Question

In cyanobacteria nitrogen fixation takes place in -

• A. Heterocyst
• B. Akinetes
• C. Nodules
• D. Hormogonia

Hint:

Associate the problem (oxygen-sensitive nitrogenase) with the solution (a specialized, oxygen-free cell). In cyanobacteria, that special cell is the {heterocyst}.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The question asks to identify the specialized cellular structure where nitrogen fixation occurs in certain types of cyanobacteria.
Step 2: Detailed Explanation:
Cyanobacteria perform oxygenic photosynthesis, which releases oxygen. However, the enzyme responsible for nitrogen fixation, nitrogenase, is irreversibly inactivated by oxygen. To solve this problem, many filamentous cyanobacteria (like Nostoc and Anabaena) have evolved specialized cells to carry out nitrogen fixation.
Heterocyst: This is a specialized, thick-walled cell found in the filaments of some cyanobacteria. It lacks photosystem II (the oxygen-producing part of photosynthesis) and has a thick wall to limit oxygen diffusion, thereby creating the necessary anaerobic environment for the nitrogenase enzyme to function.
Akinetes: These are thick-walled, dormant cells that serve as resting spores, allowing the cyanobacterium to survive harsh environmental conditions. They are not involved in nitrogen fixation.
Nodules: These are structures found on the roots of leguminous plants that house symbiotic nitrogen-fixing bacteria (Rhizobium), not structures within cyanobacteria themselves.
Hormogonia: These are short, motile filaments that break off from the main filament for dispersal and reproduction.
Step 3: Final Answer:
In cyanobacteria, nitrogen fixation takes place in specialized cells called heterocysts.