5: Digestion

• The pancreas produces enzymes such as amylase, lipase, and protease that aid in the digestion of carbohydrates, fats, and proteins, respectively.
• Starch is hydrolysed by amylase to form maltose.
• Disaccharides are hydrolysed by disaccharidase.
• Proteins are hydrolysed by exopeptidases to form amino acids.
• Proteins are also hydrolysed by endopeptidases to form polypeptides.
• Dipeptides are hydrolysed by dipeptidase enzymes to form amino acids.
• Lipids are hydrolysed by lipase to form fatty acids and glycerol.
• Maltase catalyses the hydrolysis of maltose into molecules of glucose.
• The word equation for the hydrolysis of sucrose is water + sucrose → glucose + fructose.
• The word equation for the hydrolysis of lactose is water + lactose → glucose + galactose.
• Endopeptidases hydrolyse peptide bonds between amino acids in the central region of the protein molecule, this creates more ends for the exopeptidases to act on.
• Exopeptidases hydrolyse peptide bonds at the end of the chain of amino acids, to release dipeptides or single amino acids.
• Dipeptidases are membrane bound and are found on the cell surface membrane of the small intestine epithelial cells. They hydrolyse peptide bonds holding the two amino acids of a dipeptide.
• Lipid digestion begins in the duodenum and is completed in the ileum.
• Lipids form large droplets when mixed with water. Bile salts, which are produced by the liver can break these large droplets up into smaller droplets. This process is called emulsification and increases the surface area of the lipid in order to speed up the action of lipases.
• Lipids are hydrolysed by lipases. Lipases are enzymes produced in the pancreas that hydrolyse ester bonds found in triglycerides to form fatty acids and glycerol.
• The presence of microvilli in the small intestine increases the surface area for absorption as more protein channels and more protein carriers can be present in the membrane.
• Large numbers of mitochondria within the epithelial cell of the villus will provide lots of energy in the form of ATP from aerobic respiration. Large numbers of mitochondria will increase the rate of active transport.
• Amino acids and monosaccharides such as glucose are absorbed from the gut by co-transport. This means absorption is coupled with the absoprtion of another substance, in this case sodium ions.
• A co-transport protein recognises sodium ions and glucose molecules as they have a complementary shape to a binding site on the protein, which has a specific tertiary structure.
• Sodium ions are actively transported from ileum cell to the blood. this forms a diffusion gradient for sodium to enter cells from the gut. Sodium ions enters by facilitated diffusion with glucose ( co-transport). Glucose leaves the cell and enters the blood by facilitated diffusion maintained by circulation.