THE HUMAN DIGESTIVE SYSTEM

These note have been produced by students from St Mary's College - they are pretty comprehensive the only thing I would add is that the source and site of action of exo/endopeptidases could do with revising seperately. The notes on control of digestion are fantastic concise yet comprehensive.

 

THE MOUTH
  • Chewing makes a larger surface area of the food for the enzymes to attack.
  • Salivery amylase hydrolyses some starch to maltose.
THE STOMACH
  • the walls of the stomach contain layers of muscle. the functions of which include:
  • churning, mechanical digestion, mixing, and peristalsis.
  • the gastric glands in the stomach wall secrete endopeptidase pepsin. however, it is secreted in its inactive form, pepsinogen. HCl in the stomach activates the enzyme.
  • the enzyme is secreted in its inactive form in order to prevent it from digesting the walls of the stomach, while it is in storage in the gastric glands.
  • once the enzyme has been activated, mucus, which coats the stomach walls, prevents them from being digested, and also protects the walls from acid.
  • HCl in the stomach kills bacteria which are ingested along with food, and also created a low pH environment in which stomach enzymes work at their optimum rate.
  • endopeptidases digest proteins into polypeptide chains
  • food is released from the stomach by periodic relaxation of the pyloric sphincter muscle at the lower end of the stomach.
  • after being released from the stomach, food enters the first part of the small intestine, known as the duodenum.
THE SMALL INTESTINE
  • Large Surface area
  • Moist surface
  • Thin (epithelial) surface/ short absorption pathway
  • Long/ folds (increasing surface area)
  • Villi
  • Microvilli
  • Lacteal
  • Capillary network in villus/ good blood supply
  • Mitochondria to supply ATP/ energy for active transport
  • Carrier proteins in membranes.
  • the duodenum contains the following enzymes:
  • lipase – for the digestion of lipids. lipids are hydrolysed to fatty acids and glycerol.
  • maltases – the small intestine contains maltase as part of the intestinal fluid which forms a secretion which coats the walls of the small intestine epithelial cells. maltase acts on the disaccharide sugar maltose and hydrolyses the glycoside bonds between the units of glucose. the sugar is broken down to its simplest form glucose, and can then be absorbed.
  • endopeptidases – for the digestion of proteins. these are hydrolysed to polypeptides.
  • exopeptidases digest polypeptide chains to amino acids.
  • both endo and exopeptidases are required for efficient digestion of polypeptides and proteins because endopeptidases act on the centre of polypeptide chains within proteins and hydrolyse them to smaller chains.
  • this means that more ‘ends’ are created for the exopeptidases to act upon, in order to break down polypeptide chains to amino acids.
  • enzymes in the duodenum are secreted from the pancreas, and are carried to the duodenum by the hepato-pancreatic duct which also brings bile from the liver.
  • the duodenum is the main site of absorption of all components of digestion, except water.
  • food is moved along the duodenum by peristalsis (rhythmic contraction of the muscles of the intestinal wall, cause food to be pushed along the duodenum)
  • Segmentation in the duodenum produces a to and fro movement that causes mixing of the contents of the gut and digestive juices. causing acid chyme which has a creamy consistency to be converted to chyle which is alkaline with a watery consistency.
  • segmentation also aids digestion by bringing products into contact with the mucosa –hence enabling absorption to occur.
ABSORPTION IN THE SMALL INTESTINE
  • Diffusion in capillaries
  • Active transport/ facilitated diffusion involved
  • ATP used by active transport
  • Disaccharides/ enzymes in cell surface membrane
  • Glucose/ monomers/ monosaccharides actively transported into epithelial cells via protein carriers/ channels (in membrane)
  • Facilitated diffusion from epithelial cell/ towards blood
THE ROLE OF THE LIVER IN DIGESTION
  • bile is a biological detergent, which is produced in the liver.
  • bile reduces the surface tension of the contents of the gut and increases the surface area. this allows enzymes to work. e.g. lipase.
  • in order for lipids to act upon triglycerides, the triglycerides must first be broken down into minute droplets to enable then to mix with lipases present in the pancreatic juice within the duodenum.
  • in order to do this bile is secreted from the gall bladder.
  • bile reduces the surface tension and increases the surface area /volume ratio. i.e., fats are emulsified.
  • therefore, lipases act on a larger volume of material in a shorter time, ensuring that enzymes operate at their optimum rate.
  • bile also neutralizes stomach acid, and provides the optimum pH for pancreatic digestive enzymes to work.
THE ROLE OF THE PANCREAS IN DIGESTION
  • produces pancreatic juice.
  • pancreatic juice is rich in sodium hydrogencarbonate, which:
  • neutralizes acid chyme from the stomach.
  • raises the pH to enable enzymes in the pancreatic juice to work.
THE LARGE INTESTINE
  • the large intestine is made up of the following parts:
  • ceacum and appendix – these are sack-like structures t the junction of the small and large intestines.
  • the appendix is vestigial – i.e. it has no function.
  • the colon and rectum- this is a muscular tube which contains large amounts of bacteria.
  • peristalsis moves contents along the colon, and also compacts faeces.
  • faeces are stored in the rectum.
  • mucosa in the colon secretes mucus which lubricates the mucosa and protects it from enzymes action.
  • the colon absorbs water and other soluble compounds.
  • the colon absorbs vitamins and ions.
  • bacteria contained in the colon, break down undigested food. this food is then absorbed of excreted as faeces.
  • these bacteria synthesize vitamins b and k.
  • faeces excreted via the anus. main components are:
  • undigested food, bile pigments, bacteria, and dead cells from the small intestine.

HORMONAL CONTROL OF DIGESTION

The following underlined words show the hormones involved in digestion, which must be learnt for the BYO3 exam. The bullet points underneath give the information that is asked in the exams, which you will need to know.

ROLE OF GASTRIN IN CONTROL OF GASTRIC JUICE SECRETION

  • Arrival of food in stomach
  • Causes cells to secrete gastrin
  • Gastrin transported by blood to gastric glands
  • Enzymes / acids created

GASTRIN

  • Gastrin is found in the stomach.
  • It is produced in the stomachs Gastric juice.
  • The contact of food with the stomachs lining causes the production of gastrin.

CHOLECYSTOKININ

  • The stimulus that triggers the release of cholecystokinin is the contact of food with the lining of the small intestine/duodenum.
  • It is secreted in the pancreatic juice and in bile.
  • The release of the hormone cholecystokinin causes the gall bladder to contract.
  • Certain drugs such as loxiglumide may stop the action of the hormone cholecystokinin on the gall bladder.

SECRETIN

  • Secretin is also secreted when food makes contact with the lining of the duodenum.
  • It is secreted in an alkaline fluid from the pancreas.

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