A2 1 extended response, muscle contraction · 6 marks
Describe the sliding filament theory of muscle contraction, starting from the arrival of an action potential at the neuromuscular junction.
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A2 · A2 1 Physiology, Co-ordination and Control, and Ecosystems
Muscle contraction: the sliding filament theory step by step
The short answer
In the sliding filament theory the filaments themselves do not shorten. Calcium released from the sarcoplasmic reticulum moves tropomyosin off the binding sites on actin; myosin heads bind, form cross-bridges and pull the actin past the myosin, shortening the sarcomere. ATP then detaches each head so the cycle repeats.
The cross-bridge cycle
- An action potential travels down the T-tubules and calcium ions are released from the sarcoplasmic reticulum.
- Calcium binds to troponin, changing its shape and pulling tropomyosin off the myosin-binding sites on actin.
- Myosin heads attach to actin, forming cross-bridges.
- The power stroke: heads pivot, pulling the actin filament towards the centre of the sarcomere; ADP and Pi are released.
- ATP binds to the myosin head, which detaches; ATP hydrolysis by ATPase recocks the head ready to bind again.
- When stimulation stops, calcium is actively pumped back into the sarcoplasmic reticulum and tropomyosin re-blocks the sites.
What happens to the bands
| Region | During contraction |
|---|---|
| Sarcomere (Z line to Z line) | Shortens |
| I band (light, actin only) | Shortens |
| H zone (myosin only) | Shortens |
| A band (myosin length) | Stays the same |
Where the ATP comes from
Muscle uses three sources in sequence: existing ATP for a couple of seconds, phosphocreatine to regenerate ATP anaerobically for a few more, then aerobic respiration — with anaerobic glycolysis and lactate production when oxygen supply cannot keep up. Slow-twitch fibres are adapted for the aerobic route with more mitochondria, more myoglobin and a richer capillary supply; fast-twitch fibres are adapted for short, powerful anaerobic bursts.
Phrases that earn the marks
- calcium ions released from the sarcoplasmic reticulum
- calcium binds to troponin, moving tropomyosin
- myosin-binding sites on actin are exposed
- cross-bridges form; power stroke pulls actin past myosin
- ATP binds to the myosin head causing detachment
- the A band stays the same length
Where marks get lost
- Saying the filaments shorten. They slide past each other.
- Confusing troponin (binds calcium) with tropomyosin (blocks the site).
- Giving ATP only one role — it is needed both for detachment and for pumping calcium back.
- Claiming the A band shortens.
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Exam questions on muscle contraction a level biology, marked
Two A2-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
A2 1 extended response, muscle contraction · 6 marks
Describe the sliding filament theory of muscle contraction, starting from the arrival of an action potential at the neuromuscular junction.
Command word: Describe — how to answer itMark scheme — 6 creditworthy points
- Action potential travels along the T-tubules and causes the sarcoplasmic reticulum to release calcium ions
- Calcium ions bind to troponin, changing its shape
- Tropomyosin moves, exposing the myosin binding sites on the actin filament
- Myosin heads bind to actin forming cross-bridges
- The power stroke: myosin heads flex, pulling the actin filament past the myosin, and ADP + Pi are released
- ATP binds to the myosin head causing detachment; hydrolysis of ATP recocks the head and the cycle repeats
Full-mark answer
The action potential passes along the T-tubules into the muscle fibre and stimulates the sarcoplasmic reticulum to release calcium ions into the sarcoplasm. Calcium ions bind to troponin, changing its shape so that tropomyosin moves away from the myosin binding sites on the actin filament. Myosin heads then bind to actin, forming cross-bridges. Each myosin head flexes in a power stroke, pulling the actin filament past the myosin towards the centre of the sarcomere, and ADP and Pi are released. ATP then binds to the myosin head, causing it to detach; hydrolysis of that ATP by ATPase returns the head to its original position so it can bind further along the actin and repeat the cycle. The sarcomere shortens as the I band and H zone narrow, while the A band stays the same length.
Examiner insight: Two marks are routinely thrown away: the role of ATP in detachment (not in the power stroke itself) and the fact that the filaments do not shorten — they slide. Mentioning the A band staying constant secures the last point if you have dropped an earlier one.
A2 1 application question · 3 marks
Explain why muscles cannot relax immediately after death, a condition known as rigor mortis.
Command word: Explain — how to answer itMark scheme — 4 creditworthy points
- Respiration stops, so no ATP is produced
- ATP is needed to break the cross-bridge / detach the myosin head from actin
- Cross-bridges remain attached, so the muscle stays contracted / cannot relax
- Calcium ions also cannot be pumped back into the sarcoplasmic reticulum (allow)
Full-mark answer
After death respiration stops, so no ATP is produced. ATP is required to bind to the myosin head to break the cross-bridge and detach it from actin, and it is also needed for the active transport of calcium ions back into the sarcoplasmic reticulum. Without ATP the cross-bridges stay attached, so the muscle remains contracted and cannot relax.
Examiner insight: This tests whether you know that ATP is used for detachment, not for the power stroke. It is the single most common muscle misconception in CCEA scripts.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- Why does rigor mortis happen?
- Respiration stops, so no ATP is available to detach the myosin heads from actin and the cross-bridges stay locked in place.
- What is the role of calcium in muscle contraction?
- Calcium ions bind to troponin, which shifts tropomyosin away from the myosin-binding sites on actin so cross-bridges can form.
- Which band does not change length?
- The A band, because it corresponds to the length of the myosin filaments, and myosin does not shorten.
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