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Muscle contractionEdexcel International A Level Biology: Revision notes

Section 1

The sliding filament theory

Muscle contraction is explained by the sliding filament theory: during contraction the thin actin filaments slide between the thick myosin filaments, so each sarcomere shortens. The filaments themselves do not shorten.

In a contracting sarcomere:

  • the I band and H zone become shorter
  • the A band stays the same length (it is the length of the myosin filaments)
  • the Z lines move closer together
Key termssliding filament theoryactinmyosin
Common mistake

Never say the filaments shorten. The sarcomere shortens because actin slides over myosin.

Section 2

The proteins involved

  • Actin: forms the thin filament and carries the myosin-binding sites.
  • Myosin: forms the thick filament. Each myosin molecule has a head that can bind to actin and also acts as an ATPase, hydrolysing ATP.
  • Tropomyosin: a fibrous protein wound around actin which, at rest, covers the myosin-binding sites.
  • Troponin: a protein attached to tropomyosin and actin with a binding site for Ca²⁺.

At rest (no Ca²⁺) tropomyosin blocks the binding sites, so myosin heads cannot attach.

Key termstropomyosintroponinmyosin headATPasemyosin-binding site

Section 3

Starting contraction: the role of calcium ions

  1. An action potential spreads along the sarcolemma and down the T-tubules.
  2. The sarcoplasmic reticulum releases Ca²⁺ into the sarcoplasm.
  3. Ca²⁺ bind to troponin, which changes shape.
  4. Troponin pulls tropomyosin away from the binding sites, exposing them on actin.
Key termsaction potentialCa²⁺

Section 4

The cross-bridge cycle

  1. A myosin head carrying ADP and Pi binds to an exposed site on actin, forming a cross-bridge.
  2. The head tilts in the power stroke, pulling the actin towards the centre of the sarcomere. ADP and Pi are released.
  3. A new ATP binds to the head, which causes it to detach from actin.
  4. ATPase on the head hydrolyses ATP to ADP and Pi; the energy released returns (re-cocks) the head to its original position.
  5. If Ca²⁺ is still present, the head binds again and the cycle repeats.
Key termscross-bridgepower strokeATPADPPi
Exam tip

ATP is needed to detach the myosin head, not to attach it. This is why muscles stiffen in rigor mortis.

Section 5

Relaxation and the supply of ATP

When stimulation stops, Ca²⁺ are actively transported (using ATP) back into the sarcoplasmic reticulum. Troponin returns to its original shape, tropomyosin covers the binding sites again, cross-bridges cannot form and the muscle relaxes (it is then stretched by its antagonist).

ATP is therefore needed for detachment of myosin heads, for re-cocking them and for pumping Ca²⁺. It is supplied by respiration and by phosphocreatine in short bursts.

Key termsactive transportrelaxation

Must know

  • Actin slides over myosin: I band and H zone shorten, A band unchanged
  • Ca²⁺ bind troponin, which moves tropomyosin and exposes binding sites
  • Myosin heads bind (cross-bridge), power stroke, ATP detaches the head, ATPase re-cocks it
  • Relaxation: Ca²⁺ pumped back into the sarcoplasmic reticulum by active transport

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Muscle contraction

  1. A biochemist is investigating the cause of rigor mortis, the stiffening of skeletal muscle a few hours after death. After death the supply of ATP to the muscle fibres falls to zero, but calcium ions continue to leak from the sarcoplasmic reticulum into the sarcoplasm of the fibres.
    Explain why the muscles become stiff and cannot relax in rigor mortis.2 marks
  2. In a relaxed skeletal muscle a sarcomere measures 2.4 µm in length and its A band measures 1.6 µm. When the muscle is stimulated and contracts, the sarcomere shortens to 1.8 µm.
    Calculate the percentage decrease in sarcomere length on contraction, and explain why the A band stays the same length.2 marks
  3. A researcher isolates myofibrils from rabbit skeletal muscle and places them in three solutions, measuring sarcomere length each time. Condition 1: ATP present and no calcium ions: sarcomeres stay at 2.4 µm. Condition 2: ATP and calcium ions both present: sarcomeres shorten to 1.8 µm and then lengthen again when the calcium ions are removed. Condition 3: calcium ions present but no ATP: sarcomeres shorten slightly, then stay shortened and the myofibrils become rigid and cannot be stretched.
    Explain why the sarcomeres shorten in Condition 2 but not in Condition 1.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).