Dark Reaction Nomenclature: Light Independence — NEET Biology
📚 Practice ConceptCan we, hence, say that calling the biosynthetic phase as the dark reaction is a misnomer? Discuss this amongst yourselves.
Which of the above statements are correct? S1: NADPH is primarily formed during non-cyclic photophosphorylation and functions as a key reducing agent in the biosynthetic phase of photosynthesis. S2: The chemiosmotic hypothesis explains that ATP synthesis is driven by a proton gradient where H+ ions accumulate in the stroma, leading to their diffusion into the thylakoid lumen through ATP synthase. S3: For the fixation of one molecule of CO₂ in the Calvin cycle, the energy requirement is two molecules of ATP and two molecules of NADPH. S4: Cyclic photophosphorylation exclusively involves Photosystem I, resulting in the production of ATP but not NADPH, specifically when light wavelengths beyond 680 nm are available. S5: The ATP produced during the light reaction is utilized in the Calvin cycle for the carboxylation of RuBP and the reduction of 3-PGA, but not for the regeneration of RuBP.
View solution & NCERT explanation
Correct answer: A — S1 and S4 are correct
Let's evaluate each statement: S1: NADPH is indeed formed primarily during non-cyclic photophosphorylation and serves as a powerful reducing agent for CO₂ fixation in the Calvin cycle (biosynthetic phase). This statement is correct. S2: The chemiosmotic hypothesis states that ATP synthesis is driven by a proton gradient, but H+ ions accumulate within the lumen of the thylakoids, not the stroma. The protons then move from the lumen to the stroma through the ATP synthase. This statement is incorrect. S3: For every CO₂ molecule entering the Calvin cycle, three molecules of ATP and two molecules of NADPH are required for reduction and regeneration phases, not two ATP and two NADPH. This statement is incorrect. S4: Cyclic photophosphorylation involves only Photosystem I and results only in the synthesis of ATP, not NADPH. It occurs when only light wavelengths beyond 680 nm are available. This statement is correct. S5: ATP is utilized in the Calvin cycle during the reduction of 3-PGA and crucially for the regeneration of RuBP. Carboxylation does not directly consume ATP. This statement is incorrect. Therefore, statements S1 and S4 are correct. The final answer is A.
What does NCERT say about Can we hence say?
Has this concept appeared in NEET?
Which chapter is this from?
Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.