Practise › Questions › Digestion and absorption: one tube, four chemistries

Digestion and absorption: one tube, four chemistries questions

Digestion as hydrolysis; the pH of the mouth, stomach, duodenum and ileum and which enzymes work in each; salivary and pancreatic amylase and the membrane-bound disaccharidases; endopeptidases, exopeptidases and dipeptidases and why the order matters; bile salts, emulsification as a surface-area argument, lipase and micelles; the structure of a villus; co-transport of glucose and amino acids with sodium ions; and the reassembly of triglyceride into chylomicrons that leave in the lymph.

5 original questions · 17 marks · the digestion and absorption: one tube, four chemistries notes · Nutrition, digestion and health evidence

Every question here is written for this library rather than taken from a past paper. Write your answer out before opening a mark scheme: the schemes award marks point by point, and the points are much easier to see when you have something of your own to compare against.

• 4 marks · Explain

  Explain why a mixture containing only exopeptidases would digest a protein far more slowly than one that contains endopeptidases as well.

  Mark scheme

  1. B1 an exopeptidase hydrolyses the peptide bond at an end of a chain, removing one amino acid at a time, so it can only work where there is a free end
  2. B1 a chain of three hundred amino acids has only two ends, so the number of places an exopeptidase can start is very small however much enzyme is present
  3. B1 an endopeptidase hydrolyses peptide bonds in the interior of the chain, and each internal cut produces two more ends
  4. B1 far more ends means far more sites at which the exopeptidases can act at the same time, so the overall rate of hydrolysis rises

• 4 marks · Explain

  Explain how bile salts increase the rate at which lipid is digested, and explain how micelles help the products of that digestion to be absorbed.

  Mark scheme

  1. B1 bile salts are amphipathic, coating a lipid surface with the hydrophobic face inwards, so the droplets repel one another instead of merging and the churning of the gut can break them into much smaller ones that stay broken
  2. B1 that emulsification greatly increases the total surface area of lipid, so more ester bonds are accessible to water-soluble pancreatic lipase and the rate of hydrolysis rises, although bile itself breaks no bond
  3. B1 monoglycerides and fatty acids, with bile salts and any fat-soluble vitamins, assemble into micelles that carry them through the watery layer next to the epithelium and release them at the membrane, keeping the concentration of free products high there
  4. B1 the micelle itself is not absorbed: monoglycerides and fatty acids are small and non-polar, so they diffuse straight through the phospholipid bilayer by simple diffusion

• 4 marks · Describe

  Describe how glucose is absorbed from the lumen of the ileum into the blood, making clear where in the cell ATP is used.

  Mark scheme

  1. B1 a sodium-potassium pump in the basal membrane hydrolyses ATP and moves three sodium ions out of the epithelial cell for every two potassium ions in, so the sodium concentration inside the cell falls below the concentration in the lumen
  2. B1 a co-transporter protein in the apical membrane binds one glucose molecule together with two sodium ions
  3. B1 sodium moves into the cell down the electrochemical gradient the pump created and glucose is carried in with it, against its own concentration gradient, with no ATP hydrolysed at this membrane
  4. B1 glucose then leaves the far side of the cell into the blood by facilitated diffusion through a different carrier protein, down the gradient its accumulation has created

• 3 marks · Suggest

  A patient has had a long section of the ileum removed by surgery. Suggest why the patient loses body mass, and suggest why the faeces contain an unusually high proportion of fat.

• 2 marks · Name

  Name the enzyme that hydrolyses maltose, and name the exact place in the ileum at which that enzyme is found.

Practise digestion and absorption: one tube, four chemistries one question at a time

The player marks nothing for you. It shows one question, waits, then shows the scheme so you can mark yourself, and brings a question back sooner when it went badly.