Photosynthesis involves the transfer and harvesting of energy from sunlight and the fixation of carbon dioxide into carbohydrates. This process occurs in higher plants, including Arabidopsis thaliana. Oxygenic photosynthesis requires water, which acts as an electron donor molecule. The reactions which involve the trapping of sunlight are known as "light reactions", and result in the production of NADPH, adenosine triphosphate, and molecular oxygen. The "dark reactions" are known as the Calvin cycle, and involve the use of the products of the light reactions to fix carbon dioxide and produce carbohydrates. The light-dependent reactions of photosynthesis begins with photosystem II, located in the thylakoid membrane within chloroplasts, which captures light energy to transfer electrons from water to plastoquinone. This process generates oxygen as well as a proton gradient used to synthesize ATP. The D1/D2 (psbA/psbD) reaction center heterodimer binds P680, the primary electron donor of PSII as well as several subsequent electron acceptors. Next, the cytochrome b6-f complex mediates electron transfer between photosystem II (PSII) and photosystem I (PSI). Plastoquinol shuttles electrons from PSII to cytochrome b6-f complex. Plastocyanin shuttles electrons from cytochrome b6-f complex to PSI. Photosystem I is a plastocyanin-ferredoxin oxidoreductase which uses light energy to transfer an electron from the donor P700 chlorophyll pair to the electron acceptors A0, A1, FX, FA and FB in turn. The function of PSI is to produce the NADPH necessary for the reduction of CO2 in the Calvin-Benson cycle. Finally, the proton gradient allows ATPase to synthesize ATP from ADP.

Photosynthesis (Light-Dependent Reactions) References