لخّصلي

es of the diet are either large polysaccharides or di- saccharides, which are combinations of monosaccharides bound to one another by condensation.Specific enzymes in the digestive juices of the gastrointestinal tract return the H+ and OH- from H,0 to the polysaccharides and thereby separate the monosac- charides from each other.The two monosaccha- rides then combine with each other at these sites of remov- al, and the Ht and OH - then combine to form water (H,O).When carbohydrates are digested, this process is reversed, and the carbohydrates are converted into mono- saccharides.This phenomenon means that a hydrogen ion (H+) has been removed from one of the monosaccharides, and a hydroxyl ion (OH-) has been removed from the next one.

es of the diet are either large polysaccharides or di-
saccharides, which are combinations of monosaccharides
bound to one another by condensation. This phenomenon
means that a hydrogen ion (H+) has been removed from
one of the monosaccharides, and a hydroxyl ion (OH-) has
been removed from the next one. The two monosaccha-
rides then combine with each other at these sites of remov-
al, and the Ht and OH - then combine to form water (H,O).
When carbohydrates are digested, this process is
reversed, and the carbohydrates are converted into mono-
saccharides. Specific enzymes in the digestive juices of the
gastrointestinal tract return the H+ and OH- from H,0
to the polysaccharides and thereby separate the monosac-
charides from each other. This process, called hydrolysis,
is the following (in which R"-R' is a disaccharide):
R"-R' + H,0 Digestive > R'OH + R'H
enzyme
Hydrolysis of Fats. Almost the entire fat portion of the
diet consists of triglycerides (neutral fats), which are com-
binations of three fatty acid molecules condensed with a
single glycerol molecule. During condensation, three mol-
ecules of water are removed.
Hydrolysis (digestion) of the triglycerides consists of
the reverse process: the fat-digesting enzymes return
three molecules of water to the triglyceride molecule
and thereby split the fatty acid molecules away from the
glycerol.
Hydrolysis of Proteins. Proteins are formed from multi-
ple amino acids that are bound together by peptide link-
ages. At each linkage, a OH- has been removed from one
amino acid and a H+ has been removed from the succeed-
ing one; thus, the successive amino acids in the protein
chain are also bound together by condensation, and di-
gestion occurs by the reverse effect: hydrolysis. That is,
the proteolytic enzymes return H+ and OH- from water
molecules to the protein molecules to split them into their
constituent amino acids.
Therefore, the chemistry of digestion is simple because,
in the case of all three major types of food, the same basic
process of hydrolysis is involved. The only difference lies
in the types of enzymes required to promote the hydroly-
sis reactions for each type of food.
All the digestive enzymes are proteins. Their secretion
by the different gastrointestinal glands was discussed in
Chapter 65.
DIGESTION OF CARBOHYDRATES
Carbohydrate Foods of the Diet. Only three major
sources of carbohydrates exist in the normal human diet.
They are sucrose, which is the disaccharide known popu-
larly as cane sugar; lactose, which is a disaccharide found
in milk; and starches, which are large polysaccharides
present in almost all nonanimal foods, particularly in po-
tatoes and different types of grains. Other carbohydrates
ingested to a slight extent are amylose, glycogen, alcohol,
lactic acid, pyruvic acid, pectins, dextrins, and minor
quantities of carbohydrate derivatives in meats.
The diet also contains a large amount of cellulose, which
is a carbohydrate. However, enzymes capable of hydrolyz-
ing cellulose are not secreted in the human digestive tract.
Consequently, cellulose cannot be considered a food for
humans.
Digestion of Carbohydrates Begins in the Mouth
and Stomach. When food is chewed, it is mixed with