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3.3.3 Digestion and absorption

AQA A-level Biology 3.3.3 practice on Digestion and absorption, with free MCQs, clues and worked explanations drawn from the Organisms exchange substances with their environment section of specification 7402.

Specification route
3.3.3
Question bank
21 questions
Course stage
AS / Year 12

Sample questions

QUESTION 1 · 3.3.3 · LEVEL 2

Which types of enzymes are responsible for the digestion of carbohydrates in mammals?

  • Amylases and membrane-bound disaccharidases
  • Lipases and bile salts
  • Endopeptidases and exopeptidases
  • Amylases and membrane-bound dipeptidases
Show clue

The prefix 'sacchar-' refers to sugars.

Show answer and explanation

Answer: Amylases and membrane-bound disaccharidases

Digestion in mammals of carbohydrates is carried out by amylases and membrane-bound disaccharidases. A common error is confusing disaccharidases with dipeptidases, which act on proteins.

QUESTION 2 · 3.3.3 · LEVEL 3

What is the specific role of bile salts in the digestion of lipids?

  • They emulsify lipids into smaller droplets, increasing the surface area for lipase to act upon.
  • They are powerful enzymes that directly hydrolyse the ester bonds in triglycerides.
  • They sharply decrease the pH of the small intestine to provide the optimum environment for lipase.
  • They actively transport large fatty acid chains directly across the ileum epithelium.
Show clue

Bile salts perform a physical process, not a chemical hydrolysis.

Show answer and explanation

Answer: They emulsify lipids into smaller droplets, increasing the surface area for lipase to act upon.

Digestion of lipids is by lipase, including the action of bile salts. A widespread misconception is that bile salts are enzymes. They do not break chemical bonds; they emulsify lipids to increase surface area.

QUESTION 3 · 3.3.3 · LEVEL 2

How do endopeptidases differ from exopeptidases during the digestion of proteins?

  • Endopeptidases hydrolyse internal peptide bonds within a polypeptide, while exopeptidases hydrolyse bonds at the ends.
  • Endopeptidases break glycosidic bonds, while exopeptidases break peptide bonds.
  • Endopeptidases build proteins inside the cell, while exopeptidases digest them in the gut lumen.
  • Endopeptidases only work on short dipeptides, while exopeptidases work on large polypeptides.
Show clue

Look at the prefixes: 'endo-' means inside, 'exo-' means outside/ends.

Show answer and explanation

Answer: Endopeptidases hydrolyse internal peptide bonds within a polypeptide, while exopeptidases hydrolyse bonds at the ends.

Digestion of proteins is by endopeptidases, exopeptidases and membrane-bound dipeptidases. Endopeptidases create more 'ends' for exopeptidases to work on by cutting the chain internally.

QUESTION 4 · 3.3.3 · LEVEL 2

Which mechanism is utilised for the absorption of amino acids and monosaccharides by cells lining the mammalian ileum?

  • Co-transport mechanisms
  • Simple diffusion directly through the phospholipid bilayer
  • Phagocytosis by the epithelial cells
  • Osmosis down a water potential gradient
Show clue

These polar/large molecules require help from a specific ion gradient.

Show answer and explanation

Answer: Co-transport mechanisms

Mechanisms for the absorption of the products of digestion by cells lining the ileum include co-transport mechanisms for the absorption of amino acids and of monosaccharides. Students often mistakenly believe these dissolve directly across the membrane via simple diffusion.

QUESTION 5 · 3.3.3 · LEVEL 3

What is the biological function of micelles in the absorption of lipids?

  • They transport poorly soluble monoglycerides and fatty acids to the surface of the epithelial cells.
  • They are carrier proteins embedded in the membrane that actively pump lipids into the cell.
  • They are intracellular vesicles that transport re-synthesised triglycerides into the lacteal.
  • They act as enzymes to hydrolyse the remaining ester bonds in lipids just before absorption.
Show clue

Micelles act as tiny 'ferries' in the aqueous environment of the gut lumen.

Show answer and explanation

Answer: They transport poorly soluble monoglycerides and fatty acids to the surface of the epithelial cells.

The role of micelles in the absorption of lipids is to keep insoluble lipid digestion products in solution and carry them to the cell surface. A common misconception is thinking micelles are transport proteins or intracellular chylomicrons.

QUESTION 6 · 3.3.3 · LEVEL 2

During the final stages of protein digestion, where are membrane-bound dipeptidases specifically located?

  • In the cell-surface membrane of the epithelial cells lining the mammalian ileum.
  • Floating freely within the acidic lumen of the stomach.
  • Inside the lysosomes of phagocytes circulating in the blood.
  • They are secreted directly into the bile duct by the liver.
Show clue

They act immediately before absorption occurs into the cell.

Show answer and explanation

Answer: In the cell-surface membrane of the epithelial cells lining the mammalian ileum.

Proteins are digested by endopeptidases, exopeptidases and membrane-bound dipeptidases. 'Membrane-bound' refers strictly to the epithelial cell membranes of the ileum, not free-floating enzymes in the stomach or pancreas.

QUESTION 7 · 3.3.3 · LEVEL 2

A common misconception is that salivary amylase completes the digestion of carbohydrates. What happens to the products of amylase digestion in the mammalian ileum?

  • They are further hydrolysed into monosaccharides by membrane-bound disaccharidases.
  • They are immediately absorbed into the bloodstream via simple diffusion.
  • They are broken down into fatty acids by the action of bile salts.
  • They are actively pumped into the lacteals by carrier proteins.
Show clue

Amylase only breaks starch down into disaccharides like maltose.

Show answer and explanation

Answer: They are further hydrolysed into monosaccharides by membrane-bound disaccharidases.

Digestion in mammals of: carbohydrates by amylases and membrane-bound disaccharidases. Amylase produces maltose, which must be broken down by membrane-bound maltase into glucose before absorption.

QUESTION 8 · 3.3.3 · LEVEL 1

Which class of enzymes acts to hydrolyse peptide bonds at the ends of a polypeptide chain?

  • Exopeptidases
  • Endopeptidases
  • Membrane-bound dipeptidases
  • Lipases
Show clue

The prefix 'exo-' means outside or at the extremities.

Show answer and explanation

Answer: Exopeptidases

Digestion of proteins by endopeptidases, exopeptidases and membrane-bound dipeptidases. Exopeptidases remove terminal amino acids, while endopeptidases cut within the chain.

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