All revision notes topics

Muscle contractionEdexcel A-Level Biology A: Revision notes

Section 1

The sliding filament theory

A muscle contracts because actin filaments slide between myosin filaments, pulling the Z lines closer together. The filaments themselves do not shorten.

During contraction, the sarcomere shortens, the I band gets shorter, the H zone gets narrower or disappears, and the A band stays the same length. The cause is the movement of many myosin heads forming and breaking cross-bridges with actin.

Example: a sarcomere shortening from 2.4 μm to 1.8 μm has shortened by 0.62.4×100=25%\frac{0.6}{2.4} \times 100 = 25\%.

Key termssliding filament theoryI bandH zoneA band
Common mistake

Do not say the filaments get shorter. They slide, and only the I band, H zone and sarcomere length change.

Section 2

Actin, myosin, troponin and tropomyosin

  • Actin: thin filament with binding sites for myosin heads.
  • Myosin: thick filament with globular heads that have an actin-binding site and an ATPase site.
  • Tropomyosin: a fibrous protein lying along actin that covers the myosin-binding sites when the muscle is relaxed.
  • Troponin: attached to tropomyosin; it has a binding site for calcium ions (Ca²⁺).

In a relaxed muscle the myosin-binding sites are blocked, so no cross-bridges can form.

Key termstroponintropomyosinmyosin ATPase

Section 3

Starting contraction: calcium and exposure of binding sites

  1. An action potential spreads along the sarcolemma and down the T-tubules.
  2. The sarcoplasmic reticulum releases Ca²⁺ into the sarcoplasm.
  3. Ca²⁺ binds to troponin, which changes shape.
  4. Troponin pulls tropomyosin away, exposing the myosin-binding sites on actin.
  5. Myosin heads, carrying ADP and Pi, bind to actin, forming cross-bridges.
Key termscross-bridgesarcoplasmic reticulum
Exam tip

Write the events in order: action potential, T-tubules, calcium release, troponin, tropomyosin, binding sites, cross-bridge.

Section 4

The cross-bridge cycle: role of ATP and ATPase

  1. A myosin head with ADP + Pi binds actin (cross-bridge).
  2. Power stroke: the head changes angle, pulling actin towards the centre of the sarcomere; ADP and Pi are released.
  3. ATP binds to the myosin head, breaking the cross-bridge.
  4. ATPase hydrolyses ATP to ADP + Pi; the energy released returns the head to its original high-energy position.
  5. The head binds to a new site further along actin and the cycle repeats.

ATP is therefore needed for detachment and for resetting the head, not directly for the power stroke.

Key termspower strokeATP hydrolysis
Common mistake

ATP does not 'power' the stroke itself: it detaches the head, and its hydrolysis resets the head ready for the next stroke.

Section 5

Relaxation and rigor mortis

When nerve impulses stop, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum using ATP. Troponin returns to its shape, tropomyosin covers the binding sites, the heads detach and the muscle relaxes.

After death, ATP is no longer made, so Ca²⁺ is not pumped back and heads cannot detach: the muscle stays stiff in rigor mortis until the proteins are broken down.

Key termsrigor mortisactive transport of calcium

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Muscle contraction

  1. A student studies a skeletal muscle under a microscope and measures one sarcomere. When the muscle is relaxed the sarcomere is 2.4 μm long, and each thick filament is 1.6 μm long. When the muscle is contracted the same sarcomere is 1.8 μm long.
    Explain why the H zone becomes narrower when the sarcomere contracts.2 marks
  2. Researchers study two isolated skeletal muscle fibres supplied with plenty of ATP. Fibre P is treated with a chemical that prevents calcium ions being released from the sarcoplasmic reticulum. Fibre Q is treated with a chemical that blocks the ATP-binding site of the myosin heads. Both fibres are then stimulated electrically.
    Explain how calcium ions allow myosin heads to bind to actin.2 marks
  3. In relaxed skeletal muscle, each myosin head carries ADP and an inorganic phosphate group and is held in a high-energy position. The head is part of a protein that has ATPase activity. Muscle contraction occurs when myosin heads bind to actin and move it, and this cycle needs a supply of ATP.
    Explain the role of ATP and ATPase in the cycle of movement of a myosin head.3 marks
See the full worksheet

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).