Photosynthesis – C3 and C4 Pathways: NEET MCQs with Answers

Explanation: The diagram represents the Hatch and Slack (C₄) pathway. At A, atmospheric CO₂ is fixed onto PEP by PEP carboxylase in the mesophyll cell to give a C₄ acid. This acid moves through plasmodesmata into the bundle sheath cell, where at B it undergoes decarboxylation, releasing CO₂ for the Calvin cycle. The resulting C₃ acid returns to the mesophyll cell, where at C PEP is regenerated.

Explanation: C₄ pathway – phosphoenol pyruvate (primary acceptor); light reaction – splitting of water (2H₂O → 4H⁺ + O₂ + 4e⁻); photorespiration – phosphoglycolate; Calvin cycle – ribulose 1,5-bisphosphate. Hence A-III, B-I, C-IV, D-II.

Explanation: In C₄ plants, RuBisCO works in the bundle sheath cells and the mesophyll cells lack it, so placing RuBisCO activity in the mesophyll is incorrect. RuBisCO is bifunctional (carboxylase and oxygenase) and its substrate RuBP is a 5-carbon compound. ATP and NADPH are used by the Calvin cycle in the reduction and regeneration steps; the carboxylation by RuBisCO itself does not use them.

Explanation: All four properties describe the C₄ syndrome. By concentrating CO₂ in the bundle sheath, a C₄ plant avoids photorespiration, needs less RuBisCO (and hence less nitrogen), can keep its stomata less open (saving water) and continues to photosynthesise rapidly at high temperature.

Explanation: In C₄ plants such as sugarcane, maize and Sorghum, the optimum temperature for photosynthesis is 30–45 °C, and the rate of net photosynthesis in full sunlight (40–80 mg CO₂/dm²/hr) is far higher than that of C₃ plants (15–35 mg CO₂/dm²/hr). Sugarcane is thus among the most productive crops.

Explanation: C₄ plants have dimorphic chloroplasts — granal chloroplasts in the mesophyll and agranal ones in the bundle sheath. Mesophyll cells are specialised for the light reaction, while the bundle sheath cells possess RuBisCO. The Calvin cycle therefore operates in the stroma of the bundle sheath chloroplasts.

Explanation: In C₄ plants, phosphoenol pyruvate (PEP) is the first acceptor of CO₂, while ribulose bisphosphate acts as the second acceptor (in the bundle sheath, during the Calvin cycle). Oxaloacetic acid is the first stable product, not the acceptor.

Explanation: Plants in which the first stable product of the dark reaction is a 4-carbon compound are called C₄ plants — for example sugarcane, maize and sorghum. These plants show the characteristic Kranz anatomy, and their primary CO₂ acceptor is phosphoenol pyruvate. Papaya, pea and potato are C₃ plants.

Explanation: The C₄ or dicarboxylic acid pathway is an alternative route of CO₂ fixation, discovered by M.D. Hatch and C.R. Slack in 1967. It is therefore also known as the Hatch–Slack cycle. Calvin elucidated the C₃ cycle, Hill demonstrated the light-driven reduction reaction, and Arnon worked on photophosphorylation.

Explanation: The leaves of C₄ plants such as sugarcane and maize show Kranz anatomy, in which the mesophyll is undifferentiated and its cells are arranged in concentric layers around the vascular bundles, which are themselves ringed by large chloroplast-rich bundle sheath cells.

Explanation: Maize is a C₄ plant, not a C₃ plant, so this pair is incorrect. The remaining pairs are correctly matched: C₄ plants possess Kranz anatomy, PGA is formed in the Calvin cycle, and OAA (a 4-C compound) is the first stable product of the Hatch and Slack cycle.

Explanation: In C₄ plants, the first (primary) carboxylation is catalysed by PEP carboxylase in the mesophyll cells, forming the 4-carbon oxaloacetic acid. RuBP carboxylase (RuBisCO) acts only later, in the bundle sheath cells, during the Calvin cycle.

Explanation: Sugarcane is a C₄ plant. CO₂ is taken up by phosphoenol pyruvate in a reaction catalysed by PEP carboxylase to yield oxaloacetic acid, which is then reduced by a dehydrogenase (using NADPH) to malic acid within the mesophyll cells.

Explanation: The key reason is that primary fixation is carried out by PEP carboxylase, which has a much greater affinity for CO₂ than RuBisCO and, importantly, has no oxygenase activity. This allows C₄ plants to capture CO₂ efficiently even at low atmospheric concentrations, after which it is delivered to the bundle sheath cells.

Explanation: In C₄ plants every CO₂ molecule is fixed twice — first by PEP carboxylase and then by RuBisCO — so more energy is needed. A C₃ plant requires 18 ATP for one hexose molecule, while a C₄ plant requires 30 ATP. The difference is therefore 12 additional ATP molecules.