Photosynthesis is actually oxidation reduction process in which water is oxidised and CO₂ is reduced to carbohydrates. The reduction of CO₂ to carbohydrates needs assimilatory powers, i.e., ATP and NADPH₂. The process of photosynthesis involves two steps–
(i) Light dependent phase or photochemical reaction.
(ii) Light independent phase or dark reaction.
In Calvin cycle, CO₂ acceptor molecule is RuBP or RuDP. The enzyme catalyzing this reaction is RuBPcarboxylase/oxygenase (RuBisCO). As Calvin cycle takes in only one carbon (as CO₂ ) at a time, so it takes six turns of the cycle to produce a net gain of six carbons (i.e., hexose or glucose). In this cycle, for formation of one mole of hexose sugar (Glucose), 18 ATP and 12 NADPH₂ are used. For 6 molecules of CO₂ it needs 18 ATP and 12 NADPH₂ molecules so for assimilation of one molecule of CO₂ it needs 3 ATP and 2 NADPH₂ molecules.