Explain the importance of iron ions in red blood cells and of phosphate ions in the production of ATP.

AQA A-Level Biology (7402) — 3.1.8 Inorganic Ions · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Iron ions are a component of haemoglobin, the protein that carries oxygen in red blood cells. The iron ion in each haem group binds one oxygen molecule, allowing haemoglobin to transport oxygen from the lungs to tissues. Without iron ions, haemoglobin cannot bind oxygen, so oxygen transport around the body would fail.

Phosphate ions are a component of ATP, the energy currency of the cell. When the high‑energy phosphate bond in ATP is hydrolysed, energy is released, which drives or fuels cellular processes and reactions.

Examiner tips

Use the exact phrases "component of haemoglobin" and "binds oxygen" to match the mark scheme.
Mention that iron ions are essential for oxygen transport, not just a general role.
State that phosphate ions form part of ATP and that ATP releases energy when its phosphate bond is broken.
Keep the answer concise and directly linked to the points in the scheme.

Common mistakes

Confusing iron’s role with that of oxygen itself, or saying iron simply carries oxygen instead of binding it in haemoglobin.
Omitting the phrase "high‑energy phosphate bond" when describing ATP energy release.
Using vague terms like "energy molecules" instead of specifying ATP and phosphate ions.

Mark scheme (5 marks)

Iron ions are a component of / found in haemoglobin
Iron ions allow haemoglobin to bind / carry oxygen
(Therefore) without iron ions oxygen cannot be transported around the body / to cells / tissues
Phosphate ions are used to form / are a component of ATP
ATP releases energy (when the phosphate bond is broken) to drive / fuel cellular processes / reactions

Key terms in this question

iron ions · phosphate ions · ATP