Digestion+absorption

• Digestion

  The hydrolysis of large biological molecules into smaller molecules that can be absorbed across cell membranes
• Carbohydrate digestion (starts in mouth)


  1. Amylase → starch into smaller polysaccharides

  2. Maltase → maltose into 2 x glucose

  3. Sucrase → sucrose into glucose and fructose

  4. Lactase → lactose into glucose and galactose
• Lipid digestion (chemical)


  1. Lipase (produced in pancreas)=hydrolyses ester bonds in triglycerides to form monoglycerides and fatty acids
• Lipid digestion (physical)


  1. Lipids coated in bile salts to create micelle through emulsification (water-soluble)- many droplets provide large SA for faster hydrolysis action by lipase-micelles carry fatty acids/monoglycerides through membrane to epithelial cell
• Protein digestive enzymes (starts in stomach)


  • Endopeptidases hydrolyse peptide bonds between specific amino acids in the middle of a polypeptide
  • Exopeptidases hydrolyse peptide bonds between specific amino acids at the end of a polypeptide
  • Dipeptidases hydrolyse peptide bonds between dipeptides into amino acids
• Co-transport


  Mechanism by which certain molecules are absorbed into the ileum despite a negative concentration gradient
• Molecules requiring co-transport

  • Amino acids
  • Monosaccharides
• Fatty acids and monoglycerides do not require co-transport due to non-polar nature, they can simply diffuse across cell surface membrane to enter epithelial cells
• Carbohydrate digestive enzymes

  • Amylase (produced in pancreas+salivary glands)=hydrolyses polysaccharides into disaccharide maltose by hydrolysing glycosidic bonds
  • Sucrase+lactase (membrane-bound disaccharides)=hydrolyse sucrose+lactose into monosaccharides
• Adaptions for absorption

  -ileum wall covered in villi=thin walls (decreases diffusion distance) surrounded by network of capillaries (maintains conc gradient)

  -epithelial cells have even small microvilli=increases SA
• Co-transport of glucose and amino acids

  -Sodium ion actively transported out epithelial cells into blood (reducing Na+ concentration in cell)

  -forms diffusion gradient for Na+ to enter cells from lumen

  -glucose/amino acids enter blood by facilitated diffusion against their concentration gradient with Na+
• Absorption of lipids from epithelial cell to blood

  -Golgi modifies triglycerides and combines them with proteins to form chylomicrons (packaged for release)

  -Chylomicrons are extruded (through exocytosis) from cell into lymph capillary
• What is the role of ATP hydrolase

  -releases energy

  -for active transport of ions/ allows ions to move against their concentration gradient