Digestion

• Autotrophs

  Use simple inorganic materials to manufacture complex organic compounds

  Photoautotrophic = use light energy to convert simple inorganic materials...

  Chemoautotrophic = used the energy derived from oxidation to convert simple inorganic materials...
• Heterotrophs

  Consume complex organic food material

  Holozoic feeders:

  • Food is taken into the body and digested
  • Many have a specialized digestive system
  • Carnivores, herbivores, omnivores, detritivores.

  Saprophytes:

  • Feed on dead or decaying matter
  • extracellular digestion

  Parasites:

  • Obtain nutrition from another living organism and cause it harm.
• Intestine wall structure

  .

  A) Serosa

  B) circular muscle

  C) Longitudinal muscle

  D) Sub-mucosa

  E) Mucosa

  F) Lumen

  
• Main stages of digestion

  1. Ingestion = Large food particles are taken into the mouth and broken down by the teeth, saliva, and the tongue - mechanical digestion
  2. Digestion = The chemical breakdown of large food molecules to small soluble molecules
  3. Absorption = Small soluble food molecules move from the small intestine into the blood stream, which transports them to the cells.
  4. Egestion = undigested food moves out of the body as faeces, via the colon, rectum and anus
• Amylase

  • Secretion: Saliva
  • Site of production: Salivary glands
  • Site of action: Mouth
  • Substrate: Starch
  • Product: Maltose
• Endopeptidase

  • Secretion: Gastric juice
  • Site of production: Gastric glands
  • Site of action: Stomach
  • Substrate: Protein
  • Products: Polypeptides

  Trypsin is secreted in an inactive form called trypsinogen (secreted by pancreas) which is activated by enterokinase
• Exopeptidase

  • Secretion: Pancreatic juice
  • Site of production: Pancreas
  • Site of action: Lumen of duodenum
  • substrate: Polypeptides
  • Products: Dipeptides
• Maltase and dipeptidase


  • Secretion: Intracellular enzymes
  • Site of production: duodenum and ileum
  • Site of action: Intracellular
  • Substrate: Maltose and dipeptides
  • Products: Glucose and amino acids
• Structure of Stomach

  • A sack with capacity of around 1.5 litres
  • Foldings in mucosa called rugae help with mechanical breakdown of food - food churned up against wall
  • Oblique muscle layer contracts and relaxes churning the food - mechanical digestion
  • Cardiac sphincter at upper end allow food to enter
  • Pyloric sphincter at lower end allows food to exit into duodenum
  • Mucus is secreted from goblet cells to protect the stomach from the enzymes and acid
• Functions of Stomach

  The stomach contains gastric juices (secreted in mucosa) which consists of:

  • Oxyntic cells - secrete hydrochloric acid which kills most bacteria in food and provided optimum pH for enzymes
  • Peptidase enzymes (pepsin) - hydrolyses protein into smaller peptides - secreted in an inactive form called pepsinogen by the chief cells, activated into pepsin by hydrochloric acid
• Lipid digestion

  • Area: Duodenum
  • secretion source: Pancreas via the pancreatic duct
  • Enzyme: Lipase
  • Substrate: Triglycerides
  • Product: Fatty acids and glycerol
• The liver

  The liver produces bile which is stored in the gall bladder from where it can enter the duodenum via the bile duct

  Bile:

  • Emulsifies lipids into small droplets, increasing the surface area which increases the rate of digestion
  • Aids in neutralising the stomach acid as it enters the duodenum with the food.
• The Pancreas

  The exocrine glands in the pancreas secrete pancreatic juice containing Endopeptidases, exopeptidases, amylase and lipase. This juice enter the duodenum via the pancreatic duct.

  • Endopeptidases - hydrolyses protein to polypeptides
  • Exopeptidases - hydrolyses the terminal ends of a polypeptide into dipeptides
  • Amylase - hydrolyses any remaining starch into maltose
  • Lipase - hydrolyses lipids into fatty acids and glycerol
• Buccal Cavity

  • Food is broken up into small pieces by the chewing action of teeth (mastication) - mechanical digestion
  • This increases the surface area for chemical digestion

  Saliva:

  • salivary amylase - converts starch and glycogen to maltose and smaller polysaccharides.
  • pH of 6.5-7.5 - optimum for salivary amylase
  • Mineral salts - maintain slightly alkali conditions
  • Lysozyme - an enzyme which kills bacteria
• Oesophagus

  • Food is now called bolus
  • Mucus lubricates its passage
  • The bolus moves down the stomach by a series of rhythmic contractions called peristalsis - behind the ball of food, circular muscles contract and longitudinal muscles relax, propelling food along the gut
• The duodenum

  Makes up the first 20 cm of the small intestine and receives secretions from the liver and the pancreas .

  Brunner's glands (submucosa):

  • Mucus for lubrication and protection
  • alkaline juices which helps maintain the optimum pH for the functioning enzymes in this area

  Carbohydrate and protein digestion is completed by enzymes fixed in the membrane of the epithelial cells of the mucosa:

  • Maltase - hydrolyses maltose into two glucose molecules
  • Dipeptidase - hydrolyses dipeptides into amino acids
• Digestion of Proteins

  Large polypeptides:

  • Pepsin - stomach
  • Trypsin - duodenum
  • Endopeptidases produce more ends to hydrolyse

  Small Polypeptides:

  • Exopeptidase - duodenum
  • Hydrolyse ends of polypeptides into dipeptides

  Intracellular:

  • epithelial cells lining the ileum
  • dipeptidases hydrolyse dipeptides into amino acids
• Assimilation/absorption of lipids

  1. Fatty acids and glycerol diffuse through the phospholipid bilayer into the epithelial cell from the lumen
  2. In the SER they are recombined into triglycerides + (chylomicrons)
  3. The triglycerides and chylomicrons exit the basal membrane of the epithelial cells by exocytosis and enter the lacteal
  4. Lacteal forms part of the lymphatic system and joins to the blood stream at the thoracic duct/subclavian vein
• Colon and Rectum

  Colon:

  • water and mineral salts reabsorbed along with vitamins secreted by microorganisms living in the colon
  • These bacteria are responsible for making vitamin K and folic acid

  Rectum - consists of residues of undigested cellulose, bacteria and sloughed cells