DIGESTIVE

• Ingestion 


  Eating
• mastication


  chewing food
• digestion


  breaking down of food into simple soluble molecules to encourage absorption
• absorption


  uptake of small soluble molecules to be used by cells
• emulsification 


  breaking down of ester bonds by emulsifier to make simpler molecules
• egestion


  waste elimination in the form of faeces
• the alimentary canal

  • one continuous tube that runs from the mouth to the anus
  • mouth, oesophagus, stomach, duodenum, ileum, colon, rectum, anus
• walls of the gastrointestinal system

  1. mucosa - innermost layer surrounding the lumen
  2. submucosa - dense layer of connective tissue with blood vessels, lymphatic vessels and nerves
  3. muscularis - contains inner oblique layer, a middle circular layer and an outer longitudinal layer
  4. serosa - outer layer and consists of several layers of connective tissue
• mechanical digestion


  the use of physical movement to help break apart food to increase surface area for enzymatic activity
• chemical digestion


  The use of enzymes to catalyse hydrolysis reactions between biological molecules within food leaving smaller, simpler molecules
• buccal cavity

  • Teeth initiate mechanical digestion
  • Salivary glands and saliva excretions initiate chemical digestion
• salivary glands

  • parotid - watery saliva containing enzymes
  • submandibular - enzymes and mucus
  • sublingual - mucous saliva
• oesophagus

  • tube connecting buccal cavity to stomach
  • no enzyme secretions
  • peristalsis moves food in an antagonist movement of circular and longitudinal muscles - controlled by autonomic nervous system
• stomach

  • contraction of smooth muscle causes churning action and mixes with acid and enzymes
  • takes three hours
  • secretions from gastric glands mix with food to produce chyme
  • epithelial lining has goblet cells which secrete mucus - neutralise acids and lubrication
• small intestine 


  duodenum

  • slightly alkaline continuation of digestion
  • secretes enzymes to continue the digestion of biological molecules

  ileum

  • site of nutrient absorption
  • maximal surface area to increase absorption
• large intestine


  colon

  • reabsorption of water
  • contains gut microbiome

  rectum

  • faeces storage unit before egestion out of the anus
• faecal components

  • cellulose
  • dead bacteria
  • cells from gut lining
  • brown due to bile pigments
• Liver

  • Reduces blood glucose levels by storing excess glucose as glycogen
  • Excess amino acids broken down to form urea in deamination
  • Production of bile - emulsifies fats and neutralises chyme - bile is stored in the liver
• pancreas

  • secretes enzyme-rich pancreatic juice and alkaline salts to neutralise chyme
  • insulin production - beta cells
  • glucagon production - alpha cells
• Salivary amylase

  • Carbohydrate
  • Begins digestion of starch into maltose
  • Polysaccharide to disaccharide
• pancreatic amylase

  • pancreatic fluid - duodenum
  • carbohydrate
  • completes digestion of starch to maltose
  • polysaccharide to disaccharide
• maltase

  • duodenum
  • carbohydrate
  • converts maltose into two alpha glucose molecules
  • disaccharide to monosaccharide
• sucrase

  • duodenum
  • carbohydrate
  • converts sucrose into one glucose and one fructose molecule
  • disaccharide to monosaccharide
• lactase

  • duodenum
  • carbohydrate
  • converts lactose into one glucose and one galactose molecule
  • disaccharide to monosaccharide
• Proteases

  • Stomach or duodenum
  • Proteins
  • Digestion of large protein molecules into smaller fragments
  • Polypeptides to dipeptides to amino acids
• lipases

  • duodenum
  • Lipids
  • Breaks down lipids into components
  • Lipids to fatty acids and glycerol
• Co transport

  • Coupled movement of substances across a membrane
  • As one molecule moves with the concentration gradient, the other molecule moves against it
  • Involves the use of a sodium pump which is important for glucose absorption
• glucose absorption

  • sodium pump into the plasma membrane pumps sodium ions out of epithelial cells lining the intestines and into the blood stream
  • generates a sodium concentration gradient from the intestines to the epithelial cells
• glucose is a polar molecule so it diffuses in and out of the epithelial cell via facilitated diffusion
• Bacteria

  • Most bacteria resides in the colon and make up what is known as the microbiome
  • Gut bacteria in humans also aid digestion