Q.61 Given below are two statements:
Statement–I: Cyanide results in cellular hypoxia.
Statement–II: The tight coupling of electron transport and phosphorylation is uncoupled by cyanide.
In the light of the above statements, choose the most appropriate answer from the options given below.
- Both Statement–I and Statement–II are true
- Both Statement–I and Statement–II are false
- Statement–I is true but Statement–II is false
- Statement–I is false but Statement–II is true
Cyanide causes cellular hypoxia by inhibiting mitochondrial respiration. Statement I is true, but Statement II is false since cyanide blocks electron transport rather than uncoupling it from phosphorylation.
Question Breakdown
The query tests understanding of cyanide’s effect on cellular respiration in the context of oxidative phosphorylation.
Statement Analysis
Statement I: Cyanide results in cellular hypoxia.
This is correct. Cyanide binds to cytochrome c oxidase (Complex IV), halting electron flow to oxygen and preventing ATP synthesis despite oxygen availability—this defines histotoxic hypoxia.Statement II: Tight coupling of electron transport and phosphorylation is uncoupled by cyanide.
This is incorrect. Uncoupling (e.g., by DNP) dissipates the proton gradient while allowing electron transport. Cyanide inhibits the chain itself, stopping both processes.Option Evaluation
Option Validity Reason Both true False Statement II misrepresents cyanide’s action. Both false False Statement I accurately describes hypoxia. I true, II false True Matches mechanisms: inhibition causes hypoxia; no uncoupling occurs. I false, II true False Cyanide does not uncouple processes. Cyanide cellular hypoxia occurs when cyanide binds cytochrome c oxidase, blocking the electron transport chain (ETC) and ATP production—this is histotoxic hypoxia despite normal oxygen levels. For biology students preparing for competitive exams, distinguishing cyanide’s inhibition from uncouplers is crucial.
Cyanide’s Molecular Action
Cyanide forms a stable complex with the ferric iron in Complex IV, preventing electron transfer to O₂.
This stops proton pumping, collapses the gradient, and halts ATP synthase—no uncoupling, just blockade.
Result: Cells shift to anaerobic metabolism, causing lactic acidosis and rapid ATP depletion.Vs. Uncoupling Agents
Agent Effect on ETC Proton Gradient ATP Production Heat Generation Cyanide Inhibits (blocks Complex IV) Collapses (no pumping) Stopped Minimal Uncoupling (e.g., DNP) Continues Dissipated Reduced High Exam Relevance
In assertion-reason questions, cyanide poisoning exemplifies histotoxic hypoxia, not uncoupling—common in NEET/GATE Life Sciences.
Key takeaway: Inhibition ≠ uncoupling; both impair ATP but via distinct mechanisms.
