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7.2.3Unit 6: Practical Skills in Biology II

The Process of Skeletal Muscle Contraction — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 4 parts Lesson 25 of 48 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Section 1 of 4Filament Structure

7.2.3 The Process of Skeletal Muscle Contraction
Thick filament
  • Made of many myosin molecules, each with a fibrous tail that anchors it and a globular head that projects outwards.
Thin filament
  • Two chains of globular actin molecules twisted together to form a helix.
Tropomyosin
  • A fibrous protein wound around the actin chains that covers the myosin-binding sites when the muscle is relaxed.
Troponin
  • A protein attached to the actin at intervals that binds calcium ions and moves tropomyosin out of the way.
Myosin head
  • Has an ATP-binding site and works as ATPase, binding to actin to form cross-bridges.

Section 2 of 4Sequence of Contraction

Muscle shortens because filaments slide past each other, not because the filaments themselves shorten.

1
An action potential arrives at the neuromuscular junction, spreads along the sarcolemma and T-tubules, and calcium ions are released from the sarcoplasmic reticulum.
2
Calcium ions bind to troponin, which changes shape and pulls tropomyosin aside so that myosin-binding sites on actin are exposed.
3
Myosin heads bind to actin, forming cross-bridges, and then tilt in the power stroke, pulling the thin filament toward the centre of the sarcomere.
4
ATP binds to each myosin head, which breaks the cross-bridge and lets the head detach from the actin.
5
ATPase hydrolyses ATP into ADP and Pi, and the energy returns the myosin head to its original position in the recovery stroke.
6
The head binds again to a new site further along the actin, and the cycle repeats while calcium ions and ATP remain available.

Section 3 of 4Relaxation and Rigor Mortis

Relaxation and Rigor Mortis
Calcium removal
  • When stimulation stops, calcium ions are actively transported back into the sarcoplasmic reticulum.
Blocking again
  • Troponin returns to its original shape and tropomyosin covers the binding sites, so no cross-bridges can form.
Lengthening
  • Actin filaments slide back to their relaxed positions, restoring the sarcomere to its resting length.
Rigor mortis
  • After death no ATP is made, so myosin heads cannot detach and the muscles stay stiff and contracted.

Section 4 of 4Exam Checklist & Tips

7.2.3 – Key Points for the Exam

  • Sliding filament theory The explanation that muscle contracts as actin and myosin filaments slide past each other.
  • Cross-bridge The link formed when a myosin head binds to an actin filament.
  • Power stroke The bending of the myosin head that pulls the actin filament along.
  • Tropomyosin A protein that blocks myosin-binding sites on actin in relaxed muscle.
  • Troponin A calcium-binding protein that moves tropomyosin when stimulated.

Common Mistakes

  • Claiming ATP is needed to form the cross-bridge; it is needed to detach the myosin head.
  • Stating that the actin and myosin filaments shorten during contraction.
  • Forgetting that calcium ions bind to troponin, not directly to myosin or tropomyosin.
  • Leaving out the neuromuscular junction when describing how contraction begins.

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