Q. 2 - Biology

Question

Which of the following statements about muscle contraction is true? 

1. The power stroke occurs when ATP is hydrolyzed to ADP and phosphate. 
2. The power stroke occurs when ADP and phosphate dissociate from the myosin head. 
3. The power stroke occurs when ADP and phosphate dissociate from the actin active site. 
4. The power stroke occurs when Ca2+ binds the calcium head. 

Step-by-Step Solution

Answer

The Correct answer is option (b).

The power stroke occurs when ADP and phosphate dissociate from the myosin head.

Step: 1 ATP and ADP's function in the power stroke

•  The myosin is in a low-energy state at the completion of the power stroke. The cross-bridge created is unchanged even when ADP is released after the power stroke.
• Myosin binds to ATP, it transitions to an extremely energetic condition and emits the myosin from the actin active site's head. 

Step: 2 Explanation for Correct answer

• The start of this power stroke is strongly associated with the release of phosphate.
• During ATP hydrolysis, the actin attachment is transformed from weak, nonstereospecific to strong, stereospecific by the myosin head. Thus, the option b is correct.

Step: 3 Explanation for incorrect answer

• ATP is transformed into adenosine diphosphate once a phosphate group is separated by dissolving a phosphoanhydride bond, a procedure known as hydrolysis (ADP).So, option a is wrong.
• Myosin-ADP-inorganic phosphate (M-ADP-Pi) release of phosphate is accelerated by the thin filament actin-tropomyosin (Tm)-troponin. So, option c is wrong as well.
• In the presence of calcium ions, troponin conforms and tropomyosin fails to attach to actin binding sites. The contraction is caused by the formation of a cross-bridge between actin and myosin when tropomyosin is removed. Hence, option d is wrong too.