Calvin Cycle Overview and Phases

Calvin Cycle Overview

• The Calvin cycle is a crucial part of photosynthesis
• It's a process that converts inorganic carbon dioxide into organic molecules, like glucose.
• This process occurs in three phases: carbon fixation, reduction, and regeneration.

Carbon Fixation Phase

• RuBisCo is the key enzyme in this phase
• RuBisCo catalyzes the reaction between CO₂ and RuBP (ribulose 1,5-bisphosphate)
• Forms an unstable six-carbon intermediate that quickly splits into two molecules of 3-PGA (3-phosphoglycerate)
• Marks the point where inorganic carbon is converted to organic form

Reduction Phase

• 3PGA is converted into G3P (glyceraldehyde 3-phosphate)
• This process requires ATP and NADPH, which are provided by the light-dependent reactions.
• This phase effectively reduces 3PGA to G3P, storing energy.
• Out of six G3Ps generated, only one is used to make glucose, the rest are recycled to regenerate RuBP.

Regeneration Phase

• The remaining G3P molecules are used to regenerate RuBP
• This process requires ATP
• This cycle ensures a continuous supply of RuBP to continue fixing CO₂.

Types of Photosynthesis

• The pathways for photosynthesis are C3, C4, and CAM.
• C3 photosynthesis is the most common type, fixing CO₂ directly into 3PGA
  • Efficient in cool and moist climates
  • Less effective in hot, dry conditions due to photorespiration (in hot conditions RuBisCo mistakenly fixes O₂ rather than CO₂.)
• C4 photosynthesis minimizes photorespiration by concentrating CO₂ in bundle sheath cells.
  • This adaptation allows plants to thrive in high temperatures and low water availability.
  • CO₂ is initially fixed into oxaloacetate, then converted to malate, ultimately entering the Calvin cycle
• CAM photosynthesis is a modified C4 pathway.
  • Plants open their stomata at night to take up CO₂ and store it as organic acids. During the day, CO₂ is released from these acids and used in the Calvin cycle.
  • This adaptation helps plants to conserve water in arid environments.

Key Players

• RuBisCo: Fixes inorganic carbon into usable form
• ATP: Provides energy for the Calvin cycle.
• NADPH: Provides electrons to reduce 3PGA to G3P
• Other important molecules: RuBP, 3PGA, G3P, and various intermediate compounds.

Energy Inputs and Outputs

• Energy inputs: ATP and NADPH (from light-dependent reactions)
• Energy output: G3P (glyceraldehyde-3-phosphate), used for glucose synthesis and RuBP regeneration.