THE HUMAN DIGESTIVE SYSTEM - KEY NOTES

 

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 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 by hydrolysing bonds in the centre of the protein molecule.
  • 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:
  • amylase (from pancreas) hydrolyses starch to maltose.
  • lipase (from pancreas) - for the digestion of lipids. lipids are hydrolysed to fatty acids and glycerol.
  • endopeptidases (from pancreas) - for the digestion of proteins. these are hydrolysed to polypeptides.
  • exopeptidases (from pancreas) - 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.
  • many 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.
  • 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.
  • dipeptidases - the small intestine contains dipeptidases as part of the intestinal fluid which forms a secretion which coats the walls of the small intestine epithelial cells. Dipeptidases hydrolyses the peptide bonds between amino acids. the dipeptide is broken down into 2 amino acids, and can then be absorbed.
  • 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.
  • 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
  • disaccharidases/Dipeptidases/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.
  • 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 contains many enzymes as detailled above.
  • 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 are at the junction of the small and large intestines.
  • 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.

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