Question

Dark reaction of photosynthesis does not need light. Do plants undergo dark reaction at night explain.

Answer 1

Hello dear !
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No it doesn't mean it is in night !
as we know light reaction occurs in light so it means in the presence of sunlight 
It is clear that light reaction is in day time and we know dark reaction doesn't need light rather need its by products of light reaction ATP and NADPH as we know photosynthesis occurs In day time so it means both  light and dark reaction occurs in day time . By name dark it doesn't means it is occurs in night but also it is misnomer term that dark reaction doesn't need light rather by products formed in light reaction to convert CO2 into sugar
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Answer 2

Dark reaction does not mean that it occurs in the absence of light i.e., at night. Infact, it is the reaction occurring simultaneously with light reaction. It a chemical process occurring independent of light. This reaction is completely based on enzymes.