Different Phases of Photosynthesis

Different Phases of Photosynthesis

Different phases of photosynthesis:

Photosynthesis is the procedure by which all autotrophic organisms like green plants, algae and photosynthetic bacteria uses light energy to make sugar and oxygen gas from carbon dioxide and water. In 1905 scientist Black man divided photosynthesis into two phases:

  1. Light phase or photo-chemical reaction
  2. Dark phase or photo-neutral reaction

Light phase:

Light reactions involve the conversion of sunlight to chemical energy taking on the form of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).

  • This phase needs light
  • It occurs in grana of the chloroplast
  • Here light energy is converted to chemical energy and NADH2 and ATR are found.
  • Reaction is: ADP + NADP + H2O → ATP +NADPH + H ion + O2.

This phenomenon is known as photo-phosphorylation.

Photo-phosphorylation is of two types:

(a) Cyclic Photo-phosphorylation and

(b) Non-cyclic Photo-phosphorylation.

Dark phase:

When the dark reactions are ready to take place, carbon dioxide becomes sugar with ATP and NADPH, in a process called the Calvin cycle, or carbon fixation.

  • This phase does not require light and it occurs after the light phase.
  • It occurs in the stroma of chloroplast.
  • Carbon dioxide is reduced to form carbohydrate by the action of ATP, NADPH2 etc produced during the light reaction.
  • ATP + NADPH + CO2 → ADP + NADP + glucose.


All reactions of different plants in the dark phase are not same.

Dark reaction takes place by two recognized paths:

  • Calvin and Bassham path away: – This is seen in C3
  • Hatch and stack path way: This path away is seen in C4

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