Photosynthesis in Higher Plants: NEET previous year questions

The water-splitting (oxygen-evolving) complex lies on the inner side of the membrane and is associated with Photosystem II, not PS I, so statement I is incorrect. Kranz anatomy is a feature of $\mathrm{C}_{4}$ plants, not $\mathrm{C}_{3}$ plants, so statement IV is incorrect. Statements II, III and V are correct. Hence the incorrect statements are I and IV only.

In the Calvin cycle, $\mathrm{CO}_{2}$ is fixed onto ribulose bisphosphate (RuBP) by RuBP carboxylase-oxygenase (RuBisCO), the most abundant enzyme in the biosphere. PEP carboxylase catalyses the primary $\mathrm{CO}_{2}$ fixation in mesophyll cells of $\mathrm{C}_{4}$/CAM plants but not the Calvin-cycle carboxylation, while hexokinase and carboxypeptidase have unrelated functions.

Fixing one $\mathrm{CO}_{2}$ in the Calvin cycle consumes 3 adenosine triphosphate (ATP) and 2 reduced nicotinamide adenine dinucleotide phosphate (NADPH). Synthesising one glucose requires the fixation of 6 $\mathrm{CO}_{2}$ (six turns of the cycle), so the total need is $6 \times 3 = 18$ ATP and $6 \times 2 = 12$ NADPH.

Ribulose bisphosphate carboxylase-oxygenase (RuBisCO) has affinity for both $\mathrm{CO}_{2}$ and $\mathrm{O}_{2}$ at the same active site. When oxygen wins, the five-carbon ribulose bisphosphate (RuBP) is split into one molecule of the three-carbon 3-phosphoglycerate and one molecule of the two-carbon 2-phosphoglycolate - note the carbon bookkeeping, 5 = 3 + 2. This wasteful route consumes adenosine triphosphate (ATP) without fixing carbon or producing sugar. NCERT Class 11, Photosynthesis in Higher Plants.

Only chlorophyll a occupies the reaction centre of a photosystem, and it is named for its absorption peak: P700 in photosystem I and P680 in photosystem II. All other pigments - chlorophyll b, xanthophylls and carotenoids - are ACCESSORY pigments in the light-harvesting complex that capture photons and funnel the energy to chlorophyll a. NCERT Class 11, Photosynthesis in Higher Plants.

The discoveries came in order of increasing refinement. Priestley first showed that plants restore something to 'injured' air. Ingenhousz then established that it is sunlight and the green parts that release oxygen. Sachs later demonstrated that glucose is produced and stored as starch in chloroplasts. Finally Engelmann worked out the action spectrum and the absorption properties of the chlorophylls. NCERT Class 11, Photosynthesis in Higher Plants (early experiments).

Each turn of the Calvin cycle fixes ONE molecule of $\mathrm{CO}_{2}$, so building the six-carbon skeleton of one glucose needs six turns. Three glucose molecules therefore need $3 \times 6 = 18$ turns. The common trap is to forget that the cycle handles one carbon at a time, not one sugar. NCERT Class 11, Photosynthesis in Higher Plants.

Statement III is correct: the oxygenase activity of ribulose bisphosphate carboxylase-oxygenase (RuBisCO) splits ribulose bisphosphate (RuBP) into 3-phosphoglycerate and 2-phosphoglycolate. Statement IV is also correct: photorespiration is purely wasteful, producing no sugar, no adenosine triphosphate (ATP) and no reduced nicotinamide adenine dinucleotide phosphate (NADPH) while consuming energy. Statement I is false because photorespiration is characteristic OF $\mathrm{C}_{3}$ plants - it is $\mathrm{C}_{4}$ plants that avoid it. Statement II reverses the gas exchange: photorespiration consumes $\mathrm{O}_{2}$ and releases $\mathrm{CO}_{2}$. NCERT Class 11, Photosynthesis in Higher Plants.

During photorespiration, the oxygenation activity of RuBisCo leads to the formation of one molecule of 3-phosphoglycerate (3-C) and one molecule of phosphoglycolate (2-C). Chapter: Photosynthesis in Higher Plants.

Plastoquinone facilitates the transfer of electrons from PS-II to the cytochrome $b_6f$ complex during the light reactions of photosynthesis. Chapter: Photosynthesis in Higher Plants.