Q.2 Heterocyst, a thick-walled, pale-yellow, barrel-shaped structure found in blue-green algae, helps in
The correct answer is (2) N2 fixation. Heterocysts in blue-green algae (cyanobacteria) are specialized cells that create an oxygen-free environment for the oxygen-sensitive nitrogenase enzyme to convert atmospheric N2 into ammonia.
Option Analysis
(1) Defence
Heterocysts lack defensive roles like toxin production or mechanical protection; their thick envelope primarily excludes oxygen rather than pathogens.
(2) N2 Fixation
Heterocysts perform nitrogen fixation by housing nitrogenase, degrading PSII to avoid O2 production, and generating ATP via PSI for N2 reduction to NH3 shared with vegetative cells.
(3) CO2 Fixation
CO2 fixation (Calvin cycle) occurs in vegetative cells with both photosystems; heterocysts lack PSII and prioritize N2 metabolism over carbon fixation.
(4) Sulfur Metabolism
Heterocysts do not specialize in sulfur metabolism; they focus on N2 fixation, with sulfur assimilation handled by general cellular pathways.
Heterocyst, a thick-walled, pale-yellow, barrel-shaped structure in blue-green algae, primarily enables N2 fixation by providing an anaerobic microenvironment for nitrogenase.
These cells form from vegetative cells under nitrogen limitation, developing a multi-layered envelope (laminated, homogeneous, fibrous) impermeable to O2 and CO2. They connect to vegetative cells via microplasmodesmata for metabolite exchange.
Key Features Table
| Feature | Description |
|---|---|
| Shape/Appearance | Barrel-shaped, pale-yellow |
| Envelope Layers | 3 (inner laminated, middle homogeneous, outer fibrous) |
| Key Enzyme | Nitrogenase (O2-sensitive) |
| Photosystems | PSI only (no PSII/O2 production) |
| Location | Filaments (terminal/intercalary) |
This adaptation supports diazotrophy in cyanobacteria like Anabaena and Nostoc, vital for ecological nitrogen cycling in rice paddies.
