Explain why a molecule of ATP is described as a nucleotide, and explain how the hydrolysis of ATP releases energy for use in cellular processes.

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).

ATP (adenosine triphosphate) is often called the universal energy currency of living cells. It is synthesised during respiration and used to drive a wide range of endergonic reactions throughout the cell.

Model answer (4 marks)

ATP is a nucleotide because it contains a pentose sugar (ribose), a nitrogenous base (adenine) and a phosphate group.
ATP has three phosphate groups; the bonds between them are high‑energy, unstable bonds that store potential energy.
Hydrolysis of ATP removes the terminal phosphate group, producing ADP and inorganic phosphate (Pi). This reaction releases energy.
The released energy (or the inorganic phosphate) is used to drive endergonic cellular reactions, for example by phosphorylating substrate molecules or proteins.

Examiner tips

  • Use the exact terms – pentose sugar, nitrogenous base, phosphate group; mention ribose and adenine. Explain that the high‑energy phosphoanhydride bonds store potential energy. State that hydrolysis removes the terminal phosphate, giving ADP + Pi and releases energy. Show how this energy drives endergonic reactions by phosphorylation.

Common mistakes

  • Confusing ATP with a general nucleoside or missing the ribose base. Saying the energy is stored in the sugar rather than the phosphoanhydride bonds. Not specifying that the terminal phosphate is removed, or that ADP + Pi are the products.

Mark scheme (4 marks)

  1. ATP is a nucleotide because it contains a pentose sugar (ribose), a nitrogenous base (adenine), and a phosphate group
  2. ATP has three phosphate groups and the bonds between them are unstable / store potential energy
  3. Hydrolysis of ATP removes the terminal phosphate group, producing ADP and inorganic phosphate (Pi), and this reaction releases energy
  4. The released energy (and/or the inorganic phosphate) is used to drive endergonic / energy-requiring cellular reactions by phosphorylating substrate molecules or proteins

Key terms in this question

nucleotide · hydrolysis

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