The Dark Reactions of Photosynthesis: The Calvin Cycle

The dark reactions of photosynthesis, also known as the Calvin cycle, occur in the stroma of chloroplasts and utilize the ATP and NADPH produced in the light reactions to convert carbon dioxide into glucose.

Key Steps:

  1. Carbon Fixation:

A carbon dioxide molecule (CO2) combines with a 5-carbon compound called ribulose-1,5-bisphosphate (RuBP).

This reaction, catalyzed by the enzyme RuBisCO, forms a 6-carbon intermediate.

  1. Intermediate Breakdown:

The 6-carbon intermediate is immediately broken down into two 3-carbon molecules, glycerate-3-phosphate (G3P).

  1. Reduction:

G3P molecules are reduced to glyceraldehyde-3-phosphate (G3A) using ATP and NADPH.

ATP provides the energy for the reduction process, while NADPH donates electrons.

  1. Regeneration:

Some of the G3A molecules are used to regenerate RuBP, ensuring the cycle can continue.

This regeneration step requires ATP.

  1. Glucose Synthesis:

A portion of the G3A molecules are combined to form glucose (C6H12O6).

Examples of Organisms Using the Calvin Cycle:

  • Plants: 

Most plants use the Calvin cycle to fix carbon dioxide and produce glucose.

  • Algae: 

Photosynthetic algae, such as green algae and diatoms, also use the Calvin cycle.

  • Cyanobacteria: 

These photosynthetic bacteria employ the Calvin cycle to produce organic matter.


The Calvin cycle is a highly efficient process that allows plants and other photosynthetic organisms to convert inorganic carbon dioxide into organic molecules, providing the foundation for life on Earth.

Would you like to explore specific aspects of the Calvin cycle in more detail, such as the role of RuBisCO or the energetic requirements of the process?