Digestion

• Digestion and absorption of fats, carbohydrates, and proteins are key objectives of the course
• Carbohydrates, fats, and proteins in the diet must be digested for absorption
• Major carbohydrates in the diet are polysaccharides like starch and glycogen
• Salivary amylase hydrolyses polysaccharides to produce mono and disaccharides
• Entry of acidic stomach contents into the duodenum stimulates the release of secretin and cholecystokinin
• Secretin stimulates the pancreas to release bicarbonate and water to neutralize acidic chyme
• Cholecystokinin stimulates the production of digestive enzymes, including pancreatic amylase
• Pancreatic amylase digests polysaccharides to maltose, isomaltose, and a limit dextrin
• Disaccharidases like maltase, lactase, and sucrase digest disaccharides into monosaccharide units
• Humans do not produce β 1,4 - endoglucosidase in digestive juice to digest cellulose
• Lactase deficiency leads to lactose intolerance
• Galactose and glucose are absorbed rapidly by an active process
• Fructose and mannose are absorbed by a Na+ independent facilitative transport mechanism
• Proteins do not undergo digestion in the mouth
• Gastrin stimulates the release of gastric juice containing HCl and pepsinogen in the stomach
• Pepsin digests protein polypeptides into tripeptides, dipeptides, and amino acids
• Rennin in infants is also called chymosin or rennet
• Entry of acidic stomach contents into the intestine triggers the secretion of hormones like cholecystokinin and secretin
• Secretin stimulates the release of bicarbonate and pancreatic juice from the pancreas into the small intestine
• Cholecystokinin stimulates the secretion of pancreatic endo and exopeptidases
• Endopeptidases cleave internal peptide bonds of proteins into smaller peptides
• Trypsin hydrolyses peptide bonds involving lysine and arginine
• Chymotrypsin hydrolyses peptide bonds involving aromatic amino acids and others
• Elastase hydrolyses peptide bonds formed by non-polar amino acids
• Carboxypeptidase and Aminopeptidase are exopeptidases that hydrolyse peptide bonds
• Amino acids are absorbed via Na+ dependent active transport mechanism
• Digestion of lipids depends on bile salts for emulsification
• Gastric lipase and lingual lipase are active only at neutral pH
• Short and medium-chain fatty acids are absorbed via mucosal cells, longer chain fatty acids pass into the duodenum
• Cholecystokinin helps in the contraction of the gallbladder to release bile salts into the small intestine
• Secretin causes the pancreas to release a bicarbonate-rich solution to neutralize acidic chyme
• Bile salts provide emulsifying action in the duodenum
• Pancreatic lipolytic enzymes hydrolyse lipids after emulsification
• Dietary glycerophospholipids are digested by pancreatic phospholipase-A2
• Fats are re-synthesized inside mucosal cells and transported to blood as chylomicrons
• Long-chain free fatty acids, cholesterol, monoglycerides, and lysophospholipids form mixed micelles for absorption
• Bile salts are reabsorbed in the intestine and returned to the liver for re-secretion
• Monoacylglycerols are hydrolysed by intestinal lipase in the mucosal cell
• Triacylglycerols are re-synthesized inside mucosal cells for absorption
• Absorbed lipids are transported as chylomicrons into the lymph or oxidized in the liver