Explain the roles of inorganic ions in the contraction of muscle cells and in the transport of oxygen in the blood.

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)

Calcium ions are essential for muscle contraction. When a nerve impulse reaches the muscle fibre, Ca²⁺ is released from the sarcoplasmic reticulum into the cytoplasm, allowing actin–myosin cross‑bridge formation and contraction. Iron ions are a key component of haemoglobin. In the haemoglobin molecule, Fe²⁺ binds oxygen to form oxyhaemoglobin, which transports oxygen in the blood. Without adequate iron, haemoglobin synthesis is impaired and oxygen transport to tissues is reduced.

Examiner tips

Use the exact terms ‘Ca²⁺’, ‘sarcoplasmic reticulum’, ‘actin–myosin cross‑bridges’, ‘Fe²⁺’, ‘haemoglobin’, ‘oxyhaemoglobin’.
Show the causal link: Ca²⁺ release → contraction; Fe²⁺ binding → oxygen transport.
Keep the answer concise – 5 marks, so one sentence per point is sufficient.

Common mistakes

Mixing up Ca²⁺ with other ions such as Na⁺ or K⁺; not mentioning the sarcoplasmic reticulum.
Using ‘oxygen’ instead of ‘oxyhaemoglobin’ when describing iron’s role.
Omitting the consequence of iron deficiency on oxygen transport.

Mark scheme (5 marks)

Calcium ions are required for muscle contraction
Calcium ions are released into the muscle cell (from the sarcoplasmic reticulum / during a nerve impulse) to allow contraction to occur
Iron ions are found in / are a component of haemoglobin
Iron ions bind to / carry oxygen (forming oxyhaemoglobin)
Without sufficient iron ions haemoglobin cannot be made / cannot carry oxygen effectively, so less oxygen is transported to respiring cells

Key terms in this question

inorganic ions