Calvin Cycle

Photosynthesis is often simplified into 6CO2 + 6H2O + light –> C6H12O6 + 6O2 . However, the true product is 3-phosphoglycerate that can be used to generate longer carbohydrates like glucose. 

The Calvin Cycle with ball-and-stick structures.

The light independent reactions are also known as the dark reactions or Calvin Cycle and utilize the ATP and NADPH from the light-dependent reactions to fix gaseous CO2 into carbohydrate backbones.

CO2 enters the cycle in the reaction catalyzed by RuBisCo.

  • Red: Oxygen
  • Black: Carbon
  • Purple: Phosporus

The starting point of carbon fixation is the carbohydrate Ribulose 1,5-bisphosphate. The enzyme Ribulose Bisphospate Carboxylase (RuBisCO) captures a CO2 molecule onto Ribulose 1,5-bisphosphate (RuBP) to generate 2 molecules of 3-phosphoglycerate (3-PGA) which can enter the process of gluconeogenesis to generate glucose. ATP from the light reactions can then facilitate the conversion of 3-phosphoglycerate to 1,3 bisphosphoglycerate which can be reduced by NADPH to glyceraldehyde-3-phosphate (G3P). G3P can then be used to regenerate  Ribulose 1,5-bisphosphate.

Break down of dark reactions in C3 plants

1: Carbon fixation by RuBisCO

CO2 is added to the C5 sugar RuBP, forming an C6 unstable intermediate that breaks down into 2 C3 molecules (3-PGA).

2: Reduction by NADPH

NADPH is oxidized into NADP+ while carboxyle group of 3-PGA is reduced into an aldehyde.

3: RuBP regeneration

The three steps of the Calvin cycle.

3 Calvin cycles are needed to form 6 G3P molecules. 5 G3P molecules are used, and 1 G3P molecule can be used to form glucose.

Calvin cycle video

Back to resources