Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Actin acts as an ATPase.
Reason (R) : Active sites on thin filaments are unmasked due to binding of ATP to actin.
In light of the above statements, choose the correct answer from the option given below.

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(A) is true but (R) is false.

Both (A) and (R) are true and (R) is the correct explanation of (A).

Both (A) and (R) are false.

Both (A) and (R) are true but (R) is not the correct explanation of (A).

answer is D.

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Detailed Solution

Both (A) and (R) are false.

Actin itself does not act as an ATPase. ATPase activity is associated with myosin, not actin. Myosin hydrolyzes ATP to ADP and Pi to provide energy for muscle contraction.

The active (myosin-binding) sites on actin are actually covered by tropomyosin in the absence of calcium ions. During muscle contraction, calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the active sites on actin. This allows myosin heads to bind to actin and initiate muscle contraction. The presence of ATP is required for myosin detachment after contraction, not for unmasking the active sites on actin.

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