Final answer:
The correct answer is A. Calcium binds to troponin, causing tropomyosin to move away from the actin and myosin binding sites, enabling muscle contraction. This action is part of the excitation-contraction coupling process.
Step-by-step explanation:
The correct answer is: A. Calcium binds to troponin, which moves the tropomyosin that otherwise blocks the interaction of actin and myosin.Explanation: During muscle contraction, calcium ions play a crucial role. In the relaxed state, the binding sites on actin molecules are covered by tropomyosin. When a muscle is stimulated, calcium ions (Ca2+) are released into the sarcoplasm. These calcium ions bind to troponin, a component of the thin filament complex. Troponin, upon binding calcium, undergoes a conformational change that causes tropomyosin to move away from the myosin-binding sites on actin.
With these sites exposed, myosin heads can attach to actin, forming cross-bridges, and consequently muscle contraction occurs. This process is part of what is known as the excitation-contraction coupling. The cycle of contraction continues until calcium and ATP (adenosine triphosphate) are no longer available, leading to the cessation of contraction and muscle relaxation.