Digestive System Processes

3/3/23, 1:00 AM Digestive System Processes | Biology for Majors II
Biology for Majors II
Module 21: The Digestive System
Digestive System Processes
https://courses.lumenlearning.com/wm-biology2/chapter/digestive-system-processes/ 1/13
LEARNING OUTCOMES
Detail the steps involved in the digestive system

processes
Ingestion
The large molecules found in intact food cannot pass through the cell
membranes. Food needs to be broken into smaller particles so that ani-
mals can harness the nutrients and organic molecules. The first step in
this process is ingestion. Ingestion is the process of taking in food
through the mouth. In vertebrates, the teeth, saliva, and tongue play im-
portant roles in mastication (preparing the food into bolus). While the
food is being mechanically broken down, the enzymes in saliva begin to
chemically process the food as well. The combined action of these pro-
cesses modifies the food from large particles to a soft mass that can be
swallowed and can travel the length of the esophagus.
Digestion and Absorption
Digestion is the mechanical and chemical break down of food into small
organic fragments. It is important to break down macromolecules into
smaller fragments that are of suitable size for absorption across the di-
gestive epithelium. Large, complex molecules of proteins, polysaccha-
rides, and lipids must be reduced to simpler particles such as simple
sugar before they can be absorbed by the digestive epithelial cells.
Different organs play specific roles in the digestive process. The animal
diet needs carbohydrates, protein, and fat, as well as vitamins and inor-
ganic components for nutritional balance. How each of these compo-
nents is digested is discussed in the following sections.
Carbohydrates
The digestion of carbohydrates begins in the mouth. The salivary en-
zyme amylase begins the breakdown of food starches into maltose, a

disaccharide. As the bolus of food travels through the esophagus to the
stomach, no significant digestion of carbohydrates takes place. The
esophagus produces no digestive enzymes but does produce mucous
for lubrication. The acidic environment in the stomach stops the action of
the amylase enzyme.
The next step of carbohydrate digestion takes place in the duodenum.

Recall that the chyme from the stomach enters the duodenum and mixes
with the digestive secretion from the pancreas, liver, and gallbladder.
Pancreatic juices also contain amylase, which continues the breakdown
of starch and glycogen into maltose, a disaccharide. The disaccharides
are broken down into monosaccharides by enzymes called maltases,
sucrases, and lactases, which are also present in the brush border of
the small intestinal wall. Maltase breaks down maltose into glucose.
Other disaccharides, such as sucrose and lactose are broken down by
sucrase and lactase, respectively. Sucrase breaks down sucrose (or “ta-
ble sugar”) into glucose and fructose, and lactase breaks down lactose
(or “milk sugar”) into glucose and galactose. The monosaccharides (glu-
cose) thus produced are absorbed and then can be used in metabolic
pathways to harness energy. The monosaccharides are transported
across the intestinal epithelium into the bloodstream to be transported
to the different cells in the body. The steps in carbohydrate digestion are
summarized in Figure 1 and Table 1.
Figure 1. Digestion of carbohydrates is performed by several enzymes.

Starch and glycogen are broken down into glucose by amylase and maltase.
Sucrose (table sugar) and lactose (milk sugar) are broken down by sucrase
and lactase, respectively.
Table 1. Digestion of Carbohydrates
Produced Site of Substrate

Enzyme End Produ
By Action Acting On
Disacchari
Salivary Polysaccharides
Salivary amylase Mouth (maltose),
glands (Starch)
oligosacch
Disacchari
Small Polysaccharides
Pancreatic amylase Pancreas (maltose),
intestine (starch)
monosacc
Lining of
the Monosacc
intestine; Small (e.g., gluco
Oligosaccharidases Disaccharides
brush intestine fructose,
border galactose)
membrane
Protein
A large part of protein digestion takes place in the stomach. The enzyme
pepsin plays an important role in the digestion of proteins by breaking
down the intact protein to peptides, which are short chains of four to
nine amino acids. In the duodenum, other enzymes—trypsin, elastase,
and chymotrypsin—act on the peptides reducing them to smaller pep-
tides. Trypsin elastase, carboxypeptidase, and chymotrypsin are pro-
duced by the pancreas and released into the duodenum where they act
on the chyme. Further breakdown of peptides to single amino acids is
aided by enzymes called peptidases (those that break down peptides).
Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play
important roles in reducing the peptides to free amino acids. The amino
acids are absorbed into the bloodstream through the small intestines.
The steps in protein digestion are summarized in Figure 2 and Table 2.
Figure 2. Protein digestion is a multistep process that begins in the stomach

and continues through the intestines.
Table 2. Digestion of Protein
Substrate
Produced Site of End
Enzyme Acting
By Action Products
On
Pepsin Stomach
Stomach Proteins Peptides
chief cells
Trypsin
Small
Elastase Pancreas Proteins Peptides
intestine
Chymotrypsin
Amino
Carboxypeptidase Small acids
Pancreas Peptides
intestine and
peptides
Aminopeptidase Lining of Small Amino

Peptides
Dipeptidase intestine intestine acids
Lipids
Lipid digestion begins in the stomach with the aid of lingual lipase and
gastric lipase. However, the bulk of lipid digestion occurs in the small in-
testine due to pancreatic lipase. When chyme enters the duodenum, the
hormonal responses trigger the release of bile, which is produced in the
liver and stored in the gallbladder. Bile aids in the digestion of lipids, pri-
marily triglycerides by emulsification. Emulsification is a process in which
large lipid globules are broken down into several small lipid globules.
These small globules are more widely distributed in the chyme rather
than forming large aggregates. Lipids are hydrophobic substances: in
the presence of water, they will aggregate to form globules to minimize
exposure to water. Bile contains bile salts, which are amphipathic, mean-
ing they contain hydrophobic and hydrophilic parts. Thus, the bile salts
hydrophilic side can interface with water on one side and the hydropho-
bic side interfaces with lipids on the other. By doing so, bile salts emul-
sify large lipid globules into small lipid globules.
Why is emulsification important for digestion of lipids? Pancreatic juices

contain enzymes called lipases (enzymes that break down lipids). If the
lipid in the chyme aggregates into large globules, very little surface area
of the lipids is available for the lipases to act on, leaving lipid digestion
incomplete. By forming an emulsion, bile salts increase the available sur-
face area of the lipids many fold. The pancreatic lipases can then act on
the lipids more efficiently and digest them, as detailed in Figure 3.
Lipases break down the lipids into fatty acids and glycerides. These mol-
ecules can pass through the plasma membrane of the cell and enter the
epithelial cells of the intestinal lining. The bile salts surround long-chain
fatty acids and monoglycerides forming tiny spheres called micelles. The
micelles move into the brush border of the small intestine absorptive
cells where the long-chain fatty acids and monoglycerides diffuse out of
the micelles into the absorptive cells leaving the micelles behind in the
chyme. The long-chain fatty acids and monoglycerides recombine in the
absorptive cells to form triglycerides, which aggregate into globules and
become coated with proteins. These large spheres are called chylomi-
crons. Chylomicrons contain triglycerides, cholesterol, and other lipids
and have proteins on their surface. The surface is also composed of the
hydrophilic phosphate “heads” of phospholipids. Together, they enable
the chylomicron to move in an aqueous environment without exposing
the lipids to water. Chylomicrons leave the absorptive cells via exocyto-
sis. Chylomicrons enter the lymphatic vessels, and then enter the blood
in the subclavian vein.
Figure 3. Lipids are digested and absorbed in the small intestine.
Vitamins
Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins

are absorbed in the same manner as lipids. It is important to consume
some amount of dietary lipid to aid the absorption of lipid-soluble vita-
mins. Water-soluble vitamins can be directly absorbed into the blood-
stream from the intestine.
Figure 4. Mechanical and chemical digestion of food takes place in many
steps, beginning in the mouth and ending in the rectum.
This website has an overview of the digestion of protein, fat, and

carbohydrates.
PRACTICE QUESTION
Which of the following statements about digestive processes
is true?
a. Amylase, maltase, and lactase in the mouth digest

carbohydrates.
b. Trypsin and lipase in the stomach digest protein.
c. Bile emulsifies lipids in the small intestine.
d. No food is absorbed until the small intestine.
Show Answer
Elimination
The final step in digestion is the elimination of undigested food content

and waste products. The undigested food material enters the colon,
where most of the water is reabsorbed. Recall that the colon is also
home to the microflora called “intestinal flora” that aid in the digestion
process. The semi-solid waste is moved through the colon by peristaltic
movements of the muscle and is stored in the rectum. As the rectum ex-
pands in response to storage of fecal matter, it triggers the neural signals
required to set up the urge to eliminate. The solid waste is eliminated
through the anus using peristaltic movements of the rectum.
Common Problems with Elimination
Diarrhea and constipation are some of the most common health con-
cerns that affect digestion. Constipation is a condition where the feces
are hardened because of excess water removal in the colon. In contrast,
if enough water is not removed from the feces, it results in diarrhea.
Many bacteria, including the ones that cause cholera, affect the proteins
involved in water reabsorption in the colon and result in excessive
diarrhea.
Emesis
Emesis, or vomiting, is elimination of food by forceful expulsion through

the mouth. It is often in response to an irritant that affects the digestive
tract, including but not limited to viruses, bacteria, emotions, sights, and
food poisoning. This forceful expulsion of the food is due to the strong
contractions produced by the stomach muscles. The process of emesis
is regulated by the medulla.
IN SUMMARY: DIGESTIVE SYSTEM PROCESSES
Digestion begins with ingestion, where the food is taken in the

mouth. Digestion and absorption take place in a series of steps
with special enzymes playing important roles in digesting car-
bohydrates, proteins, and lipids. Elimination describes removal
of undigested food contents and waste products from the
body. While most absorption occurs in the small intestines, the
large intestine is responsible for the final removal of water that
remains after the absorptive process of the small intestines.
The cells that line the large intestine absorb some vitamins as
well as any leftover salts and water. The large intestine (colon)
is also where feces is formed.
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3/3/23, 1:00 AM Digestive System Processes | Biology for Majors II

Biology for Majors II

Module 21: The Digestive System

Detail the steps involved in the digestive system

Digestion and Absorption

The digestion of carbohydrates begins in the mouth. The salivary en-

zyme amylase begins the breakdown of food starches into maltose, a

The next step of carbohydrate digestion takes place in the duodenum.

Figure 1. Digestion of carbohydrates is performed by several enzymes.

Table 1. Digestion of Carbohydrates

Produced Site of Substrate

Figure 2. Protein digestion is a multistep process that begins in the stomach

Table 2. Digestion of Protein

Aminopeptidase Lining of Small Amino

Why is emulsification important for digestion of lipids? Pancreatic juices

Figure 3. Lipids are digested and absorbed in the small intestine.

Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins

This website has an overview of the digestion of protein, fat, and

Which of the following statements about digestive processes

a. Amylase, maltase, and lactase in the mouth digest

b. Trypsin and lipase in the stomach digest protein.

c. Bile emulsifies lipids in the small intestine.

d. No food is absorbed until the small intestine.

The final step in digestion is the elimination of undigested food content

Common Problems with Elimination

Emesis, or vomiting, is elimination of food by forceful expulsion through

IN SUMMARY: DIGESTIVE SYSTEM PROCESSES

Digestion begins with ingestion, where the food is taken in the

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