Muscle contractionEdexcel A-Level Biology A: Subtopic test

1. 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.

   (a)

   Which part of the sarcomere stays the same length when the muscle contracts?

   [1 mark]

   • AI band
   • BH zone
   • CThe distance between adjacent Z lines
   • DA band

   (b)

   By what percentage does the sarcomere shorten on contraction?

   [1 mark]

   • A0.6%
   • B25%
   • C33%
   • D75%

   (c)

   Explain why the H zone becomes narrower when the sarcomere contracts.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   What is observed in fibre P after electrical stimulation, and why?

   [1 mark]

   • ANo contraction, because tropomyosin continues to cover the binding sites on actin
   • BNo contraction, because myosin heads cannot bind ATP
   • CNormal contraction, because ATP is available
   • DContraction followed by slow relaxation, because calcium ions are not pumped back

   (b)

   In fibre Q, which of the following is most likely after the first power stroke?

   [1 mark]

   • AThe myosin heads detach and return to their original position
   • BThe myosin heads detach but cannot bind actin again
   • CThe myosin heads remain attached to actin
   • DThe myosin heads never bind to actin

   (c)

   Explain how calcium ions allow myosin heads to bind to actin.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Explain the role of ATP and ATPase in the cycle of movement of a myosin head.

   [3 marks]

   (b)

   Describe how myosin heads cause actin filaments to slide past myosin filaments and the sarcomere to shorten.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   After death, the cells of skeletal muscle stop producing ATP. The muscles then gradually become stiff, a condition called rigor mortis, and many hours later they become flexible again. In living muscle, once nerve impulses to a fibre stop, the fibre relaxes.

   (a)

   Explain how an action potential arriving at a muscle fibre leads to the formation of cross-bridges between myosin and actin.

   [6 marks]

   (b)

   Explain why the muscles become stiff after death, and why a living muscle relaxes when nerve impulses stop.

   [6 marks]

   Total for question 4: 12 marks

End of questions