Dentistry department
Lec. 12
Dijlah university college
Biochemistry
Ass. Lec. Noor Kadhim
Digestion and absorption
Digestion
Is the breakdown of food into smaller components that can be more easily
absorbed and assimilated by the body. In certain organisms, these smaller
substances are absorbed through the small intestine into the blood stream.
Digestion is a form of catabolism that is often divided into two processes based on
how food is broken down: mechanical and chemical digestion. The term
mechanical digestion refers to the physical breakdown of large pieces of food into
smaller pieces which can subsequently be accessed by digestive enzymes. In
chemical digestion, enzymes break down food into the small molecules the body
can use.
In the human digestive system, food enters the mouth and mechanical digestion of
the food starts by the action of mastication, a form of mechanical digestion, and
the wetting contact of saliva. Saliva, a liquid secreted by the salivary glands,
contains salivary amylase, an enzyme which starts the digestion of starch in the
food. After undergoing mastication and starch digestion, the food will be in the
form of a small, round slurry mass called a bolus. It will then travel down the
esophagus and into the stomach by the action of peristalsis.
Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains
hydrochloric acid and pepsin. As these two chemicals may damage the stomach
wall, mucus is secreted by the stomach, providing a slimy layer that acts as a
shield against the damaging effects of the chemicals. At the same time protein
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Dentistry department
Dijlah university college
digestion is occurring, mechanical mixing occurs by peristalsis, which is waves of
muscular contractions that move along the stomach wall. This allows the mass of
food to further mix with the digestive enzymes.
After some time (typically 1–2 hours in humans, 4–6 hours in dogs, 3–4 hours in
house cats) the resulting thick liquid is called chyme. When the pyloric sphincter
valve opens, chyme enters the duodenum where it mixes with digestive enzymes
from the pancreas, and then passes through the small intestine, in which digestion
continues When the chyme is fully digested, it is absorbed into the blood. 95% of
absorption of nutrients occurs in the small intestine. Water and minerals are
reabsorbed back into the blood in the colon (large intestine) where the pH is
slightly acidic about 5.6 ~ 6.9. Some vitamins, such as biotin and vitamin K
(K2MK7) produced by bacteria in the colon are also absorbed into the blood in the
colon. Waste material is eliminated from the rectum during defecation.
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Dentistry department
Dijlah university college
 Metabolism consist of two types of reactions: catabolism and anabolism
 Catabolism: process in which macromolecules are broken down to yield
energy and simple organic compounds. Catabolic reactions mainly involve
oxidative degradation.
 Anabolism: process in which simple organic compound and energy are used
to form macromolecules. Anabolic reactions mainly involve reductive
biosynthesis.
 Digestion: is the process in which complex food substances (carbohydrates,
proteins and lipids) are degraded to products that can be absorbed.
 Absorption: (or uptake) of these products occur when they are transported
into absorptive cells in the small intestine.
Main organs associated with metabolism
Mouth: carbohydrates
Stomach: proteins, lipids
Small intestine: proteins, lipids and nucleic acids.
Which organ contain which enzyme (or substances)?
Mouth: amylase, saliva and mucins.
Stomach: HCl, pepsinogen (pepsin), gastric lipase and mucins.
Liver: bile
Gall bladder: bile (concentrated)
Pancreas: pancreatic juice (ex. Lipase, peptidases, (chymo) trypsinogen,
amylase and etc…..)
Small intestine: bile (from the gall bladder), pancreatic juice.
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Dentistry department
Dijlah university college
Carbohydrates digestion:
 Carbohydrates digestion involve the enzymatic degradation of di-, tri and
polysaccharide to monosaccharide (glucose, fructose, galactose ).
 Digestion begin in the mouth with salivary amylase (hydrolyze α (1-4) bond .
 Polysaccharide are then further broken down by pancreatic amylase (also
hydrolyzes α (1-4) bonds in the small intestine.
 Final degradation occurs on the surface (brush border) of the absorptive
cells in the jejunum. The brush border secrets amylo-(1-6) glucosidase
(hydrolyze α (1-6) bonds, lactase, sucrase and maltase.
 Once carbohydrates are broken down into monosaccharides, then absorbed
into the intestinal cells by facilitated (membrane carrier) or active transport
(membrane carrier, Na+, and ATP)
 Fructose- facilitative transport
 Glucose and galactose- active transport.
 Lactose intolerance: caused by deficiency in lactase. Lactose is not
hydrolyzed and accumulates in the intestine where it is fermented by
intestinal microorganisms.
Lipid digestion and absorption:
 Lipid digestion is the enzymatic degradation of fats (TAGS ) to
monoacylglycerols and fatty acids.
 Degradation (lipolysis) begins in the stomach with (slow acting). Gastric
lipases.
 Pancreatic lipases and bile are mixed with hydrolyzed products (TAG) in the
duodenum. The bile emulsified the fat into fat droplets (micelles). While the
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Dentistry department
Dijlah university college
action of colipase and pancreatic lipase act on the product to produce
monoacylglycerol and 2 fatty acids chains.
 Monoacylglycerol and long chain fatty acids are packaged into mixed
micelles (cholesterol, bile salts and fat soluble vitamins) and diffuse across
the cellular membrane (of the jejunum).
 Fatty acids with 10 carbon atoms are absorbed across the membrane.
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