The big idea: In the sliding-filament model, myosin heads pull on actin to shorten the muscle. But a muscle must only contract when it is told to — so there has to be a switch.
That switch is calcium (Ca²⁺). Two thin-filament proteins do the gatekeeping:
Tropomyosin — a thread that lies along the actin and physically covers the myosin-binding sites.
Troponin — a small protein clamped onto tropomyosin that holds it in place and carries the Ca²⁺ binding site.
At rest the sites are blocked, so myosin cannot attach. A nerve impulse floods the cell with Ca²⁺, the block is lifted, and contraction begins.
The cross-bridge cycle is the engine behind sliding-filament contraction: myosin heads grab the exposed sites on actin and pull the thin filaments toward the centre, shortening the sarcomere.
Interactive diagram
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- Actin (thin filament)
- The thin filament that myosin pulls on; it carries the myosin-binding sites.
- Myosin (thick filament)
- The thick filament whose heads attach to actin and perform the power stroke.
- Tropomyosin
- A thread-like protein that lies along actin and BLOCKS the myosin-binding sites when the muscle is at rest.
- Troponin
- A protein attached to tropomyosin; it holds tropomyosin in the blocking position and has a binding site for Ca²⁺.
- Calcium (Ca²⁺)
- The ion released on stimulation; it binds troponin and acts as the switch that exposes the binding sites.
- Sarcoplasmic reticulum
- The internal store of Ca²⁺ in a muscle fibre; it releases Ca²⁺ on a nerve impulse and pumps it back to end contraction.
- Cross-bridge
- A temporary link formed when a myosin head attaches to an exposed binding site on actin.
Don't confuse the two 'trop-' proteins: Tropomyosin = the long thread that covers the binding sites (it does the blocking).
Troponin = the small clamp that holds tropomyosin and binds Ca²⁺ (it does the sensing).
Memory hook: troponin binds the ion (Ca²⁺); tropomyosin sits over the myosin sites.
Read the mechanism as a chain of cause and effect. It starts with the sites blocked, the Ca²⁺ switch flips them open, and then the myosin head runs a repeating four-step cross-bridge cycle as long as Ca²⁺ and ATP are both present.
Turning the switch ON
- At rest: tropomyosin (held by troponin) lies over the actin and blocks the myosin-binding sites, so no cross-bridges can form.
- A nerve impulse reaches the fibre and Ca²⁺ is released from the sarcoplasmic reticulum.
- Ca²⁺ binds troponin, which changes shape and pulls tropomyosin away from the binding sites.
- The myosin-binding sites are now EXPOSED — the cross-bridge cycle can run.
| At rest (relaxed) | When stimulated (contracting) | |
|---|---|---|
| Ca²⁺ level in the cytoplasm | Low — Ca²⁺ is stored in the sarcoplasmic reticulum | High — Ca²⁺ released from the sarcoplasmic reticulum |
| Troponin | Holds tropomyosin in the blocking position | Ca²⁺ binds it → it changes shape and shifts tropomyosin |
| Tropomyosin | Covers (BLOCKS) the myosin-binding sites on actin | Moved AWAY → binding sites are EXPOSED |
| Myosin heads | Cannot attach — no cross-bridges form | Attach to actin → cross-bridges + power strokes |
| Outcome | No sliding — muscle stays relaxed | Filaments slide → sarcomere shortens (contracts) |
One cross-bridge cycle (repeats)
- Bind: a myosin head attaches to an exposed site on actin, forming a cross-bridge.
- Power stroke: the head pivots and pulls the actin filament toward the centre of the sarcomere — this is the stroke that shortens the muscle.
- Detach: a new ATP binds the myosin head, which breaks the cross-bridge so the head lets go of actin.
- Re-cock: the ATP is hydrolysed (ATP → ADP + Pi); the energy released re-cocks the head back to its 'ready' position, set for the next cycle.
| Step | What happens | What is used / needed |
|---|---|---|
| 1. Bind | The myosin head attaches to the exposed site on actin — a cross-bridge | Sites already exposed (needs Ca²⁺) |
| 2. Power stroke | The head pivots, pulling the actin filament toward the sarcomere centre | Energy stored from the previous ATP hydrolysis |
| 3. Detach | A new ATP molecule binds the head, breaking the cross-bridge so it lets go | 1 ATP binds |
| 4. Re-cock | ATP is hydrolysed (ATP → ADP + Pi); the head returns to its 'ready' position | ATP hydrolysed — energy released |
The two jobs: Ca²⁺ is the switch, ATP is the energy: Keep the two molecules' roles separate — examiners love to test this:
Ca²⁺ = the switch. It binds troponin and decides whether the binding sites are open. It controls if the cycle can run, not the movement itself.
ATP = the energy. It is needed twice in each cycle: one ATP binds to detach the head, and the same ATP is then hydrolysed to re-cock the head for the next pull.
So Ca²⁺ unlocks the door; ATP does the work of letting go and resetting.
| Calcium (Ca²⁺) | ATP | |
|---|---|---|
| What it is | An ion released from the sarcoplasmic reticulum on stimulation | The cell's energy molecule, hydrolysed to ADP + Pi |
| What it binds | Troponin | The myosin head |
| Its job | The SWITCH — controls whether the binding sites are exposed | The ENERGY — powers detachment and re-cocking of the head |
| When missing… | Tropomyosin re-blocks the sites → muscle relaxes | The myosin head cannot let go → it stays stuck (rigor) |
How the muscle relaxes: When the nerve impulse stops, the Ca²⁺ is actively pumped back into the sarcoplasmic reticulum.
With Ca²⁺ gone, troponin returns to its original shape, tropomyosin slides back over the binding sites, and no new cross-bridges form. The muscle relaxes.
This is why a dead body goes stiff (rigor mortis): with no ATP left, the heads cannot detach from actin, so the cross-bridges stay locked.
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How this is tested: HL muscle questions love the cross-bridge cycle. A common Outline/Explain asks for the role of calcium ions in starting contraction (Ca²⁺ → troponin → tropomyosin moves → sites exposed), or the role of ATP (binds to detach the head; hydrolysed to re-cock it).
A longer Explain asks you to put the whole molecular cycle in order. Score points for: blocked sites at rest → Ca²⁺ released → Ca²⁺ binds troponin → tropomyosin moves → sites exposed → myosin binds (cross-bridge) → power stroke pulls actin → ATP binds to detach → ATP hydrolysed to re-cock → repeats.
Watch the role-split: don't say ATP opens the binding sites (that's Ca²⁺), and don't say Ca²⁺ powers the power stroke (the energy comes from ATP).
IB-style question — the molecular roles of Ca²⁺ and ATP in contraction
Explain the roles of calcium ions and ATP in the contraction of a muscle fibre. [6]
How to score all six marks
- Blocked at rest. At rest, tropomyosin (held by troponin) covers the myosin-binding sites on actin, so myosin cannot attach.
- Ca²⁺ released and binds troponin. On stimulation, Ca²⁺ is released from the sarcoplasmic reticulum and binds to troponin.
- Sites exposed (Ca²⁺ = the switch). Troponin changes shape and moves tropomyosin away, so the myosin-binding sites are exposed — this is calcium's role: the switch that allows cross-bridges to form.
- Cross-bridge + power stroke. Myosin heads bind the exposed sites (cross-bridges) and the power stroke pulls actin toward the centre of the sarcomere.
- ATP detaches the head. A new ATP molecule binds the myosin head, breaking the cross-bridge so the head detaches from actin.
- ATP re-cocks the head. The ATP is hydrolysed (ATP → ADP + Pi); the energy re-cocks the head for the next cycle — so ATP's role is detachment and re-cocking, and the cycle repeats while Ca²⁺ and ATP are present. (Award 1 mark per distinct point, up to 6.)
Final answer
At rest tropomyosin (held by troponin) blocks the binding sites. Ca²⁺ released from the sarcoplasmic reticulum binds troponin, moving tropomyosin so the sites are exposed (Ca²⁺ = the switch). Myosin heads bind and the power stroke pulls actin inward. A new ATP binds the head to detach it, then ATP is hydrolysed to re-cock it (ATP = energy for detachment and re-cocking). The cycle repeats while Ca²⁺ and ATP are present.
✓ Why this scores full marks: It keeps the roles separate — Ca²⁺ as the switch (binds troponin, exposes the sites) and ATP as the energy (binds to detach, then is hydrolysed to re-cock) — and it puts the molecular steps in the right order.
A common way to lose marks is to mix the roles up: saying ATP exposes the binding sites, or that Ca²⁺ powers the power stroke.
| Calcium (Ca²⁺) | ATP | |
|---|---|---|
| What it is | An ion released from the sarcoplasmic reticulum on stimulation | The cell's energy molecule, hydrolysed to ADP + Pi |
| What it binds | Troponin | The myosin head |
| Its job | The SWITCH — controls whether the binding sites are exposed | The ENERGY — powers detachment and re-cocking of the head |
| When missing… | Tropomyosin re-blocks the sites → muscle relaxes | The myosin head cannot let go → it stays stuck (rigor) |