132 15.3 Digestion and Absorption

Created by CK-12 Foundation/Adapted by Christine Miller

15.3.1 Hot Dog Eating Contest
Figure 15.3.1 Now that’s a mouthful.

Competitive Eating

This 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’t bother doing much chewing. Chewing is normally the first step in the process of digestion.


Digestion of food is a form of catabolism, 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 mouth and ends in the small intestine. 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: mechanical digestion and chemical digestion. 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.

15.3.2 Mechanical and Chemical Digestion
Figure 15.3.2 Mechanical and chemical digestion along the GI tract.

Mechanical Digestion


Figure 15.3.3 The teeth play an important role in the mechanical digestion of food, starting with the first bite.

Mechanical digestion 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.

Although some mechanical digestion also occurs in the small 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 chyme. Mechanical digestion is necessary so that chemical digestion can be effective. Mechanical digestion tremendously increases the surface area of food particles so they can be acted upon more effectively by digestive enzymes.

Chemical Digestion

Chemical digestion is the biochemical process in which macromolecules in food are changed into smaller molecules that can be absorbed into body fluids and transported to cells throughout the body. Substances in food that must be chemically digested include carbohydratesproteinslipids, and nucleic acids. Carbohydrates must be broken down into simple sugars, proteins into amino acids, lipids into fatty acids 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 small intestine (duodenum).

Digestive Enzymes

Chemical digestion could not occur without the help of many different digestive enzymes. Enzymes are proteins that catalyze, or speed up, biochemical reactions. Digestive enzymes are secreted by exocrine glands or by the mucosal layer of epithelium lining the gastrointestinal tract. In the mouth, digestive enzymes are secreted by salivary glands. The lining of the stomach secretes enzymes, as does the lining of the small intestine. Many more digestive enzymes are secreted by exocrine cells in the pancreas 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.

Table 15.3.1: Digestive Enzymes
Digestive Enzyme Source Organ Site of Action Reactant and Product Optimal pH
Salivary Amylase Salivary Glands Mouth starch + water ⇒ maltose Neutral
Pepsin Stomach Stomach protein + water ⇒ peptides Acidic
Pancreatic Amylase Pancreas Duodenum starch + water ⇒ maltose Basic
Maltase Small intestine Small intestine maltose + water ⇒ glucose Basic
Sucrase Small intestine Small intestine sucrose + water ⇒ glucose + fructose Basic
Lactase Small intestine Small intestine lactose + water ⇒  glucose + galactose Basic
Lipase Pancreas Duodenum fat droplet and water ⇒  glycerol and fatty acids Basic
Trypsin Pancreas Duodenum protein + water ⇒ peptides Basic
Chymotrypsin Pancreas Duodenum protein + water ⇒ peptides Basic
Peptidases Small intestine Small intestine peptides + water ⇒ Basic
Deoxyribonuclease Pancreas Duodenum DNA + water ⇒ nucleotide fragments Basic
Ribonuclease Pancreas Duodenum RNA + water ⇒ nucleotide fragments Basic
Nuclease Small intestine Small intestine nucleic acids + water ⇒ nucleotide fragments Basic
Nucleosidases Small intestine Small intestine nucleotides + water ⇒ nitrogen base + phosphate sugar Basic

Chemical Digestion of Carbohydrates

About 80% of digestible carbohydrates in a typical Western diet are in the form of the plant polysaccharide amylose, which consists mainly of long chains of glucose and is one of two major components of starch. Additional dietary carbohydrates include the animal polysaccharide glycogen, along with some sugars, which are mainly disaccharides.

The process of chemical digestion for some carbohydrates is illustrated Figure 15.3.4. To chemically digest amylose and glycogen, the enzyme amylase is required. The chemical digestion of these polysaccharides begins in the mouth, aided by amylase in saliva. Saliva also contains mucus — which lubricates the food — 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.

Other 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.

Figure 15.3.4 The process of chemical digestion for some carbohydrates.

Chemical Digestion of Proteins

Proteins consist of polypeptides, which must be broken down into their constituent amino acids 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: pepsin (secreted by the stomach), and trypsin and chymotrypsin (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. Bile from the liver and bicarbonate from the pancreas neutralize the acidic chyme 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 peptidases, also secreted by the pancreas.

Figure 15.3.5 Chemical digestion of proteins.

Chemical Digestion of Lipids

The chemical digestion of lipids begins in the mouth. The salivary glands secrete the digestive enzyme lipase, which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of lipid digestion may take place in the stomach, but most lipid digestion occurs in the small intestine.

Digestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas, as well as bile secreted by the liver. 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 (“water-loving”) heads of the fatty acids outward into the watery chyme. Lipase can then access and break down the micelles into individual fatty acid molecules.

15.3.6 Fat Digestion
Figure 15.3.6 Bile from the liver and lipase from the pancreas help digest lipids in the small intestine.

Chemical Digestion of Nucleic Acids

Nucleic acids (DNA and 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 RNA and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases.

Bacteria in the Digestive System

Your large intestine is not just made up of cells. It is also an ecosystem, 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.

Gut bacteria have several roles in the body. For example, intestinal bacteria:

  • Produce vitamin B12 and vitamin K.
  • Control the growth of harmful bacteria.
  • Break down poisons in the large intestine.
  • Break down some substances in food that cannot be digested, such as fibre and some starches and sugars. Bacteria produce enzymes that digest carbohydrates in plant cell walls. Most of the nutritional value of plant material would be wasted without these bacteria. These help us digest plant foods like spinach.
Figure 15.3.7 Commensal (good) bacteria (shown in red) reside among the mucus (green) and epithelial cells (blue) of a small intestine.

A wide range of friendly bacteria live in the gut. Bacteria begin to populate the human digestive system right after birth. Gut bacteria include Lactobacillus, the bacteria commonly used in probiotic foods such as yogurt, and E. coli bacteria. About a third of all bacteria in the gut are members of the Bacteroides species. Bacteroides are key in helping us digest plant food.

It is estimated that 100 trillion bacteria live in the gut. This is more than the human cells that make up you. It has also been estimated that there are more bacteria in your mouth than people on the planet — there are over 7 billion people on the planet!

The bacteria in your digestive system are from anywhere between 300 and 1,000 species. As these bacteria are helpful, your body does not attack them. They actually appear to the body’s immune system as cells of the digestive system, not foreign invaders. The bacteria actually cover themselves with sugar molecules removed from the actual cells of the digestive system. This disguises the bacteria and protects them from the immune system.

As the bacteria that live in the human gut are beneficial to us, and as the bacteria enjoy a safe environment to live, the relationship that we have with these tiny organisms is described as mutualism, a type of symbiotic relationship.

Lastly, keep in mind the small size of bacteria. Together, all the bacteria in your gut may weigh just about two pounds.

Control of the Digestive Process

The process of digestion is controlled by both hormones and nerves. Hormonal control is mainly by endocrine hormones secreted by cells in the lining of the stomach and small intestine. These hormones stimulate the production of digestive enzymes, bicarbonate, and bile. The hormone secretin, for example, is produced by endocrine cells lining the duodenum of the small intestine. Acidic chyme entering the duodenum from the stomach triggers the release of secretin into the bloodstream. When the secretin returns via the circulation to the digestive system, it signals the release of bicarbonate from the pancreas. The bicarbonate neutralizes the acidic chyme.  See Table 15.3.2 for a summary of the major hormones governing the process of chemical digestion.

Table 15.3.2: Major Hormones Governing Chemical Digestion
Hormone Source Organ Target Organ Trigger Result
Gastrin Stomach walls Stomach High protein intake HCL and pepsin release, stomach churning
Secretin Duodenum Pancreas


Acidic chyme entering the duodenum Release sodium bicarbonate, release bile
Cholecystokinin (CCK) Duodenum Pancreas


Partially digested fat and protein in duodenum Release lipase, trypsin, release bile

Nerves involved in digestion include those that connect digestive organs to the central nervous system, as well as nerves inside the walls of the digestive organs. Nerves connecting the digestive organs to the central nervous system cause smooth muscles in the walls of digestive organs to contract or relax as needed, depending on whether or not there is food to be digested. Nerves within digestive organs are stimulated when food enters the organs and stretches their walls. These nerves trigger the release of substances that speed up or slow down the movement of food through the GI tract and the secretion of digestive enzymes.


When digestion is finished, it results in many simple nutrient molecules that must go through the process of absorption from the lumen of the GI tract to blood or lymph vessels, so they can be transported to and used by cells throughout the body. A few substances are absorbed in the stomach and large intestine. Water is absorbed in both of these organs, and some minerals and vitamins are also absorbed in the large intestine, but about 95% of nutrient molecules are absorbed in the small intestine. Absorption of the majority of these molecules takes place in the second part of the small intestine, called the jejunum. There are, however, a few exceptions — for example, iron is absorbed in the duodenum, and vitamin B12 is absorbed in the last part of the small intestine, called the ileum. After being absorbed in the small intestine, nutrient molecules are transported to other parts of the body for storage or further chemical modification. Amino acids, for instance, are transported to the liver to be used for protein synthesis.

The epithelial tissue lining the small intestine is specialized for absorption. It is highly enfolded and is covered with villi and microvilli, creating an enormous surface area for absorption. As shown in Figure 15.3.8, each villus also has a network of blood capillaries and fine lymphatic vessels called lacteals close to its surface. The thin surface layer of epithelial cells of the villi transports nutrients from the lumen of the small intestine into these capillaries and lacteals. Blood in the capillaries absorbs most of the molecules, including simple sugars, amino acids, glycerol, salts, and water-soluble vitamins (vitamin C and the many B vitamins). Lymph in the lacteals absorbs fatty acids and fat-soluble vitamins (vitamins A, D, E, and K).

Figure 15.3.8 This simplified drawing of an intestinal villus shows the capillaries and lacteals within it that carry away absorbed substances. Note that each cell in the thin surface layer of the villus is actually covered with microvilli that greatly increase the surface area for absorption.

Feature: My Human Body

The process of digestion does not always go as it should. Many people suffer from indigestion, or dyspepsia, a condition of impaired digestion. Symptoms may include upper abdominal fullness or pain, heartburn, nausea, belching, or some combination of these symptoms. The majority of cases of indigestion occur without evidence of an organic disease that is likely to explain the symptoms. Anxiety or certain foods or medications (such as aspirin) may be contributing factors in these cases. In other cases, indigestion is a symptom of an organic disease, most often gastroesophageal reflux disease (GERD) or gastritis. In a small minority of cases, indigestion is a symptom of a peptic ulcer of the stomach or duodenum, usually caused by a bacterial infection. Very rarely, indigestion is a sign of cancer.

An occasional bout of indigestion is usually nothing to worry about, especially in people less than 55 years of age. However, if you suffer frequent or chronic indigestion, it’s a good idea to see a doctor. If an underlying disorder such as GERD or an ulcer is causing the indigestion, this can and should be treated. If no organic disease is discovered, the doctor can recommend lifestyle changes or treatments to help prevent or soothe the symptoms of acute indigestion. Lifestyle changes might include modifications in eating habits, such as eating more slowly, eating smaller meals, or avoiding fatty foods. You also might be advised to refrain from taking certain medications, especially on an empty stomach. The use of antacids or other medications to relieve symptoms may also be recommended.

15.3 Summary

  • Digestion is a form of catabolism, in which food is broken down into small molecules that the body can absorb and use for energy, growth, and repair. Digestion occurs when food moves through the gastrointestinal (GI) tract. The digestive process is controlled by both hormones and nerves.
  • Mechanical digestion is a physical process in which food is broken into smaller pieces without becoming chemically changed. It occurs mainly in the mouth and stomach.
  • Chemical digestion is a chemical process in which macromolecules — including carbohydrates, proteins, lipids, and nucleic acids — in food are changed into simple nutrient molecules that can be absorbed into body fluids. Carbohydrates are chemically digested to sugars, proteins to amino acids, lipids to fatty acids, and nucleic acids to individual nucleotides. Chemical digestion requires digestive enzymes. Gut flora carry out additional chemical digestion.
  • Absorption occurs when the simple nutrient molecules that result from digestion are absorbed into blood or lymph.

15.3 Review Questions

  1. Define digestion. Where does it occur?
  2. Identify two organ systems that control the process of digestion by the digestive system.
  3. What is mechanical digestion? Where does it occur?
  4. Describe chemical digestion.
  5. What is the role of enzymes in chemical digestion?
  6. What is absorption? When does it occur?
  7. Where does most absorption occur in the digestive system? Why does most of the absorption occur in this organ, and not earlier in the GI tract?

15.3 Explore More

Food for thought: How your belly controls your brain | Ruairi Robertson | TEDxFulbrightSantaMonica, TEDx Talks, 2015.

How the food you eat affects your gut – Shilpa Ravella, TED-Ed, 2017.

What causes heartburn? – Rusha Modi, TED-Ed, 2018.



Figure 15.3.1

Patrick_Bertoletti_eating_hot_dogs by Michael on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 15.3.2

2426_Mechanical_and_Chemical_DigestionN by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 15.3.3

Eating tacos [photo] by DeMorris Byrd on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 15.3.4

Carbohydrate digestion by Nutritional Doublethink on Flickr is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.

Figure 15.3.5

Peptide Digestion by Nutritional Doublethink on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 15.3.6

Bile from the liver and lipase from the pancreas help digest lipids in small intestine 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 CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 15.3.7

Gut Flora by NIH Image Gallery on Flickr by NIH Image Gallery on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

Figure 15.3.8

Figure_34_01_11f by CNX OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.


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 23.28 Digestion and absorption [digital image].  In Anatomy and Physiology (Section 23.7). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/23-7-chemical-digestion-and-absorption-a-closer-look

Brainard, J/ CK-12 Foundation. (2016). Figure 6 Both bile from the liver and lipase from the pancreas help digest lipids in the small intestine [digital image]. In CK-12 College Human Biology (Section 17.3) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/17.3/

OpenStax. (2016, May 27) Figure 11 Villi are folds on the small intestine lining that increase the surface area to facilitate the absorption of nutrients. [digital image]. In OpenStax, Biology (Section 34.1). OpenStax CNX.  https://cnx.org/contents/GFy_h8cu@10.53:Oestf0YE@6/Digestive-Systems

TED-Ed. (2017, March 23). How the food you eat affects your gut – Shilpa Ravella. YouTube. https://www.youtube.com/watch?v=1sISguPDlhY&feature=youtu.be

TED-Ed. (2018, November 1). What causes heartburn? – Rusha Modi. YouTube. https://www.youtube.com/watch?v=jP-9AD0wMOk&feature=youtu.be

TEDx Talks. (2015, December 7). Food for thought: How your belly controls your brain | Ruairi Robertson | TEDxFulbrightSantaMonica. YouTube. https://www.youtube.com/watch?v=awtmTJW9ic8&feature=youtu.be




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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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