Photosynthesis: The Calvin Cycle

Overview

• Calvin Cycle: Second set of reactions in photosynthesis; follows light-dependent reactions.
• Other Names: Also known as "light-independent reactions" or "dark reactions".
• Location: Occurs in the stroma, the fluid surrounding the thylakoids.
• Key Inputs: ATP and NADPH from the light-dependent reactions.

Phases of the Calvin Cycle

Phase 1: Carbon Fixation

• Enzyme: Rubisco - catalyzes the reaction.
• Process: CO2 is added to a five-carbon compound called RuBP (ribulose bisphosphate).
• Result: Forms a six-carbon compound that splits into two molecules of 3-phosphoglycerate.
• Note: Carbon fixation is done for three CO2 molecules.

Phase 2: Reduction

• Energy Source: ATP provides the energy.
• Reduction Process: NADPH donates electrons and hydrogen to reduce the intermediate compound.
• Key Product: Glyceraldehyde 3-phosphate (G3P), also known as triose phosphate.
• Output: For every three CO2 molecules, there is a net gain of one G3P.

Phase 3: Regeneration

• Process: Five G3P molecules are converted back into RuBP.
• Purpose: To regenerate the starting compound and continue the cycle.
• Carbon Counting: Five G3P (15 carbons) can regenerate three RuBP molecules (5 carbons each).
• Energy: ATP is required to drive this process.

Overall Process

• Photosynthesis Equation: Uses six CO2 molecules to make one glucose molecule.
• G3P Requirement: Two G3P molecules are needed to form one glucose.
• Importance: Glucose and other organic compounds support plant growth and life on Earth.

Conclusion

• The Calvin Cycle is essential for synthesizing glucose using products from light-dependent reactions.
• To deepen understanding, engage with interactive tools like the photosynthesis interactive at BioMan Biology.

Additional Notes

• Biological Importance: The Calvin Cycle contributes significantly to the biomass of plants and supports ecosystems globally.