Light-dependent reactions

Light-dependent reactions are the chemical reactions involved in photosynthesis induced by light; all light-dependent reactions occur in thylakoids. There are two light-dependent reactions: the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI).

In non-cyclic photophosphorylation, PSII absorbs a photon to produce a so-called high energy electron which transfers via an electron transport chain to cytochrome b₆f and then to PSI. The then-reduced PSI absorbs another photon producing a more highly reducing electron, which converts NADP⁺ to NADPH. In the cyclic form, only PSI is involved, and the electron is moved through cytochrome b₆f before returning to PSI; NADPH is not produced. Thus, the balance between the two types of photophosphorylation is important to maintain the concentrations of ATP and NADPH in the right proportion for the light-independent reactions. For both cyclic and non-cyclic photophosphorylation, cytochrome b₆f produces a proton gradient across the thylakoid membrane that creates a proton-motive force, which is then used by ATP synthase to form ATP.

In oxygenic photosynthesis, the first electron donor is water, creating oxygen (O₂) as a by-product. In anoxygenic photosynthesis, various electron donors are used.

The net-reaction of the light-dependent reactions using non-cyclic photophosphorylation in oxygenic photosynthesis is:

2H₂O + 2NADP⁺ + 3ADP + 3P_i → O₂ + 2H⁺ + 2NADPH + 3ATP

PSI and PSII are light-harvesting complexes. If a special pigment molecule in a photosynthetic reaction center absorbs a photon, an electron in this pigment attains the excited state and then is transferred to another molecule in the reaction center. This reaction, called photoinduced charge separation, is the start of the electron flow and transforms light energy into chemical forms.