Primary Fixation of CO2

The primary fixation of carbon dioxide (CO2) is the first step in the Calvin cycle, also known as the C3 cycle, which is the most common pathway of photosynthesis in plants. It involves the conversion of CO2 into an organic molecule that can be further processed to produce sugars.

RuBP Carboxylase and PEP Carboxylase

The primary fixation of CO2 is catalyzed by two enzymes: RuBP carboxylase (Ribulose-1,5-bisphosphate carboxylase/oxygenase, or Rubisco) and PEP carboxylase (Phosphoenolpyruvate carboxylase).

RuBP Carboxylase
- RuBP carboxylase is the enzyme responsible for the majority of CO2 fixation in plants.
- It catalyzes the carboxylation of Ribulose-1,5-bisphosphate (RuBP) with CO2 to form an unstable intermediate compound.
- RuBP carboxylase is highly specific to CO2 and plays a crucial role in the Calvin cycle, as it provides the starting point for the synthesis of organic molecules.
- This enzyme is present in the chloroplasts of plant cells and is considered one of the most abundant proteins on Earth.

PEP Carboxylase
- PEP carboxylase is an enzyme found in plants and some bacteria.
- It catalyzes the carboxylation of Phosphoenolpyruvate (PEP) with CO2 to form Oxaloacetate.
- PEP carboxylase has a higher affinity for CO2 compared to RuBP carboxylase and operates under conditions of low CO2 concentration or high oxygen concentration.
- It is an important enzyme in C4 plants, where it helps to minimize photorespiration and increase the efficiency of CO2 fixation.

Comparison
- While both RuBP carboxylase and PEP carboxylase are involved in the primary fixation of CO2, their roles differ.
- RuBP carboxylase is the main enzyme responsible for CO2 fixation in the Calvin cycle and is found in all photosynthetic organisms.
- PEP carboxylase, on the other hand, plays a significant role in C4 plants by enabling efficient CO2 fixation even under high oxygen or low CO2 conditions.
- Both enzymes contribute to the overall process of carbon fixation and are essential for the production of organic molecules in plants.

Therefore, option 'B' (RuBP carboxylase and PEP carboxylase) is the correct answer as both enzymes are involved in the primary fixation of CO2, with RuBP carboxylase being the main enzyme in most plants and PEP carboxylase playing a specific role in C4 plants.

Bcoz RUBisCo is used in C3 plant and PEP CARBOXYLASE in C4 plants.