3.3 digestion and absorption

• explain what happens in digestion

  large (insoluble) biological molecules hydrolysed to smaller (soluble) molecules

  that are small enough be absorbed across cell membranes into blood
• describe digestion of starch in mammals

  amylase (produced by salivary glands / pancreas) hydrolyses starch to maltose

  membrane-bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose

  hydrolysis of glycosidic bond
• describe the digestion of disaccharides in mammals

  membrane-bound disccharidases hydrolyse disaccharides to 2 monosaccharides ->

  maltase - maltose → glucose + glucose

  sucrase - sucrose → fructose + glucose

  lactase - lactose → galactose + glucose

  hydrolysis of glycosidic bond
• describe the digestion of lipids in mammals including action of bile salts

  bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets

  this increases surface area of lipids for increased / faster lipase activity

  lipase (made in pancreas) hydrolyses lipids (eg. triglycerides) → monoglycerides + fatty acids

  hydrolysis of ester bond
• describe the digestion of proteins by a mammal

  endopeptidases - hydrolyse internal (peptide) bonds within a polypeptide → smaller peptides

  so more ends / surface area for exopeptidases

  exopeptidases - hydrolyse terminal (peptide) bonds at

  ends of polypeptide → single amino acids

  membrane-bound dipeptidases - hydrolyse (peptide)

  bond between a dipeptide → 2 amino acids

  hydrolysis of peptide bond
• suggest why membrane-bound enzymes are important in digestion

  membrane-bound enzymes are located on cell membranes of epithelial cells lining ileum

  (By hydrolysing molecules at the site of absorption they) maintain concentration gradients for absorption
• describe the pathway for absorption of products of digestion in mammals

  lumen (inside) of ileum → cells lining ileum (part of small intestine) → blood
• describe the absorption of amino acids and monosaccharides in mammals

  Na+ actively transported from epithelial cells lining ileum to blood (by Na+/K+ pump)

  establishing a concentration gradient of Na+ (higher in lumen than epithelial cell)

  Na+ enters epithelial cell down its concentration gradient with glucose against it's concentration gradient

  via a co-transporter protein

  glucose moves down a concentration gradient into blood via facilitated diffusion
• describe the absorption of lipids by a mammal including the role of micelles (1)

  micelles contain bile salts, monoglycerides and fatty acids

  make monoglycerides and fatty acids (more) soluble in water

  carry / release fatty acids and monoglycerides to cell / lining of ileum

  maintain high concentration of fatty acids to cell / lining

  monoglycerides / fatty acids absorbed (into epithelial cell) by diffusion (lipid soluble)
• describe the absorption of lipids by a mammal including the role of micelles (2)


  triglycerides reformed in (epithelial) cells and aggregate into globules

  globules coated with proteins forming chylomicrons which are then packaged into vesicles

  vesicles move to cell membrane and leave via exocytosis

  enter lymphatic vessels and eventually return to blood circulation