digestion and absorption

• digestion

  process where large biological molecules which are to big and insoluble to be absorbed from gut into blood are broken down, hydrolysed into smaller, soluble molecules so can be absorbed across cell membrane
• proteins hydrolysed into amino acids

  carbohydrates hydrolysed into monosaccharides

  lipids hydrolysed into mixture of glycerol and fatty acids
• Digestive system


  A system of organs in the body that processes food
• Salivary glands

  • Near mouth
  • Secrete via duct
  • Contain enzyme amylase (hydrolyse starch)
• Digestion

  1. Food carried from mouth to stomach via oesophagus

  2. Muscular sac, inner layer produces enzymes to digest and store food

  3. Proteases in stomach digest proteins

  4. Pancreas secretes pancreatic juices, contains proteases (hydrolyse proteins), lipase (hydrolyse lipids), amylase (hydrolyse starch)

  5. Food digested further in ileum by enzymes

  6. Inner walls of ileum are folded into villi (gives large surface area)

  7. Microvilli on epithelial cells in ileum further increase surface area

  8. Large intestine absorbs water remaining in food not able to be digested

  9. Faeces stored in rectum before removed by anus
• Digestion

  1. Food carried from mouth to stomach via oesophagus

  2. Food digested further in ileum by enzymes

  3. Large intestine absorbs water remaining in food not able to be digested

  4. Faeces stored in rectum before removed by anus
• Stomach

  Muscular sac, inner layer produces enzymes, its role to digest and store food
• Proteases

  • Digest proteins
• Pancreas


  secretes pancreatic juices, contains proteases (hydrolyse proteins), lipase (hydrolyse lipids), amylase (hydrolyse starch)
• Ileum


  Inner walls are folded into villi (gives large surface area) for its purpose to absorb products of digestion in bloodstream
• Villi

  Microvilli on epithelial cells of each villus in ileum further increase surface area
• structure of digestive system

  A) mouth

  B) salivary glands

  C) oesophagus

  D) stomach

  E) liver

  F) bile duct

  G) gall bladder

  H) pancreas

  I) large intestine

  J) small intestine

  K) appendix

  L) rectum

  M) anus

  
• carbohydrases

  carbohydrate digestion takes place in the mouth and small intestine

  • amylase produced in mouth and pancreas, hydrolyses glycosidic bonds of starch and produce disaccharide maltose
  • maltase hydrolyse maltose into monosaccharide alpha glucose
• Carbohydrate digestion

  1. Saliva from salivary gland mixed with food during chewing, has salivary amylase- hydrolyse starch in food to maltose, and mineral salt (maintains pH)

  2. Food swallowed enters stomach (acidic conditions) denatures amylase to prevent further hydrolysis of starch

  3. Food enters small intestine

  4. Mixes with pancreatic juices, contains pancreatic amylase, hydrolysis of starch to maltose continue, alkaline salts produced by intestinal wall and pancreas maintains pH around neutral so amylase can function

  5. Intestinal wall push food along ileum, epithelial lining produce disaccharidase maltase released in epithelial lining if ileum (membrane bound enzyme)

  6. Maltase hydrolyse maltose from starch into alpha- glucose

  7. Other disaccharides are also digested by enzymes: sucrase: hydrolyse glycosidic bond in sucrose, produce glucose and fructose

  8. Lactase: hydrolyse glycosidic bond in lactose, produce glucose and galactose
• digestion of proteins by proteases

  in stomach protein digested by proteases

  • endopeptidases: hydrolyse peptide bonds within proteins in smaller proteins- trypsin (synthesised by pancreas secreted into small intestine), pepsin (released into stomach by stomach lining works in acidic condition- HCl in stomach)
  • exopeptidases: hydrolyse peptide bonds at ends of protein molecules, remove single amino acid. dipeptidases are exo- located on cell surface membrane of epithelial cells in small intestine
• lipid digestion by lipases

  Emulsification

  lipids broken down by lipases with bile salts

  • lipase catalyse breakdown of lipids into monoglycerides and fatty acids by hydrolysis of ester bond in lipids
  • lipases secreted from pancreas and used in small intestine
  • bile salts produced in liver and emulsify lipids, causes lipids to form small droplets
  • emulsification increase SA of fatty acid droplets for action of digestive enzymes
  • lipid broken down to monoglycerides and fatty acids join to bile salts and form micelles
• absorption of amino acids

  • specific amino acid co-transport are found within cell-surface membrane of epithelial cells in ileum
  • transport amino acid when sodium ions present
  • occurs via facilitated diffusion- molecules move down a concentration gradient
  • amino acid diffuse across epithelial cells and pass into capillaries via facilitated diffusion
  • concentration of gradient of sodium ions maintained by active transport of sodium ions out of cell into blood via sodium-potassium pump
• absorption of monosaccharides

  • sodium ions and glucose molecules co-transportedd into epithelial via faciliated diffusion
  • gucose diffuse across epithelial cell and enter capillary by facilitated diffusion
  • concentration of gradient of sodium ion maintained by active tranport of sodium ions out of cells into blood
• Absorption of lipids

  1. Monoglycerides and fatty acids combine with bile salts and form micelles

  2. Monoglycerides and fatty acids can break up and release monoglycerides and fatty acids, non polar molecules and easily diffuse across cell surface membrane to epithelial cells

  3. Transported to ER, recombined to triglycerides

  4. ER to Golgi apparatus, triglycerides associate with cholesterol and lipoproteins forms chylomicrons

  5. Moves out of cell by exocytosis, enter lymphatic capillaries- lacteals found at centre of each villus

  6. Via lymphatic system triglycerides in chylomicrons hydrolyse by enzymes in endothelial cells of blood capillaries and diffuse out of cells
• label diagram of absorption of lipids:

  A) micelles

  B) epithelial cells

  C) lacteal