Biology of Disease

can also be absorbed by a paracellular route where they pass between entero-
cytes, rather than being absorbed across the luminal surface. Relatively large
peptides can be absorbed by this method and may initiate an allergic reaction
leading to food allergies (Chapter 5 and 10 ).

Digestion of RNA produces nucleotides that are further degraded to nucle-
osides at the brush border and which, again, are absorbed in a Na+-dependent
manner. Catabolism within the cytoplasm converts the nucleotides to ribose
phosphate and bases. Eventually the purine bases are converted to urate and
the pyrimidines to uracil as shown in Figure 11.19.

Fatty acids, monoacylglycerols, monoacylphospholipids and cholesterol are
absorbed as mixed micelles by the brush border of the enterocytes (Figure
11.20). The triacylglycerols and phospholipids are reformed within the ente-
rocyte cytoplasm and packaged into large lipoprotein complexes called chy-
lomicrons (Chapter 14) that are transported from the GIT in lacteals of the
lymphatic system. This ensures the lipids bypass the liver and are delivered to
the blood through the thoracic duct.

Water-soluble vitamins are taken up by enterocytes by a variety of mecha-
nisms. Vitamins B 1 (thiamin) and B 2 (riboflavin) are absorbed in the upper
portion of the small intestine. Thiamin is actively transferred to the portal
system. Specific transporter proteins actively accumulate niacin (nicotinic
acid and nicotinamide), folic acid and biotin (vitamin H) in Na+-dependent
fashions. Pantothenic acid and the vitamers of vitamin B 6 are absorbed by
diffusion. Vitamin C is absorbed in the jejunum by a Na+-dependent mecha-
nism, similar to that described for glucose. The fat-soluble vitamins, A, D, E
and K, are absorbed within the mixed micelles of fatty acids, monoacylglyc-

ABSORPTION OF THE PRODUCTS OF DIGESTION

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Na+/K+-ATPase

PepT1

Na+/H+ transporter

Na+/amino acid transporter

Tight junction

Capillary

GIT lumen

3Na+2K+

3Na+2K+

ATP ADP

Na+ Na+Amino acids

Na+

Amino acids

Amino acids

H+

H+

H+

Peptide

Peptide

H 2 O

Figure 11.18Overview of the absorption of

amino acids and peptides by an enterocyte.

Transport movements are denoted by colored

lines, chemical transformations in black.See text

for general details.