Photosystem I

Photosystem I is a proteinaceous transmembrane structure composed of several proteins and embedded with pigment molecules. This structure is located inside chloroplasts and secured within the thylakoid membrane with exposure to the thylakoid lumen on one side and to the chloroplast stroma on the other side. PS I acts as an energy converter for various photosynthetic organisms.

Mechanics of Photosystem I

Light energy in the form of photons is converted into electrons to power the generation of ATP or the reduction of NADP+ to NADPH.[4] Photons are received by an antenna complex of pigment molecules. Antenna molecules become photoexcited and pass the energy as resonance energy (text). The resonance energy is transferred to the reaction center pigment chlorophyll a. The reaction center in turn transfers electrons to a primary electron acceptor and subsequent electron acceptors and carriers. Finally, the electrons reduce NADP+ or help generate ATP. Electrons may be recycled to increase the proton concentration in the thylakoid lumen in a process called cyclic electron flow. In cyclic electron flow electrons are passed from the PS I reaction center and then carried to a cytochromeb6f complex where they help transport protons into the thylakoid lumen thus creating ATP.Plastocyanin may accept electrons from the cytochrome b6f complex and pass them along to the reaction center in the antenna complex beginning the cycle again.

Cyclic Photophosphorylation

In cyclic electron flow, the electron begins in a pigment complex called photosystem I, passes from the primary acceptor to plastoquinone, then to cytochrome b6f (a similar complex to that found in mitochondria), and then to 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 during chemiosmosis. This pathway is known as cyclic photophosphorylation, and it does not produce O2, as well as ATP. Unlike non-cyclic photophosphorylation, NADP+ does not accept the electrons, but they are sent back to photosystem I. NADPH is NOT produced in cyclic photophosphorylation. In bacterial photosynthesis, a single photosystem is used, and therefore is involved in cyclic photophosphorylation.