{"id":5162,"date":"2019-06-24T17:27:20","date_gmt":"2019-06-24T17:27:20","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/17-3-digestion-and-absorption-3\/"},"modified":"2023-11-30T23:11:04","modified_gmt":"2023-11-30T23:11:04","slug":"17-3-digestion-and-absorption-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/17-3-digestion-and-absorption-3\/","title":{"raw":"15.3\u00a0Digestion and Absorption","rendered":"15.3\u00a0Digestion and Absorption"},"content":{"raw":" \r\n\r\n[caption id=\"attachment_4590\" align=\"aligncenter\" width=\"400\"]\"15.3.1 Figure 15.3.1 Now that's a mouthful.<\/em>[\/caption]\r\n\r\n
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Competitive Eating<\/h1>\r\n<\/div>\r\nThis man is on his way to coming in third in an international hot dog eating contest (Figure 15.3.1). It may look as though he is regurgitating his hot dogs, but in fact, he is trying to get them into his mouth and down his throat as quickly as he can. In order to eat as many hot dogs as possible in the allotted time, he pushes several into his mouth at once, and doesn\u2019t bother doing much chewing. Chewing is normally the first step in the process of digestion.\r\n
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Digestion<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"4549\"]Digestion[\/pb_glossary]<\/strong>\u00a0of food is a form of [pb_glossary id=\"5721\"]catabolism[\/pb_glossary], in which the food is broken down into small molecules that the body can absorb and use for energy, growth, and repair. Digestion occurs when food is moved through the digestive system. This process begins in the [pb_glossary id=\"4297\"]mouth[\/pb_glossary] and ends in the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary]. The final products of digestion are absorbed from the digestive tract, primarily in the small intestine. There are two different types of digestion that occur in the digestive system: [pb_glossary id=\"4550\"]mechanical digestion[\/pb_glossary] and [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary]. Figure 15.3.2 summarizes the roles played by different digestive organs in mechanical and chemical digestion, both of which are described in detail below.\r\n
\r\n\r\n[caption id=\"attachment_4591\" align=\"aligncenter\" width=\"446\"]\"15.3.2 Figure 15.3.2 Mechanical and chemical digestion along the GI tract.<\/em>[\/caption]\r\n\r\n<\/div>\r\n
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Mechanical Digestion<\/h2>\r\n<\/div>\r\n \r\n\r\n[caption id=\"attachment_4592\" align=\"alignright\" width=\"192\"]\"15.3.3\" Figure 15.3.3 The teeth play an important role in the mechanical digestion of food, starting with the first bite.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4550\"]Mechanical digestion[\/pb_glossary]<\/strong> is a physical process in which food is broken into smaller pieces without becoming changed chemically. It begins with your first bite of food (see Figure 15.3.3) and continues as you chew food with your teeth into smaller pieces. The process of mechanical digestion continues in the stomach. This muscular organ churns and mixes the food it contains, an action that breaks any solid food into still smaller pieces.\r\n\r\nAlthough some mechanical digestion also occurs in the\u00a0small intestine, it is mostly completed by the time food leaves the stomach. At that stage, food in the GI tract has been changed to the thick semi-fluid called [pb_glossary id=\"4582\"]chyme[\/pb_glossary]. Mechanical digestion is necessary so that chemical digestion can be effective. Mechanical digestion tremendously increases the [pb_glossary id=\"4593\"]surface area[\/pb_glossary] of food particles so they can be acted upon more effectively by digestive\u00a0enzymes.\r\n

Chemical Digestion<\/h2>\r\n[pb_glossary id=\"4551\"]Chemical digestion[\/pb_glossary]<\/strong>\u00a0is the biochemical process in which [pb_glossary id=\"5777\"]macromolecules<\/span>[\/pb_glossary]<\/span> in food are changed into smaller molecules that can be absorbed into body fluids and transported to\u00a0cells throughout the body. Substances in food that must be chemically digested include\u00a0[pb_glossary id=\"5655\"]carbohydrates[\/pb_glossary],\u00a0[pb_glossary id=\"5813\"]protein<\/span>s<\/span>[\/pb_glossary]<\/span>,\u00a0[pb_glossary id=\"5651\"]lipid<\/span>s<\/span>[\/pb_glossary]<\/span>, and\u00a0[pb_glossary id=\"5475\"]nucleic acids[\/pb_glossary]. Carbohydrates must be broken down into simple [pb_glossary id=\"316\"]sugar<\/span>s<\/span>[\/pb_glossary]<\/span>, proteins into\u00a0[pb_glossary id=\"5707\"]amino acid<\/span>s<\/span>[\/pb_glossary]<\/span>, lipids into [pb_glossary id=\"5657\"]fatty acids[\/pb_glossary] and glycerol, and nucleic acids into nitrogen bases and sugars. Some chemical digestion takes place in the mouth and stomach, but most of it occurs in the first part of the\u00a0small intestine\u00a0([pb_glossary id=\"4576\"]duodenum[\/pb_glossary]).<\/span>\r\n

Digestive\u00a0Enzymes<\/h3>\r\nChemical digestion could not occur without the help of many different digestive enzymes.\u00a0[pb_glossary id=\"5757\"]Enzymes[\/pb_glossary]<\/strong>\u00a0are\u00a0proteins\u00a0that catalyze, or\u00a0speed\u00a0up,\u00a0biochemical reactions. Digestive enzymes are secreted by [pb_glossary id=\"6001\"]exocrine glands<\/span>[\/pb_glossary]<\/span>\u00a0or by the [pb_glossary id=\"4561\"]mucosal layer[\/pb_glossary] of epithelium lining the gastrointestinal tract. In the [pb_glossary id=\"4297\"]mouth[\/pb_glossary], digestive enzymes are secreted by [pb_glossary id=\"4577\"]salivary gland<\/span>s<\/span>[\/pb_glossary]<\/span>. The lining of the [pb_glossary id=\"4558\"]stomach[\/pb_glossary] secretes enzymes, as does the lining of the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary]. Many more digestive enzymes are secreted by exocrine\u00a0cells\u00a0in the\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary] and carried by ducts to the small intestine. The following table lists several important digestive enzymes, the organs and\/or glands that secrete them, the compounds they digest, and the pH necessary for optimal functioning. You can read more about them below.<\/span>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table 15.3.1: Digestive Enzymes<\/caption>\r\n
Digestive\u00a0Enzyme<\/th>\r\nSource Organ<\/th>\r\nSite of Action<\/th>\r\nReactant and Product<\/th>\r\nOptimal\u00a0pH<\/th>\r\n<\/tr>\r\n
Salivary Amylase<\/td>\r\nSalivary Glands<\/td>\r\nMouth<\/td>\r\nstarch + water \u21d2 maltose<\/td>\r\nNeutral<\/td>\r\n<\/tr>\r\n
Pepsin<\/td>\r\nStomach<\/td>\r\nStomach<\/td>\r\nprotein + water \u21d2 peptides<\/td>\r\nAcidic<\/td>\r\n<\/tr>\r\n
Pancreatic Amylase<\/td>\r\nPancreas<\/td>\r\nDuodenum<\/td>\r\nstarch + water \u21d2 maltose<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Maltase<\/td>\r\nSmall intestine<\/td>\r\nSmall intestine<\/td>\r\nmaltose + water \u21d2 glucose<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Sucrase<\/td>\r\nSmall intestine<\/td>\r\nSmall intestine<\/td>\r\nsucrose + water \u21d2 glucose + fructose<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Lactase<\/td>\r\nSmall intestine<\/td>\r\nSmall intestine<\/td>\r\nlactose + water \u21d2\u00a0 glucose + galactose<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Lipase<\/td>\r\nPancreas<\/td>\r\nDuodenum<\/td>\r\nfat droplet and water \u21d2\u00a0 glycerol and fatty acids<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Trypsin<\/td>\r\nPancreas<\/td>\r\nDuodenum<\/td>\r\nprotein + water \u21d2 peptides<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Chymotrypsin<\/td>\r\nPancreas<\/td>\r\nDuodenum<\/td>\r\nprotein + water \u21d2 peptides<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Peptidases<\/td>\r\nSmall intestine<\/td>\r\nSmall intestine<\/td>\r\npeptides + water \u21d2<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Deoxyribonuclease<\/td>\r\nPancreas<\/td>\r\nDuodenum<\/td>\r\nDNA + water \u21d2 nucleotide fragments<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Ribonuclease<\/td>\r\nPancreas<\/td>\r\nDuodenum<\/td>\r\nRNA + water \u21d2 nucleotide fragments<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Nuclease<\/td>\r\nSmall intestine<\/td>\r\nSmall intestine<\/td>\r\nnucleic acids + water \u21d2 nucleotide fragments<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n
Nucleosidases<\/td>\r\nSmall intestine<\/td>\r\nSmall intestine<\/td>\r\nnucleotides + water \u21d2 nitrogen base + phosphate sugar<\/td>\r\nBasic<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

Chemical Digestion of\u00a0Carbohydrates<\/h3>\r\nAbout 80% of digestible carbohydrates in a typical Western diet are in the form of the plant [pb_glossary id=\"5683\"]polysaccharide[\/pb_glossary] amylose, which consists mainly of long chains of [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and is one of two major components of [pb_glossary id=\"5459\"]starch[\/pb_glossary]. Additional dietary carbohydrates include the animal polysaccharide [pb_glossary id=\"327\"]glycogen[\/pb_glossary], along with some sugars, which are mainly [pb_glossary id=\"4594\"]disaccharides<\/span>[\/pb_glossary]<\/span>.<\/span>\r\n\r\nThe process of chemical digestion for some carbohydrates is illustrated Figure 15.3.4. To chemically digest amylose and glycogen, the enzyme [pb_glossary id=\"4578\"]amylase[\/pb_glossary] is required. The chemical digestion of these polysaccharides begins in the mouth, aided by amylase in saliva. [pb_glossary id=\"4595\"]Saliva[\/pb_glossary] also contains mucus \u2014 which lubricates the food \u2014 and hydrogen carbonate, which provides the ideal alkaline conditions for amylase to work. Carbohydrate digestion is completed in the small intestine, with the help of amylase secreted by the pancreas. In the digestive process, polysaccharides are reduced in length by the breaking of bonds between glucose monomers. The macromolecules are broken down to shorter polysaccharides and disaccharides, resulting in progressively shorter chains of glucose. The end result is molecules of the simple sugars glucose and maltose (which consists of two glucose molecules), both of which can be absorbed by the small intestine.\r\n\r\nOther sugars are digested with the help of different enzymes produced by the small intestine. Sucrose (or table sugar), for example, is a disaccharide that is broken down by the enzyme sucrase to form glucose and fructose, which are readily absorbed by the small intestine. Digestion of the sugar lactose, which is found in milk, requires the enzyme lactase, which breaks down lactose into glucose and galactose. Glucose and galactose are then absorbed by the small intestine. Fewer than half of all adults produce sufficient lactase to be able to digest lactose. Those who cannot are said to be lactose intolerant.\r\n\r\n[caption id=\"attachment_4596\" align=\"aligncenter\" width=\"853\"]\"15.3.4\" Figure 15.3.4 The process of chemical digestion for some carbohydrates.<\/em>[\/caption]\r\n

Chemical Digestion of\u00a0Proteins<\/h3>\r\n[pb_glossary id=\"1593\"]Proteins[\/pb_glossary] consist of polypeptides, which must be broken down into their constituent\u00a0[pb_glossary id=\"5707\"]amino acids<\/span>[\/pb_glossary]<\/span> before they can be absorbed. An overview of this process is shown in Figure 15.3.5. Protein digestion occurs in the stomach and small intestine through the action of three primary enzymes: [pb_glossary id=\"4581\"]pepsin[\/pb_glossary] (secreted by the stomach), and [pb_glossary id=\"4597\"]trypsin[\/pb_glossary] and [pb_glossary id=\"4598\"]chymotrypsin[\/pb_glossary] (secreted by the pancreas). The stomach also secretes hydrochloric acid (HCl), making the contents highly acidic, which is a required condition for pepsin to work. Trypsin and chymotrypsin in the small intestine require an alkaline (basic) environment to work. [pb_glossary id=\"4599\"]Bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver [\/pb_glossary]and bicarbonate from the pancreas neutralize the acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] as it empties into the small intestine. After pepsin, trypsin, and chymotrypsin break down proteins into peptides, these are further broken down into amino acids by other enzymes called [pb_glossary id=\"4600\"]peptidase<\/span>s<\/span>[\/pb_glossary]<\/span>, also secreted by the pancreas.<\/span>\r\n\r\n[caption id=\"attachment_4601\" align=\"aligncenter\" width=\"316\"]\"15.3.5\" Figure 15.3.5 Chemical digestion of proteins.<\/em>[\/caption]\r\n

Chemical Digestion of\u00a0Lipids<\/h3>\r\nThe chemical digestion of\u00a0lipids\u00a0begins in the mouth. The salivary glands secrete the digestive enzyme [pb_glossary id=\"4602\"]lipase[\/pb_glossary], which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of\u00a0lipid\u00a0digestion may take place in the stomach, but most lipid digestion occurs in the small intestine.\r\n\r\nDigestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas, as well as bile secreted by the [pb_glossary id=\"2989\"]liver[\/pb_glossary]. As shown in the diagram below (Figure 15.3.6), bile is required for the digestion of lipids, because lipids are oily and do not dissolve in the watery chyme. Bile emulsifies (or breaks up) large globules of food lipids into much smaller ones, called micelles, much as dish detergent breaks up grease. The micelles provide a great deal more surface area to be acted upon by lipase, and also point the hydrophilic (\u201cwater-loving\u201d) heads of the fatty acids outward into the watery chyme. Lipase can then access and break down the micelles into individual fatty acid molecules.\r\n\r\n[caption id=\"attachment_4603\" align=\"aligncenter\" width=\"500\"]\"15.3.6 Figure 15.3.6 <\/em>Bile from the liver and lipase from the pancreas help digest lipids in the small intestine.<\/em>[\/caption]\r\n\r\n
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Chemical Digestion of Nucleic Acids<\/h3>\r\n<\/div>\r\nNucleic acids (DNA\u00a0and RNA) in foods are digested in the small intestine with the help of both pancreatic enzymes and enzymes produced by the small intestine itself. Pancreatic enzymes called ribonuclease and deoxyribonuclease break down\u00a0RNA\u00a0and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases.\r\n

Bacteria\u00a0in the\u00a0Digestive System<\/h3>\r\n

Your\u00a0large intestine\u00a0is not just made up of\u00a0cells. It is also an\u00a0[pb_glossary id=\"5743\"]ecosystem<\/strong>[\/pb_glossary], home to trillions of bacteria known as the \"gut flora\" (Figure 15.3.7). But don't worry, most of these bacteria are helpful. Friendly bacteria live mostly in the large intestine and part of the small intestine. The acidic environment of the stomach does not allow bacterial growth.<\/p>\r\n

Gut bacteria have several roles in the body. For example, intestinal bacteria:<\/p>\r\n\r\n