mammalian physiology : nutrition

digestion : breakdown of large insoluble molecules of food into small soluble molecules of food so that it can be absoarbed into the body.

ingestion : taking in food via the mouth into the alimentary canal

egestion : removal of faeces (undigested food) from alimentary canal

mechanical digestion : physical breakdown of food pieces into smaller pieces of food i.e. mouth and churning of stomach walls

chemical digestion : hydrolysis of ester,peptide and glycocidic bonds ( by addition of water ) by enzymes

endopeptidase : an enzyme that catalyses the hydrolysis of peptide bonds within a protein molecule

exopeptidase : an enzyme that breaks peptides into induvidual amino acids.

Mouth

ingestion : taking food via the mouth alimentary canal

1. saliva secreted by salivary glabd = containing amylase.

starch is broken by amylase to maltose (disaccharide)

the saliva helps to lubricate bolus which makes it easier to swallow

2. mechanical digestion : chewing of food large pieces of food broken down to smaller pieces of food.

Oesophagus

food moves down by peristalisis

peristalisis: rhythmic muscular movement of wall of alimentary canal pushes food along through its lumen.

Stomach

1. stomach is a sac with capacity = 5dm3

2. sphincter muscles control entry and exit of food from stomach when bolus arrives from oesophagus. cardiac sphincter relaxes to allow food in whereas pyloric sphincter remains contracted.

3. mucosa is very folded= forming gastric pits = secrete gastric juice

cells in gastric pit

* epithelium of mucosa is made up columnar cells. Epithelium = surface layer of cells making up stomach wall.

* oxyntic cell secrete HCL , they have numerous mithochondria and invaginations on the surface.

* goblet cells : secrete mucus containing hydrogencarbonate ions(alkaline). The alkaline mucus protects epithelium of stomach from acidity + enzymes by coating it.

*chief cells secrete precursor of pepsin known as pepsinogen( inactive pepsin). It's converted to acitve pepsin by removal of short lenght of amino acid. this is partly achieved by = HCl and pepsin itself.

gastric juice

mainly water and contains the HCl secreted by the oxyntic cells

acidic enviroment may kill the bacteria in food

it also contains gastric lipase which breaks lipids down to fatty acids and glycerol

enzymes and optimum ph

pepsin and gasic lipase both have an optimum ph below 7 the acidic enviroment of the stimach provides this.

Aborption in stomach

stomach isnt adpated for abosrption

= fats, carbohydrates and proteins arent absorbed

however small lipid soluble molecules are absorbed= alcohol and aspirin

high doses of these can result in painful potentially dangerous ulcers