Final Answer:
In relaxed muscle tissue, the myosin binding site on actin is blocked by d) Tropomyosin.
Step-by-step explanation:
In relaxed muscle tissue, the myosin binding site on actin is obstructed by tropomyosin. Tropomyosin is a regulatory protein that is part of the thin filaments in muscle fibers. Its role is to cover the myosin binding sites on actin, preventing the interaction between myosin and actin.
During muscle contraction, the availability of calcium ions plays a crucial role. When the muscle is stimulated, calcium ions bind to troponin, causing a conformational change in troponin-tropomyosin complex. This change exposes the myosin binding sites on actin, allowing the myosin heads to bind and initiate the contraction process.
In the absence of stimulation or during muscle relaxation, tropomyosin returns to its blocking position, covering the myosin binding sites and preventing unnecessary muscle contractions. This regulatory mechanism ensures precise control over muscle contraction and relaxation, with tropomyosin acting as a key player in the modulation of muscle activity.
To summarize, the correct answer is d) Tropomyosin, as it blocks the myosin binding sites on actin in relaxed muscle tissue, regulating muscle contraction and ensuring controlled movement.