In Depth Notes on the Light Independent Reactions (Calvin Cycle)

The Light Independent Reactions (Calvin Cycle)

• Definition: Also known as "Dark Reactions" because they do not require light; however, they do not occur exclusively in the dark.

• Discovery: Identified in the 1960s by Melvin Calvin.

• Main Function: Convert carbon dioxide (CO₂) into glucose through a series of reactions. This process includes:

  • Carbon fixation
  • Reduction of carbon molecules to produce glucose

• Energy Sources:

  • ATP is utilized to power the reactions.
  • NADPH provides electrons necessary for the reduction process.

Key Components of the Calvin Cycle

• Takes place in the stroma of chloroplasts.
• Utilizes CO₂ as a starting reactant.

Cycle Overview

• Three Turns: Each cycle produces six glyceraldehyde 3-phosphate (G3P) molecules, with one G3P being utilized to synthesize half a glucose. Therefore, six turns are required for one full glucose molecule.
• Electron Delivery: Post-reaction, NADPH gets converted into NADP⁺ and ATP into ADP, which are recycled back to the light reactions.

Detailed Phases of the Calvin Cycle

1. Phase 1 – Carbon Fixation:

   • Reaction: 6 CO₂ + 6 Ribulose 1,5-bisphosphate (RuBP) → 12 3-phosphoglycerate (PGA)
   • Mechanism: CO₂ is absorbed from the atmosphere via stomata and reacts with RuBP. The intermediate product is highly unstable and quickly splits into twelve 3-PGA molecules.
   • Enzyme Involved: Catalyzed by RUBISCO, the most abundant protein on Earth, though it is relatively slow compared to other enzymes.

2. Phase 2 – Reduction:

   • Process: Each 3-PGA is phosphorylated using ATP, leading to the formation of 12 1,3-bisphosphoglycerates (1,3-BPG).
   • Reductive Step: Each 1,3-BPG is reduced by NADPH, attaching a hydrogen ion (H⁺), resulting in the formation of glyceraldehyde 3-phosphate (G3P).

3. Phase 3 – Regeneration of RuBP:

   • Process: From the 12 G3P produced, 10 molecules proceed to regenerate RuBP, while 2 are used to form glucose.
   • Regeneration: 6 RuBP molecules are regenerated by phosphorylation, rearranging 30 carbons into six 5-carbon molecules of RuBP using ATP.

Summary of the Calvin Cycle

• Utilization of Energy: ATP and NADPH are vital for converting CO₂ into sugars.
• Cyclical Nature: The cycle begins and ends with RuBP, which is continuously regenerated.
• Product Formation: Primarily produces G3P, which can be converted into glucose and other sugars.