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Light Reaction
Inside the thylakoid membranes of the granum, water is split to produce oxygen (which is evolved as a by-product), electrons (e-) and hydrogen ions (H+).  Each electron is then absorbed by chlorophyll, which also absorbs light (radiant energy) to bring the electron to a high energy state.  The energized electron is passed from chlorophyll to the electron carriers of the electron transport chain.  As the electron is transported, an energy gradient is generated (which actually uses the H+) which allows an enzyme (ATPase) to make ATP.  When the electron gets to the end of the chain, it is pretty well drained of the added energy, and NADP+ acts as a terminal electron acceptor to utilize the last bit of energy in the electron to produce NADPH.  Thus, radiant energy (e.g. light) is used to produce metabolic chemical energy (ATP and NADPH).  "Eureka", the plant has made chemical energy from light energy!  But there is a problem.  ATP and NADPH are short lived and cannot be stored or easily transported to where needed.  The plant must figure out a way to save this precious chemical energy and that is where the Dark Reaction comes into the picture.
Dark Reaction
A very important enzyme (ribulose-bisphosphate carboxylase or rubisco) combines a 5-C sugar with a CO2 molecule to produce a 6-C compound that immediately breaks into  2 3-C sugar acids.  This 3-C sugar acid is not very useable by the plant so through a series of reactions it is converted into sugars called triose phosphates.  This requires metabolic energy, which is derived from the ATP and NADPH produced by the Light Reaction.  The triose phosphate is used to produce glucose and other sugars or is stored as glucose in starch.  Finally, "Eureka" for real, the plant has produced a stable, transportable, and storable form of chemical energy (e.g. sugars).  For the cycle to continue, some of the triose phosphate must be used to replenish the original 5-C sugar, and this takes metabolic energy which is supplied by ATP from the Light Reaction.  The Dark Reaction continues as long as the Light Reaction supplies it with energy in the form of ATP and NADPH, thus the Dark Reaction only occurs in the light!