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Sliding filaments: shortening a muscle without shortening anything questions

Skeletal muscle structure from muscle to myofibril to sarcomere, the sliding filament mechanism with the roles of calcium, tropomyosin, ATP and the myosin head, what happens to the A band, I band and H zone during contraction, slow and fast twitch fibres, and ATP supply from phosphocreatine and respiration.

6 original questions · 24 marks · the sliding filaments: shortening a muscle without shortening anything notes · Nervous coordination, receptors, muscles and behaviour

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• 5 marks · Describe

  Describe how the release of calcium ions from the sarcoplasmic reticulum causes the thin filaments of a sarcomere to slide inwards.

  Mark scheme

  1. B1 calcium ions bind to troponin, which changes shape
  2. B1 troponin pulls tropomyosin aside, exposing the myosin binding sites on the actin, which were covered before
  3. B1 a myosin head carrying ADP and phosphate attaches to an exposed binding site and forms a cross-bridge
  4. B1 the head bends in a power stroke, releasing the ADP and phosphate and pulling the thin filament about 10 nm towards the centre of the sarcomere
  5. A1 ATP binds to the head so that it detaches, and hydrolysis of that ATP returns it to its upright position to attach further along; heads work out of step so the filament is never released

• 4 marks · Explain

  Explain the three ways ATP is used by a contracting and relaxing muscle fibre, and explain why the muscles of a body stiffen after death.

  Mark scheme

  1. B1 ATP binding to the myosin head causes it to detach from the actin at the end of a power stroke
  2. B1 hydrolysis of that ATP by the head's own ATPase activity returns the head to its upright position, ready to attach further along the thin filament
  3. B1 ATP is used to pump calcium ions back into the sarcoplasmic reticulum during relaxation, so tropomyosin can slide back over the binding sites
  4. A1 after death respiration stops and ATP runs out, so the heads cannot detach and calcium cannot be pumped back, and the cross-bridges stay formed until the proteins themselves break down

• 4 marks · Compare

  Compare slow twitch muscle fibres with fast twitch muscle fibres, linking each structural difference to the way the fibre works.

  Mark scheme

  1. B1 slow twitch fibres contract more slowly but in a sustained way, whereas fast twitch fibres contract rapidly and powerfully for a short time
  2. B1 slow twitch fibres have many mitochondria and respire aerobically, whereas fast twitch fibres have few and rely on anaerobic glycolysis with a large glycogen store
  3. B1 slow twitch fibres have a dense capillary supply and much myoglobin, so oxygen is delivered and stored, whereas fast twitch fibres have a sparse supply and little myoglobin and are paler
  4. A1 slow twitch fibres therefore resist fatigue and dominate postural muscles and the calf muscles of distance runners, whereas fast twitch fibres accumulate lactate and tire quickly, and dominate the leg muscles of sprinters

• 4 marks · Calculate

  A relaxed muscle fibre is 3.0 cm long and its myofibrils are made of sarcomeres each 2.4 micrometres long, joined end to end. Calculate the number of sarcomeres in one myofibril of this fibre, and calculate the length of the fibre in centimetres when every sarcomere has shortened to 2.0 micrometres.

• 4 marks · Suggest

  A sprinter's leg muscles fail after about 40 seconds of maximal effort. A student says this is because the muscles have used up all their ATP. Suggest why that explanation is wrong, and suggest what does limit the muscles over that time.

• 3 marks · State

  State what happens to the width of the A band, the width of the I band and the width of the H zone when a sarcomere contracts.

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