MICROANATOMY

Detailed explanation-1: -(1) Calcium binds to troponin C, causing the conformational shift in tropomyosin that reveals myosin-binding sites on actin. (2) ATP then binds to myosin. (3) ATP is then hydrolyzed. (4) A cross-bridge forms and myosin binds to a new position on actin.

Detailed explanation-2: -Muscle contraction is driven by the interaction between two proteins, myosin and actin, and fueled by the hydrolysis of ATP.

Detailed explanation-3: -Once the myosin-binding sites are exposed, and if sufficient ATP is present, myosin binds to actin to begin cross-bridge cycling. Then the sarcomere shortens and the muscle contracts. In the absence of calcium, this binding does not occur, so the presence of free calcium is an important regulator of muscle contraction.

Detailed explanation-4: -Answer and Explanation: Calcium is required for cross-bridge formation process of muscle contraction. Whereas, the ATP is required to detach the myosin head from the actin-binding site for the next contraction cycle to begin. The tropomyosin molecules block the myosin-binding site on the actin filament.