difference between C3 plants and C3 cycle

• In some plants, the first product of CO2 fixation is a 3-carbon compound called 3-phosphoglyceric acid (PGA). These plants are known as C₃ plants.

• C₃ cycle or Calvin cycle is the pathway of fixation of CO2 in plants.
   

Calvin Cycle (C ₃  Cycle)

• The path of carbon in the dark reaction was traced by Melvin Calvin using radioactive carbon ( ¹⁴ C).

• Calvin pathway occurs in all photosynthetic plants, irrespective of whether they have C ₃  or C ₄ pathway.

• The primary acceptor of CO ₂  was found to be a 5-carbon ketose sugar called Ribulose bisphosphate (RuBP). This was surprising as it was believed that for the formation of a 3-carbon product, CO₂  would have to be accepted by a 2-carbon compound.

• RuBP is used in a cyclic manner (regenerated) and a sugar is synthesised.

• 3 stages of Calvin cycle: Carboxylation, Reduction and Regeneration

• Carboxylation

  • Ribulose 1, 5-bisphosphate combines with CO ₂ , and fixes it to a stable organic intermediate (3-phosphoglycerate − 2 molecules).

  • Reaction catalysed by the enzyme RuBisCO (RuBP Carboxylase-Oxygenase)

  • 3 PGA is the first stable product of this cycle.

• Reduction

  • Here, 2 molecules each of ATP and NADPH are required for fixing 1 molecule of CO ₂ .

  • This stage contains a series of reactions.

  • Glucose is formed as a result of this series of reactions.

• Regeneration

  • RuBP regenerates to enable the cycle to continue uninterrupted.

  • 1 ATP molecule is required.

• For the formation of one molecule of glucose, six molecules of CO ₂  need to be fixed; hence, six cycles are required.