Food and Digestion

Chemical

•  Chemical - Speeded up by enzymes => produced in tissues/glands in the gut, pass out of cells on to gut contents  => act on food

Mechanical

• Mechanical - Physical breakdown => teeth biting and chewing food, cutting into smaller pieces that have a larger surface area

Muscles (intestine)

• Circular layer => fibres running in rings around the gut
• Longitudinal layer => fibres running down the gut
• Both act to push food along the gut = waves of muscle contraction pass along the gut = Peristalsis

Stomach

• Stomach wall secretes HCl => makes stomach contents highly acidic => kills bacteria that are taken into the gut along with food => helps us prevent food poisoning
• Semi-digested food is held back in the stomach by a ring of muscle at outlet of stomach = sphincter muscle = when it relaxes, releases food into the first part of the small intestine = duodenum
• Several digestive enzymes are added to the food in the duodenum, made by the pancreas = digest starch, proteins and lipids
• Liver produces digestive juice = bile = stored in the gall bladder + passes down bile duct on to the food 
• Bile turns any large liquid globules in food into an emulsion of tiny droplets = increases surface area so lipase enzymes can break it down more easily
• Bile + pancreatic juice are also alkaline = neutralise the acidic content from the stomach before it moves on in the gut
• Food passes along intestine + more enzymes are added until food has been broken into soluble end products 
• Move on to the ileum of the small intestine

Ileum

• Highly adapted for absorbing digested food = has a very large surface area = can efficiently absorb soluble products of digestion into the blood
• Greates increase in surface area is due to tiny projections of the lining = Villi
• Villi come in the millions = total area of lining thought to be about 300m2 = massive surface area
• Villi have surface cells called Microvilli = extend surface area even more