(a) A striated muscle cell (muscle fibre) will contract when stimulated by a motor neurone at a neuromuscular junction. Air temperature can affect the temperature of striated muscle cells. Investigations have shown that the efficiency of contraction of striated muscle cells decreases when the air temperature decreases. Suggest reasons why a reduction in temperature can decrease the efficiency of contraction of striated muscle cells. [Figure 7.1]

Mark scheme

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

1. [4] any four from: 1 respiration rate decreases / less ATP produced ; 2 reduces activity of (named) enzymes / description ; 3 reduces movement of, Ca2+ / Na+ / ACh / neurotransmitter ; 4 (in neuromuscular junction) less/no, acetylcholine broken down acetylcholine remains attached to receptors acetylcholinesterase, less active / inactive ; 5 (in sarcomere) fewer/no, Ca2+ bind to troponin fewer/no, cross bridges formed fewer/no, power stokes ; 6 (in sarcomere) less/no, detachment of myosin heads (from actin) fewer/no, cross bridges broken ATPase/myosin head, less active / inactive hydrolysis of ATP, slower / stops; 7 AVP; e.g. shivering / reduced blood flow to muscle / energy diverted for thermoregulation (idea)
2. [3] any three from: 1 Na+ channels don't open A Na+ do not, diffuse / move, into neurone or stay open; A Na+ always, diffuse / move, into neurone 2 (axon) membrane, cannot be depolarised or (axon) membrane stays depolarised ; 3 no action potentials, generated / transmitted (along motor neurone) or continuous action potentials, generated / transmitted (along motor neurone); 4 Ca2+ does not enter presynaptic knob / vesicles not stimulated to move / no ACh release (at neuromuscular junction) or Ca2+ always enters presynaptic knob / vesicles always stimulated to move / ACh continually released (at neuromuscular junction); 5 consequence e.g. paralysis / described or continued muscle contraction;