Page 281 - Color Atlas of Biochemistry
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272 Tissues and organs
Resorption acids. Individual amino acid groups have
group–specific amino acid transporters, some
Enzymatic hydrolysis in the digestive tract of which transport the amino acids into the
+
breaks down foodstuffs into their resorbable enterocytes in cotransport with Na ions (sec-
components. Resorption of the cleavage prod- ondary active transport), while others trans-
+
ucts takes place primarily in the small intes- port them in an Na –independent manner
tine. Only ethanol and short–chain fatty acids through facilitated diffusion. Small peptides
are already resorbed to some extent in the can also be taken up.
stomach.
The resorption process is facilitated by the
large inner surface of the intestine, with its B. Lipids
brush–border cells. Lipophilic molecules pen- Fats and other lipids are poorly soluble in
etrate the plasma membrane of the mucosal water. The larger the accessible surface
cells by simple diffusion, whereas polar mol- is—i. e., the better the fat is emulsified—the
ecules require transporters (facilitated diffu- easier it is for enzymes to hydrolyze it (see
sion; see p. 218). In many cases, carrier-medi- p. 270). Due to the special properties of milk,
+
ated cotransport with Na ions can be ob- milk fats already reach the gastrointestinal
served. In this case, the difference in the con- tract in emulsified form. Digestion of them
centration of the sodium ions (high in the therefore already starts in the oral cavity
intestinal lumen and low in the mucosal cells) and stomach, where lipases in the saliva and
drives the import of nutrients against a con- gastric juice are available. Lipids that are less
centration gradient (secondary active trans- accessible—e. g., from roast pork—are emulsi-
port; see p. 220). Failure of carrier systems in fied in the small intestine by bile salts and bile
the gastrointestinal tract can result in dis- phospholipids. Only then are they capable of
eases. being attacked by pancreatic lipase [4] (see
p. 270).
Fats (triacylglycerols) are mainly attacked
A. Monosaccharides
by pancreatic lipase at positions 1 and 3 of the
The cleavage of polymeric carbohydrates by glycerol moiety. Cleavage of two fatty acid
a–amylase [1] leads to oligosaccharides, residues gives rise to fatty acids and 2-mono-
which are broken down further by exoglyco- acylglycerols, which are quantitatively the
sidases (oligosaccharidases and disacchari- most important products. However, a certain
dases [2]) on the membrane surface of the amount of glycerol is also formed by complete
brush border. The monosaccharides released hydrolysis. These cleavage products are re-
in this way then pass with the help of various sorbed by a non-ATP-dependent process
sugar–specific transporters into the cells of that has not yet been explained in detail.
intestinal epithelium. Secondary active In the mucosal cells, long-chain fatty acids
transport serves for the uptake of glucose are resynthesized by an ATP-dependent ligase
and galactose, which are transported against [5] to form acyl-CoA and then triacylglycerols
a concentration gradient in cotransport with (fats; see p. 170). The fats are released into the
+
+
Na .The Na gradient is maintained on the lymph in the form of chylomicrons (see
+
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basal side of the cells by Na /K -ATPase [3]. p. 278) and, bypassing the liver, are deposited
Another passive transporter then releases in thethoracic duct—i. e., theblood system.
glucose and galactose into the blood. Cholesterol also follows this route.
Fructose is taken up by a special type of trans- By contrast, short-chain fatty acids (with
porter using facilitated diffusion. chain lengthsof lessthan 12Catoms) pass
directly into the blood and reach the liver via
the portal vein. Resorbed glycerol can also
Amino acids (not illustrated)
take this path.
Protein degradation is initiated by proteina-
ses—bypepsins in thestomach andbytrypsin,
chymotrypsin, and elastase in the small intes-
tine. The resulting peptides are then further
hydrolyzed by various peptidases into amino
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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