Digestion and absorption

• Define digestion

  The hydrolysis of large, insoluble molecules into smaller molecules that can be absorbed across cell membranes
• Examples of large biological molecules

  Starch, proteins
• Examples of smaller molecules

  Glucose, amino acids
• Which enzymes are involved in carbohydrate digestion?

  Amylase

  Maltase, sucrase and lactase
• Where is amylase found


  Mouth - produced by salivary glands

  Pancreas
• Where is maltase, sucrase and lactase found

  Membrane of small intestine
• What are the substrates and products of the amylase digestive enzyme

  Amylase - starch into smaller polysaccharides
• What are the substrates and products of the maltase digestive enzyme

  Maltase - maltose into two glucose
• What are the substrates and products of the sucrase digestive enzyme

  Sucrase - sucrose into glucose and fructose
• What are the substrates and products of the lactase digestive enzyme

  Lactase - lactose into glucose and galactose
• Where are lipids digested?

  Small intestine
• What needs to happen before lipids can be digested?

  Emulsified by bile salts produced by the liver

  This breaks down large fat molecules into smaller, soluble molecules called micelles
• What do micelles do?

  Increase SA for absorption
• How are lipids digested?

  Lipase hydrolyses the ester bond between monoglycerides and fatty acids
• What are membrane-bound dissacharidases?

  Enzymes attached to the cell membrane of epithelial cells lining the ileum

  Help break down disaccharides into monosaccharides
• Where is the ileum found

  Small intestine
• What are bile salts

  Produced by the liver

  Emulsify lipids
• What enzymes are involved in protein digestion?

  Endopeptidases

  Exopeptidases

  Dipeptidases
• Endopeptidase?

  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?

  Break dipeptides into amino acids by hydrolysing peptide bonds
• Examples of endopeptidases?

  Trypsin, Chymotrypsin, Pepsin
• Where is trypsin and chymotrypsin synthesised?

  Pancreas and secreted into the small intestine
• Where is pepsin released?

  Into the stomach by cells in stomach lining

  Only works in acidic conditions - HCl in stomach
• Example of an exopeptidase and where is it found?

  Dipeptidases located in the cell surface membrane of epithelial cells in the small intestine
• How are certain molecules absorbed into the ileum despite a negative concentration gradient?

  Co-transport
• Which molecules require co-transport?

  Monosaccharides and amino acids
• Explain how sodium ions are involved in co-transport?

  Sodium ions are actively transported out of the cell into the lumen which creates a diffusion gradient

  Glucose and galactose (NOT fructose - facilitated diffusion) are then taken up into cells along with sodium ions
• Why do fatty acid and monoglycerides not require co-transport?

  They're lipid soluble and non-polar so they can directly diffuse across the cell membrane of epithelial cells by simple diffusion
• How do micelles help move monoglycerides and fatty acids towards the epithelium?

  Micelles constantly break and reform - so they can release monoglycerides and fatty acids so that they can be absorbed
• How are amino acids absorbed?

  Sodium ions are actively transported out of epithelium cells into the ileum

  Then they diffuse back into cells by sodium-dependent transporter proteins in the epithelial cell membranes and carry amino acid with them