Page 275 - Color Atlas of Biochemistry

266 Tissues and organs
Digestion: overview
Most components of food (see p. 360) cannot
be resorbed directly by the organism. It is only jejunum.
after they have beenbrokendowninto
smaller molecules that the organism can
take up the essential nutrients. Digestion re-
fers to the mechanical and enzymatic break-
down of food and the resorption of the result-
ing products.
A. Hydrolysis and resorption of food
components
Following mechanical fragmentation of food
during chewing in the mouth, the process of
enzymatic degradation starts in the stomach.
For this purpose, the chyme is mixed with
digestive enzymes that occur in the various
digestive secretions or in membrane-bound
form on the surface of the intestinal epithe-
lium (see p. 268). Almost all digestive en-
zymes are hydrolases (class 3 enzymes; see
p. 88); they catalyze the cleavage of compo-
site bonds with the uptake of water.
Proteins are first denatured by the
stomach’s hydrochloric acid (see p. 270), mak-
ing them more susceptible to attack by the
endopeptidases (proteinases) present in gas-
tric and pancreatic juice. The peptides re-
leased by endopeptidases are further de-
graded into amino acids by exopeptidases.Fi-
nally, the amino acids are resorbed by the
intestinal mucosa in cotransport with Na + peristalsis.
ions (see p. 220). There are separate transport
systems for each of the various groups of
amino acids.
Carbohydrates mainly occur in food in the
form of polymers (starches and glycogen).
They are cleaved by pancreatic amylase into
oligosaccharides and are then hydrolyzed by
glycosidases, whichare located onthe surface
of the intestinal epithelium, to yield mono-
saccharides. Glucose and galactose are taken
up into the enterocytes by secondary active
+
cotransport with Na ions (see p. 220). In ad-
dition, monosaccharides also have passive
transport systems in the intestine.
Nucleic acids are brokendownintotheir
components by nucleases from the pancreas
and small intestine (ribonucleases and deoxy-
ribonucleases). Further breakdown yields the
nucleobases (purine and pyrimidine deriva-
tives), pentoses (ribose and deoxyribose),
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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phosphate, and nucleosides (nucleobase pen-
tose). These cleavage products are resorbed
by the intestinal wall in the region of the
Lipids are a special problem for digestion,
as they are not soluble in water. Before enzy-
maticbreakdown, they haveto beemulsified
by bile salts and phospholipids in the bile (see
p. 314). At the water–lipid interface, pancre-
atic lipase then attacks triacylglycerols with
the help of colipase (see p. 270). The cleavage
products include fatty acids, 2–monoacylgly-
cerols, glycerol, and phosphate from phospho-
lipid breakdown. After resorption into the
epithelial cells, fats are resynthesized from
fatty acids, glycerol and 2–monoacylglycerols
and passed into the lymphatic system (see
p. 272). The lipids in milk are more easily
digested, as they are already present in emul-
sion; on cleavage, they mostly provide short-
chain fatty acids.
Inorganic components such as water, elec-
trolytes, and vitamins are directly absorbed by
the intestine.
High-molecular-weight indigestible com-
ponents, such as the fibrous components of
plant cell walls, which mainly consist of cel-
lulose and lignin, pass through the bowel un-
changed and form the main component of
feces, in addition to cells shed from the intes-
tinal mucosa. Dietary fiber makes a positive
contribution to digestion as a ballast material
by binding water and promoting intestinal
The food components resorbed by the epi-
thelialcells of theintestinalwallinthe region
of the jejunum and ileum are transported
directly to the liver via the portal vein.Fats,
cholesterol, and lipid–soluble vitamins are
exceptions. These are first released by the
enterocytes in the form of chylomicrons (see
p. 278) into the lymph system, and only reach
the blood via the thoracic duct.