Explain the sliding-filament model
Explain contraction through calcium, troponin, tropomyosin, myosin cross-bridges and ATP.
Section 15.1 · Control and coordination in mammals
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- A control pathway links a stimulus, receptor, coordination system and response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which protein moves away from actin-binding sites when calcium binds troponin?
What you will learn
- Explain how calcium exposes actin-binding sites
- Describe the ATP-dependent attachment, power-stroke, detachment and reset cycle
Sliding-filament cycle
| Feature | Meaning |
|---|---|
| Expose sites | Calcium binds troponin; tropomyosin moves away from actin-binding sites |
| Attach and pull | Myosin heads bind actin and perform a power stroke |
| Detach | A new ATP molecule binds myosin, allowing it to detach from actin |
| Reset | ATP hydrolysis re-cocks the head for another cycle |
Build the mechanism
Calcium ions released into the sarcoplasm bind to troponin. This changes troponin shape and moves tropomyosin away from myosin-binding sites on actin. Energised myosin heads bind actin and perform a power stroke, pulling thin filaments towards the sarcomere centre. A new ATP molecule binds myosin, allowing detachment; ATP hydrolysis re-cocks the head. Repeated cycles slide filaments past one another, shortening the sarcomere without shortening actin or myosin.
Keep the sequence precise
ATP is needed for myosin detachment and re-cocking; calcium exposes binding sites but does not supply the power stroke directly. The filaments slide and overlap more; they do not shrink.
Pause and explain
A learner says calcium pulls actin directly towards the centre. Correct the role of calcium.
Worked example
What happens to myosin when a new ATP molecule binds its head?
Show the worked solution
- ATP binding changes the interaction between myosin and actin.
- The cross-bridge detaches.
Answer Myosin detaches from actin when a new ATP molecule binds.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace one cross-bridge cycle from calcium release to myosin reset.
Give me a hint
Name the first event, then follow each link in order.
Compare my reasoning
- Calcium binds troponin and exposes actin sites.
- Myosin pulls, ATP causes detachment and hydrolysis resets the head.
Calcium binds troponin, moving tropomyosin to expose actin sites. Myosin binds and performs a power stroke. New ATP causes detachment, and ATP hydrolysis re-cocks the myosin head.
Look for these in your work
- Mechanism stated
- Steps linked in order
A muscle runs out of ATP while calcium remains in the sarcoplasm. Explain why cross-bridge cycling cannot continue normally.
Give me a hint
Use the terms in the question and state what changes.
Compare my reasoning
- ATP is needed for myosin detachment.
- Without detachment, heads cannot reset and repeat the cycle.
Without ATP, myosin cannot detach normally from actin and cannot be re-cocked by ATP hydrolysis, so repeated cross-bridge cycling is disrupted.
Look for these in your work
- Evidence interpreted
- Biological direction explained
A learner says calcium pulls actin directly towards the centre. Correct the role of calcium.
Give me a hint
Apply the mechanism to the new context without adding unsupported claims.
Compare my reasoning
- Calcium binds to troponin.
- This moves tropomyosin and permits myosin binding.
Calcium binds to troponin and causes tropomyosin to move away from actin-binding sites. Myosin heads then bind actin and generate the pulling power stroke.
Look for these in your work
- Mechanism applied
- Limits preserved
Common mistakes
- Saying calcium breaks down ATP to power the stroke
- Saying actin and myosin filaments shorten
What can you explain now?
Which protein moves away from actin-binding sites when calcium binds troponin?
Compare with the explanation
Tropomyosin
Calcium-bound troponin changes tropomyosin position.
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Make it stick
What roles do calcium, troponin, tropomyosin and ATP have in sliding-filament contraction?
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