Explain how ATP is used during the cross-bridge cycle in skeletal muscle contraction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
ATP binds to the myosin head, causing it to detach from actin and break the cross‑bridge.
ATP is hydrolysed by myosin ATPase to ADP + Pi, providing energy that cocks the myosin head into a high‑energy, pre‑power‑stroke position.
The cocked myosin head (with ADP + Pi still bound) attaches to actin, forming a new cross‑bridge; release of Pi triggers the power stroke, pulling actin and shortening the muscle.
ATP is also required by the sarcoplasmic reticulum Ca²⁺‑ATPase pumps to actively transport Ca²⁺ back into the SR, terminating contraction.
ATP is hydrolysed by myosin ATPase to ADP + Pi, providing energy that cocks the myosin head into a high‑energy, pre‑power‑stroke position.
The cocked myosin head (with ADP + Pi still bound) attaches to actin, forming a new cross‑bridge; release of Pi triggers the power stroke, pulling actin and shortening the muscle.
ATP is also required by the sarcoplasmic reticulum Ca²⁺‑ATPase pumps to actively transport Ca²⁺ back into the SR, terminating contraction.
Examiner tips
- Use the word ‘binds’ and ‘hydrolysed’ to show sequence; include the role of Pi release in the power stroke; mention Ca²⁺‑ATPase to cover all four marks.
- Show the cycle as a clear step‑by‑step chain: detach → hydrolysis → cock → attach → power stroke → Ca²⁺ re‑uptake.
- Use correct terminology: myosin ATPase, sarcoplasmic reticulum, Ca²⁺‑ATPase, cross‑bridge, power stroke.
Common mistakes
- Confusing ATP hydrolysis with the power stroke; forgetting that Pi release triggers the power stroke.
- Omitting the role of Ca²⁺‑ATPase in ending contraction.
- Using ‘binds’ for both ATP and Ca²⁺ pumps without distinguishing their functions.
Mark scheme (4 marks)
- ATP binds to the myosin head, causing it to detach from the actin filament (breaking the cross-bridge).
- ATP is hydrolysed (to ADP + Pi) by myosin ATPase, providing energy that cocks/re-energises the myosin head into the high-energy position.
- The cocked myosin head (with ADP + Pi bound) attaches to actin, forming a new cross-bridge; release of Pi triggers the power stroke.
- ATP is also required by the sarcoplasmic reticulum Ca²⁺-ATPase pumps to actively transport Ca²⁺ back into the sarcoplasmic reticulum, ending contraction.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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