18.2 DIGESTION OF FOODS

Digestion of foods:

• stage 1 of catabolism:

  • food undergoes digestion: a process that converts large molecules to smaller ones that can be absorbed by the body

Digestion of Carbohydrates:

• enzymes produced in salvary glands hydrolyze some of the a- (alpha) glycosidic bonds in amylose and amylopectin

  • produces:

    • maltose

    • glucose

    • dextrins (may contain 3-8 glucose units)

• After being swallowed, primaraly digested starches enter the acidic enviornment of the stomach, where the low pH stops carbohydrate digestion

• in the small intestine (has a pH of about 8 )

  • enzymes produced in the pancreas hydrolyze the remaining dextrins to maltose and glucose

  • enzymes produced in the mucosal cells that line the small intestine hydrolyze maltose as well as lactose and sucrose into monosaccharides

• Monosaccharides are then absorbed through the intestinal wall to the blood stream and carried to the liver, where any fructose and galactose are converted to glucose

• Stage 1 of catabolism: the digestion of carbohydrated behins in the mouth and is completed in the small intestine

Digestion of Fats:

• digestion of fats (triacylglycerols) begins in the small intestine, where bile salts break fat globules into smaller particles called micelles

Digestion of Triacylglycerols:

• the triacylglycerols are hydrolyzed in the small intestine and re-formed in the intestinal lining, where they bind to proteins for transport through the lymphatic system and bloodstream to the cells

Digestion of Proteins:

• starts in the stomach

  • HCI at pH 2 denatures proteins and activated enzymes such as pepsin to hydrolyze peptide bonds

• moves out of the stomach to the small intestine, where trypsin and chymotrypsin hydrolyze the polypeptides to amino acids

• ends as amino acids are absorbed through the intestinal walls and enter the bloodstream for transport to the cells

• Proteins are hydrolyzed in the stomach and the small intestine