Digestion

DigestionIB Core Topic V

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

The Alimentary Canal

Basic 4-layered structure:1. Inner lining – contains goblet cells that secrete

mucus for lubrication2. Nerves, blood and lymph vessels3. Involuntary muscle layer that mixes food and

causes peristalsis (pushing food along)4. Peritoneum – outer, lubricated layer

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

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

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.)

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

Gall Bladder Produces bile, which contains:1. Bile pigments2. 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

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

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