Figure 1631

Diagram showing the digestion and absorption of protein by an enterocyte. Proteins entering the alimentary canal are completely digested into free amino acids and small dipeptide or tripeptide fragments. Protein digestion starts in the stomach with pepsin, which hydrolyzes proteins to large polypeptides. The next step occurs in the small intestine by the action of pancreatic proteolytic enzymes. The activation process is shown in Figure 16.29. Free amino acids are transported by four different amino acid transporters and several dipeptide and tripeptide transporters into the cell and then from the cell into the underlying capillaries of the portal circulation.

externa exhibits three thickened, equally spaced bands known as the teniae coli.

• The external surface of the cecum and colon exhibits sacculations known as haustra that are visible between the teniae. The mucosa has a "smooth" surface; neither plicae circulares nor villi are present.

• Small fatty projections of the serosa known as omental appendices are visible on the outer intestinal surface.

Mucosa

The mucosa of the large intestine contains numerous straight tubular intestinal glands (crypts of Lieberkiihn) that extend through the full thickness of the mucosa (Fig. 16.33a). The glands consist of simple columnar epithelium, as does the intestinal surface from which they invaginate. Examination of the luminal surface of the large intestine at

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