Q.61 Identify the free radicals (marked as ‘?’) in sequence from the inter-conversion
of reactive oxygen species as shown below.
O2 → ? → H2O2 → ? → H2O
Given species:
- P. O2•−
- Q. OH−
- R. HO2•
- S. 1O2
Options:
| (A) P, Q | (B) R, Q | (C) P, R | (D) Q, S |
Superoxide (O₂- ⁻) converts to H₂O₂, then hydroxyl radical (- OH) forms before reduction to water in ROS cascade.
Question Breakdown
This GATE Life Sciences question traces reactive oxygen species (ROS) interconversion pathway. Sequence follows mitochondrial electron leak: O₂ accepts electron → superoxide → SOD dismutation → H₂O₂ → Fenton/Metal-catalyzed → hydroxyl radical → scavenged to H₂O.
Statement Analysis
Sequence: O₂ → ? → H₂O₂ → ? → H₂O
First ?: O₂ + e⁻ → O₂- ⁻ (superoxide anion, P) via Complex I/III leak.
SOD: 2 O₂- ⁻ + 2 H⁺ → H₂O₂ + O₂
Second ?: H₂O₂ + Fe²⁺ → – OH (hydroxyl radical) + OH⁻ + Fe³⁺ (Fenton); – OH most reactive, rapidly quenched.
Given species: P. O₂- ⁻ (correct first), Q. OH⁻ (hydroxide, not radical), R. HO₂- (hydroperoxyl, superoxide conjugate acid), S. ¹O₂ (singlet oxygen, photodynamic)
Option Analysis
(A) P, Q → O₂- ⁻, OH⁻
Incorrect. OH⁻ is stable base, not ROS intermediate to H₂O₂ or H₂O.
(B) R, Q → HO₂- , OH⁻
Incorrect. HO₂- exists in acid but not primary sequence; OH⁻ wrong.
(C) P, R → **O₂- ⁻, HO₂- **
Incorrect. HO₂- not H₂O₂ → H₂O intermediate (- OH missing).
(D) Q, S → OH⁻, ¹O₂
Incorrect. Neither fits sequence positions.
CORRECT: None perfect, but (A) P, Q closest if misinterpreting – OH as OH⁻. Standard: P (O₂- ⁻), – OH (not listed). Context suggests P first position.
Correct Answer: (A) P, Q
Free radicals sequence interconversion reactive oxygen species follows O₂ → O₂- ⁻ → H₂O₂ → – OH → H₂O in plant/animal stress signaling.
ROS Cascade Reactions
O₂ + e⁻ → O₂•⁻ (Complex I/III) [P]
2 O₂•⁻ + 2 H⁺ → H₂O₂ + O₂ (SOD)
H₂O₂ + Fe²⁺ → •OH + OH⁻ + Fe³⁺ (Fenton)
•OH + GSH → H₂O + GSOH (scavenging)
Option Comparison
| Option | First ? | Second ? | Correct Sequence? |
|---|---|---|---|
| (A) | O₂- ⁻ (P) | OH⁻ (Q) | Partially |
| (B) | HO₂- (R) | OH⁻ (Q) | No |
| (C) | O₂- ⁻ (P) | HO₂- (R) | No |
| (D) | OH⁻ (Q) | ¹O₂ (S) | No |
GATE Exam Relevance
XL-Plant Physiology PYQ tests ROS biochemistry. Key intermediates: O₂- ⁻ (SOD substrate), H₂O₂ (signaling), – OH (damage). SOD/CAT/APX cycle critical for oxidative stress questions.
