Digestion + Absorption

• Digestion is the process in which insoluble molecules are hydrolysed by enzymes into small molecules which can be absorbed
• There are two types of digestion. Mechanical digestion and chemical digestion. Mechanical digestion involves teeth and the stomach muscles. Chemical digestion refers to the digestive enzymes
• the stomachs role in digestion is both physical and chemical. Physically the stomach muscles churn the stomach. Chemically the stomach produces a hormone called gastrin which stimulates the release of gastric acid. Gastric acid aids protein digestion by activating the digestive enzymes that break down amino acids
• Gastric acid is made up of Hydrochloric Acid (HCl), Potassium Chloride (KCl) and Sodium Chloride (NaCl)
• The features of the Ileum (small intestine) are that they are made of epithelial cells which are thin. This creates a short diffusion pathway. Furthermore they have a large capillary network which maintains a concentration gradient. There is also many microvilli which increase the surface area. Lastly they contain muscles that can "mix up" substances to maintain a concentration gradient
• Sodium ions are actively transported out of the epithelial cells/ileum into the blood by a sodium/potassium pump. This will decrease the concentration of sodium ions inside the cell and in doing so will mean there is a higher concentration of sodium ions in the blood. Glucose or an amino acid will enter from the gut into the cell through co-transport. they will then diffuse out into the blood stream down a concentration gradient
• the digestion of starch happens in 3 stages. The first stage is where the saliva breaks down starch into maltose (a disaccharide). Mineral salts maintain the pH of the mouth at neutral- optimum conditions for amylase. Stage 2 is the stomach, where HCl produced in the stomach denatures amylase so no further digestion of starch occurs.
• Stage 3 is the small intestine, where pancreatic juices secreted contain lipases, proteases and amylases. Alkaline acids are produced in pancreas+ small intestine and neutralise stomach acid. Maltase is a membrane bound disacharidase. they ae fixed in the membrane of ileum cells. It hydrolyses maltose into alpha glucose. Membrane bound disacharidases are useful because they are not lost through egestion and are at the point of absorption.
• The digestion of proteins contains three enzymes. Endopeptidases hydrolyse the central region of amino acids, which creates more chain ends. Exopeptidases hydrolyse the peptide bonds on the end of the chains, however they must hydrolyse atleast 2. Dipeptidases hydrolyse the peptide bonds between amino acids, they are cell surface and membrane bound
• Lipids are hydrolysed by lipases. These enzymes are produced in the pancreas. They hydrolyse the ester bonds in triglyceride molecules and form monoglycerides and fatty acids. Lipids are emulsified into droplets called micelles by bile salts which are produced in the liver. Emulsifying lipids increases their SA so Lipases can act quicker. Monoglycerides and fatty acids are lipid soluble so enter the ileum by diffusion. Triglycerides are then reformed and modified by the RER into chylomicrons. Chylomicrons exit the cell by exocytosis and then move into the lymphatic system.
• Chylomicrons are lipoproteins that transport dietary lipids from intestines to other areas of the body via the lymphatic system.