The absorption of light by chloroplast pigments excites a protein and sets off a chain of reactions that end in the production of the molecule nicotinamide adenine dinucleotide phosphate in its reduced form. NADPH is essential for the next step of photosynthesis, the dark reactions, to take place.
The chloroplast pigment is chlorophyll, which forms two photosystems to harvest light energy. Initially, chlorophyll in photosystem II captures a photon of light energy which excites an electron to a higher energy level. The now unstable electron becomes part of an electron transport chain; it passes between a series of molecules as it loses energy and moves towards photosystem I. In photosystem I the electron gains additional energy from a second photon.
Typically, the source of the electron in photosynthesis is water. Water gives up an electron and splits to form oxygen. The plant's stomata, the pores in its leaves, open to release the oxygen into the atmosphere. The final step of this portion of photosynthesis occurs when an NADP molecule accepts the electron to become reduced, or NADPH.
The absorption of light energy and the subsequent electron transport chain occurs during the light reactions of photosynthesis. The NADPH molecules from the light reactions go on to the dark reactions and play an important role in helping the plant create glucose for energy.