Photosynthesis takes place in two stages: the light reactions and the dark reactions. The light reactions take their name because they require the presence of direct light, while the dark reactions do not have the same proviso. Light reactions occur in the thylakoid stacks in the grana for the most part, while dark reactions do not, although they also most frequently happen during the daytime.
Light reactions involve the conversion of sunlight to chemical energy taking on the form of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). The chlorophyll in the plant absorbs the energy from the light, beginning a sequence of events that leads to the production of ATP, NADPH and oxygen, with the splitting of H2O molecules. The stomata release oxygen, and the other two by-products produce sugar as a part of the dark reactions.
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 cycle has three steps: carbon fixation, reduction and regeneration. When these reactions take place, the plant is all set to start the cycle over again, allowing the plant to keep converting carbon dioxide into a useful substance.