Digestion and Carbohydrates

• Digestion mainly involves polysaccharides and begins in the mouth.
• Salivary amylase, secreted by salivary glands, hydrolyzes α(1,4) glycosidic linkages in starch and glycogen.
• This process yields a mixture of maltose, glucose, and oligosaccharides.
• The process is completed in the small intestines by pancreatic amylase.
• All digestible carbohydrates are converted to glucose and other monosaccharides.
• Most mammals cannot digest cellulose, which provides essential fiber for intestinal mobility.
• Ruminants have bacteria in their rumen that hydrolyze cellulose to glucose, which is then fermented to lactate, acetate, and propionate.
• The liver converts lactate and propionate to blood sugar.
• Enzymes on the epithelial cells of the small intestines hydrolyze disaccharides.
• Sucrase (invertase) hydrolyzes sucrose into glucose and fructose.
• Lactase hydrolyzes lactose into glucose and galactose.
• Maltase hydrolyzes maltose into glucose units.
• Lactose intolerance results from a lack of lactase activity.
• Undigested lactose ferments in the intestine by microorganisms causing diarrhea and gas.
• Protein digestion begins in the stomach with pepsin.
• Proteins stimulate gastrin secretion, which then stimulates HCl secretion by parietal cells in gastric glands.
• Gastric juice acidity acts as an antiseptic, killing most bacteria, and converts pepsinogen into pepsin.
• Pepsin hydrolyzes peptide bonds of ingested proteins, producing small peptides.
• The low pH of stomach contents entering the small intestine stimulates the pancreas to secrete bicarbonate which neutralizes gastric HCl.

Digestion of Proteins

• Cholecystokinin stimulates the pancreas to secrete trypsin, chymotrypsin, and carboxypeptidase.
• These hormones are released as inactive forms and later activated.
• Trypsin hydrolyzes peptide bonds with C=O contributed by lysine and arginine residues.
• Chymotrypsin hydrolyzes peptide bonds involving phenylalanine, tyrosine, and tryptophan residues.
• Carboxypeptidase, containing zinc, splits carboxy terminal residues from peptides.
• Aminopeptidase, secreted by the small intestines, cleaves NH2 terminal residues from short peptides.
• Fibrous proteins like keratin and plant proteins in cereal grains are only partially digested.
• Celiac disease is a rare condition where intestinal enzymes cannot digest certain water-insoluble wheat proteins, especially gliadin.
• Gliadin is injurious to the lining cells of the small intestine in people with Celiac disease.
• In acute pancreatitis, zymogens are prematurely converted into their active forms inside the pancreas, thus destroying the normal secretion pathway.

Fat Digestion

• Fat digestion begins in the small intestine involving prolipase secreted by the pancreas.
• Prolipase is converted to active lipase which, with bile salts, acts on a specific protein called colipase.
• Lipase binds to triacylglycerol droplets and catalyzes removal of one or both external fatty acid residues, producing free fatty acids and 2-monoacylglycerols.
• Bile salts are essential for absorption of fat-soluble nutrients.
• Deficient bile salt formation or secretion leads to undigested fats in the stool.
• Incomplete absorption of fat-soluble vitamins A, D, E, and K can occur, leading to nutritional deficiencies, such as vitamin A deficiency.