lons that must be present for binding the cross bridges are

Options

A. `Na^(+)`
B. `Ca^(2+)`
C. `K^(+)`
D. `Mg^(+)`

Answer

To answer the question regarding the ions that must be present for binding the cross bridges, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Cross Bridges**: Cross bridges refer to the connections formed between myosin heads and actin filaments during muscle contraction. This process is crucial for muscle movement and force production.

2. **Role of Calcium Ions**: The binding of myosin heads to actin filaments is regulated by calcium ions (Ca²⁺). When a muscle is stimulated, calcium ions are released from the sarcoplasmic reticulum.

3. **Binding Mechanism**: The released calcium ions bind to troponin, a regulatory protein associated with actin. This binding causes a conformational change in another protein, tropomyosin, which exposes the binding sites on actin for the myosin heads.

4. **Cross Bridge Formation**: Once the binding sites on actin are exposed, the myosin heads can attach to actin, forming cross bridges. This is essential for the contraction of muscle fibers.

5. **Conclusion**: Therefore, the ion that is specifically required for the binding of cross bridges is calcium ion (Ca²⁺).

### Final Answer:
The ion that must be present for binding the cross bridges is **Calcium ion (Ca²⁺)**.

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