Carbon Fixation: Calvin Cycle & C₄ Pathway — NEET Biology
📚 Practice ConceptIn the carbon fixation cycle, CO₂ is added by the enzyme, RuBisCO₂ to a 5-carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. This is then converted to sugar by the Calvin cycle, and the RuBP is regenerated. During this process ATP and NADPH synthesised in the light reaction are utilised. RuBisCO also catalyses a wasteful oxygenation reaction in C₃ plants: photorespiration. Some tropical plants show a special type of photosynthesis called C₄ pathway. In these plants the first product of CO₂ fixation that takes place in the mesophyll, is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carried out for the synthesis of carbohydrates.
Which of the following statements about photorespiration and associated enzymes are NOT correct? S1: RuBisCO is unique in that its active site has a much greater affinity for O₂ than for CO₂, leading to inefficient carbon fixation in C₃ plants. S2: In the photorespiratory pathway, RuBP binds with O₂ to form one molecule of phosphoglycerate and phosphoglycolate, which results in the release of CO₂ and utilization of ATP. S3: The wasteful oxygenation activity of RuBisCO is completely avoided in C₄ plants due to the absence of RuBisCO enzyme in their mesophyll cells. S4: The biological function of photorespiration is currently understood to be an adaptive mechanism that provides an important advantage to C₃ plants in dry and hot conditions. S5: Cyclic photophosphorylation occurs to generate additional ATP required to meet the higher ATP to NADPH ratio needed for the Calvin cycle (3 ATP: 2 NADPH per CO₂ fixed). Choose the option with the NOT correct statements:
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