Digestion and Absorption

• ➢Larger molecules are hydrolysed into smaller molecules

  ➢ smaller molecules – absorbed into blood in the small

  intestine

  ➢These are hydrolysis reactions

  ➢Add molecule of water to break the bond

  ➢Use specific enzymes to catalyse…

  ➢ these are specific due to the shape of their active site

  which is only complementary to shape of the substrate. Only

  this molecule can bind with the active site to form an

  enzyme - substrate complex

  ➢Break polymers down into monomers

  ➢ starch, cellulose, glycogen, polypeptides/proteins
• Digestion of lipids

  Hydrolysis of ester bonds

  Lipase – from pancreas

  Hydrolyses triglycerides to fatty

  acids & monoglyceride
• Large lipid droplets are emulsified into smaller droplets

  by bile salts

  Providing a larger surface area for lipase to act – faster

  hydrolysis/digestion
• Digestion of carbohydrates

  Hydrolysis of glycosidic bonds

  Amylase – from salivary glands

  & pancreas

  Hydrolyses starch to maltose

  Disaccharidases – embedded in csm of epithelial cells

  (of ileum)

  Including …

  maltase, sucrase & lactase

  for hydrolysis of maltose, sucrose & lactose
• Digestion of proteins/polypeptides

  Hydrolysis of peptide bonds

  Three types of protease involved

  • Exopeptidases

  • Endopeptidases

  • Dipeptidase
• Endopeptidases

  Hydrolyse internal

  peptide bonds

  creating more

  smaller

  polypeptides

  Giving more “ends”

  for exopeptidases

  to act on

  = faster hydrolysis
• Exopeptidases

  Hydrolyse end

  peptide bonds

  creating single

  amino acids
• Combined action of both leaves some dipeptides

  These are hydrolysed by dipeptidases embedded in the csm

  (like disaccharidases
• Blood flow away from ileum

  maintains concentration

  gradients so products of

  digestion continue to diffuse

  across
• • Glucose and sodium ions bind to a carrier protein

  which facilitates their diffusion into the epithelial cell.

  The sodium ions are then moved across into the blood

  by active transport

  • This ensures there is always a lower concentration of

  ions in the cell.

  • There is a steep concentration gradient of sodium ions

  between the cell and lumen so the ions always diffuse

  into the cell.

  • Glucose is carried across into the cell with the sodium

  ions by the co-transport protein.

  • It then diffuses across into the blood with the aid of a

  carrier protein