Muscle contractionEdexcel International A Level Biology: Mind map

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