Question
A tricky NEET-style problem on photosynthesis: given that 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O, and that one of the supporting components (light reactions) is blocked by a specific inhibitor, predict the downstream effect on the whole system.
Solution — Step by Step
The inhibitor targets light reactions, which normally occur in thylakoid; produce ATP, NADPH, O₂. So the function carried out by this component is now unavailable to the system.
Without light reactions, the next step in the pathway cannot proceed. Substrate piles up upstream; product falls downstream. This is the classic “bottleneck” scenario that NEET loves to test.
Biology usually has redundancy. Does the system have an alternative route — perhaps using dark reactions (Calvin cycle in stroma; fix CO₂ into sugars)? If yes, the effect is partial. If no, the whole pathway halts.
At the organism level, blocking light reactions in photosynthesis typically produces a visible phenotype — slowed growth, reduced response, or metabolic imbalance. This is exactly the kind of prediction NEET asks you to make.
Final prediction: Upstream substrate accumulates, downstream product drops, and the organism shows a measurable defect unless a backup pathway (e.g., involving dark reactions) can compensate.
Why This Works
Tricky NEET and JEE-Advanced-style biology problems almost always reduce to pathway reasoning: break a step, predict the pileup, predict the deficit. If you have the pathway diagram in your head for photosynthesis, these questions are free marks.
Alternative Method
Instead of reasoning forward, reason backward from the observed symptom. If you’re told “organism shows X”, ask “which step in photosynthesis would fail to produce X?” This reverse approach is faster for multiple-choice questions.
NEET has asked pathway-inhibitor questions on photosynthesis in at least two recent years. Expect one every cycle — it’s a scoring topic if you know the pathway, brutal if you don’t.
Common Mistake
Assuming a blocked component means the whole system dies. Most biological pathways have redundancy or alternative routes. Always mention the backup before concluding.
For any pathway chapter, memorise: (1) the sequence, (2) one inhibitor per step, (3) the resulting phenotype. Three bullet points per step covers every trick question.