Introductory Crop Physiology

CARBON DIOXIDE FIXATION-- Dark reaction or Blackman’s reaction

Carbon dioxide fixation

Dark reaction or Blackman’s reaction or Path of carbon in photosynthesis
This is the second step in the mechanism of photosynthesis.

• The chemical processes of photosynthesis occurring independent of light is called dark reaction. It takes place in the stroma of chloroplast.
• The dark reaction is purely enzymatic and it is slower than the light reaction. The dark reactions occur also in the presence of light.
• In dark reaction, the sugars are synthesized from CO2.
• The energy poor CO2 is fixed to energy rich carbohydrates using the energy rich compound, ATP and the assimilatory power, NADPH2 of light reaction.
• The process is called carbon fixation or carbon assimilation.
• Since Blackman demonstrated the existence of dark reaction, the reaction is also called as Blackman’s reaction.
Two types of cyclic reactions occur in this reaction, which are as follows

1. Calvin cycle or C3 cycle
2. Hatch and Slack pathway or C4 cycle