Digestion in mammals of: carbohydrates by amylases and membrane-bound…

The idea

Salivary and pancreatic amylase hydrolyse the glycosidic bonds in starch to maltose. Membrane-bound disaccharidases such as maltase, sucrase and lactase sit in the cell-surface membrane of the epithelial cells on the microvilli of the ileum, completing digestion to monosaccharides at the site of absorption. Pancreatic lipase hydrolyses the ester bonds of a triglyceride to a monoglyceride and two fatty acids. Bile salts, made in the liver, do not hydrolyse anything: they emulsify lipid droplets into many tiny droplets, greatly increasing the surface area on which lipase can act, raising the rate of digestion. Proteins are hydrolysed in stages: endopeptidases hydrolyse peptide bonds inside the chain, creating shorter peptides with more ends; exopeptidases remove terminal amino acids; membrane-bound dipeptidases hydrolyse dipeptides to single amino acids at the epithelial surface.

What earns the marks

• Amylase hydrolyses glycosidic bonds in starch to produce maltose; a membrane-bound disaccharidase such as maltase then hydrolyses maltose to glucose.
• State that the disaccharidase is membrane-bound on the microvilli of the epithelial cells, not secreted into the lumen.
• Lipase hydrolyses the ester bonds of a triglyceride to produce a monoglyceride and two fatty acids.
• Bile salts emulsify lipids into smaller droplets, increasing the surface area on which lipase can act and so raising the rate of hydrolysis; they do not hydrolyse bonds themselves.
• Endopeptidases hydrolyse internal peptide bonds, creating shorter peptides with more ends; exopeptidases remove terminal amino acids; membrane-bound dipeptidases hydrolyse dipeptides to single amino acids.
• For each enzyme, link bond hydrolysed to product formed, e.g. peptide bond to amino acids, ester bond to monoglyceride and fatty acids.

Common mistakes and how to put them right

• Saying bile salts digest or hydrolyse lipids.

  Put it right: Bile salts emulsify lipids, increasing surface area for lipase; they hydrolyse nothing.

• Writing that amylase breaks starch straight into glucose.

  Put it right: Amylase gives maltose, then a membrane-bound disaccharidase gives glucose.

• Saying disaccharidases are secreted into the lumen.

  Put it right: They are membrane-bound on the microvilli of epithelial cells.

• Swapping endopeptidase and exopeptidase.

  Put it right: Endopeptidases cut internal peptide bonds; exopeptidases remove terminal amino acids.

• Writing 'protease' throughout when the question names specific enzyme groups.

  Put it right: Name endopeptidases, exopeptidases and dipeptidases as appropriate.

Exam tips

• Build each digestion point from bond plus enzyme plus product, so write all three each time.
• Place membrane-bound enzymes (disaccharidases, dipeptidases) in the cell-surface membrane of the epithelial cells, not in the lumen.
• For proteins, sequence endopeptidases, exopeptidases and dipeptidases, stating the bond each hydrolyses and why the order speeds digestion.