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.

Key concepts

Power Stroke

The power stroke is a critical phase in muscle contraction, characterized by a dramatic movement of the myosin head. This movement occurs when the myosin head, which is tightly bound to actin, releases the ADP and phosphate that were originally attached to it. The dissociation of these molecules enables the myosin head to pivot forward. This pivoting is the essence of the power stroke, pulling the actin filament along and causing the muscle fiber to contract.

The power stroke is essentially the 'pulling' action within muscle cells that generates force. It is important to distinguish this event from ATP hydrolysis, which only prepares the myosin head for the power stroke by placing it in a high-energy state. Without the release of ADP and phosphate, the power stroke can't occur, making this release vital to muscle contraction.

• Initiated by the detachment of ADP and phosphate.
• Causes the myosin head to swivel, pulling actin.
• Results in muscle fiber shortening.

Cross-Bridge Cycling

Cross-bridge cycling is a repeating cycle of interactions between myosin heads and actin filaments during muscle contraction. It is this cycle that allows for continuous muscle movement and contraction. Initially, the cycle begins when ATP binds to the myosin head, causing it to detach from actin. Once detached, ATP is hydrolyzed, converting into ADP and phosphate. This conversion triggers the myosin head to cock into a high-energy position.

Next, the myosin head forms a cross-bridge by binding to a new site on the actin filament. Following attachment, the power stroke occurs, and the cycle repeats until signaling proteins cease this activity. Each phase of cross-bridge cycling ensures that muscles can tighten repeatedly, allowing for sustained contractions.

• Begins with ATP binding and hydrolysis.
• Involves recurring attachments of myosin to actin.
• Ends with the power stroke and muscle contraction.

Actin and Myosin Interaction

The interaction between actin and myosin filaments is the foundation of muscle contraction. Myosin, a motor protein, possesses a head that binds to specific sites on actin, a protein forming thin filaments in muscle fibers. This attachment, termed a cross-bridge, is crucial for transmitting force during contraction.

The interaction is fueled by ATP, which binds to the myosin head and gets hydrolyzed. This process prepares the myosin head to bind to actin. Upon binding, series of events ensue leading to the power stroke where actin is pulled by myosin, causing the muscle to contract. This cycle of binding, movement, and release happens numerously during muscle contractions.

• Crucial for the power stroke and muscle movement.
• ATP and actin-myosin interaction are intertwined processes.
• Forms the basis of muscle contraction mechanics.