Knowledge-10 EOS

• During digestion the addition of water molecules in hydrolysis reactions breaks down large biological molecules into smaller molecules.
• Smaller biological molecules are easier to absorb across a membrane.
• Digestion is a two-step process in mammals:

  1. physical breakdown-food is broken down into smaller pieces by teeth to increase the surface area available for enzyme action.
  2. chemical digestion- large, insoluble molecules are broken down into small, soluble ones by enzymes (hydrolase).
• There are three types of hydrolases are: carbohydrates , lipases and peptidases.
• Enzyme action for carbohydrates:

  • is hydrolyzed by amylase and membrane-bound disaccharides.
  • carbohydrates become maltose, glucose and other monosaccharides.
• Exopeptidases hydrolyze amino acids at the ends of peptides, reducing them to amino acids.
• Endopeptidases hydrolyze peptide bonds in the central region of the protein, reducing it to series of peptides.
• Role of bile:

  bile is a greenish fluid produced by the liver. It does not contain enzymes but is made up of two important digestive chemicals.

  • mineral salts- neutralize stomach hydrochloric acid to provide neutral pH for small intestine enzymes.
  • bile salts- emulsify lipids into tiny droplets thus increasing the surface area for lipases to act on.
• Absorption:

  the products of digestion must be absorbed by cells lining the ileum in the mammalian small intestine.

  • amino acids and monosaccharides are absorbed using co-transport mechanisms.
  • micelles are involved in the absorption of lipids.
• The ileum is adapted to maximize absorption:

  • it has very large surface area- it is about 6m long , lined with 1mm long villi which greatly increase the surface area. The villi are covered in cells which have microscopic projections called microvilli.
  • it has good blood supply to remove products of digestion and maintain a steep conc gradient.
  • it has thin walls so there are short diffusion pathways.
• Co-transport in the ileum:

  amino acids and glucose are absorbed in the ileum through co-transport.

  sodium ions and glucose bind to a co-transporter protein in cell-surface membrane of an epithelial cell.

  As the sodium ions move into the cell by facilitated diffusion, the glucose molecules are mainly carried through against a concentration gradient.

  glucose and sodium ions pass through the cell and into the blood by facilitated diffusion (glucose) or active transport. (sodium ions).
• After digestion , fatty acids and monoglycerides , along with bile salts form tiny round complexes called micelles.
• Micelles have a hydrophobic outer layer which enables them to dissolve.
• When a micelle reaches the ileum epithelium, it breaks down , releasing the fatty acids and monoglycerides which can diffuse through the epithelial cell surface membrane . the bile salts are reused.
• In the epithelial cell , monoglycerides combine with fatty chains and glycerol molecules to reform triglycerides. these triglycerides package with cholesterol and phospholipids to form a water soluble fat droplet called a chylomicron. This is then transferred by exocytosis to the lacteal.