Starch is a carbohydrate found in foods such as bread and pasta. Describe how the body uses enzymes to break down starch, including where these enzymes are produced, what they break starch down into, and how the 'lock and key' hypothesis explains their action.

Eduqas A-Level Biology — 1.1 Chemical elements and biological molecules · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Amylase (a carbohydrase) is the enzyme that breaks down starch. It is produced in the salivary glands and in the pancreas. The enzyme hydrolyses the α‑1,4‑glycosidic bonds in starch, producing simple sugars such as maltose and glucose. The active site of amylase has a specific, complementary shape that matches the starch substrate; only starch can fit into this site, which explains the enzyme’s specificity and is illustrated by the lock‑and‑key hypothesis.

Examiner tips

  • Mention both salivary and pancreatic amylase to cover all production sites
  • State the bond type hydrolysed and the simple sugars produced
  • Explain the lock‑and‑key model in one sentence
  • Use the exact terminology: amylase, carbohydrase, active site, substrate

Common mistakes

  • Confusing amylase with other digestive enzymes (e.g. lipase)
  • Failing to specify the simple sugars produced
  • Over‑generalising the lock‑and‑key explanation to all enzymes

Mark scheme (5 marks)

  1. Amylase / carbohydrase is the enzyme that breaks down starch
  2. Amylase is produced in the salivary glands and/or pancreas
  3. Starch is broken down into simple sugars
  4. The active site of the enzyme has a specific / complementary shape to the starch substrate
  5. Only starch (and not other substrates) can fit into amylase's active site, explaining enzyme specificity / the lock and key hypothesis

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