Answer :

because in photosynthesis Calvin's cycle is part if the light-independent phase, also called dark phase. it is the stage where light is not required for the process.
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Calvin cycle is a series of biochemical reactions which reduces carbon dioxide to three carbon sugar. It is also known as reductive pentose phosphate cycle.

The Calvin cycle takes place-

  • In the cytosol of prokaryotic photosynthetic organism.
  • In the stroma of chloroplasts in eukaryotic photosynthetic organisms.

In Calvin cycle glyceraldehyde-3-phosphate ( three carbon sugar) is produced.

The Calvin cycle is referred to as dark reaction because it is a light independent reaction and products of the light reaction ( ATP and NADPH) are used to make glyceraldehyde-3-phosphate from reduction of carbon dioxide.

Learn more about Calvin cycle-

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