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A2 · Practice questions · Striated muscle and contraction

Bind, pull, release.

Six original Cambridge-style questions on striated muscle, from identifying bands in an electron micrograph to explaining every step of the cross-bridge cycle.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Bands, calcium, ATP.

01
Recall
[4 marks]

Describe the ultrastructure of a sarcomere as seen in an electron micrograph.

  • A sarcomere runs from one Z line to the next. ✓
  • Light I bands contain thin actin filaments only, attached to the Z lines. ✓
  • The dark A band is the length of the thick myosin filaments and includes regions where actin and myosin overlap. ✓
  • The paler H zone in the centre of the A band contains myosin only, with the M line at its centre. ✓
02
Explain
[4 marks]

Describe how an impulse in a motor neurone leads to the release of calcium ions inside a muscle fibre.

  • At the neuromuscular junction, ACh is released and binds to receptors on the sarcolemma. ✓
  • Na+ channels open and the sarcolemma depolarises; an action potential spreads along it. ✓
  • The action potential travels down the T-tubules into the fibre. ✓
  • This causes the sarcoplasmic reticulum to release Ca2+ into the sarcoplasm (Ca2+ channels in the SR open). ✓
03
Explain
[6 marks]

Explain the sliding filament model of muscle contraction, including the roles of troponin, tropomyosin, calcium ions and ATP.

  • Ca2+ binds to troponin, which changes shape. ✓
  • This moves tropomyosin away from the myosin-binding sites on the actin filaments. ✓
  • Myosin heads (with ADP and Pi attached) bind to actin, forming cross-bridges. ✓
  • ADP and Pi are released; the heads tilt (power stroke), pulling actin towards the M line. ✓
  • ATP binds to the myosin heads, causing them to detach from actin. ✓
  • Myosin ATPase hydrolyses ATP, re-cocking the heads, which bind further along; the cycle repeats while Ca2+ and ATP are present. ✓
04
Data
[3 marks]

A relaxed sarcomere is 2.6 μm long, with an A band of 1.6 μm and an H zone of 0.6 μm. After contraction the sarcomere is 2.2 μm long. Calculate the new total I band length and the new H zone length.

  • The A band stays at 1.6 μm, so new I band (both halves) = 2.2 - 1.6 = 0.6 μm. ✓
  • The sarcomere shortened by 2.6 - 2.2 = 0.4 μm, and the H zone shortens by the same amount as the sarcomere. ✓
  • New H zone = 0.6 - 0.4 = 0.2 μm. ✓
05
Explain
[2 marks]

Explain why a muscle fibre relaxes when stimulation by its motor neurone stops.

  • Ca2+ is actively pumped back into the sarcoplasmic reticulum (using ATP). ✓
  • Troponin loses its Ca2+, so tropomyosin covers the binding sites again; no cross-bridges form and the filaments can slide apart. ✓
06
Analysis
[2 marks]

Suggest why muscle fibres contain large numbers of mitochondria.

  • Contraction needs large amounts of ATP, for detaching and re-cocking myosin heads. ✓
  • ATP is also needed to pump Ca2+ back into the sarcoplasmic reticulum (and to make ACh at the junction); aerobic respiration in mitochondria supplies it. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.