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Lecture07 - Lecture 7 Chapter 4 Carbohydrates(continued...

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Lecture 7: Chapter 4 Carbohydrates (continued) Fibers: Their Solubility, Sources and Actions -Table 4-2 - Soluble fibers versus insoluble fibers Digestion and Absorption of Carbohydrates CARBOHYDRATE DIGESTION Figure 4-10: Carbohydrate digestion in the GI tract In the mouth : Salivary amylases begin starch digestion. In the stomach : Stomach acid and stomach proteases inactivate the salivary amylases, halting starch digestion. In the small intestine : Here is where most carbohydrate digestion takes place, via the action of pancreatic amylase and disaccharidases : maltase, sucrase, and lactase (Note: maltase and sucrase are α -glycosidases , but lactase is a β -glycosidase ). Fibers delay the absorption of carbohydrates and fats in the small intestine. Within 1-4 hours, all the sugars and most of the starches have been digested. In the large intestine : The small amount of undigested starch that gets to the large intestine is called resistant starch . The resistant starch and indigestible fiber attract water, which softens the stools. Some bacteria can ferment ( metabolize without the requirement for oxygen) these materials. Absorption into the bloodstream : Except for a small amount of glucose absorbed by the tissues of the mouth, carbohydrate absorption takes place solely in the small intestine via active transport and are carried away in the blood to the liver, via the portal vein. See Figure 4-11. Summary : Digestion of carbohydrates by the body converts starches into disaccharides and then monosaccharides (principally glucose, fructose, and galactose). Glucose is the prominent monosaccharide, since the others (fructose and galactose) are readily converted into glucose.
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Lactose Intolerance : Lactose = "milk sugar" = a disaccharide of glucose joined to galactose, via a β -glycosidic bond . Lactase is the enzyme that hydrolyzes this β -glycosidic bond, converting lactose to glucose + galactose: lactose + H 2 O glucose + galactose Lactase is produced in abundance immediately after birth, but it declines in amount during childhood and adolescence.
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