Chapter 24 Module 5 Digestion of Macromolecules

Chapter 24 Module 5 Digestion of Macromolecules


This is Chapter 24,
Module Five– Digestion and Absorption of Carbohydrates,
Lipids, and Proteins. The learning objectives
of this module are one, describe
the chemical events responsible for the digestion
of carbohydrates, lipids, and proteins. And number two,
describe the mechanisms involved in the absorption
of carbohydrates, lipids, and proteins. We’ll start with the
life of a carbohydrate. Keep in mind that a
complex carbohydrate is called a polysaccharide. This must be broken down
into small complexes called disaccharides and
trisaccharides, which must then be broken down into the smallest
unit, called a monosaccharide. It is the monosaccharide
that will be absorbed. Carbohydrates begin
their chemical digestion in the mouth. The salivary amylase secreted
by the salivary glands will begin the breakdown
of carbohydrates. For the most part, no
absorption of carbohydrates takes place in the mouth. The carbohydrates
enter the esophagus where the salivary amylase will
continue to chemically digest the carbohydrates. These polysaccharides
will be broken down into disaccharides
and trisaccharides by these enzymes. The disaccharides are
carried to the stomach. In the stomach, after
about one to two hours, the hydrochloric acid drops
the pH to 2.0 or below, and this stops the activity
of the salivary amylase. So chemical digestion of the
remaining polysaccharides stops temporarily
in the stomach. In the duodenum,
pancreatic amylase is released into the
duodenum and will break down the remaining complex
carbohydrates. Any disaccharides and
trisaccharides produced will not be broken
down any further until they contact
the intestinal mucosa. Brush Border enzymes of
the intestinal microvilli break these disaccharides
and trisaccharides into monosaccharides,
or simple sugars, that are then small
enough to be absorbed. The intestinal epithelium then
absorbs the monosaccharides by facilitated diffusion
and co-transport mechanisms. The simple sugars that are
transported into the cell will diffuse into the cytoplasm
of the epithelial cells, then carried back by
facilitated diffusion across the basement membrane. From there, the
monosaccharides will diffuse into the
capillaries of the villi where they will be
transported to the liver through blood vessels of
the hepatic portal system. Now it’s your turn. Draw a concept map
of a carbohydrate as it begins as a
polysaccharide and ends up as a monosaccharide that
diffuses into the bloodstream and transported to the liver. Next we’ll look at
the life of a lipid. Keep in mind that
the lipid is usually ingested as a triglyceride. The triglycerides congregate
together as a large lipid drop. Triglycerides get broken down
into monoglycerides and fatty acids. Lipids begin their chemical
digestion in the mouth. The most important and
abundant dietary lipids are the triglycerides. These triglycerides tend
to form large lipid drops as their hydrophobic ends
repel away from water. These lipid drops are large
enough that only about 20% of them will be digested by the
time they enter the duodenum. Small amounts of lingual
lipase from the small glands of the tongue begin breaking
down these triglycerides that are ingested. Two fatty acid tails are
broken up the glycerol head, leaving one fatty acid
tail and a glycerol, which we now call
a monoglyceride. Bile salts from the
liver and the gallbladder will help us to emulsify these
large drops into tiny droplets, making it easier for
further breakdown. This emulsification occurs only
after the chyme has been mixed with the bile in the duodenum. Pancreatic lipase
released from the pancreas will enter the
duodenum and break down the tiny droplets
of triglycerides into a mixture of fatty
acids and monoglycerides. These fatty acids
and monoglycerides will react with bile
salts to form micelles. When a micelle contacts
the intestinal epithelium, the lipids diffuse across the
cell membrane of the epithelium and enter the cytoplasm. The intestinal cell walls
synthesize new triglycerides from these monoglycerides
and fatty acids. These new triglycerides are
then coated with proteins and the resulting complexes
are called chylomicrons. The protein coating keeps
the chylomicron suspended in the interstitial
fluid, but they are too large to enter
into the blood capillaries. Instead, they diffuse into
the intestinal lacteals and from there they
are transported through larger lymphatic
vessels toward the heart. Close to the heart, the
lymphatic vessels deposit their fluid into the subclavian
veins, and the lymphatic fluid along with the chylomicrons
are deposited into the blood. Most of the chylomicrons
and bile salts are reabsorbed by the body. Only about 1% of the bile salts
leave the body in the feces. Now it’s your turn. Draw a concept map of a lipid
as it begins as triglycerides bundled together in
a large lipid, then breaks down into
monoglycerides and fatty acids, and then reacts with bile
salts to form a micelle. Then diffuses across
the cell membrane where new triglycerides
are formed from the monoglycerides and
fatty acids, then coated with proteins. And finally, then diffused
into the lacteals where they will be carried
close to the heart and deposited into the
subclavian veins and mix with blood. Next we’re going to talk
about the life of a protein. Proteins are complex,
so protein digestion is complex and time consuming. First, mechanical breakdown by
the teeth occurs in the mouth. Then chemical processing
begins in the stomach. There’s no chemical digestion
of proteins in the mouth. The strong acid in the
stomach kills pathogens and breaks down plant cell walls
and animal connective tissues. This acid also disrupts the
tertiary and secondary protein structures, exposing
the peptide bonds. The acidic contents
of the stomach also help increase the
activity of pepsin, which is secreted by the
chief cells of the stomach. This is a proteolytic enzyme. In other words, it
breaks the peptide bonds within the proteins, or
within the polypeptides. This enzyme works well at the
low pH of the stomach, which is 1.5 to 2.0. When the chyme
reaches the duodenum, the proteases from the
pancreas can begin working. These proteases are trypsin,
chymotrypsin, and elastase. They act like pepsin and
break specific peptide bonds with any polypeptide, creating
shorter polypeptide chains. The epithelial surface
of the small intestine contain several peptidases,
most notably dipeptidases. These are enzymes that will
break down short polypeptide chains into individual
amino acids. These amino acids are absorbed
through facilitated diffusion and co-transport mechanisms
into the blood capillaries found within the villi. The amino acids enter
the blood, which will then be
transported to the liver through the hepatic
portal system. Now it’s your turn. Draw a concept map of how a
large protein or a polypeptide is mechanically broken
down in the mouth, enters the stomach
to be broken down into a shorter polypeptide chain
by the acid and the pepsin. Enters the duodenum to
be further broken down into small peptide
chains, and then broken apart by dipeptidases
into individual amino acids. And finally, transported
into the blood capillaries to be transported to the liver. This ends Chapter 24,
Module Five– Digestion and Absorption of Carbohydrates,
Lipids, and Proteins.

Leave a Reply

Your email address will not be published. Required fields are marked *