Name the ion responsible for the unmasking of active sites for myosin for cross- bridge activity during muscle contraction

Options

A. Calcium
B. Magnesium
C. Sodium
D. Potassium

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Contraction**: Muscle contraction involves the interaction between two main proteins: actin (thin filament) and myosin (thick filament). For myosin to interact with actin, certain active sites on actin must be exposed.

2. **Role of Calcium Ions**: The process of muscle contraction is initiated by a neural signal that leads to the release of calcium ions (Ca²+) into the muscle fibers. This release occurs at the neuromuscular junction when a neurotransmitter called acetylcholine is released.

3. **Binding of Calcium to Troponin**: The calcium ions bind to troponin, a regulatory protein that is part of the actin filament complex. Troponin is associated with tropomyosin, which covers the active sites on actin.

4. **Unmasking of Active Sites**: When calcium binds to troponin, it causes a conformational change that moves tropomyosin away from the active sites on actin. This unmasking allows myosin to bind to actin, forming cross-bridges necessary for muscle contraction.

5. **Conclusion**: Therefore, the ion responsible for unmasking the active sites for myosin during muscle contraction is calcium (Ca²+).

**Final Answer**: Calcium ion (Ca²+).

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