Photosynthesis – Dark Reaction: NEET MCQs with Answers

Explanation: In the Calvin cycle, A is carboxylation (RuBP + CO₂ → 3-PGA), B is reduction of 3-PGA to triose phosphate using ATP and NADPH, and C is regeneration of RuBP using ATP.

Explanation: Fixation of one CO₂ gives two molecules of 3-PGA, and their reduction to triose phosphate uses 2 ATP and 2 NADPH. The third ATP needed for each CO₂ (of the total 3 ATP + 2 NADPH) is used in regeneration of RuBP. Hence the reduction phase uses 2 ATP + 2 NADPH per CO₂ fixed.

Explanation: The enzymes of the Calvin cycle, including RuBisCO, are dissolved in the stroma. The light-independent reactions therefore occur in the stromal matrix, while the light reactions are confined to the thylakoid membranes.

Explanation: CO₂ is fixed during the dark (biosynthetic) reaction. Here the reducing agent NADPH and the energy source ATP, both generated in the light reaction, are used to convert carbon dioxide into carbohydrate. The light reaction itself involves no carbon fixation.

Explanation: Calvin, Benson and Bassham traced the path of carbon in photosynthesis by supplying algae with ¹⁴CO₂ and identifying the labelled intermediates at successive time intervals by chromatography and autoradiography. This radioactive isotope tracer technique revealed the entire cycle of carbon assimilation.

Explanation: The Calvin cycle involves carboxylation of RuBP followed by reduction of the resulting acid to carbohydrate, and finally regeneration of RuBP. Because carbon is both carboxylated and reduced, it is termed reductive carboxylation; it is also called the reductive pentose phosphate pathway or photosynthetic carbon reduction (PCR) cycle.

Explanation: In C₃ plants, CO₂ combines with the 5-carbon acceptor ribulose 1,5-bisphosphate (RuBP/RuDP) in the presence of RuBisCO, producing two molecules of 3-phosphoglyceric acid. PEP is the primary acceptor only in C₄ plants.

Explanation: RuBisCO (ribulose bisphosphate carboxylase-oxygenase) is the most abundant protein on the planet, constituting about 16% of total chloroplast protein. It catalyses the carboxylation of RuBP with CO₂ in the Calvin cycle, and its sheer abundance reflects its comparatively low catalytic efficiency.

Explanation: In the Calvin cycle, 2 ATP and 2 NADPH are consumed in reducing the two molecules of PGA to glyceraldehyde-3-phosphate, and one additional ATP is spent in phosphorylating ribulose-5-phosphate back to RuBP. The total per CO₂ fixed is therefore 3 ATP and 2 NADPH₂.

Explanation: Exactly one molecule of CO₂ is fixed in each turn of the Calvin cycle. A glucose molecule contains six carbon atoms, so six turns of the cycle are required to assemble it.

Explanation: The light reaction produces oxygen, ATP and NADPH. While oxygen is released into the atmosphere, ATP carries the chemical energy and NADPH the reducing power needed by the dark reaction. Of the options offered, ATP is the energy carrier that links the two phases.

Explanation: CO₂ combines with the 5-carbon acceptor RuBP in the presence of RuBisCO to yield an unstable 6-carbon intermediate that immediately splits into two molecules of the 3-carbon acid, 3-phosphoglyceric acid (PGA). Because the first stable product has three carbons, the pathway is called the C₃ pathway.

Explanation: Melvin Calvin, working with Benson and Bassham, used radioactive ¹⁴C to trace the path of carbon in the unicellular green alga Chlorella. These studies established that the first stable product of CO₂ fixation is the 3-carbon compound 3-phosphoglyceric acid (PGA). Calvin received the Nobel Prize in 1961.

Explanation: Six turns of the Calvin cycle are needed to fix the six carbon atoms of one glucose molecule. Since each turn requires 3 ATP and 2 NADPH₂, the total is 6 × 3 = 18 ATP and 6 × 2 = 12 NADPH₂.

Explanation: Each turn of the Calvin cycle fixes one CO₂ and consumes 3 ATP and 2 NADPH. Two ATP and two NADPH are used in converting 2 molecules of PGA into 2 molecules of glyceraldehyde-3-phosphate, and one further ATP is required to regenerate RuBP from ribulose-5-phosphate.