Question

The enzyme that is not found in C₃ plants is

• A. ATP synthase
• B. RUBP carboxylase
• C. NADP reductase
• D. PEP carboxylase

Answer 1

The Correct Option is D

C₃ plants are those that utilize the C₃ pathway (Calvin cycle) as their sole mechanism for carbon fixation during photosynthesis. This pathway occurs in the mesophyll cells.

Let's analyze the enzymes listed: 

• ATP synthase: This enzyme is essential for producing ATP during photophosphorylation in the light-dependent reactions of photosynthesis, which occur in all photosynthetic plants, including C₃ plants. It is also found in mitochondria for ATP production during respiration. So, ATP synthase is found in C₃ plants.
• RUBP carboxylase (Ribulose-1,5-bisphosphate carboxylase/oxygenase, or RuBisCO): This is the key enzyme of the Calvin cycle (C₃ pathway). It catalyzes the initial fixation of atmospheric CO₂ by combining it with Ribulose-1,5-bisphosphate (RuBP). This enzyme is characteristic and highly abundant in C₃ plants. So, RuBP carboxylase is found in C₃ plants.
• NADP reductase (Ferredoxin-NADP⁺ reductase): This enzyme catalyzes the final step in the electron transport chain of the light-dependent reactions, reducing NADP⁺ to NADPH. NADPH is required for the reduction phase of the Calvin cycle. Therefore, NADP reductase is found in C₃ plants.
• PEP carboxylase (Phosphoenolpyruvate carboxylase): This enzyme catalyzes the reaction between phosphoenolpyruvate (PEP) and CO₂ (as bicarbonate) to form oxaloacetate. While C₃ plants do possess PEP carboxylase in their cytoplasm for various metabolic functions (like replenishing TCA cycle intermediates), it does not play the primary role in photosynthetic carbon fixation as it does in C₄ and CAM plants. In C₄ and CAM plants, PEP carboxylase is responsible for the initial fixation of atmospheric CO₂, often in different cells (C₄) or at different times (CAM) than RuBisCO activity. The primary photosynthetic carbon fixation in C₃ plants is solely mediated by RuBisCO.

Therefore, considering the context of primary photosynthetic carbon fixation pathways, PEP carboxylase is the enzyme listed that is characteristic of C₄/CAM pathways but not the primary photosynthetic carboxylase in C₃ plants.

Answer 2

Let's analyze each enzyme in the context of C₃ plants:

• ATP synthase: This enzyme is present in C₃ plants and is involved in the synthesis of ATP during cellular respiration and photosynthesis.
• RUBP carboxylase (RuBisCO): This enzyme is present in C₃ plants and plays a crucial role in the Calvin cycle by fixing carbon dioxide into ribulose bisphosphate (RuBP).
• NADP reductase: This enzyme is also present in C₃ plants and is involved in the reduction of NADP+ to NADPH during the light reactions of photosynthesis.
• PEP carboxylase: This enzyme is not present in C₃ plants. It is involved in the initial fixation of carbon dioxide in the C₄ pathway and CAM pathway, not in C₃ plants.

Thus, the enzyme PEP carboxylase is not found in C₃ plants.

The correct answer is (D) : PEP carboxylase.