The ion that must be present in adequate amount for the
binding of cross bridges with actin is

Options

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

Answer

To answer the question regarding the ion that must be present in adequate amounts for the binding of cross bridges with actin, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Cross Bridges**: Cross bridges refer to the connections formed between myosin heads (part of thick filaments) and actin filaments (part of thin filaments) during muscle contraction.

2. **Role of Calcium Ions**: The binding of cross bridges is regulated by calcium ions. When a muscle is stimulated, nerve impulses trigger the release of calcium ions from the sarcoplasmic reticulum, an organelle in muscle cells.

3. **Calcium Binding to Troponin**: The released calcium ions bind to a protein called troponin, which is located on the actin filaments. This binding is crucial for the next step.

4. **Change in Tropomyosin Configuration**: When calcium binds to troponin, it causes a conformational change in another protein called tropomyosin. Tropomyosin normally blocks the binding sites on actin for myosin heads.

5. **Exposure of Binding Sites**: The change in tropomyosin's configuration exposes the binding sites on actin, allowing the myosin heads to attach to actin, forming cross bridges.

6. **Muscle Contraction**: Once the cross bridges are formed, the myosin heads pull on the actin filaments, leading to muscle contraction.

7. **Conclusion**: Therefore, the ion that must be present in adequate amounts for the binding of cross bridges with actin is **calcium**.

### Final Answer:
The ion that must be present in adequate amount for the binding of cross bridges with actin is **calcium**.