Q.84 Transformation of normal cyanobacterial cells into heterocysts involves
(A) synthesis of nitrogenase and retention of photosystem I
(B) synthesis of nitrogenase and loss of photosystem I
(C) loss of nitrogenase but retention of photosystem I
(D) loss of both nitrogenase and photosystem I

The correct answer is (A) synthesis of nitrogenase and retention of photosystem I. Heterocyst transformation in cyanobacteria like Anabaena involves nitrogenase induction for N₂ fixation under microoxic conditions, while retaining PSI for cyclic photophosphorylation to generate ATP, but losing PSII to prevent O₂ production.

Heterocysts differentiate from vegetative cells under nitrogen starvation (~20-24h process), forming a pattern every 10-15 cells. Thick cell walls and glycolipid layers create anaerobiosis for oxygen-sensitive nitrogenase.

Option Analysis

(A) Synthesis of nitrogenase and retention of photosystem I

Correct. Nitrogenase genes (nifHDK) are expressed post-differentiation. PSII (psb genes) is degraded, but PSI (psa genes) persists for ATP via cyclic electron flow, supporting N₂ fixation energetics.

(B) Synthesis of nitrogenase and loss of photosystem I

Incorrect. PSI is retained in heterocysts; losing both photosystems would eliminate photophosphorylation, starving nitrogenase (requires 16 ATP + 8e⁻ per N₂).​

(C) Loss of nitrogenase but retention of photosystem I

Wrong. Heterocysts synthesize nitrogenase specifically; vegetative cells lack it. PSI retention supports energy needs.​

(D) Loss of both nitrogenase and photosystem I

False. Both are gained/retained: nitrogenase upregulated, PSI maintained. Complete photosystem loss would make heterocysts non-viable.​

Introduction

Transformation of normal cyanobacterial cells into heterocysts is a classic GATE Life Sciences topic. This differentiation requires synthesis of nitrogenase and retention of photosystem I to enable N₂ fixation in oxygen-free microenvironments.

Heterocyst Differentiation Process

1. Nitrogen starvation → 2-oxoglutarate signal → NtcA/HetR activation

2. 20-24h timeline: Vegetative → committed (8h) → mature heterocyst

3. Envelope formation: Polysaccharide + glycolipid layers block O₂ diffusion

4. Photosystem changes: PSII degraded (no O₂ evolution), PSI retained (ATP via cyclic flow)

5. Nitrogenase expression: nif genes in microoxic envelope

Key Molecular Changes

GATE Exam Strategy

Remember the triad: Nitrogenase ↑, PSII ↓, PSI → (stable). Pattern formation via HetR/PatS signaling (every 10-15 cells). Cross-links with N-fixing bacteria.​