign to your body and cause a reaction called agglutination. In this reaction, the transfused red blood cells will clump together. The agglutination reaction is serious and potentially fatal.
Knowing the antigens and antibodies present in each of the ABO blood types will help you understand which type(s) of blood you can safely receive if you ever need a transfusion. This information is shown in Figure 5.14.7 for all of the ABO blood types. If you have blood type A, this means that your red blood cells have the A antigen and that your blood plasma contains anti-B antibodies. If you were to receive a transfusion of type B or type AB blood, both of which have the B antigen, your anti-B antibodies would attack the transfused red blood cells, causing agglutination.
You may have heard that people with blood type O are called "universal donors," and that people with blood type AB are called universal recipients. People with type O blood have neither A nor B antigens in their blood, so if their blood is transfused into someone with a different ABO blood type, it causes no immune reaction, meaning they can donate blood to anyone. On the other hand, people with type AB blood have no anti-A or anti-B antibodies in their blood, so they can receive a transfusion of blood from anyone. Which blood type(s) can safely receive a transfusion of type AB blood, and which blood type(s) can be safely received by those with type O blood?
5.14 Summary
- refers to the inheritance of traits that have a more complex genetic basis than one with two and complete .
- Multiple allele traits are controlled by a single gene with more than two alleles. An example of a human multiple allele trait is ABO blood type, for which there are three common alleles: A, B, and O.
- occurs when two alleles for a gene are expressed equally in the of heterozygotes. A human example of codominance also occurs in the ABO blood type, in which the A and B alleles are codominant.
- is the case in which the dominant allele for a gene is not completely dominant to a recessive allele for the gene, so an intermediate phenotype occurs in who inherit both alleles. A human example of incomplete dominance is Tay Sachs disease, in which heterozygotes produce half as much functional enzyme as normal homozygotes.
- are controlled by more than one gene, each of which has a minor additive effect on the phenotype. This results in a whole continuum of phenotypes. Examples of human polygenic traits include skin color and adult height.
- Many traits are affected by the environment, as well as by genes. This may be especially true for polygenic traits. Skin color, for example, may be affected by exposure to UV light, and adult stature may be affected by diet or childhood disease.
- refers to the situation in which a gene affects more than one phenotypic trait. A human example of pleiotropy occurs with sickle cell anemia. People who inherit two alleles for this disorder have abnormal red blood cells and may exhibit multiple other phenotypic effects, such as stunting of physical growth, kidney failure, and strokes.
- is the situation in which one gene affects the expression of other genes. An example of epistasis is albinism, in which the albinism mutation negates the expression of skin color genes.
5.14 Review Questions
- What is non-Mendelian inheritance?
- Explain why the human ABO blood group is an example of a multiple allele trait with codominance.
- What is incomplete dominance? Give an example of this type of non-Mendelian inheritance in humans.
- Explain the genetic basis of human skin color.
- How can the human trait of adult height be influenced by the environment?
- Define pleiotropy, and give a human example.
- Compare and contrast epistasis and dominance.
- What is the difference between pleiotropy and epistasis?
5.14 Explore More
https://www.youtube.com/watch?time_continue=1&v=YJHGfbW55l0&feature=emb_logo
Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!,
Amoeba Sisters, 2015.
https://www.youtube.com/watch?v=-4vsio8TZrU&feature=emb_logo
Non-Mendelian Genetics, Teacher's Pet, 2015.
Attributes
Figure 5.14.1
- Woman's Face from Iran by Omid Armin on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Woman Wearing Black Coat by Anastasiya Pavlova on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Dark haired man, Queretaro, México by Leonel Hernandez Arteaga on Unsplash is used under the Unsplash License (https://unsplash.com/license). <not found on Unsplash>
- Man in White V-Neck T-Shirt (self) by Joseph Gonzalez on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Natural Redhead in Brazil by Gabriel Silvério on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Dark-Skinned Woman with Large White Rose by Oladimeji Oduns on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 5.14.2
ABO Blood Types Per Genotype by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 5.14.3
Three Phenotypes of Hair Based on Inheritance/ Incomplete Dominance Hair by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 5.14.4
Average height /Human Adult Height by CK-12 Foundation is used under a CC BY 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Figure 5.14.5
Tan lines by katiebordner on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Figure 5.14.6
Sickle cell anemia by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) ©
Figure 5.14.7
ABO_blood_type.svg by InvictaHOG on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Amoeba Sisters. (2015, May 25). Incomplete dominance, codominance, polygenic traits, and epistasis! YouTube. https://www.youtube.com/watch?v=YJHGfbW55l0
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 18.9 Sickle cells [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/18-3-erythrocytes
Brainard, J/ CK-12 Foundation. (2016). Figure 2 Human adult height [digital image]. In CK-12 College Human Biology (Section 5.13) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/5.13/
Mayo Clinic Staff. (n.d.). Tay-Sachs disease [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/tay-sachs-disease/symptoms-causes/syc-20378190
Mayo Clinic Staff. (n.d.). Sickle cell anemia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/sickle-cell-anemia/symptoms-causes/syc-20355876
Teacher's Pet. (2015, January 25). Non-mendelian genetics. YouTube. https://www.youtube.com/watch?v=-4vsio8TZrU
Created by CK-12 Foundation/Adapted by Christine Miller
Case Study Conclusion: Needing to Relax
As you learned in the beginning of this chapter, botulinum toxin — one form of which is sold under the brand name — does much more than smooth out wrinkles. It can be used to treat a number of disorders involving excessive muscle contraction, including cervical dystonia. You also learned that cervical dystonia, which Edward suffers from, causes abnormal, involuntary muscle contractions of the neck. This results in jerky movements of the head and neck, and/or a sustained abnormal tilt to the head. It is often painful and can significantly interfere with a person’s life.
How could a toxin actually help treat a muscular disorder? The botulinum toxin is produced by the soil bacterium, Clostridium botulinum, and it is the cause of the potentially deadly disease called botulism. Botulism is often a foodborne illness, commonly caused by foods that are improperly canned. Other forms of botulism are caused by wound infections, or occur when infants consume spores of the bacteria from soil or honey.
Botulism can be life-threatening, because it paralyzes muscles throughout the body, including those involved in breathing. When a very small amount of botulinum toxin is injected carefully into specific muscles by a trained medical professional, however, it can be useful in inhibiting unwanted muscle contractions.
For cosmetic purposes, botulinum toxin injected into the facial muscles relaxes them to reduce the appearance of wrinkles. When used to treat cervical dystonia, it is injected into the muscles of the neck to inhibit excessive muscle contractions. For many patients, this helps relieve the abnormal positioning, movements, and pain associated with the disorder. The effect is temporary, so the injections must be repeated every three to four months to keep the symptoms under control.
How does botulinum toxin inhibit muscle contraction? First, recall how skeletal muscle contraction works. A motor neuron instructs skeletal muscle fibres to contract at a synapse between them called the neuromuscular junction. A nerve impulse called an action potential travels down to the axon terminal of the motor neuron, where it causes the release of the neurotransmitter acetylcholine (ACh) from synaptic vesicles. The ACh travels across the synaptic cleft and binds to ACh receptors on the muscle fibre, signaling the muscle fibre to contract. According to the sliding filament theory, the contraction of the muscle fibre occurs due to the sliding of myosin and actin filaments across each other. This causes the Z discs of the sacromeres to move closer together, shortening the sacromeres and causing the muscle fibre to contract.
If you wanted to inhibit muscle contraction, at what points could you theoretically interfere with this process? Inhibiting the action potential in the motor neuron, the release of ACh, the activity of ACh receptors, or the sliding filament process in the muscle fibre would all theoretically impair this process and inhibit muscle contraction. For example, in the disease myasthenia gravis, the function of the ACh receptors is impaired, causing a lack of sufficient muscle contraction. As you have learned, this results in muscle weakness that can eventually become life-threatening. Botulinum toxin works by inhibiting the release of ACh from the motor neurons, thereby removing the signal instructing the muscles to contract.
Fortunately, Edward’s excessive muscle contractions and associated pain improved significantly thanks to botulinum toxin injections. Although cervical dystonia cannot currently be cured, botulinum toxin injections have improved the quality of life for many patients with this and other disorders involving excessive involuntary muscle contractions.
As you have learned in this chapter, our muscular system allows us to do things like make voluntary movements, digest our food, and pump blood through our bodies. Whether they are in your arm, heart, stomach, or blood vessels, muscle tissue works by contracting. But as you have seen here, too much contraction can be a very bad thing. Fortunately, scientists and physicians have found a way to put a potentially deadly toxin — and wrinkle-reducing treatment — to excellent use as a medical treatment for some muscular system disorders.
Chapter 12 Summary
In this chapter, you learned about the muscular system. Specifically, you learned that:
- The consists of all the muscles of the body. There are three types of muscle: (which is attached to bones by tendons and enables voluntary body movements), (which makes up the walls of the heart and makes it beat) and (which is found in the walls of internal organs and other internal structures and controls their movements).
- Muscles are organs composed mainly of muscle cells, which may also be called or . Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy from . Muscle tissue is the only type of tissue that has cells with the ability to contract.
- Muscles can grow larger, or . This generally occurs through increased use, although hormonal or other influences can also play a role. Muscles can also grow smaller, or . This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size — but not the number — of muscle fibres changes. The size of muscles is the main determinant of muscle strength.
- Skeletal muscles need the stimulus of to contract, and to move the body, they need the to act upon.
- Skeletal muscle is the most common type of muscle tissue in the human body. To move bones in opposite directions, skeletal muscles often consist of pairs of muscles that work in opposition to one another to move bones in different directions at .
- Skeletal muscle fibres are bundled together in units called muscle fascicles, which are bundled together to form individual skeletal muscles. Skeletal muscles also have connective tissue supporting and protecting the muscle tissue.
-
- Each skeletal muscle fibre consists of a bundle of , which are bundles of protein filaments. The filaments are arranged in repeating units called , which are the basic functional units of skeletal muscles. Skeletal muscle tissue is striated, because of the pattern of sarcomeres in its fibres.
- Skeletal muscle fibres can be divided into two types, called and fibres. Slow-twitch fibres are used mainly in endurance activities (such as long-distance running). Fast-twitch fibres are used mainly for non-aerobic, strenuous activities (such as sprinting). Proportions of the two types of fibres vary from muscle to muscle and person to person.
- Smooth muscle tissue is found in the walls of internal organs and vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. The pattern of smooth muscle contraction to move substances through body tubes is called . Contractions of smooth muscles are and controlled by the , , and other substances.
-
- Cells of smooth muscle tissue are not striated because they lack sarcomeres, but the cells contract in the same basic way as striated muscle cells. Unlike striated muscle, smooth muscle can sustain very long-term contractions and maintain its contractile function, even when stretched.
- Cardiac muscle tissue is found only in the wall of the . When cardiac muscle contracts, the heart beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. They are controlled by electrical impulses from specialized cardiac cells.
-
- Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres. The exact arrangement, however, differs, making cardiac and skeletal muscle tissues look different from one another.
- The heart is the muscle that performs the greatest amount of physical work in the course of a lifetime. Its cells contain a great many to produce ATP for energy and to help the heart resist fatigue.
- A muscle contraction is an increase in the tension or a decrease in the length of a muscle. A muscle contraction is if muscle tension changes, but muscle length remains the same. It is if muscle length changes, but muscle tension remains the same.
-
- A skeletal muscle contraction begins with electrochemical stimulation of a muscle fibre by a motor neuron. This occurs at a chemical synapse called a neuromuscular junction. The neurotransmitter acetylcholine diffuses across the synaptic cleft and binds to receptors on the muscle fibre. This initiates a muscle contraction.
- Once stimulated, the protein filaments within the skeletal muscle fibre slide past each other to produce a contraction. The is the most widely accepted explanation for how this occurs. According to this theory, thick filaments repeatedly attach to and pull on thin filaments, thus shortening sarcomeres.
- is a cycle of molecular events that underlies the sliding filament theory. Using energy in ATP, myosin heads repeatedly bind with and pull on actin filaments. This moves the actin filaments toward the center of a sarcomere, shortening the sarcomere and causing a muscle contraction.
- The ATP needed for a muscle contraction comes first from ATP already available in the cell, and more is generated from . These sources are quickly used up. and glycogen can be broken down to form ATP and pyruvate. Pyruvate can then be used to produce ATP in if oxygen is available, or it can be used in if oxygen is not available.
- Physical exercise is defined as any bodily activity that enhances or maintains physical fitness and overall health. Activities such as household chores may even count as physical exercise! Current recommendations for adults are 30 minutes of moderate exercise a day.
- is any physical activity that uses muscles at less than their maximum contraction strength, but for long periods of time. This type of exercise uses a relatively high percentage of slow-twitch muscle fibres that consume large amounts of oxygen. Aerobic exercises increase cardiovascular endurance, and include cycling and brisk walking.
- is any physical activity that uses muscles at close to their maximum contraction strength, but for short periods of time. This type of exercise uses a relatively high percentage of fast-twitch muscle fibres that consume small amounts of oxygen. Anaerobic exercises increase muscle and bone mass and strength, and they include push-ups and sprinting.
- is any physical activity that stretches and lengthens muscles, thereby improving range of motion and reducing risk of injury. Examples include stretching and yoga.
- Many studies have shown that physical exercise is positively correlated with a diversity of physical, mental, and emotional health benefits. Physical exercise also increases quality of life and life expectancy.
-
- Many of the benefits of exercise may come about because contracting muscles release hormones called , which promote tissue repair and growth and have anti-inflammatory effects.
- Physical exercise can reduce risk factors for cardiovascular disease, including and . Physical exercise can also increase factors associated with cardiovascular health, such as mechanical efficiency of the heart.
- Physical exercise has been shown to offer protection from and other cognitive problems, perhaps because it increases blood flow or neurotransmitters in the brain, among other potential effects.
- Numerous studies suggest that regular aerobic exercise works as well as pharmaceutical antidepressants in treating mild-to-moderate , possibly because it increases synthesis of natural in the brain.
- Research shows that physical exercise generally improves sleep for most people, and helps sleep disorders, such as insomnia. Other health benefits of physical exercise include better immune system function and reduced risk of type 2 diabetes and obesity.
- There is great variation in individual responses to exercise, partly due to genetic differences in proportions of slow-twitch and fast-twitch muscle fibres. People with more slow-twitch fibres may be able to develop greater endurance from aerobic exercise, whereas people with more fast-twitch fibres may be able to develop greater muscle size and strength from anaerobic exercise.
- Some adverse effects may occur if exercise is extremely intense and the body is not given proper rest between exercise sessions. Many people who overwork their muscles develop delayed onset muscle soreness (DOMS), which may be caused by tiny tears in muscle fibres.
- are injuries that occur in muscles or associated tissues (such as tendons) because of biomechanical stresses. The disorders may be caused by sudden exertion, over-exertion, repetitive motions, and similar stresses.
-
- A is an injury in which muscle fibres tear as a result of overstretching. First aid for a muscle strain includes the five steps represented by the acronym PRICE (protection, rest, ice, compression, and elevation). Medications for inflammation and pain (such as NSAIDs) may also be used.
- is inflammation of a tendon that occurs when it is over-extended or worked too hard without rest. Tendinitis may also be treated with PRICE and NSAIDs.
- is a biomechanical problem that occurs in the wrist when the median nerve becomes compressed between carpal bones. It may occur with repetitive use, a tumor, or trauma to the wrist. It may cause pain, numbness, and eventually — if untreated — muscle wasting in the thumb and first two fingers of the hand.
- are systemic disorders that occur because of problems with the nervous control of muscle contractions, or with the muscle cells themselves.
-
- is a genetic disorder caused by defective proteins in muscle cells. It is characterized by progressive skeletal muscle weakness and death of muscle tissues.
- is a genetic neuromuscular disorder characterized by fluctuating muscle weakness and fatigue. More muscles are affected, and muscles become increasingly weakened as the disorder progresses. Myasthenia gravis most often occurs because immune system antibodies block acetylcholine receptors on muscle cells, and because of the actual loss of acetylcholine receptors.
- is a degenerative disorder of the central nervous system that mainly affects the muscular system and movement. It occurs because of the death of neurons in the midbrain. Characteristic signs of the disorder are muscle tremor, muscle rigidity, slowness of movement, and postural instability. Dementia and depression also often characterize advanced stages of the disease.
As you saw in this chapter, muscles need oxygen to provide enough ATP for most of their activities. In fact, all of the body’s systems require oxygen, and also need to remove waste products, such as carbon dioxide. In the next chapter, you will learn about how the respiratory system obtains and distributes oxygen throughout the body, as well as how it removes wastes, such as carbon dioxide.
Chapter 12 Review
- What are tendons? Name a muscular system disorder involving tendons
- Describe the relationship between muscles, muscle fibres, and fascicles.
- The biceps and triceps muscles are shown above. Answer the following questions about these arm muscles.
- When the biceps contract and become shorter (as in the picture above), what kind of motion does this produce in the arm?
- Is the situation described in part (a) more likely to be an isometric or isotonic contraction? Explain your answer.
- If the triceps were to then contract, which way would the arm move?
- What are Z discs? What happens to them during muscle contraction?
- What is the function of mitochondria in muscle cells? Which type of muscle fibre has more mitochondria — slow-twitch or fast-twitch?
- What is the difference between primary and secondary Parkinson’s disease?
- Why can carpal tunnel syndrome cause muscle weakness in the hands?
Attributions
Figure 12.7.1
Botox, he whispered by Michael Reuter on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Figure 12.7.2
botulism by jason wilson on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Reference
Pearson Scott Foresman. (2020, April 14). File:Biceps (PSF).jpg [digital image]. Wikimedia Commons. https://commons.wikimedia.org/w/index.php?title=File:Biceps_(PSF).jpg&oldid=411251538. [Public Domain (https://en.wikipedia.org/wiki/Public_domain)]
Created by CK-12 Foundation/Adapted by Christine Miller
Oxygen Bar
Belly up to the bar and get your favorite... oxygen? That’s right — in some cities, you can get a shot of pure oxygen, with or without your choice of added flavors. Bar patrons inhale oxygen through a plastic tube inserted into their nostrils, paying up to a dollar per minute to inhale the pure gas. Proponents of the practice claim that breathing in extra oxygen will remove toxins from the body, strengthen the immune system, enhance concentration and alertness, increase energy, and even cure cancer! These claims, however, have not been substantiated by controlled scientific studies. Normally, blood leaving the lungs is almost completely saturated with oxygen, even without the use of extra oxygen, so it’s unlikely that a higher concentration of oxygen in air inside the lungs would lead to significantly greater oxygenation of the blood. Oxygen enters the blood in the lungs as part of the process of gas exchange.
What is Gas Exchange?
is the biological process through which gases are transferred across cell membranes to either enter or leave the blood. Oxygen is constantly needed by cells for aerobic cellular respiration, and the same process continually produces carbon dioxide as a waste product. Gas exchange takes place between the blood and cells throughout the body, with oxygen leaving the blood and entering the cells, and carbon dioxide leaving the cells and entering the blood. Gas exchange also takes place between the blood and the air in the lungs, with oxygen entering the blood from the inhaled air inside the lungs, and carbon dioxide leaving the blood and entering the air to be exhaled from the lungs.
Gas Exchange in the Lungs
Alveoli are the basic functional units of the lungs where gas exchange takes place between the air and the blood. are tiny air sacs that consist of connective and epithelial tissues. The connective tissue includes elastic fibres that allow alveoli to stretch and expand as they fill with air during inhalation. During exhalation, the fibres allow the alveoli to spring back and expel the air. Special cells in the walls of the alveoli secrete a film of fatty substances called . This substance prevents the alveolar walls from collapsing and sticking together when air is expelled. Other cells in alveoli include , which are mobile scavengers that engulf and destroy foreign particles that manage to reach the lungs in inhaled air.
As shown in Figure 13.4.2, alveoli are arranged in groups like clusters of grapes. Each alveolus is covered with epithelium that is just one cell thick. It is surrounded by a bed of capillaries, each of which has a wall of epithelium just one cell thick. As a result, gases must cross through only two cells to pass between an alveolus and its surrounding capillaries.
The pulmonary artery (also shown in Figure 13.4.2) carries deoxygenated blood from the heart to the lungs. Then, the blood travels through the pulmonary capillary beds, where it picks up oxygen and releases carbon dioxide. The oxygenated blood then leaves the lungs and travels back to the heart through pulmonary veins. There are four pulmonary veins (two for each lung), and all four carry oxygenated blood to the heart. From the heart, the oxygenated blood is then pumped to cells throughout the body.
Mechanism of Gas Exchange
Gas exchange occurs by across cell membranes. Gas molecules naturally move down a concentration gradient from an area of higher concentration to an area of lower concentration. This is a process that requires no energy. To diffuse across cell membranes, gases must first be dissolved in a liquid. Oxygen and carbon dioxide are transported around the body dissolved in blood. Both gases bind to the protein hemoglobin in red blood cells, although oxygen does so more effectively than carbon dioxide. Some carbon dioxide also dissolves in blood plasma.
As shown in Figure 13.4.3, oxygen in inhaled air diffuses into a pulmonary capillary from the alveolus. Carbon dioxide in the blood diffuses in the opposite direction. The carbon dioxide can then be exhaled from the body.
Gas exchange by diffusion depends on having a large surface area through which gases can pass. Although each alveolus is tiny, there are hundreds of millions of them in the lungs of a healthy adult, so the total surface area for gas exchange is huge. It is estimated that this surface area may be as great as 100 m2 (or approximately 1,076 ft²). Often we think of lungs as balloons, but this type of structure would have very limited surface area and there wouldn't be enough space for blood to interface with the air in the alveoli. The structure alveoli take in the lungs is more like a giant mass of soap bubbles — millions of tiny little chambers making up one large mass — this is what increases surface area giving blood lots of space to come into close enough contact to exchange gases by diffusion.
Gas exchange by diffusion also depends on maintaining a steep concentration gradient for oxygen and carbon dioxide. Continuous blood flow in the capillaries and constant breathing maintain this gradient.
- Each time you inhale, there is a greater concentration of oxygen in the air in the alveoli than there is in the blood in the pulmonary capillaries. As a result, oxygen diffuses from the air inside the alveoli into the blood in the capillaries. Carbon dioxide, in contrast, is more concentrated in the blood in the pulmonary capillaries than it is in the air inside the alveoli. As a result, carbon dioxide diffuses in the opposite direction.
- The cells of the body have a much lower concentration of oxygen than does the oxygenated blood that reaches them in peripheral capillaries, which are the capillaries that supply tissues throughout the body. As a result, oxygen diffuses from the peripheral capillaries into body cells. The opposite is true of carbon dioxide. It has a much higher concentration in body cells than it does in the blood of the peripheral capillaries. Thus, carbon dioxide diffuses from body cells into the peripheral capillaries.
13.4 Summary
- is the biological process through which gases are transferred across cell membranes to either enter or leave the blood. Gas exchange takes place continuously between the blood and cells throughout the body, and also between the blood and the air inside the lungs.
- Gas exchange in the lungs takes place in , which are tiny air sacs surrounded by networks of . The pulmonary artery carries deoxygenated blood from the heart to the lungs, where it travels through pulmonary capillaries, picking up oxygen and releasing carbon dioxide. The oxygenated blood then leaves the lungs through pulmonary veins.
- Gas exchange occurs by diffusion across cell membranes. Gas molecules naturally move down a concentration gradient from an area of higher concentration to an area of lower concentration. This is a passive process that requires no energy.
- Gas exchange by diffusion depends on the large surface area provided by the hundreds of millions of alveoli in the lungs. It also depends on a steep concentration gradient for oxygen and carbon dioxide. This gradient is maintained by continuous blood flow and constant breathing.
13.4 Review Questions
- What is gas exchange?
- Summarize the flow of blood into and out of the lungs for gas exchange.
- Describe the mechanism by which gas exchange takes place.
- Identify the two main factors upon which gas exchange by diffusion depends.
- If the concentration of oxygen were higher inside of a cell than outside of it, which way would the oxygen flow? Explain your answer.
- Why is it important that the walls of the alveoli are only one cell thick?
13.4 Explore More
https://www.youtube.com/watch?v=nRpwdwm06Ic&feature=emb_logo
Oxygen movement from alveoli to capillaries | NCLEX-RN | Khan Academy, khanacademymedicine, 2013.
https://www.youtube.com/watch?v=KmgIqVwytwA&feature=emb_logo
About Carbon Monoxide and Carbon Monoxide Poisoning, EMDPrepare, 2009.
https://www.youtube.com/watch?v=GVU_zANtroE
Oxygen’s surprisingly complex journey through your body - Enda Butler, TED-Ed, 2017.
Attributions
Figure 13.4.1
Oxygen Bar by Farrukh on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.
Figure 13.4.2
Alveolus_diagram.svg by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 13.4.3
Gas_exchange_in_the_aveolus.svg by domdomegg on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.
References
EMDPrepare. (2009, December 21). About carbon monoxide and carbon monoxide poisoning. YouTube. https://www.youtube.com/watch?v=KmgIqVwytwA&feature=youtu.be
khanacademymedicine. (2013, February 25). Oxygen movement from alveoli to capillaries | NCLEX-RN | Khan Academy. YouTube. https://www.youtube.com/watch?v=nRpwdwm06Ic&feature=youtu.be
TED-Ed. (2017, April 13). Oxygen’s surprisingly complex journey through your body - Enda Butler. YouTube. https://www.youtube.com/watch?v=GVU_zANtroE&feature=youtu.be
Created by: CK-12/Adapted by Christine Miller
Got Lactase?
Do you remember the American “got milk?” slogan from the 1990s? It was used in advertisements for milk in which celebrities were pictured wearing milk “mustaches.” While the purpose of the “got milk?” ads was to sell more milk, there is no denying that drinking milk can actually be good for one’s health. Milk is naturally high in and minerals. It can also be low in fat or even fat-free if treated to remove the that naturally occur in milk. However, before you reach for a tall, cold glass of milk, you might want to ask yourself another question: “got lactase?”
Adaptation to Lactose
Do you drink milk? Or do you avoid drinking milk and consuming milk products because they cause you discomfort? If the latter is the case, then you may have trouble digesting milk.
Milk, Lactose, and Lactase
Milk naturally contains more than just proteins and lipids — it also contains carbohydrates. Specifically, milk contains the sugar lactose, which is a disaccharide (two-sugar) compound that consists of one molecule each of galactose and glucose, as shown in the structural formula below (Figure 6.8.2). Lactose makes up between two and eight per cent of milk by weight. The exact amount varies both within and between species.
Lactose in milk must be broken down into its two component sugars to be absorbed by the small intestine. The enzyme lactase is needed for this process, as shown in Figure 6.8.3. Human infants are almost always born with the ability to synthesize lactase. This allows them to readily digest the lactose in their mother’s milk (or infant formula). In the majority of children, however, lactase synthesis begins to decline at about two years of age, and less and less lactase is produced throughout childhood.
Lactose Intolerance
Lactose intolerance is the inability of older children and adults to digest lactose in milk. People who are lactose intolerant may be able to drink small quantities of milk without any problems, but if they try to consume larger amounts, they are likely to suffer adverse effects. For example, they may have abdominal bloating and cramping, flatulence (gas), diarrhea, nausea, and vomiting. The symptoms may occur from 30 minutes to two hours after milk is consumed, and they're generally worse when the quantity of milk consumed is greater. The symptoms result from the small intestine's inability to digest and absorb lactose, so the lactose is passed on to the large intestine, where normal intestinal bacteria start breaking it down through the process of fermentation. This process releases gas and causes the other symptoms of lactose intolerance.
Lactose intolerance is actually the original and normal condition of the human species, as well as all other mammalian species. Early humans were hunter-gatherers that subsisted on wild plant and animal foods. The animal foods may have included meat and eggs, but did not include milk because animals had not been domesticated. Therefore, beyond the weaning period, milk was not available for people to drink in early human populations. It makes good biological sense to stop synthesizing an enzyme that the body does not need. After a young child is weaned, it is a waste of materials and energy to keep producing lactase when milk is no longer likely to be consumed.
Overall, an estimated 60 per cent of the world’s adult human population is thought to be lactose intolerant today. You can see the geographic distribution of modern human lactose intolerance on the map in Figure 6.8.4. Lactose intolerance (dark blue) approaches 100 per cent in populations throughout southern South America, southern Africa, and East and Southeast Asia.
Lactose intolerance is not considered a medical problem, because its symptoms can be avoided by avoiding milk or milk products. Dietary control of lactose intolerance may be a matter of trial and error, however, because different people may be able to consume different quantities of milk or milk products before symptoms occur. If you are lactose intolerant, be aware that low-fat and fat-free milk may contain somewhat more lactose than full-fat milk because the former often have added milk solids that are relatively high in lactose.
Lactase Persistence
Lactase persistence is the opposite of lactose intolerance. People who are lactase persistent continue to produce the enzyme lactase beyond infancy and generally throughout life. As a consequence they are able to digest lactose and drink milk at older ages without adverse effects. The map in Figure 6.8.4 can also be read to show where lactase persistence occurs today. Populations with a low percentage of lactose intolerance (including most North Americans and Western and Northern Europeans) have high percentages of lactase-persistent people.
Lactase persistence is a uniquely human trait that is not found in any other mammalian species. Why did lactase persistence evolve in humans? When some human populations began domesticating and keeping herds of animals, animal milk became a potential source of food. Animals such as cows, sheep, goats, camels, and even reindeer (see Figure 6.8.5) can be kept for their milk. These animal milks also contain lactose, so natural selection would be strong for any individuals who kept producing lactase beyond infancy and could make use of this nutritious food. Eventually, the trait of lactase persistence would increase in frequency and come to be the predominant trait in dairying populations.
It is likely that lactase persistence occurs as a result of both genes and environment. Some people inherit genes that help them keep producing lactase after infancy. Geneticists think that several different mutations for lactase persistence arose independently in different populations within the last ten thousand years. Part of lactase persistence may be due to continued exposure to milk in the childhood and adulthood diet. In other words, a person may be genetically predisposed to synthesize lactase at older ages because of a mutation, but they may need the continued stimulation of milk drinking to keep producing lactase.
Thrifty Gene or Drifty Gene?
Besides variation in lactase persistence, human populations may vary in how efficiently they use calories in the foods they consume. People in some populations seem able to get by on quantities of food that would be inadequate for others, so they tend to gain weight easily. What explains these differences in people?
Thrifty Gene Hypothesis
In 1962, human geneticist James Neel proposed the . According to this hypothesis, so-called “thrifty genes” evolved in some human populations because they allowed people to get by on fewer calories and store the rest as body fat when food was plentiful. According to Neel’s hypothesis, thrifty genes would have increased in frequency through natural selection, because they would help people survive during times of famine. People with the genes would be fatter and able to rely on their stored body fat for calories when food was scarce.
Such thrifty genes would have been advantageous in early human populations of hunter-gatherers if food scarcity was a recurrent stress. However, in modern times, when most people have access to enough food year-round, thrifty genes would no longer be advantageous. In fact, under conditions of plentiful food, having thrifty genes would predispose people to gain weight and develop obesity. They would also tend to develop the chronic diseases associated with obesity, particularly type II diabetes. is a disease that occurs when there are problems with the pancreatic hormone insulin, which normally helps cells take up glucose from the blood and controls blood glucose levels. In type II diabetes, body cells become relatively resistant to insulin, leading to high blood glucose. This causes symptoms like excessive thirst and urination. Without treatment, diabetes can lead to serious consequences, such as blindness and kidney failure.
Neel proposed his thrifty gene hypothesis not on the basis of genetic evidence for thrifty genes, but as a possible answer to the mystery of why genes that seem to promote diabetes have not been naturally selected out of some populations. The mystery arose from observations that certain populations — such as South Pacific Islanders, sub-Saharan Africans, and southwestern Native Americans — developed high levels of obesity and diabetes after they abandoned traditional diets and adopted Western diets.
Assessing the Thrifty Gene Hypothesis
One of the assumptions underlying the thrifty gene hypothesis is that human populations that recently developed high rates of obesity and diabetes after Western contact had a long history of recurrent famine. Anthropological evidence contradicts this assumption for at least some of the populations in question. South Pacific Islanders, for example, have long lived in a “land of plenty,” with lush tropical forests year-round on islands surrounded by warm waters full of fish. Another assumption underlying the thrifty gene hypothesis is that hunter-gatherer people became significantly fatter during periods of plenty. Again, there is little or no evidence that hunter-gatherers traditionally deposited large fat stores when food was readily available.
Some geneticists have searched directly for so-called thrifty genes. Studies have revealed many genes with small effects associated with obesity or diabetes. However, these genes can explain only a few percentage points of the total population variation in obesity or diabetes.
The Drifty Gene and Other Hypotheses
Given the lack of evidence for the thrifty gene hypothesis, several researchers have suggested alternative hypotheses to explain population variation in obesity and diabetes. One hypothesis posits that susceptibility to obesity and diabetes may be a side effect of heat . According to this idea, some populations evolved lower metabolic rates as an adaptation to heat stress, because lower metabolic rates reduced the amount of heat that the body produced. The lower metabolic rates also predisposed people to gain excess weight and develop obesity and diabetes.
A thrifty phenotype hypothesis has also been proposed. This hypothesis suggests that individuals who have inadequate nutrition during fetal development might develop an insulin-resistant phenotype. The insulin-resistant phenotype would supposedly prepare these individuals for a life of famine, based on the environment within the womb. In a famine-free environment, however, the thrifty phenotype would lead to the development of diabetes.
The most recent alternative to the thrifty gene hypothesis is the drifty gene hypothesis, which was proposed by biologist John Speakman. He argues that genes protecting humans from obesity were under strong natural selection pressure for a very long period of time while human ancestors were subject to the risk of predation. According to this view, being able to outrun predators would have been an important factor selecting against fatness. When the risk of predation was lessened — perhaps as early as two million years ago — genes keeping fatness in check would no longer be selected for. Without selective pressure for these genes, their frequencies could change randomly due to genetic drift. In some populations, by chance, frequencies of the genes could decrease to relatively low levels, whereas in other populations the frequencies could be much higher.
Feature: Myth vs. Reality
Myth: Lactose intolerance is an allergy to milk.
Reality: Lactose intolerance is not an allergy because it is not an immune system response. It is a sensitivity to milk caused by lactase deficiency so the sugar in milk cannot be digested. Milk allergy does exist, but it is a different condition that occurs in only about four per cent of people. It results when milk proteins (not milk sugar) trigger an immune reaction. How can you determine whether you have lactose intolerance or milk allergy? If you can drink lactose-free milk without symptoms, it is likely that you are lactose intolerant and not allergic to milk. However, if lactose-free milk also produces symptoms, it is likely that you have milk allergy. Note that it is possible to have both conditions.
Myth: If you are lactose intolerant, you will never be able to drink milk or consume other dairy products without suffering adverse physical symptoms.
Reality: Lactose intolerance does not mean that consuming milk and other dairy products is out of the question. Besides lactose-free milk, which is widely available, many dairy products have relatively low levels of lactose, so you may be able to consume at least small amounts of them without discomfort. You may be able to consume milk in the form of yogurt without any problems because the bacteria in yogurt produce lactase that breaks down the lactose. Greek yogurt may be your best bet, because it is lower in lactose to begin with. Aged cheeses also tend to have relatively low levels of lactose, because of the cheese-making process. Finally, by gradually adding milk or milk products to your diet, you may be able to increase your tolerance to lactose.
6.8 Summary
- Milk contains the sugar lactose, a disaccharide. Lactose must be broken down into its two component sugars to be absorbed by the small intestine, and the enzyme lactase is needed for this process.
- In about 60 per cent of people worldwide, the ability to synthesize lactase and digest lactose declines after the first two years of life. These people become lactose intolerant and cannot consume much milk without suffering symptoms of bloating, cramps, and diarrhea.
- In populations that herded milking animals for thousands of years, lactase persistence evolved. People who were able to synthesize lactase and digest lactose throughout life were strongly favored by natural selection. People — including many Europeans and European-Americans — who descended from these early herders generally still have lactase persistence.
- Human populations may vary in how efficiently they use calories in food. Some people (especially South Pacific Islanders, Native Americans, and sub-Saharan Africans) seem to be able to get by on fewer calories than would be adequate for others, so they tend to easily gain weight, become obese, and develop diseases such as diabetes.
- The thrifty gene hypothesis answers the question of how genes for this ability could have evolved. It proposes that “thrifty genes” were selected for because they allowed people to use calories efficiently and store body fat when food was plentiful so they had a reserve to use when food was scarce. Thrifty genes become detrimental and lead to obesity and diabetes when food is consistently plentiful.
- Several assumptions underlying the thrifty gene hypothesis have been called into question, and genetic research has been unable to actually identify thrifty genes. Alternate hypotheses to the thrifty gene hypothesis have been proposed, including the drifty gene hypothesis. The latter hypothesis explains variation in the tendency to become obese by genetic drift on neutral genes.
6.8 Review Questions
- Distinguish between the terms lactose and lactase.
- What is lactose intolerance, and what percentage of all people have it?
- Where and why did lactase persistence evolve?
- What is the thrifty gene hypothesis?
- How well is the thrifty gene hypothesis supported by evidence?
- Describe an alternative hypothesis to the thrifty gene hypothesis.
- Do you think that a lack of exposure to dairy products might affect a person’s lactase level? Why or why not?
- Describe an experiment you would want to do or data you would want to analyze that would help to test the thrifty phenotype hypothesis. Remember, you are studying people, so be sure it is ethical! Discuss possible confounding factors that you should control for, or that might affect the interpretation of your results.
- Explain the relationship between insulin, blood glucose, and type II diabetes.
6.8 Explore More
https://www.youtube.com/watch?v=G1NGzycaQV0&feature=emb_logo
Why Are People Lactose Intolerant?, Super Scienced, 2016.
https://www.youtube.com/watch?v=UMhLBPPtlrY
Peter Attia: What if we're wrong about diabetes?, TED, 2013.
https://www.youtube.com/watch?v=4O8k9qe8fjI
The Last Nomadic Reindeer Herders in the World, Great Big Story, 2018.
https://www.youtube.com/watch?v=XIYag5MWhPU
Experience a Traditional Whale Hunt in Northern Alaska | Short Film Showcase, National Geographic, 2018.
Attributions
Figure 6.8.1
IMG_4325 Milk Mustache licking 3 by Cedar Summit Farm on Flickr is used under a CC BY SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.
Figure 6.8.2
Lactose Haworth by NEUROtiker on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 6.8.3
Lactase by Boghog2 on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 6.8.4
Lactose Intolerance by Rainer Z ... on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 6.8.5
Reindeer_herding by Mats Andersson on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.
Figure 6.8.6
Milk Photo [photo] by Eiliv-Sonas Aceron on Unsplash is used under the Unsplash License (https://unsplash.com/license).
References
Great Big Story. (2018, November 29). The last nomadic reindeer herders in the world. YouTube. https://www.youtube.com/watch?v=4O8k9qe8fjI&feature=youtu.be
Super Scienced. (2016, February 26). Why are people lactose intolerant? YouTube. https://www.youtube.com/watch?v=G1NGzycaQV0&feature=youtu.be
National Geographic. (2018, November 27). Experience a traditional whale hunt in northern Alaska | Short film showcase. YouTube. https://www.youtube.com/watch?v=XIYag5MWhPU&feature=youtu.be
TED. (2013, June 25). Peter Attia: What if we're wrong about diabetes? YouTube. https://www.youtube.com/watch?v=UMhLBPPtlrY&feature=youtu.be
Wikipedia contributors. (2019, December 15). James V. Neel. In Wikipedia. https://en.wikipedia.org/w/index.php?title=James_V._Neel&oldid=930860629
Wikipedia contributors. (2020, June 9). John Speakman. In Wikipedia. https://en.wikipedia.org/w/index.php?title=John_Speakman&oldid=961610417
Created by CK-12/Adapted by Christine Miller
Case Study: Diet Dilemma
Joseph is a college student who has watched his father suffer from complications of type 2 diabetes for the past few years. For people with type 2 diabetes, the hormone insulin does not transmit its signal sufficiently. Insulin normally removes sugar from the bloodstream and brings it into the body’s cells. Diabetes prevents blood sugar levels from being properly regulated, and this can cause damage to the cells.
Diabetes can be treated with insulin injections, as shown above, as well as with dietary modifications, but complications can still occur. Joseph’s father has some nerve damage (or neuropathy) in his feet which makes his feet numb. He didn’t notice when he developed minor injuries to his feet which caused some serious infections.
Joseph is obese and knows that his weight — along with a family history of diabetes — increases his risk of getting the disease himself. He wants to avoid the health issues that his father suffered, so he begins walking every day for exercise and starts to lose weight. Joseph also wants to improve his diet in order to lose more weight, lower his risk of diabetes, and improve his general health, but he is overwhelmed with all of the different dietary advice he reads online and hears from his friends and family.
Joseph’s father tells him to limit refined carbohydrates, such as white bread and rice, because that is what he does to help keep his blood sugar at an acceptable level, but Joseph’s friend tells him that eating a diet high in carbohydrates and low in fat is a good way to lose weight. Joseph reads online that “eating clean” by eating whole, unprocessed foods and avoiding food with “chemicals” can help with weight loss. One piece of advice that everyone seems to agree on is that drinking enough water is good for overall health.
All this dietary advice may sound confusing, but you can better understand health conditions, such as diabetes, and the role of diet and nutrition by understanding chemistry. Chemistry is much more than chemical reactions in test tubes in a lab — it is the atoms, molecules, and reactions that make us who we are and keep us alive and functioning properly. Our diets are one of the main ways our bodies take in raw materials that are needed for the important chemical reactions that take place inside of us.
Chapter Overview: Chemistry
As you read this chapter, you will learn more about how chemistry relates to our lives, health, and the foods we eat. Specifically, you will learn about:
- The nature of chemical substances, including elements, compounds, and their component and molecules.
- The structures and functions of biochemical compounds, including carbohydrates, lipids, proteins, and nucleic acids (such as and RNA).
- What chemical reactions are, how energy is involved in chemical reactions, how enzymes assist in chemical reactions, and some types of biochemical reactions in living organisms.
- Properties of water and the importance of water for most biochemical processes.
- What pH is, and why maintaining a proper pH in the body is important for biochemical reactions.
As you read the chapter, think about the following questions regarding Joseph’s situation, as well as how diabetes and diet relate to the chemistry of life:
- Why do you think Joseph’s father's diabetes increases Joseph's risk of getting diabetes?
- What is the difference between refined (simple) carbohydrates and complex carbohydrates? Why are refined carbohydrates particularly problematic for people with diabetes?
- Insulin is a peptide hormone. In which class of biochemical compounds would you categorize insulin?
- Why is drinking enough water important for overall health? Can you drink too much water?
- Sometimes “eating clean” is described as avoiding “chemicals” in food. Think about the definition of “chemicals” and how it relates to what we eat.
Attributions
Figure 3.1.1
Diabetes-equipment by Steve Buissinne [stevepb] on Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).
Figure 3.1.2
Early Morning Hike, by Luke Pamer on Unsplash, is used under the Unsplash license (https://unsplash.com/license).
References
Mayo Clinic Staff. (n.d.). Peripheral neuropathy [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/peripheral-neuropathy/symptoms-causes/syc-20352061
Mayo Clinic Staff. (n.d.). Type 2 diabetes [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/type-2-diabetes/symptoms-causes/syc-20351193