all the following statement are correct except

Options

A. (a) Mitochondria are rich in manganese.
B. (b) Molybdenum in necessary for the fixation of nitrogen catalyzed by the enzyme nitrogenase .
C. (c) Magnesium is essential for a large number of enzymes , particulaly those utilizing ATP.
D. (d) Calcium and magnesium have effect on the excitability of nerves and muscles .

Answer

To solve the question "All the following statements are correct except," we need to analyze each option provided and identify the incorrect statement.

### Step-by-Step Solution:

1. **Evaluate Option A**:
- Statement: "Mitochondria are rich in manganese."
- Explanation: This statement is true. Manganese superoxide dismutase, an important antioxidant enzyme, is abundant in mitochondria and helps reduce free radicals in the cell.

2. **Evaluate Option B**:
- Statement: "Molybdenum is necessary for the fixation of nitrogen catalyzed by the enzyme nitrogenase."
- Explanation: This statement is also true. Molybdenum acts as a cofactor for the enzyme nitrogenase, which is crucial for nitrogen fixation.

3. **Evaluate Option C**:
- Statement: "Magnesium is essential for a large number of enzymes, particularly those utilizing ATP."
- Explanation: This statement is true as well. Magnesium (in its divalent form, Mg²⁺) is essential for ATP synthesis and utilization, acting as a cofactor for many enzymes.

4. **Evaluate Option D**:
- Statement: "Calcium and magnesium have an effect on the excitability of nerves and muscles."
- Explanation: This statement is false. While calcium ions are indeed responsible for the excitability of nerves and muscles, magnesium does not have the same effect. Therefore, this statement is incorrect.

### Conclusion:
The incorrect statement among the options is **Option D**.

### Final Answer:
**Option D** is the statement that is not correct.

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