Cell in some filamentous cyanobacteria which is specialised for nitrogen fixation is called:

Options

A. Heterocyst
B. Mesosome
C. Volutin
D. Phycobillisome

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the specific type of cell found in some filamentous cyanobacteria that is specialized for nitrogen fixation.

2. **Identifying the Cell Type**: In filamentous cyanobacteria, certain cells are adapted to perform nitrogen fixation, which is the conversion of atmospheric nitrogen into a form that can be used by living organisms.

3. **Recognizing the Correct Term**: The specialized cells in cyanobacteria responsible for nitrogen fixation are called **heterocysts**. These cells are distinct from other cells in the filament and are designed to create an anaerobic environment that is conducive to nitrogen-fixing bacteria.

4. **Understanding the Function of Heterocysts**: Heterocysts facilitate the process of nitrogen fixation by providing a low-oxygen environment, which is essential for the nitrogen-fixing enzyme, nitrogenase, to function effectively.

5. **Eliminating Incorrect Options**:
- **Mesosome**: This is a structure involved in cell wall formation and enzyme secretion, not nitrogen fixation.
- **Volutene**: This refers to cytoplasmic storage of inorganic phosphate, unrelated to nitrogen fixation.
- **Phycobilisome**: This is a light-harvesting complex involved in photosynthesis, not in nitrogen fixation.

6. **Conclusion**: Based on the information, the correct answer to the question is **heterocyst**.

### Final Answer:
The cell in some filamentous cyanobacteria which is specialized for nitrogen fixation is called **heterocyst**.

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