B3.3 Muscle and motility (HL)

The question asks which pairing of an event and its immediate cause is correctly stated in the context of the cross-bridge cycle and calcium regulation.

When ${\text{Ca}}^{2+}$ is released from the sarcoplasmic reticulum, it binds to troponin. This causes a conformational change in the troponin–tropomyosin complex, shifting tropomyosin away from the myosin binding sites on actin, thereby exposing those sites.

The power stroke is triggered by Pᵢ release (not ATP binding). Cross-bridge detachment requires new ATP binding to myosin (not Pᵢ release). Re-cocking of the myosin head is driven by ATP hydrolysis (ADP + Pᵢ formation), not Ca²⁺ re-uptake.

Only the pairing of Ca²⁺ binding troponin → binding sites exposed accurately reflects the immediate biochemical cause and its effect. This is the correct row.

The question tests knowledge of the cross-bridge cycle and calcium regulation, specifically matching events with their correct immediate causes.

"Power stroke occurs immediately after new ATP binds" is incorrect. New ATP binding causes cross-bridge detachment, not the power stroke. The power stroke follows Pᵢ release.

"Cross-bridge detachment occurs after Pᵢ release" is incorrect. Pᵢ release initiates the power stroke (transition from weak to strong binding). Detachment requires a fresh ATP molecule binding to the myosin head.

"Myosin head is re-cocked after Ca²⁺ is pumped back" is incorrect. Ca²⁺ re-uptake leads to relaxation (tropomyosin re-covers binding sites). Re-cocking of the myosin head is driven by ATP hydrolysis, which can occur independently of Ca²⁺ levels.

"Myosin binding sites on actin are exposed immediately after Ca²⁺ binds to troponin" is correct. ${\text{Ca}}^{2+}$ → troponin conformational change → tropomyosin shifts → binding sites uncovered. This is the established regulatory mechanism.

The question asks which sarcomere region remains constant in width during contraction, requiring knowledge of the sliding filament theory and sarcomere anatomy.

During contraction, actin filaments slide toward the centre of the sarcomere. The filament lengths themselves do not change — only the degree of overlap changes. The A-band represents the full length of the myosin (thick) filaments, which are unchanged in length.

The I-bands (P and S) narrow because actin slides further into the A-band, reducing the zone containing only thin filaments. The H-zone (R) narrows because actin filaments from both sides encroach on the central myosin-only region.

Region Q (the A-band) remains constant in width because it spans the entire length of the myosin filament, which does not shorten. This is a key prediction of the sliding filament theory.

The question asks which of four labelled sarcomere regions does not change width during contraction. The key concept is that the A-band (full myosin filament length) is constant.

P and S are the I-bands, which contain only thin (actin) filaments. As actin slides inward during contraction, these regions become narrower. Neither P nor S remains constant — both can be eliminated.

R is the H-zone, the central pale region containing only thick filaments. As actin filaments from both sides slide inward, they encroach on this zone, making it narrower (and eventually absent in full contraction). R changes — eliminate it.

Q, the A-band, spans the full length of the myosin filament. Since filament lengths do not change during the sliding process, the A-band width is constant regardless of contraction state. Q is correct.