Muscle contractionEdexcel International A Level Biology: Subtopic test

1. 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.

   (a)

   Which protein binds calcium ions to start muscle contraction?

   [1 mark]

   • AActin
   • BMyosin
   • CTropomyosin
   • DTroponin

   (b)

   Which statement describes the myosin heads in a muscle in rigor mortis?

   [1 mark]

   • AThey are detached from actin and bound to ATP
   • BThey remain attached to actin as cross-bridges and cannot detach
   • CThey are bound to troponin
   • DThey are bound to tropomyosin, blocking the actin binding sites

   (c)

   Explain why the muscles become stiff and cannot relax in rigor mortis.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which band does not change in length when the muscle contracts?

   [1 mark]

   • AI band
   • BH zone
   • CA band
   • DSarcomere

   (b)

   Which is the role of ATPase on the myosin head?

   [1 mark]

   • AIt hydrolyses ATP to ADP and phosphate, releasing energy for the myosin head to return to its original position
   • BIt synthesises ATP from ADP and phosphate during the power stroke
   • CIt binds calcium ions so that troponin can change shape
   • DIt moves tropomyosin away from the actin binding sites

   (c)

   Calculate the percentage decrease in sarcomere length on contraction, and explain why the A band stays the same length.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Explain why the sarcomeres shorten in Condition 2 but not in Condition 1.

   [3 marks]

   (b)

   Use the results of Condition 3 to explain the role of ATP in muscle contraction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A toxin from a marine organism is found to block the active transport of calcium ions from the sarcoplasm back into the sarcoplasmic reticulum of skeletal muscle fibres. The toxin does not affect the release of calcium ions from the sarcoplasmic reticulum, the supply of ATP or the transmission of action potentials along the sarcolemma.

   (a)

   Describe how an action potential arriving at a skeletal muscle fibre leads to the shortening of its sarcomeres.

   [6 marks]

   (b)

   Predict and explain the effect of the toxin on a muscle fibre after a single action potential.

   [6 marks]

   Total for question 4: 12 marks

End of questions