Compare the structure of ATP with the structure of ADP, and explain what happens during the interconversion of these two molecules.

AQA A-Level Biology (7402) — 3.1.6 ATP · Compare · 5 marks · View as Markdown

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

Model answer (5 marks)

ATP contains three phosphate groups, ADP contains two. Both have a ribose sugar and an adenine base.

When ATP is hydrolysed to ADP + Pi, the high‑energy bond between the terminal phosphate and the rest of the molecule is broken, releasing energy.

ADP is phosphorylated to regenerate ATP, a process that requires energy input (e.g. from glycolysis or photosynthesis). The same ATP molecules can then be reused, allowing continuous recycling.

Examiner tips

  • Use the word ‘compare’ to list similarities and differences first, then explain the energy change. Include the key terms: phosphate, ribose, adenine, hydrolysis, phosphorylation, energy released/required.
  • Show the reaction equations: ATP + H₂O → ADP + Pi + energy; ADP + Pi + energy → ATP + H₂O.

Common mistakes

  • Confusing the number of phosphate groups (e.g. saying ATP has two).
  • Forgetting to mention that ATP regeneration requires energy input.

Mark scheme (5 marks)

  1. ATP has three phosphate groups whereas ADP has two phosphate groups
  2. Both ATP and ADP contain a ribose sugar and an adenine base
  3. When ATP is broken down / hydrolysed to ADP (and phosphate), energy is released
  4. ADP is converted back to ATP by adding a phosphate group, which requires energy
  5. ATP can be continuously recycled / regenerated so the same ATP molecules can be reused

Key terms in this question

ATP · ADP · interconversion

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