Question

Figure 16.5 Which of the following statements about muscle contraction is true? a. The power stroke occurs when ATP is hydrolyzed to ADP and phosph ate. b. The power stroke occurs when ADP and phosphate disso ciate from the myosin head. c. The power stroke occurs when ADP and phosphate disso ciate from the actin active site.

Short answer

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

Step-by-step solution

Understanding Muscle Contraction and Power Stroke

Muscle contraction occurs due to the cross-bridge cycling between actin and myosin filaments. The power stroke is the process where the myosin head pulls the actin filament to slide overlapping the myosin filament, thus shortening the muscle. Let's analyze each statement and find the answer.

Option a: The power stroke occurs when ATP is hydrolyzed to ADP and phosphate.

When ATP is hydrolyzed to ADP and phosphate, the myosin head binds to the actin active site, forming a cross-bridge. This process is called the cross-bridge attachment, and it sets the stage for a power stroke to happen. However, it is not the power stroke itself. So, option a is incorrect.

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

When the myosin head has both ADP and phosphate bound to it, the head is in its high-energy state, ready to produce a power stroke. When ADP and phosphate dissociate from the myosin head, the myosin head undergoes a conformational change, and this change results in a power stroke. Therefore, option b is correct.

Option c: The power stroke occurs when ADP and phosphate dissociate from the actin active site.

ADP and phosphate are not bound to the actin active site during the cross-bridge cycling process. They are bound to the myosin head, as explained in option b. Therefore, this statement is false, making option c incorrect. Based on our analysis, the correct answer is option b: The power stroke occurs when ADP and phosphate dissociate from the myosin head.