Muscle

In skeletal muscle contraction, calcium plays a crucial role by facilitating the interaction between the contractile proteins, actin and myosin. Here's a step-by-step explanation of how calcium functions in muscle contraction and why the given option is correct:

1. In a relaxed muscle, the active sites on actin filaments are blocked by the protein tropomyosin. This prevents myosin heads from binding to actin and thus keeps the muscle relaxed.
2. Upon stimulation, the sarcoplasmic reticulum releases calcium ions into the cytoplasm of the muscle cell.
3. Calcium ions bind to another protein called troponin, which is attached to tropomyosin on the actin filament.
4. This binding causes a conformational change in troponin, which leads to the shifting of tropomyosin away from the active sites on actin.
5. Once the active sites are exposed, myosin heads can attach to them, forming cross-bridges and initiating muscle contraction through the sliding filament mechanism.

Now, let us examine the options:

• Option 1: Binds to troponin to remove the masking of active sites on actin for myosin. This is correct because calcium ions bind to troponin to expose the active sites on actin by moving tropomyosin.
• Option 2: Detaches the myosin head from the actin filament. This is incorrect. Detachment of myosin heads from actin requires ATP, not calcium.
• Option 3: Activates the myosin ATPase by binding to it. This is incorrect. Calcium does not directly activate myosin ATPase; it exposes the active sites on actin.
• Option 4: Prevents the formation of bonds between the myosin cross bridges and the actin filament. This is incorrect. Calcium actually facilitates the formation of these bonds by exposing actin's active sites.

Therefore, the correct answer is: binds to troponin to remove the masking of active sites on actin for myosin.