The Calvin Cycle in Photosynthesis

Overview

• The Calvin cycle is the second set of reactions in photosynthesis, following the light-dependent reactions.
• Also known as light-independent or dark reactions since it doesn't require light directly.
• Requires ATP and NADPH from the light reactions to proceed.
• Occurs in the stroma, the fluid surrounding the thylakoids.

Key Inputs and Outputs

• Inputs: ATP, NADPH, and CO2
• Outputs: G3P (glyceraldehyde 3-phosphate), which can be used to form glucose and other sugars.

Phases of the Calvin Cycle

Phase 1: Carbon Fixation

• Enzyme RuBisCO captures CO2 from the air.
• CO2 is added to a 5-carbon compound called RuBP, forming a 6-carbon compound.
• The 6-carbon compound splits into two molecules of 3-phosphoglycerate (3-PGA).
• This process is repeated for three molecules of CO2.

Phase 2: Reduction

• ATP from the light reactions provides energy to convert 3-PGA to an intermediate compound.
• NADPH donates electrons and hydrogen to reduce this intermediate to G3P.
• Net gain: 1 G3P for every three CO2 molecules fixed.

Phase 3: Regeneration

• Out of the G3P produced, one is a net gain, and five are used to regenerate RuBP.
• Energy from ATP is used to convert 5 G3P molecules back into 3 RuBP molecules.
• This regeneration is crucial to keep the cycle continuous.

Understanding Carbon Counting

• 5 G3P molecules, each containing 3 carbons, provide 15 carbon atoms.
• These are rearranged to form 3 RuBP molecules, each with 5 carbon atoms.

Linking to Glucose Production

• To synthesize one glucose molecule (6 carbons), two G3Ps are required.
• As the Calvin cycle proceeds, it works iteratively to build glucose.
• Overall photosynthesis equation: 6 CO2 + light energy → C6H12O6 + 6 O2.

Importance of the Calvin Cycle

• Produces glucose and other organic compounds necessary for plant and algae growth.
• Supports most life forms on Earth, directly or indirectly.

Learning and Exploration

• Further understanding can be enhanced by engaging with interactive tools such as Photosynthesis Interactive from Bioman Biology.