A - Calcium ions (Ca^{2+}) activate the inter | vedclass

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(C) Assertion $(A)$ is correct: Calcium ions $(Ca^{2+})$ bind to the troponin subunit on actin filaments,which causes a conformational change that exp

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$A$ is correct and $R$ is incorrect.

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(C) Assertion $(A)$ is correct: Calcium ions $(Ca^{2+})$ bind to the troponin subunit on actin filaments,which causes a conformational change that exposes the active sites for myosin on the actin filament,thereby activating the interaction.Reason $(R)$ is incorrect: While the formation of the cross-bridge requires $ATP$ hydrolysis,the breaking or detachment of the cross-bridge specifically requires the binding of a new $ATP$ molecule to the myosin head. The statement claims breaking occurs in the absence of $ATP,$ which is false; in the absence of $ATP,$ the cross-bridge remains locked (a state known as rigor mortis).Therefore,$A$ is correct and $R$ is incorrect.

$A -$ Calcium ions $(Ca^{2+})$ activate the interaction between myosin and actin filaments.
$R -$ The cross-bridge is formed in the presence of $ATP,$ and the breaking of the cross-bridge also requires the binding of a new $ATP$ molecule.

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$A -$ Calcium ions $(Ca^{2+})$ activate the interaction between myosin and actin filaments.$R -$ The cross-bridge is formed in the presence of $ATP,$ and the breaking of the cross-bridge also requires the binding of a new $ATP$ molecule.