CAIE A Level Biology 9700 · Control and coordination
A student examines a stained longitudinal section of skeletal muscle under a light microscope. The sarcomeres show distinct A-bands, I-bands, and H-zones. The muscle was fixed immediately after electrical stimulation. Using the sliding filament model, explain how an action potential arriving at a neuromuscular junction leads to shortening of a sarcomere. Include in your answer the roles of calcium ions, troponin, tropomyosin, and ATP.
6 marks, so the mark scheme is looking for about 6 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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