Digestion Digestion Assimilation Egestion 5 What is digestion and why do we do it? (Remember the five stages of nutrition.) Digestion is breaking down large, insoluble molecules into small, soluble molecules, so that they can be absorbed into the bloodstream. Small Molecules Small molecules are soluble and do not need to be digested. They can be absorbed into the blood easily. e.g. glucose, minerals, vitamins, water Large Molecules Large molecules are insoluble and must be digested to small soluble molecules before they can pass into the blood. e.g. starch, protein, lipid Fibre Fibre is the carbohydrate cellulose, which humans cannot digest. It cannot cross the gut wall and so remain as faeces. Faeces is mostly fibre and bacteria. e.g. cellulose cell walls of plants Does all food need to be digested? Digestion of large molecules What are the three large molecules? What small molecules are they made of? Digestive Enzymes Large molecules are digested to small molecules by enzymes. Enzymes are specific, so there are different enzymes for different substrate molecules. mouth rectum pancreas liver salivary glands large intestine oesophagus small intestine stomach anus Enzymes are made in the glands amylase amylase protease lipase mouth rectum pancreas liver salivary glands large intestine oesophagus small intestine stomach anus amylase protease amylase maltase protease lipase Enzymes are made in the glands, amylase amylase protease lipase but they work in the alimentary canal Digestive Enzymes Digestive Enzyme SubstrateProduct Where is it made? Where does it work? amylase maltase protease lipase Copy and complete this table: Digestive Enzymes Digestive Enzyme SubstrateProduct Where is it made? Where does it work? amylasestarchmaltose salivary glands, pancreas mouth, small intestine maltase protease lipase Digestive Enzymes Digestive Enzyme SubstrateProduct Where is it made? Where does it work? amylasestarchmaltose salivary glands, pancreas mouth, small intestine maltasemaltoseglucosesmall intestine protease lipase Digestive Enzymes Digestive Enzyme SubstrateProduct Where is it made? Where does it work? amylasestarchmaltose salivary glands, pancreas mouth, small intestine maltasemaltoseglucosesmall intestine proteaseproteinamino acids stomach, pancreas stomach, small intestine lipase Digestive Enzymes Digestive Enzyme SubstrateProduct Where is it made? Where does it work? amylasestarchmaltose salivary glands, pancreas mouth, small intestine maltasemaltoseglucosesmall intestine proteaseproteinamino acids stomach, pancreas stomach, small intestine lipaselipids fatty acids + glycerol pancreassmall intestine What does the liver do in digestion? The liver is a gland, but it does not make digestive enzymes. The liver makes bile, which is stored in the gall bladder then secreted into the small intestine. Bile has two components: 1.An alkali, which neutralises the stomach acid. This means that enzymes from the pancreas can work at their optimum pH of 7. 2.Bile salts, which emulsify lipids. This means breaking up large globules of fat into tiny droplets, which have a large surface area, so lipase enzymes can digest the lipid more quickly. What does the liver do in digestion? Lipids form large insoluble globules Emulsification of Lipids by Bile Emulsification from Saul Lipid digestion from Purves Absorption and Assimilation The small soluble products of digestion are absorbed into the bloodstream in the small intestine. The wall of the small intestine is folded into millions of villi to increase the surface area. Each villus has a network of blood capillaries close to the thin wall. Absorption and Assimilation The epithelial cells lining the villi are covered in microvilli to increase the surface area further. VilliMicrovilli Digestion Animations Digestion from Starr