Digestion and absorption

• What happens during digestion?

  Large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes.
• Digestion of starch (polysaccharide)

  - Hydrolysis of glycosidic bonds

  - Amylase hydrolyses starch to maltose

  - Maltose is hydrolyses to glucose by disaccharidase/maltase

  - Membrane-bound (disaccharidase/maltase)
• Digestion of lipids

  -Bile salts emulsify lipids to form small droplets which increase the surface area for lipases to work on.

  - One hydrolysed the monoglycerides and fatty acids stick with the bile salts to form the micelles which help with absorption by moving them towards the intestinal epithelium.
• Digestion of proteins

  >Endopeptidases

  - Hydrolyse the peptide bond between amino acids in the central region of the protein

  > Exopeptidases

  - Hydrolyse the peptide bonds on the terminal amino acids of peptide molecules

  > Dipeptidase

  - Hydrolyse the bond between 2 amino acids of a dipeptide.
• Role of micelles in absorption of lipids

  -Monoglycerides and fatty acids diffuse out of micelles in lumen and into epithelial cells because they are lipid soluble

  - Monoglycerides and triglycerides recombine to triglycerides which aggregate into globules

  - Globules coated with proteins to form chylomicrons

  - Leave via exocytosis and enter lymphatic vessels

  - Return to blood circulation
• Glucose uptake by co-transport

  1. An Na+ ion actively transported out of the cell into the blood

  - Maintains diffusion gradient for the initial co-transport and then K+ is actively transported into the cell to maintain the charge.

  2. With each glucose molecule one Na+ ion is absorbed through a carrier protein by co-transport into the epithelial cells

  3. Glucose molecules then pass through a carrier protein by facilitated diffusion into the blood.
• Adaptations of the small intestine wall

  1. Microvilli on epithelial cells- large SA for absorption

  2. Single layer of cells- short diffusion pathway

  3. Extensive capillary network- Maintain diffusion gradient

  4. Many mitochondria and carrier proteins- active uptake and facilitated diffusion
• Why is is more efficient to have both endo and exo peptidases?

  - Endopeptidases hydrolyse internal peptide bonds and exopeptidases remove amino acids by hydrolysis terminal bonds

  - Endopeptidases provide more ends or an increased surface area for exopeptidases.