The light-dependent sequence of photosynthetic reactions that, taking place on the thylakoid membranes of chloroplasts, produce the ATP and NADPH used in the carbon dioxide fixation of the subsequent * dark reactions. Incident light is absorbed by pigments, distributed between two separate photosystems, I and II. The energy absorbed causes excitation of chlorophyll a molecules, which emit high-energy electrons that are passed down an electron transport chain, their energy being used to produce ATP and the reduced form of NADP. Unlike respiration, the transfer of electrons in photosynthesis is from less electronegative molecules to more electronegative molecules. The energy needed to achieve this reversal of normal electron flow is provided by light. See photosystems I and II. |
