Muscle contractionAQA A-Level Biology: Subtopic test

1. 1
   A biochemist is investigating how protein filaments interact within the sarcomeres of skeletal muscle. She works with isolated myofibrils from a mammal, to which she can add ATP and calcium ions as she chooses.

   (a)

   Calcium ions are added to myofibrils at a low concentration. What is their direct effect on the thin filaments?

   [1 mark]

   • AThey bind to the myosin heads and cause them to detach
   • BThey hydrolyse ATP to provide energy for the filaments to slide
   • CThey cause tropomyosin to move, exposing the myosin-binding sites on actin
   • DThey bind to actin and pull it towards the centre of the sarcomere

   (b)

   After a myosin head has bent and pulled the actin filament, what causes the head to detach from actin?

   [1 mark]

   • ACalcium ions binding to the head
   • BHydrolysis of ATP to ADP and phosphate, releasing the head
   • CTropomyosin moving back over the binding site
   • DA molecule of ATP binding to the myosin head

   (c)

   Explain how the movement of myosin heads causes the sarcomere to shorten.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Within a few hours of death, the skeletal muscles of an animal become stiff and cannot be stretched (rigor mortis). During life, a resting muscle is soft and its myofibrils are easily stretched. After death, ATP is no longer made in the muscle and calcium ions leak out of the sarcoplasmic reticulum.

   (a)

   Why do the muscles become stiff after death?

   [1 mark]

   • ATropomyosin permanently binds to the myosin heads
   • BMyosin heads stay attached to actin because there is no ATP to detach them
   • CCalcium ions are no longer present to move tropomyosin
   • DActin filaments are broken down by acetylcholinesterase

   (b)

   In a living, resting muscle, what prevents myosin heads from attaching to actin?

   [1 mark]

   • ATropomyosin blocks the myosin-binding sites on actin
   • BATP is attached to the actin
   • CCalcium ions bind to the myosin heads
   • DThe actin filaments are too far from the myosin

   (c)

   When stimulation of a living muscle stops, the muscle relaxes. Explain how.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a 100 m sprint, a muscle uses ATP at a rate of 4 mmol kg⁻¹ s⁻¹. At the start of the race the muscle contains an ATP store of 5 mmol kg⁻¹ and a phosphocreatine store of 25 mmol kg⁻¹. One molecule of phosphocreatine can regenerate one molecule of ATP.

   (a)

   Calculate for how long the ATP and phosphocreatine stores together could supply the sprint. State what the muscle must rely on after this time.

   [3 marks]

   (b)

   Explain the role of phosphocreatine in muscle contraction and why it is important in a sprint.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Researchers studying the mechanism of contraction use myofibrils isolated from rabbit muscle. They also record the events in an intact muscle fibre stimulated through its motor neurone. They compare how isolated myofibrils respond in different solutions.

   (a)

   Describe and explain the sequence of events by which a stimulated muscle fibre contracts, from the arrival of the action potential to the shortening of the sarcomere.

   [6 marks]

   (b)

   Isolated myofibrils are placed in three solutions. In solution X, ATP is present but calcium ions are absent: the myofibrils do not shorten. In solution Y, ATP and calcium ions are both present: the myofibrils shorten. In solution Z, calcium ions are present but ATP is absent: the myofibrils become rigid after a small initial shortening. Explain these results.

   [6 marks]

   Total for question 4: 12 marks

End of questions