3.10 Chemical Reactions in Living Things
Assembly Line
We stay alive because millions of different are taking place inside our bodies all the time. Each of our is like the busy auto assembly line pictured in Figure 3.10.1. Raw materials, half-finished products, and waste materials are constantly being used, produced, transported, and excreted. The “workers” on the cellular assembly line are mainly enzymes. These are the that make happen.
What Are Biochemical Reactions?
that take place inside living things are called . The sum of all the biochemical reactions in an organism is called . Metabolism includes both (energy-releasing) chemical reactions and (energy-absorbing) chemical reactions.
Catabolic Reactions
Exothermic reactions in organisms are called . These reactions break down molecules into smaller units and release energy. An example of a catabolic reaction is the breakdown of glucose during cellular respiration, which releases energy that cells need to carry out life processes.
Anabolic Reactions
Endothermic reactions in organisms are called . These reactions build up bigger molecules from smaller ones and absorb . An example of an anabolic reaction is the joining of to form a . Which type of reactions — catabolic or anabolic — do you think occur when your body digests food?
Enzymes
Most of the biochemical reactions that happen inside of living organisms require help. Why is this the case? For one thing, temperatures inside living things are usually too low for biochemical reactions to occur quickly enough to maintain life. The concentrations of reactants may also be too low for them to come together and react. Where do the biochemical reactions get the help they need to proceed? From the enzymes.
An is a that speeds up a . It is a biological . An enzyme generally works by reducing the amount of needed to start the reaction. The graph in Figure 3.10.2 shows the activation energy needed for glucose to combine with oxygen. Less activation energy is needed when the correct is present than when it is not present.
An enzyme speeds up the reaction by lowering the required activation energy. Compare the activation energy needed with and without the enzyme.
How Well Enzymes Work
Enzymes are involved in most biochemical reactions, and they do their jobs extremely well. A typical biochemical reaction that would take several days or even several centuries to happen without an enzyme is likely to occur in just a split second with the proper enzyme! Without enzymes to speed up biochemical reactions, most organisms could not survive.
Enzymes are substrate-specific. The of an enzyme is the specific substance it affects. Each enzyme works only with a particular substrate, which explains why there are so many different enzymes. In addition, for an enzyme to work, it requires specific conditions, such as the right temperature and pH. Some enzymes work best under acidic conditions, for example, while others work best in neutral environments.
Enzyme-Deficiency Disorders
There are hundreds of known inherited metabolic disorders in humans. In most of them, a single enzyme is either not produced by the body at all, or is otherwise produced in a form that doesn’t work. The missing or defective enzyme is like an absentee worker on the cell’s assembly line. Imagine the auto assembly line from the image at the start of this section. What if the worker who installed the steering wheel was absent? How would this impact the overall functioning of the vehicle? When an enzyme is missing, toxic chemicals build up, or an essential product isn’t made. Generally, the normal enzyme is missing because the individual with the disorder inherited two copies of a gene mutation, which may have originated many generations previously.
Any given inherited metabolic disorder is generally quite rare in the general population. However, there are so many different metabolic disorders that a total of one in 1,000 to 2,500 newborns can be expected to have one.
3.10 Summary
- Biochemical reactions are chemical reactions that take place inside of living things. The sum of all of the biochemical reactions in an organism is called metabolism.
- Metabolism includes catabolic reactions, which are energy-releasing (exothermic) reactions, as well as anabolic reactions, which are energy-absorbing (endothermic) reactions.
- Most biochemical reactions need a biological catalyst called an enzyme to speed up the reaction. Enzymes reduce the amount of activation energy needed for the reaction to begin. Most enzymes are proteins that affect just one specific substance, which is called the enzyme’s substrate.
- There are many inherited metabolic disorders in humans. Most of them are caused by a single defective or missing enzyme.
3.10 Review Questions
- What are biochemical reactions?
- Define metabolism.
- Compare and contrast catabolic and anabolic reactions.
- Explain the role of enzymes in biochemical reactions.
- What are enzyme-deficiency disorders?
- Explain why the relatively low temperature of living things, along with the low concentration of reactants, would cause biochemical reactions to occur very slowly in the body without enzymes.
- Answer the following questions about what happens after you eat a sandwich.
- Pieces of the sandwich go into your stomach, where there are digestive enzymes that break down the food. Which type of metabolic reaction is this? Explain your answer.
- During the process of digestion, some of the sandwich is broken down into glucose, which is then further broken down to release energy that your cells can use. Is this an exothermic endothermic reaction? Explain your answer.
- The proteins in the cheese, meat, and bread in the sandwich are broken down into their component amino acids. Then your body uses those amino acids to build new proteins. Which kind of metabolic reaction is represented by the building of these new proteins? Explain your answer.
- Explain why your body doesn’t just use one or two enzymes for all of its biochemical reactions.
- A ________ is the specific substance that an enzyme affects in a biochemical reaction.
- An enzyme is a biological _____________ .
- catabolism
- form of activation energy
- catalyst
- reactant
3.10 Explore More
Enzymes (Updated), by The Amoeba Sisters, 2016.
What triggers a chemical reaction? – Kareem Jarrah, TED-Ed, 2015.
Figure 3.10.1
Auto Assembly line by Brian Snelson on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 3.10.2
Enzyme_activation_energy by G. Andruk [IMeowbot at the English language Wikipedia], is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.
References
Amoeba Sisters. (2016, August 28). Enzymes (updated). YouTube. https://www.youtube.com/watch?v=qgVFkRn8f10&feature=youtu.be
TED-Ed. (2015, January 15). What triggers a chemical reaction? – Kareem Jarrah. YouTube. https://www.youtube.com/watch?v=8m6RtOpqvtU&feature=youtu.be
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another.
The smallest unit of life, consisting of at least a membrane, cytoplasm, and genetic material.
A class of biological molecule consisting of linked monomers of amino acids and which are the most versatile macromolecules in living systems and serve crucial functions in essentially all biological processes.
The transformation of one molecule to a different molecule inside a cell.
The chemical processes that occur in a living organism to sustain life.
A chemical reaction that releases energy through light or heat.
Created by CK-12 Foundation/Adapted by Christine Miller
Double Jointed?
Is this woman double jointed? No, there is actually no such thing — at least as far as humans are concerned. However, some people, like the woman pictured in Figure 11.6.1, are much more flexible than others, generally because they have looser ligaments. Physicians call this condition joint hypermobility. Regardless of what it’s called, the feats of people with highly mobile joints can be quite impressive.
What Are Joints?
are locations at which bones of the skeleton connect with one another. A joint is also called an articulation. The majority of joints are structured in such a way that they allow movement. However, not all joints allow movement. Of joints that do allow movement, the extent and direction of the movements they allow also vary.
Classification of Joints
Joints can be classified structurally or functionally. The structural classification of joints depends on the manner in which the bones connect to each other. The functional classification of joints depends on the nature of the movement the joints allow. There is significant overlap between the two types of classifications, because function depends largely on structure.
Structural Classification of Joints
The structural classification of joints is based on the type of tissue that binds the bones to each other at the joint. There are three types of joints in the structural classification: fibrous, cartilaginous, and synovial joints.
- are joints in which bones are joined by dense connective tissue that is rich in collagen fibres. These joints are also called sutures. The joints between bones of the cranium are fibrous joints.
- are joints in which bones are joined by cartilage. The joints between most of the vertebrae in the spine are cartilaginous joints.
- are characterized by a fluid-filled space (called a synovial cavity) between the bones of the joints. You can see a drawing of a typical synovial joint in Figure 11.6.2. The cavity is enclosed by a membrane and filled with a fluid (called synovial fluid) that provides extra cushioning to the ends of the bones. Cartilage covers the articulating surfaces of the two bones, but the bones are actually held together by ligaments. The knee is a synovial joint.
Functional Classification of Joints
The functional classification of joints is based on the type and degree of movement that they allow. There are three types of joints in the functional classification: immovable, partly movable, and movable joints.
- allow little or no movement at the joint. Most immovable joints are fibrous joints. Besides the bones of the cranium, immovable joints include joints between the tibia and fibula in the lower leg, and between the radius and ulna in the lower arm.
- permit slight movement. Most partly movable joints are cartilaginous joints. Besides the joints between vertebrae, they include the joints between the ribs and sternum (breast bone).
- allow bones to move freely. All movable joints are synovial joints. Besides the knee, they include the shoulder, hip, and elbow. Movable joints are the most common type of joints in the body.
Types of Movable Joints
Movable joints can be classified further according to the type of movement they allow. There are six classes of movable joints: pivot, hinge, saddle, plane, condyloid, and ball-and-socket joints. An example of each class — as well as the type of movement it allows — is shown in Figure 11.6.3.
- A allows the greatest range of movement of any movable joint. It allows forward and backward motion, as well as upward and downward movement. It also allows rotation in a circle. The hip and shoulder are the only two ball-and-socket joints in the human body.
- A allows one bone to rotate around another. An example of a pivot joint is the joint between the first two vertebrae in the spine. This joint allows the head to rotate from left to right and back again.
- A allows back and forth movement like the hinge of a door. An example of a hinge joint is the elbow. This joint allows the arm to bend back and forth.
- A allows two different types of movement. An example of a saddle joint is the joint between the first metacarpal bone in the hand and one of the carpal bones in the wrist. This joint allows the thumb to move toward and away from the index finger, and also to cross over the palm toward the little finger.
- A (also called a gliding joint) allows two bones to glide over one another. The joints between the tarsals in the ankles and between the carpals in the wrists are mainly gliding joints. In the wrist, this type of joint allows the hand to bend upward at the wrist, and also to wave from side to side while the lower arm is held steady.
- A is one in which an oval-shaped head on one bone moves in an elliptical cavity in another bone, allowing movement in all directions, except rotation around an axis. The joint between the radius in the lower arm and carpal bones of the wrist is a condyloid joint, as is the joint at the base of the index finger.
Feature: My Human Body
Of all the parts of the skeletal system, the joints are generally the most fragile and subject to damage. If the cartilage that cushions bones at joints wears away, it does not grow back. Eventually, all of the cartilage may wear away. This causes , which can be both painful and debilitating. In serious cases of osteoarthritis, people may lose the ability to climb stairs, walk long distances, perform routine daily activities, or participate in activities they love, such as gardening or playing sports. If you protect your joints, you can reduce your chances of joint damage, pain, and disability. If you already have joint damage, it is equally important to protect your joints and limit further damage. Follow these five tips:
- Maintain a normal, healthy weight. The more you weigh, the more force you exert on your joints. When you walk, each knee has to bear a force equal to as much as six times your body weight. If a person weighs 200 pounds, each knee bears more than half a ton of weight with every step. Seven in ten knee replacement surgeries for osteoarthritis can be attributed to obesity.
- Avoid too much high-impact exercise. Examples of high-impact activities include volleyball, basketball, and tennis. These activities generally involve running or jumping on hard surfaces, which puts tremendous stress on weight-bearing joints, especially the knees. Replace some or all of your high-impact activities with low-impact activities, such as biking, swimming, yoga, or lifting light weights.
- Reduce your risk of injury. Don’t be a weekend warrior, sitting at a desk all week and then crowding all your physical activity into two days. Get involved in a regular, daily exercise routine that keeps your body fit and your muscles toned. Building up muscles will make your joints more stable, allowing stress to spread across them. Be sure to do some stretching every day to keep the muscles around joints flexible and less prone to injury.
- Distribute work over your body, and use your largest, strongest joints. Use your shoulder, elbow, and wrist to lift heavy objects — not just your fingers. Hold small items in the palm of your hand, rather than by the fingers. Carry heavy items in a backpack, rather than in your hands. Hold weighty objects close to your body, instead of at arms’ length. Lift with your hips and knees, not your back.
- Respect pain. If it hurts, stop doing it. Take a break from the activity — at least until the pain stops. Try to use joints only to the point of mild fatigue, not pain.
11.6 Summary
- are spots at which of the skeleton connect with one another. A joint is also called an articulation.
- Joints can be classified structurally or functionally, and there is significant overlap between the two types of classifications.
- The structural classification of joints depends on the type of tissue that binds the bones to each other at the joint. There are three types of joints in the structural classification: , , and .
- The functional classification of joints is based on the type and degree of movement that they allow. There are three types of joints in the functional classification: , , and .
- Movable joints can be classified further according to the type of movement they allow. There are six classes of movable joints: , , , , , and .
11.6 Review Questions
- What are joints?
- What are two ways that joints are commonly classified?
- How are joints classified structurally?
- Describe the functional classification of joints.
- How are movable joints classified?
- Name the six classes of movable joints. Describe how they move and give an example of each.
- Which specific type of moveable joint do you think your knee joint is? Explain your reasoning.
- Explain the difference between cartilage in a cartilaginous joint and cartilage in a synovial joint.
- Why are fibrous joints immovable?
- What is the function of synovial fluid?
11.6 Explore More
https://www.youtube.com/watch?v=IjiKUmfaZr4
Why do your knuckles pop? - Eleanor Nelsen, TED-Ed, 2015.
https://www.youtube.com/watch?v=FWsBm3hr3B0
Why haven’t we cured arthritis? - Kaitlyn Sadtler and Heather J. Faust, TED-Ed, 2019.
Attributions
Figure 11.6.1
Tags: Sports Gymnastics Fitness Woman Preparation by nastya_gepp on Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).
Figure 11.6.2
Synovial_Joints by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.6.3
Types_of_Synovial_Joints by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
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 9.8 Synovial joints [digital image]. In Anatomy and Physiology (Section 9.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/9-4-synovial-joints
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 9.10 Types of synovial joints [digital image]. In Anatomy and Physiology (Section 9.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/9-4-synovial-joints
Mayo Clinic Staff. (n.d.). Osteoarthritis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/osteoarthritis/symptoms-causes/syc-20351925
TED-Ed. (2015, May 5). Why do your knuckles pop? - Eleanor Nelsen. YouTube. https://www.youtube.com/watch?v=IjiKUmfaZr4
TED-Ed. (2019, November 7). Why haven’t we cured arthritis? - Kaitlyn Sadtler and Heather J. Faust. YouTube. https://www.youtube.com/watch?v=FWsBm3hr3B0
A type of metabolic reaction that takes place within a cell in which larger molecules are separated to form smaller molecules.
Anabolic reactions are endergonic, meaning they require an input of energy to progress and are not spontaneous. They involve creation of larger molecules from smaller units.
The ability to do work.
Amino acids are organic compounds that combine to form proteins.
Biological molecules that lower amount the energy required for a reaction to occur.
A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.
The minimum energy required to cause a reaction to occur.
A specific reactant in a chemical reaction which works with a specific enzyme.
