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

Options

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

Answer

To solve the question regarding the ion necessary 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 the myosin heads and actin filaments during muscle contraction.

2. **Role of Tropomyosin**: Tropomyosin is a protein that covers the binding sites on actin filaments, preventing myosin from attaching to actin when the muscle is relaxed.

3. **Troponin Complex**: The troponin complex, which includes three subunits, plays a crucial role in muscle contraction. One of these subunits (often referred to as TPC) binds to calcium ions.

4. **Calcium Ion Function**: When calcium ions are present, they bind to the troponin complex. This binding causes a conformational change in the troponin, which then moves tropomyosin away from the binding sites on actin.

5. **Exposure of Binding Sites**: With tropomyosin moved aside, the binding sites on actin become exposed, allowing myosin heads to attach and form cross bridges.

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

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

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