A-LevelBiologyControl and coordination9700/42 · Feb/Mar 2024 · Q1010 Marks

Striated muscle is composed of myofibrils. Myofibrils contain several structural proteins including troponin, tropomyosin, actin and myosin.

(a)Outline the roles of these four structural proteins in the contraction of a sarcomere.[4]
(b)Drugs that cause muscle paralysis (paralytic drugs) are used during surgery to stop the patient moving. One commonly used paralytic drug is succinylcholine, which works by preventing contraction of muscles. Succinylcholine is able to prevent muscles contracting because it has a similar shape to acetylcholine. Suggest how succinylcholine is able to prevent muscles contracting.[3]
(c)Duchenne muscular dystrophy (DMD) is a genetic disease that is caused by a single gene. DMD affects striated muscle, and symptoms of the disease first appear at an early age. A fibrous protein, dystrophin, stabilises muscle fibres during contraction. A person with DMD produces non-functioning dystrophin or no dystrophin at all. The disease occurs in about four in 100000 people and mainly affects boys. Suggest and explain why boys are more likely to have DMD than girls.[3]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2024). The mark or mark type is shown in brackets.

[1] troponin binds Ca²+ and changes shape ;
[1] (so) tropomyosin moves away from the binding sites on actin ;
[1] myosin heads, bind to / form cross-bridges, with actin ;
[1] myosin head, pulls actin / performs power stroke, to, either shorten / contract, sarcomere or cause Z lines to move closer together / Z lines move closer to M lines ;
[1] myosin head, is ATPase / binds to ATP, to allow detachment ;
[1] succinylcholine binds to ACh receptors or succinylcholine acts as a competitive inhibitor ; R if binds to active site of receptor
[1] (sodium ions) channel proteins do not open; R if voltage gated
[1] sodium ions do not enter, muscle cell / sarcoplasm ;
[1] sarcolemma / post-synaptic membrane, not depolarised ;
[1] sex-linked / gene on X chromosome ;
[1] recessive (allele);
[1] males only have one, copy of gene / allele / X chromosome ;
[1] female heterozygotes, do not have DMD / are carriers or only homozygous females have DMD ;

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