Stomach important features

Figure 3.12 The mammalian gastrointestinal tract showing important features of the small intestine, the major site of absorption for orally administered compounds (A) liver (B) stomach (C) duodenum (D) ileum (E) colon (F) longitudinal section of the ileum showing folding, which increases surface area (G) detail of fold showing villi with circular and longitudinal muscles, (H) and (I) respectively, bounded by (J) the serosal membrane (K) detail of villi showing network of (L) epithelial cells, (M) capillaries, and (N) lacteals (O) detail of epithelial cells showing brush border or (P) microvilli. The folding, vascularization, and microvilli all facilitate absorption of substances from the lumen. Source From Ref. 1.

In this section, we describe the factors that influence the movement of water in and out of cells, an important feature of the life of both plants and animals. The following section discusses other transport phenomena that are critical to essential physiological processes, focusing on the asymmetrical distribution of certain transport proteins in epithelial cells. We will see how this permits absorption of nutrients from the intestinal lumen and acidification of the stomach lumen. 

A structural feature worth mentioning pertains only to the important minority of globular proteins confined to fluids outside cells. These include the proteins of the blood serum and of the pancreatic and stomach juices (like chymotrypsin, shown above). To enable them to survive in an unsheltered milieu (hostile even, as in the hydrochloric acid-rich stomach juice), they have acquired an additional source of structural stability they have one or several disulphide bonds. These arise from one of the twenty types of amino acid, cysteine, the side chain of which emits sulphydryl group, —SH. This group is very easily oxidized (oxidation being taken to mean chemical addition of oxygen or elimination of hydrogen the converse of oxidation is reduction ). 

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