Explain why a molecule of ATP can release energy rapidly to drive cellular processes, whereas glucose cannot release its energy directly without first undergoing a series of enzyme-controlled reactions.

OCR A-Level Biology A (H420) — 2.3 Nucleotides and nucleic acids · Explain · 4 marks · View as Markdown

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

Cells require a universal energy currency to couple energy-releasing reactions to energy-requiring ones. ATP fulfils this role in all living organisms, from bacteria to mammals.

Model answer (4 marks)

ATP can release energy rapidly because the hydrolysis of its terminal phosphoanhydride bond is a single‑step reaction that liberates a large amount of free energy (≈−30.5 kJ mol⁻¹). The energy released is just enough to drive the next reaction, so it is not wasted as heat. In contrast, glucose must first be broken down through a multi‑step pathway (glycolysis, the citric‑acid cycle and oxidative phosphorylation) before its energy can be coupled to ATP synthesis. Thus glucose cannot directly supply energy to cellular processes; its energy is only released after a series of enzyme‑controlled reactions that generate ATP, which then acts as the universal, immediate energy currency.

Examiner tips

  • Mention the phosphoanhydride bond and single‑step hydrolysis for ATP
  • Explain the small, matched energy release of ATP to avoid waste
  • State that glucose requires multi‑step catabolism before energy can be coupled to ATP
  • Highlight ATP as the universal energy currency that directly couples to energy‑requiring reactions

Common mistakes

  • Confusing ATP with ADP or AMP as the energy source
  • Saying glucose releases energy directly without mentioning the need for ATP synthesis
  • Over‑emphasising the amount of energy released by ATP instead of its rapid, controlled release

Mark scheme (4 marks)

  1. ATP releases energy through hydrolysis of a phosphoanhydride bond, which is a single-step reaction
  2. The energy released per hydrolysis of ATP is small and matched to the needs of cellular reactions, preventing wasteful heat loss
  3. Glucose must first be broken down through a multi-step pathway (glycolysis / respiration) before its energy can be coupled to ATP synthesis
  4. ATP acts as a universal / immediate energy currency, directly coupling to energy-requiring processes via phosphorylation

Key terms in this question

ATP · enzyme-controlled reactions

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