Chemical Digestion and Absorption - A Closer Look - Anatomy and Physiology
Chemical Digestion and Absorption - A Closer Look - Anatomy and Physiology
Chemical Digestion and Absorption - A Closer Look - Anatomy and Physiology
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CONTENTS
Learning Objectives
Identify the locations and primary secretions involved in the chemical digestion of
carbohydrates, proteins, lipids, and nucleic acids
As you have learned, the process of mechanical digestion is relatively simple. It involves the
physical breakdown of food but does not alter its chemical makeup. Chemical digestion, on the
other hand, is a complex process that reduces food into its chemical building blocks, which are
thenPrevious:
absorbed to nourish the cells of the body (Figure 1). In this section, you will look more
23.6 Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder
closely at the processes of chemical digestion and absorption.
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Chemical Digestion
Large food molecules (for example, proteins, lipids, nucleic acids, and starches) must be broken
down into subunits that are small enough to be absorbed by the lining of the alimentary canal.
This is accomplished by enzymes through hydrolysis. The many enzymes involved in chemical
digestion are summarized in Table 8.
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Enzyme
Category Enzyme Name Source Substrate Product
Aminopeptidase: Aminopeptidase:
amino acids at the amino acids and
Brush
Small amino end of peptides peptides
border Peptidases
intestine
enzymes
Dipeptidase: Dipeptidase: amino
dipeptides acids
Brush
Small
border Sucrase Sucrose Glucose and fructose
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Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder
enzymes
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Enzyme
Category Enzyme Name Source Substrate Product
Pancreatic
Pancreatic Carboxy- Amino acids at the carboxyl
acinar Amino acids and peptides
enzymes peptidase* end of peptides
cells
Pancreatic
Pancreatic
Chymotrypsin* acinar Proteins Peptides
enzymes
cells
Pancreatic
Pancreatic
Elastase* acinar Proteins Peptides
enzymes
cells
Ribonuclease:
ribonucleic acids
Pancreatic
Pancreatic
Nucleases acinar Deoxyribonuclease: Nucleotides
enzymes
cells
deoxyribonucleic
acids
Carbohydrate Digestion
The average American diet is about 50 percent carbohydrates, which may be classified according
to the number of monomers they contain of simple sugars (monosaccharides and disaccharides)
and/or complex sugars (polysaccharides). Glucose, galactose, and fructose are the three
monosaccharides that are commonly consumed and are readily absorbed. Your digestive system is
also able to break down the disaccharide sucrose (regular table sugar: glucose + fructose), lactose
(milk sugar:23.6
Previous: glucose + galactose),
Accessory Organs in and maltose
Digestion: The(grain sugar: glucose
Liver, Pancreas, + glucose), and the
and Gallbladder
polysaccharides glycogen and starch (chains of monosaccharides). Your bodies do not produce
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enzymes that can break down most fibrous polysaccharides, such as cellulose.Increase Font Size
While indigestible
polysaccharides do not provide any nutritional value, they do provide dietary fiber, which helps
propel food through the alimentary canal.
The chemical digestion of starches begins in the mouth and has been reviewed above.
In the small intestine, pancreatic amylase does the ‘heavy lifting’ for starch and carbohydrate
digestion (Figure 2). After amylases break down starch into smaller fragments, the brush border
enzyme α-dextrinase starts working on α-dextrin, breaking off one glucose unit at a time. Three
brush border enzymes hydrolyze sucrose, lactose, and maltose into monosaccharides. Sucrase
splits sucrose into one molecule of fructose and one molecule of glucose; maltase breaks down
maltose and maltotriose into two and three glucose molecules, respectively; and lactase breaks
down lactose into one molecule of glucose and one molecule of galactose. Insufficient lactase can
lead to lactose intolerance.
Protein Digestion
Proteins are polymers composed of amino acids linked by peptide bonds to form long chains.
Digestion reduces them to their constituent amino acids. You usually consume about 15 to 20
percent of your total calorie intake as protein.
ThePrevious:
digestion ofAccessory
23.6 protein starts
Organsininthe stomach,
Digestion: Thewhere HCl and and
Liver, Pancreas, pepsin break proteins into smaller
Gallbladder
polypeptides, which then travel to the small intestine (Figure 3). Chemical digestion in the small
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intestine is continued by pancreatic enzymes, including chymotrypsin and trypsin, eachFont Size
of which
act on specific bonds in amino acid sequences. At the same time, the cells of the brush border
secrete enzymes such as aminopeptidase and dipeptidase, which further break down peptide
chains. This results in molecules small enough to enter the bloodstream (Figure 4).
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Lipid Digestion
A healthy diet limits lipid intake to 35 percent of total calorie intake. The most common dietary
lipids are triglycerides, which are made up of a glycerol molecule bound to three fatty acid chains.
Small amounts of dietary cholesterol and phospholipids are also consumed.
The three lipases responsible for lipid digestion are lingual lipase, gastric lipase, and pancreatic
lipase. However, because the pancreas is the only consequential source of lipase, virtually all
lipid digestion occurs in the small intestine. Pancreatic lipase breaks down each triglyceride into
two free fatty acids and a monoglyceride. The fatty acids include both short-chain (less than 10 to
12 carbons) and long-chain fatty acids.
The nucleic acids DNA and RNA are found in most of the foods you eat. Two types of
pancreatic nuclease are responsible for their digestion: deoxyribonuclease, which digests DNA,
and ribonuclease, which digests RNA. The nucleotides produced by this digestion are further
broken down by two intestinal brush border enzymes (nucleosidase and phosphatase) into
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pentoses, phosphates, and nitrogenous bases, which can be absorbed through the alimentary canal
Next: Introduction
wall. The large food molecules that must be broken down into subunits are summarized Table 9
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Source Substance
Absorption
The mechanical and digestive processes have one goal: to convert food into molecules small
enough to be absorbed by the epithelial cells of the intestinal villi. The absorptive capacity of the
alimentary canal is almost endless. Each day, the alimentary canal processes up to 10 liters of
food, liquids, and GI secretions, yet less than one liter enters the large intestine. Almost all
ingested food, 80 percent of electrolytes, and 90 percent of water are absorbed in the small
intestine. Although the entire small intestine is involved in the absorption of water and lipids,
most absorption of carbohydrates and proteins occurs in the jejunum. Notably, bile salts and
vitamin B12 are absorbed in the terminal ileum. By the time chyme passes from the ileum into the
large intestine, it is essentially indigestible food residue (mainly plant fibers like cellulose), some
water, and millions of bacteria (Figure 5).
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Absorption can occur through five mechanisms: (1) active transport, (2) passive diffusion, (3)
facilitated diffusion, (4) co-transport (or secondary active transport), and (5) endocytosis. As you
will recall from Chapter 3, active transport refers to the movement of a substance across a cell
membrane going from an area of lower concentration to an area of higher concentration (up the
concentration gradient). In this type of transport, proteins within the cell membrane act as
“pumps,” using cellular energy (ATP) to move the substance. Passive diffusion refers to the
movement of substances from an area of higher concentration to an area of lower concentration,
while facilitated diffusion refers to the movement of substances from an area of higher to an area
of lower concentration using a carrier protein in the cell membrane. Co-transport uses the
movement of one molecule through the membrane from higher to lower concentration to power
the movement of another from lower to higher. Finally, endocytosis is a transportation process in
which the cell membrane engulfs material. It requires energy, generally in the form of ATP.
Previous:
Because the23.6 Accessory
cell’s plasmaOrgans in Digestion:
membrane is madeThe
upLiver, Pancreas, andphospholipids,
of hydrophobic Gallbladder water-soluble
nutrients must use transport molecules embedded in the membrane to enter cells. Moreover,
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substances cannot pass between the epithelial cells of the intestinal mucosa because Font
these Sizeare
cells
bound together by tight junctions. Thus, substances can only enter blood capillaries by passing
through the apical surfaces of epithelial cells and into the interstitial fluid. Water-soluble nutrients
enter the capillary blood in the villi and travel to the liver via the hepatic portal vein.
In contrast to the water-soluble nutrients, lipid-soluble nutrients can diffuse through the plasma
membrane. Once inside the cell, they are packaged for transport via the base of the cell and then
enter the lacteals of the villi to be transported by lymphatic vessels to the systemic circulation via
the thoracic duct. The absorption of most nutrients through the mucosa of the intestinal villi
requires active transport fueled by ATP. The routes of absorption for each food category are
summarized in Table 10.
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Breakdown Entry to
Food products Absorption mechanism bloodstream Destination
Liver via
Capillary
Carbohydrates Glucose Co-transport with sodium ions hepatic
blood in villi
portal vein
Liver via
Capillary
Carbohydrates Galactose Co-transport with sodium ions hepatic
blood in villi
portal vein
Liver via
Capillary
Carbohydrates Fructose Facilitated diffusion hepatic
blood in villi
portal vein
Liver via
Capillary
Protein Amino acids Co-transport with sodium ions hepatic
blood in villi
portal vein
Systemic
circulation
Diffusion into intestinal cells,
Long-chain fatty Lacteals of via lymph
Lipids where they are combined with
acids villi entering
proteins to create chylomicrons
thoracic
duct
Systemic
circulation
Diffusion into intestinal cells,
Lacteals of via lymph
Lipids Monoacylglycerides where they are combined with
villi entering
proteins to create chylomicrons
thoracic
duct
Liver via
Short-chain fatty Capillary
Lipids Simple diffusion hepatic
acids blood in villi
portal vein
Liver via
Capillary
Lipids Glycerol Simple diffusion hepatic
blood in villi
portal vein
Liver via
Nucleic acid Active transport via membrane Capillary
Lipids hepatic
digestion products carriers blood in villi
portal vein
Carbohydrate Absorption
All carbohydrates are absorbed in the form of monosaccharides. The small intestine is highly
Previous:
efficient 23.6 absorbing
at this, Accessory Organs in Digestion: The
monosaccharides Liver,
at an Pancreas,
estimated andofGallbladder
rate 120 grams per hour. All
normally digested dietary carbohydrates are absorbed; indigestible fibers are eliminated in the
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feces. The monosaccharides glucose and galactose are transported into the epithelial Font
cells bySize
common protein carriers via secondary active transport (that is, co-transport with sodium ions).
The monosaccharides leave these cells via facilitated diffusion and enter the capillaries through
intercellular clefts. The monosaccharide fructose (which is in fruit) is absorbed and transported by
facilitated diffusion alone. The monosaccharides combine with the transport proteins immediately
after the disaccharides are broken down.
Protein Absorption
Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as
their breakdown products, amino acids. Almost all (95 to 98 percent) protein is digested and
absorbed in the small intestine. The type of carrier that transports an amino acid varies. Most
carriers are linked to the active transport of sodium. Short chains of two amino acids (dipeptides)
or three amino acids (tripeptides) are also transported actively. However, after they enter the
absorptive epithelial cells, they are broken down into their amino acids before leaving the cell and
entering the capillary blood via diffusion.
Lipid Absorption
About 95 percent of lipids are absorbed in the small intestine. Bile salts not only speed up lipid
digestion, they are also essential to the absorption of the end products of lipid digestion. Short-
chain fatty acids are relatively water soluble and can enter the absorptive cells (enterocytes)
directly. Despite being hydrophobic, the small size of short-chain fatty acids enables them to be
absorbed by enterocytes via simple diffusion, and then take the same path as monosaccharides
and amino acids into the blood capillary of a villus.
The large and hydrophobic long-chain fatty acids and monoacylglycerides are not so easily
suspended in the watery intestinal chyme. However, bile salts and lecithin resolve this issue by
enclosing them in a micelle, which is a tiny sphere with polar (hydrophilic) ends facing the
watery environment and hydrophobic tails turned to the interior, creating a receptive environment
for the long-chain fatty acids. The core also includes cholesterol and fat-soluble vitamins. Without
micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive
surfaces of the epithelial cells. Micelles can easily squeeze between microvilli and get very near
the luminal cell surface. At this point, lipid substances exit the micelle and are absorbed via
simple diffusion.
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The free fatty acids and monoacylglycerides that enter the epithelial cells are reincorporated into
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triglycerides. The triglycerides are mixed with phospholipids and cholesterol, Increase Font Size
and surrounded
with a protein coat. This new complex, called a chylomicron, is a water-soluble lipoprotein. After
being processed by the Golgi apparatus, chylomicrons are released from the cell (Figure 6). Too
big to pass through the basement membranes of blood capillaries, chylomicrons instead enter the
large pores of lacteals. The lacteals come together to form the lymphatic vessels. The
chylomicrons are transported in the lymphatic vessels and empty through the thoracic duct into
the subclavian vein of the circulatory system. Once in the bloodstream, the enzyme lipoprotein
lipase breaks down the triglycerides of the chylomicrons into free fatty acids and glycerol. These
breakdown products then pass through capillary walls to be used for energy by cells or stored in
adipose tissue as fat. Liver cells combine the remaining chylomicron remnants with proteins,
forming lipoproteins that transport cholesterol in the blood.
The products of nucleic acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—
are transported by carriers across the villus epithelium via active transport. These products then
enter the bloodstream.
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The electrolytes absorbed by the small intestine are from both GI secretions and ingested foods.
Since electrolytes dissociate into ions in water, most are absorbed via active transport throughout
the entire small intestine. During absorption, co-transport mechanisms result in the accumulation
of sodium ions inside the cells, whereas anti-port mechanisms reduce the potassium ion
concentration inside the cells. To restore the sodium-potassium gradient across the cell
membrane, a sodium-potassium pump requiring ATP pumps sodium out and potassium in.
In general, all minerals that enter the intestine are absorbed, whether you need them or not. Iron
and calcium are exceptions; they are absorbed in the duodenum in amounts that meet the body’s
current requirements, as follows:
Iron—The ionic iron needed for the production of hemoglobin is absorbed into mucosal cells via
active transport. Once inside mucosal cells, ionic iron binds to the protein ferritin, creating iron-
ferritin complexes that store iron until needed. When the body has enough iron, most of the stored
iron is lost when worn-out epithelial cells slough off. When the body needs iron because, for
example, it is lost during acute or chronic bleeding, there is increased uptake of iron from the
intestine and accelerated release of iron into the bloodstream. Since women experience significant
iron loss during menstruation, they have around four times as many iron transport proteins in their
intestinal epithelial cells as do men.
Calcium—Blood levels of ionic calcium determine the absorption of dietary calcium. When blood
levels of ionic calcium drop, parathyroid hormone (PTH) secreted by the parathyroid glands
stimulates the release of calcium ions from bone matrices and increases the reabsorption of
calcium by the kidneys. PTH also upregulates the activation of vitamin D in the kidney, which
then facilitates intestinal calcium ion absorption.
Vitamin Absorption
The small intestine absorbs the vitamins that occur naturally in food and supplements. Fat-soluble
vitamins (A, D, E, and K) are absorbed along with dietary lipids in micelles via simple diffusion.
This is why you are advised to eat some fatty foods when you take fat-soluble vitamin
supplements. Most water-soluble vitamins (including most B vitamins and vitamin C) also are
absorbed by simple diffusion. An exception is vitamin B12, which is a very large molecule.
Intrinsic factor secreted in the stomach binds to vitamin B12, preventing its digestion and creating
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a complex that binds to mucosal receptors in the terminal ileum, where it is taken up by
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Each day, about nine liters of fluid enter the small intestine. About 2.3 liters are ingested in foods
and beverages, and the rest is from GI secretions. About 90 percent of this water is absorbed in
the small intestine. Water absorption is driven by the concentration gradient of the water: The
concentration of water is higher in chyme than it is in epithelial cells. Thus, water moves down its
concentration gradient from the chyme into cells. As noted earlier, much of the remaining water is
then absorbed in the colon.
Chapter Review
The small intestine is the site of most chemical digestion and almost all absorption. Chemical
digestion breaks large food molecules down into their chemical building blocks, which can then
be absorbed through the intestinal wall and into the general circulation. Intestinal brush border
enzymes and pancreatic enzymes are responsible for the majority of chemical digestion. The
breakdown of fat also requires bile.
Most nutrients are absorbed by transport mechanisms at the apical surface of enterocytes.
Exceptions include lipids, fat-soluble vitamins, and most water-soluble vitamins. With the help of
bile salts and lecithin, the dietary fats are emulsified to form micelles, which can carry the fat
particles to the surface of the enterocytes. There, the micelles release their fats to diffuse across
the cell membrane. The fats are then reassembled into triglycerides and mixed with other lipids
and proteins into chylomicrons that can pass into lacteals. Other absorbed monomers travel from
blood capillaries in the villus to the hepatic portal vein and then to the liver.
Review Questions
A. mouth
B. esophagus
C. stomach
D. small intestine
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C. sucrase
D. pancreatic nuclease
A. small intestine
B. gallbladder
C. liver
D. pancreas
A. glucose
B. iron
C. sodium
D. water
1. Explain the role of bile salts and lecithin in the emulsification of lipids (fats).
Glossary
α-dextrin
breakdown product of starch
α-dextrinase
brush border enzyme that acts on α-dextrins
aminopeptidase
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deoxyribonuclease
pancreatic enzyme that digests DNA
dipeptidase
brush border enzyme that acts on proteins
lactase
brush border enzyme that breaks down lactose into glucose and galactose
lipoprotein lipase
enzyme that breaks down triglycerides in chylomicrons into fatty acids and monoglycerides
maltase
brush border enzyme that breaks down maltose and maltotriose into two and three molecules
of glucose, respectively
micelle
tiny lipid-transport compound composed of bile salts and phospholipids with a fatty acid and
monoacylglyceride core
nucleosidase
brush border enzyme that digests nucleotides
pancreatic amylase
enzyme secreted by the pancreas that completes the chemical digestion of carbohydrates in the
small intestine
pancreatic lipase
enzyme secreted by the pancreas that participates in lipid digestion
pancreatic nuclease
enzyme secreted by the pancreas that participates in nucleic acid digestion
phosphatase
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brush border enzyme that digests nucleotides
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sucrase
brush border enzyme that breaks down sucrose into glucose and fructose
Solutions
1. A
2. B
3. D
4. B
1. Bile salts and lecithin can emulsify large lipid globules because they are
amphipathic; they have a nonpolar (hydrophobic) region that attaches to the large fat
molecules as well as a polar (hydrophilic) region that interacts with the watery chime
in the intestine.
2. Intrinsic factor secreted in the stomach binds to the large B12 compound, creating a
combination that can bind to mucosal receptors in the ileum.
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