In Nostoc, enzyme nitrogenase occurs in:

Options

A. Vegetative cells
B. Heterocysts
C. Both (a) and (b)
D. Only in hormogones

Explanation

To answer the question "In Nostoc, enzyme nitrogenase occurs in:", we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Organism**: Nostoc is a type of cyanobacteria, often referred to as blue-green algae. It plays a significant role in nitrogen fixation.

2. **Identify the Function of Nitrogenase**: Nitrogenase is an enzyme that facilitates the conversion of atmospheric nitrogen (N2) into ammonia (NH3), a process known as nitrogen fixation. This is crucial for making nitrogen available to plants.

3. **Locate the Site of Nitrogenase**: In Nostoc, nitrogenase is specifically found in specialized cells known as heterocysts. These cells are adapted for nitrogen fixation and provide an anaerobic environment necessary for the enzyme to function.

4. **Conclusion**: Therefore, the enzyme nitrogenase in Nostoc occurs in heterocysts.

### Final Answer:
In Nostoc, the enzyme nitrogenase occurs in heterocysts.

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