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