The first stage of the photosynthetic system is the light-dependent reaction, which converts solar energy into chemical energy. Light absorbed by chlorophyll or other photosynthetic pigments is used to drive a transfer of electrons and hydrogen from water (or some other donor molecule) to an acceptor called NADP+, reducing it to the form of NADPH by adding a pair of electrons and a single proton (hydrogen nucleus). The water or other donor molecule is split in the process; it is the light reaction which produces waste oxygen.
The light reaction also generates ATP by powering the addition of a phosphate group to ADP, a process called photophosphorylation. ATP is a versatile source of chemical energy used in most biological processes. Note, however, that the light reaction produces no carbohydrates such as sugars.
Both of these processes are accomplished via the mechanism of an electron transport chain. This is a series of proteins embedded in a biological membrane that transfers high-energy electrons from one to another, accomplishing various activities along the way as the electron drops in energy level. The electrons originate when a photon of sunlight strikes a chlorophyll molecule contained within a photosystem (cluster of associated pigment molecules) and excites one of its electrons, which is then captured by a primary acceptor protein.
The chlorophyll's electron can follow either of two different pathways, cyclic or non-cyclic.
In cyclic electron flow, the electron originates in a pigment complex called photosystem I, passes from the primary acceptor to ferredoxin, then to plastoquinone (a complex of two cytochromes similar to those found in mitochondria), and then plastocyanin before returning to chlorophyll. This transport chain produces a proton-motive force, pumping H+ ions across the membrane; this produces a concentration gradient which can be used to power ATP synthase. This pathway is known as cyclic photophosphorylation, and it produces neither O2 nor NADPH.Cyclic photophosphorylation