BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.3. The Human Alimentary Canal

8.3.8. Enzymatic Digestion in the Small Intestine

Figure 8.22 illustrates the General Pathways of carbohydrate, protein, and lipid Digestion. All digestive Enzymes of the Small Intestine, except for pancreatic enzymes, are bound to Cell/30.html">The Plasma Membrane of the epithelial microvilli (Fig. 8.21, E) or located within the epithelial Cells themselves. It is in these locations that the final Hydrolysis of Disaccharides, dipeptides, and certain tripeptides takes place (Fig. 8.23). The End products of this hydrolysis are Monosaccharides and Amino Acids, respectively. A list of the enzymes involved in digestion is given in Table 8.3.

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Fig. 8.22. General pathways of enzymatic DIGESTION IN THE human alimentary tract.

Fig. 8.23. Cytology/cytology/92.html">SCHEMATIC Structure OF an ileal epithelial cell bearing microvilli. The left side shows the final stages of Protein Digestion and subsequent Absorption of Amino acids. The right side shows the corresponding processes for CARBOHYDRATES.

Table 8.3. Digestive enzymes and their action

In addition to its own enzymes, the small intestine receives alkaline pancreatic juice from the pancreas and bile from the liver. Bile is produced by hepatocytes and stored in the Gallbladder. It contains a mixture of salts (bile salts) which, upon entering the small intestine, act as natural detergents by reducing the surface tension of fat globules. This results in The formation of smaller droplets, thereby increasing their total surface area. (This process is called emulsification.) These smaller droplets are more effectively acted upon by lipases (lipid-digesting enzymes). Further information regarding the Structure and function of the liver can be found in Chapter 19.

The pancreas is a large gland located behind The Stomach (Fig. 8.11). It contains groups of cells that secrete a range of digestive enzymes, which enter the duodenum via the pancreatic duct (Fig. 5.29). These include the following enzymes:

1) amylase — converts amylose into maltose;

2) lipase — breaks down lipids (fats and oils) into Fatty Acids and glycerol;

3) trypsinogen — converted by enterokinase into trypsin, which breaks down proteins into shorter polypeptides and also converts excess trypsinogen into trypsin;

4) chymotrypsinogen — upon conversion into chymotrypsin, breaks down proteins into amino acids;

5) carboxypeptidase — converts peptides into amino acids.