Describe different phases of Calvin cycle.

Introduction

The Calvin cycle, also known as the Calvin-Benson cycle, is the set of chemical reactions that take place in the chloroplasts during photosynthesis. It is a part of the light-independent reactions (also called the dark reactions) and is responsible for the fixation of carbon dioxide (CO₂) into organic compounds like glucose. The Calvin cycle does not require light directly, but it depends on the ATP and NADPH produced in the light-dependent reactions.

Overview of the Calvin Cycle

The Calvin cycle occurs in the stroma of chloroplasts and can be divided into three main phases:

1. Carbon Fixation
2. Reduction
3. Regeneration of RuBP

1. Carbon Fixation

This is the first step of the Calvin cycle, where atmospheric CO₂ is fixed into a more usable form. The key events include:

• CO₂ molecules enter the cycle and combine with ribulose-1,5-bisphosphate (RuBP), a 5-carbon compound.
• This reaction is catalyzed by the enzyme Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
• The result is an unstable 6-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA), each containing three carbon atoms.

2. Reduction Phase

In this phase, the 3-PGA molecules are converted into glyceraldehyde-3-phosphate (G3P), which is a more energy-rich molecule. This process involves the following steps:

• Each molecule of 3-PGA receives a phosphate group from ATP and becomes 1,3-bisphosphoglycerate.
• Then, 1,3-bisphosphoglycerate is reduced using electrons from NADPH to form G3P (glyceraldehyde-3-phosphate).
• Out of every six G3P molecules produced, only one is used to form glucose and other carbohydrates. The other five are recycled to regenerate RuBP.

3. Regeneration of RuBP

This is the final step in the cycle, where the molecule that accepts CO₂ (RuBP) is regenerated so the cycle can continue. The main points are:

• Five molecules of G3P are rearranged using energy from ATP to form three molecules of RuBP.
• This regeneration process ensures that the Calvin cycle is a continuous loop.
• The ATP used in this phase is supplied by the light-dependent reactions.

Overall Reaction of the Calvin Cycle

The Calvin cycle uses the following input to produce glucose:

• 6 CO₂ molecules
• 18 ATP molecules
• 12 NADPH molecules

And it produces:

• 1 molecule of glucose (C₆H₁₂O₆)
• 18 ADP and 18 Pi (inorganic phosphate)
• 12 NADP⁺

Importance of the Calvin Cycle

• It helps plants convert inorganic carbon (CO₂) into organic compounds like glucose.
• It provides the sugar molecules that are used for energy and as building blocks for other important molecules.
• The Calvin cycle is also vital for the carbon cycle on Earth and helps reduce the levels of CO₂ in the atmosphere.

Conclusion

The Calvin cycle is a critical process in photosynthesis where carbon dioxide is converted into glucose. It has three major phases: carbon fixation, reduction, and regeneration of RuBP. Each step plays an important role in ensuring that plants can produce their own food and supply energy to the entire ecosystem. Without the Calvin cycle, life on Earth would not be possible as we know it.