A-LevelBiologyControl and coordination9700/43 · May/June 2023 · Q78 Marks

(a) Sarcomeres are the functioning units of the myofibrils of the muscle fibres (muscle cells) of striated muscle. The arrival of an action potential at the sarcoplasmic reticulum of a striated muscle fibre can lead to the shortening of a sarcomere. This shortening occurs by the sliding filament model. Outline the sequence of events occurring after stimulation of the sarcoplasmic reticulum that results in the shortening of a sarcomere.

(a)Outline the sequence of events occurring after stimulation of the sarcoplasmic reticulum that results in the shortening of a sarcomere.[4]
(b)Scientists compared the diameter of samples of striated muscle fibres taken from young mice and adult mice. The results are shown in Fig. 7.1. 40 30 number of 20 muscle fibres 10 0+ 10 30 50 70 90 muscle fibre diameter/µm Fig. 7.1 Key young mice adult mice With reference to Fig. 7.1: : describe two differences between the muscle fibres of young mice and adult mice suggest how these differences may affect the sliding filament model.[4]

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Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

any three from:
[1] opens (voltage-gated) Ca2+ channels in sarcoplasmic reticulum or calcium ions leave sarcoplasmic reticulum ;
[1] calcium ions bind to troponin ;
[1] troponin changes shape / tropomyosin moves ;
[1] exposes binding site on actin ;
[1] myosin head, binds to (binding) site / forms cross bridge ;
plus
[1] myosin head, tilts / AW ;
[1] pulls actin/ power stroke (so sarcomere shortens) ;
any two from (description):
[1] young mice have more (muscle fibres) that are smaller (in diameter) ; ora
[1] young mice have smaller range (of diameters of muscle fibres) ; ora
[1] comparative data quote ; e.g. young mice 35 at 30, adult mice 16 at 50, spread of young mice 16-44, adult mice 20-80
plus any two from (suggestion):
[1] fewer, (muscle) fibres / myofibrils / sarcomeres ; ora
[1] less, muscle protein / actin and myosin ; ora
[1] so, weaker contraction / AW ; ora

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