Digestion

1. Digestion IB Core Topic V

2. Processing Food • Ingestion: taking food into the digestive system • Digestion: breaking down large molecules into smaller molecules that can be absorbed • Absorption: Transferring nutrients from the digestive system to the circulatory system for transporting around the body • Assimilation: The body’s processing of digested, absorbed substances (e.g. transamination and deamination of animo acids) • Egestion: Passing indigestible matter out of the body

4. The Alimentary Canal • Basic 4-layered structure: • Inner lining – contains goblet cells that secrete mucus for lubrication • Nerves, blood and lymph vessels • Involuntary muscle layer that mixes food and causes peristalsis (pushing food along) • Peritoneum – outer, lubricated layer

6. Alimentary Canal Sections • Oesophagus – “food pipe” • Epiglottis stops food going down the trachea • Uses peristalsis to carry food to the stomach • Stomach – muscular bag for storing and digesting food • Cardiac sphincter relaxes to let food in • Holds ~ 1litre of food for ~ 4hrs

7. The Stomach • Stomach lining is heavily folded, containing gastric glands • Glands secrete gastric juice • Mucus – lines stomach to prevent self-digestion • Pepsinogen – converted to pepsin (protease) in low pH conditions and by pepsin (autocatalytic) • Hydrochloric acid – kills micro-organisms, gives optimum pH for pepsin, loosens fibrous/cellular food and deactivates salivary amylase • Young mammals have rennin, and enzyme that converts caesinogen (soluble milk protein) to caesin (insoluble) so it can be digested by pepsin • Food + gastric juice = CHYME

8. Duodenum • 1st section of the small intestine • Passage of food from stomach controlled by pyloric sphincter • Surface is highly folded, containing villi (finger-like projections) • Cells on surface of villi have further projections – microvilli. These form the brush border (massive S.A.)

9. The Pancreas • Contains the following: • water and bicarbonate ions to neutralise stomach acids • Pancreatic amylase, to continue the breakdown of starch to maltose • Lipase, to break down lipids to fatty acids and glycerol • Trypsinogen, precursor of the protease trypsin

10. Gall Bladder • Produces bile, which contains: • Bile pigments • Bile salts • alkali, to neutralise stomach acid • Emulsify fats, to increase SA for lipase action • Assists in absorption of fat-soluble vitamins (A, D and K) The acidic chyme is converted to chyle by the addition of bile and pancreatic juice

11. Jejunum and Ileum • Length of the small intestines are covered in villi • The structure of the villi reflect their function (absorption of nutrients). • Large surface area • Very long (~8m) • Extremely thin lining (1 cell thick) • Rich blood supply – lots of capillaries Capillaries absorb small substances (e.g. glucose, water, amino acids, vitamins, minerals) by a combination of diffusion (with the concentration gradient) and active transport (against the concentration gradient

13. Large intestines • Consists of caecum, appendix, colon, rectum and anus • Caecum and appendix are thought to be vestigial in humans: herbivores host cellulase-secreting bacteria here • Large intestines host bacteria that produce vitamins (K, B2 group), but some can cause infection (appendicitis) • Colon reabsorbs water – undigested liquid from small intestines is converted to semi-solid faeces (colour due to bile pigments) • Faeces are stored in the rectum and pass out of the anus under the control of the anal sphincter