Skeletal muscle structureAQA A-Level Biology: Subtopic test

1. 1
   A physiotherapist is explaining arm movement to a patient who is recovering from an elbow injury. She uses the biceps muscle on the front of the upper arm and the triceps muscle on the back of the upper arm, both of which are attached by tendons to bones of the forearm and shoulder.

   (a)

   The biceps contracts and bends the elbow. Why is a second muscle needed to straighten the arm again?

   [1 mark]

   • AMuscles can contract and lengthen without any help from other muscles
   • BMuscles can only exert a force by contracting, so a second muscle must pull the bone back the other way
   • CBones are compressible, so they spring back after the biceps contracts
   • DTendons are elastic and contract to straighten the arm

   (b)

   What is the importance of the skeleton being incompressible when a muscle contracts?

   [1 mark]

   • AIt acts as a rigid lever, so the force of contraction moves the bone rather than squashing it
   • BIt stores the calcium ions needed for contraction
   • CIt makes the muscle fibres contract more quickly
   • DIt allows the muscle to push as well as pull

   (c)

   The biceps and triceps are described as an antagonistic pair. Explain how they work together to bend and then straighten the elbow.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a relaxed sarcomere from a mammalian skeletal muscle, the distance from one Z line to the next is 2.4 µm. The dark A band is 1.6 µm long. In the centre of the A band is a lighter region, the H zone, which is 0.4 µm long.

   (a)

   What is the combined length of the two I bands (one half either side of the Z lines) in this relaxed sarcomere?

   [1 mark]

   • A0.4 µm
   • B0.6 µm
   • C0.8 µm
   • D1.2 µm

   (b)

   Which filaments are present in the H zone of a relaxed sarcomere?

   [1 mark]

   • AActin only
   • BBoth actin and myosin, overlapping
   • CNeither actin nor myosin
   • DMyosin only

   (c)

   Explain why the A band appears darker than the I band in an electron micrograph.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A physiologist takes muscle biopsies from the thigh of a marathon runner and of a sprinter. About 80% of the fibres in the runner's sample are slow fibres, compared with about 25% in the sprinter's sample. The sprinter's fast fibres have thicker myosin filaments, a large store of phosphocreatine, high concentrations of glycolytic enzymes and few mitochondria.

   (a)

   Explain why a high proportion of slow fibres is an advantage in the leg muscles of a marathon runner.

   [3 marks]

   (b)

   Use the information to explain how the features of the sprinter's fast fibres suit them to a short, powerful burst of activity.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student studies a thin section of rabbit skeletal muscle. Under the light microscope she sees long, striped muscle fibres, each with several nuclei. An electron micrograph of one myofibril shows a repeating pattern of light and dark bands between dark lines.

   (a)

   Describe the structure of a skeletal muscle fibre and explain how its structure is adapted for contraction.

   [6 marks]

   (b)

   Describe the arrangement of the filaments in a sarcomere and explain the pattern of light and dark bands.

   [6 marks]

   Total for question 4: 12 marks

End of questions