Calvin cycle consists of three phases. What are they? Explain the significance of each of them.

The Calvin cycle, also known as the Calvin-Benson cycle, is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It is the set of chemical reactions that occur during the light-independent reactions of photosynthesis. The Calvin cycle can be divided into three main phases: carbon fixation, reduction, and regeneration of ribulose-1,5-bisphosphate (RuBP). Each phase plays a crucial role in synthesizing glucose from carbon dioxide and other molecules.

1. Carbon Fixation
Significance:

Primary Function: The initial phase captures atmospheric carbon dioxide and incorporates it into an organic molecule.
Process Details: Carbon dioxide is combined with a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO).
Outcome: The resulting six-carbon compound is unstable and immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. This marks the beginning of carbon being fixed into an organic molecule that can be used by the plant.
2. Reduction
Significance:

Primary Function: The reduction phase converts the 3-PGA molecules into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
Process Details: This phase involves two major steps:
Phosphorylation: Each 3-PGA molecule receives an additional phosphate group from ATP, resulting in 1,3-bisphosphoglycerate.
Reduction: The 1,3-bisphosphoglycerate molecules are then reduced by NADPH (donating electrons), forming G3P.
Outcome: G3P is a versatile molecule that can be used to form glucose and other carbohydrates, which are essential for plant energy storage and structure. One out of every six molecules of G3P produced exits the cycle to contribute to the formation of glucose and other carbohydrates.
3. Regeneration of RuBP
Significance:

Primary Function: The final phase regenerates RuBP, the molecule that accepts CO₂, enabling the cycle to continue.
Process Details: The remaining five molecules of G3P (from the initial six produced in the reduction phase) undergo a series of complex reactions. These reactions involve the rearrangement of carbon atoms and the consumption of ATP.
Outcome: Three molecules of RuBP are regenerated from the five molecules of G3P, allowing the cycle to begin anew. This regeneration is critical because it ensures a continuous supply of RuBP to capture more carbon dioxide and maintain the cycle.
Summary
Carbon Fixation: Converts inorganic CO₂ into an organic molecule (3-PGA).
Reduction: Produces G3P from 3-PGA, using ATP and NADPH, forming the building blocks for glucose and other carbohydrates.
Regeneration of RuBP: Reforms the CO₂ acceptor molecule, RuBP, ensuring the cycle's continuity.
The Calvin cycle is fundamental for converting atmospheric carbon dioxide into organic molecules, ultimately supporting the energy needs of the plant and, indirectly, the energy needs of almost all living organisms on Earth through the food chain.