4 pancreatic and intestinal brush border enzymes

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4. Pancreatic and intestinal brush-border enzymes break down starches into maltose, maltotriose, and α-dextrins (pancreatic amylase), α-dextrins into glucose (α-dextrinase), maltose to glucose (maltase), sucrose to glucose andfructose (sucrase), lactose to glucose and galactose (lactase), and proteins into peptides (trypsin, chymotrypsin, and elastase). Also, enzymes break off amino acids at the carboxyl ends of peptides (carboxypeptidases) and break off amino acids at the amino ends of peptides (aminopeptidases). Finally, enzymes split dipeptides into amino acids (dipeptidases), triglycerides to fatty acids and monoglycerides (lipases), and nucleotides to pentoses and nitrogenous bases (nucleosidases and phosphatases).5. Mechanical digestion in the small intestine involves segmentation and migrating motility complexes.6. Absorption occurs via simple diffusion (vitamins), facilitated diffusion, osmosis (water), and active transport (electrolytes); most absorption occurs in the small intestine.7. Monosaccharides, amino acids, and short-chain fatty acids pass into the blood capillaries.8. Long-chain fatty acids and monoglycerides are absorbed from micelles, resynthesized to triglycerides, and formed into chylomicrons.9. Chylomicrons move into lymph in the lacteal of a villus.10. The small intestine also absorbs electrolytes, vitamins, and water.24.13 Large Intestine1. The large intestine extends from the ileocecal sphincter to the anus.2. Its regions include the cecum, colon, rectum, and anal canal.3. Typical four layers found in GI tract: mucosa, submucosa, muscularis, and serosa.The mucosa contains many goblet cells, and the muscularis consists of teniae coli (3 bands) and haustra (pouches). mucosa consists of sim-ple columnar epithelium, lamina propria (areolar connective tissue), and muscularis mucosae (smooth muscle) (Figure 24.24a). The epithelium contains mostly absorptive and goblet cells (Figure 24.24b, d). The absorptive cells function primarily in water absorption; the goblet cells secrete mucus that lubricates the passage of the colonic contents.There are no circular folds or villi; however, microvilli are present on the absorptive cells =more absorption occurs in the small intestine than in the large intestine.submucosa of the large intestine con-sists of areolar connective tissue. The muscularis consists of an external layer of longitudinal smooth muscle andan internal layer of circular smooth muscle.The serosa of the large intestine is part of the visceral peritoneum. Small pouches of visceral peritoneum filled with fat are attached to teniae coli and are called omental (fatty) ap-pendices.4. Mechanical movements of the large intestine include haustral churning, peristalsis, and mass peristalsis. One movement characteristic of the large intestine is haustral churning. In this process, the haustra remain relaxed and become distended while they fill up. When the distension reaches a certain point, the walls contract and squeeze the contents into the next haustrum. Peristalsis also occurs, although at a slower rate (3–12 contractions per minute) than in more proximal portions of the tract. A final type of movement is mass peristalsis, a strong

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