Light reaction

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.

Cyclic photophosphorylation

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.

Noncyclic photophosphorylation

The other pathway, noncyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems. First, a photon is absorbed by the chlorophyll core of photosystem II, exciting two electrons which are transferred to the primary acceptor protein. The deficit of electrons is made up for by taking electrons from a molecule of water, splitting it into O₂ and H⁺. The electrons transfer from the primary acceptor to plastoquinone, then to plastocyanin, producing proton-motive force as with cyclic electron flow and driving ATP synthase.

Since the photosystem II complex replaced its lost electrons from an external source, however, these electrons are not returned to photosystem II as they would in the analogous cyclic pathway. Instead, the still-excited electrons are transferred to a photosystem I complex, which boosts their energy level to a higher level using a second solar photon. The highly excited electrons are transferred to the primary acceptor protein, but this time are passed on to ferredoxin, and then to an enzyme called NADP⁺ reductase which uses them to drive the reaction

NADP⁺ + H⁺ + 2e^- → NADPH

This consumes the H⁺ ions produced by the splitting of water, leading to a net production of O², ATP, and NADPH with the consumption of solar photons and water.