The Calvin Cycle in Photosynthesis

Overview

• The Calvin Cycle is the second set of reactions in photosynthesis, occurring after the light-dependent reactions.
• Also known as light-independent reactions or dark reactions because they do not directly require light.
• Key products from light reactions that are vital for the Calvin Cycle:
  • ATP: Provides energy.
  • NADPH: Provides electrons and hydrogens.

Location

• Takes place in the stroma, the fluid surrounding the thylakoids.

Phases of the Calvin Cycle

1. Carbon Fixation

   • Enzyme Rubisco captures CO2 from the air and adds it to Ribulose Bisphosphate (RuBP), a 5-carbon compound.
   • Results in a 6-carbon compound that splits into two molecules of 3-Phosphoglycerate (3-PGA).
   • Example: 3 molecules of CO2 lead to carbon fixation.

2. Reduction Phase

   • ATP provides energy to create an intermediate compound.
   • NADPH supplies electrons and hydrogen to reduce the intermediate to Glyceraldehyde 3-Phosphate (G3P).
   • G3P is the main product and can be used to synthesize sugar.
   • For every 3 CO2 molecules, there is a net gain of 1 G3P (3 carbons in, 3 carbons out).

3. Regeneration Phase

   • From 6 G3P molecules, only 1 is used for sugar synthesis while 5 are recycled.
   • Those 5 G3P (15 carbons) are converted back into 3 RuBP molecules (5 carbons each).
   • Requires energy, sourced from ATP.

Cycle Repetition and Glucose Formation

• To synthesize one glucose molecule (C6H12O6), the cycle must run twice.
• Total requirement: 6 CO2 molecules result in 2 G3P molecules to form one glucose.

Importance

• Provides glucose and other organic compounds essential for plant/algal growth.
• Supports all life forms by supplying energy.

Further Learning

• Explore the Calvin Cycle interactively at BioMan Biology.

These notes summarize the key concepts of the Calvin Cycle in photosynthesis, highlighting each phase and its significance in forming glucose.