Calcium is the ion responsible for unmasking the active sites for myosin during muscle contraction.

Muscle contraction is a complex process that involves the interaction between actin and myosin, the two main proteins found in muscle fibers. These proteins form cross-bridges, which allow the muscle fibers to generate force and produce movement.

Role of Calcium in Muscle Contraction:
During muscle contraction, calcium ions play a crucial role in regulating the interaction between actin and myosin. Here's how calcium is involved in the process:

1. Excitation-Contraction Coupling: When a muscle is stimulated by a nerve impulse, an action potential is generated along the muscle fiber. This action potential travels deep into the muscle fiber via the T-tubules, which are invaginations of the cell membrane.

2. Release of Calcium from the Sarcoplasmic Reticulum: The action potential in the T-tubules causes the release of calcium ions from the sarcoplasmic reticulum, a specialized network of membrane-bound tubules within the muscle fiber. This release is mediated by a protein called ryanodine receptor.

3. Binding of Calcium to Troponin: The calcium ions released from the sarcoplasmic reticulum bind to the protein troponin, which is located on the actin filaments. This binding causes a conformational change in troponin, exposing the active sites on the actin filaments.

4. Formation of Cross-Bridges: Once the active sites on actin are exposed, the myosin heads, which are projections from thick filaments, can bind to the actin. This forms cross-bridges between the actin and myosin.

5. Power Stroke and Muscle Contraction: The myosin heads undergo a power stroke, during which they pull the actin filaments towards the center of the sarcomere, the contractile unit of the muscle. This shortens the sarcomere and results in muscle contraction.

6. Removal of Calcium: After muscle contraction, the calcium ions are actively transported back into the sarcoplasmic reticulum by a calcium pump. This process requires ATP.

In summary, calcium ions are essential for muscle contraction as they initiate the exposure of active sites on actin, allowing for the formation of cross-bridges with myosin. Without calcium, the muscle cannot contract effectively.