Photosystem I

This photosystem is known as PSI because it was discovered before Photosystem II, although future experiments showed that Photosystem II is actually the first enzyme of the photosynthetic electron transport chain. Aspects of PSI were discovered in the 1950s, but the significance of these discoveries was not yet recognized at the time.[4] Louis Duysens first proposed the concepts of Photosystems I and II in 1960, and, in the same year, a proposal by Fay Bendall and Robert Hill assembled earlier discoveries into a coherent theory of serial photosynthetic reactions.^[4] Hill and Bendall's hypothesis was later confirmed in experiments conducted in 1961 by the Duysens and Witt groups.^[4]

Components and action

Two main subunits of PSI, PsaA and PsaB, are closely related proteins involved in the

Photoexcitation of the pigment molecules in the antenna complex induces electron and energy transfer.^[10]

Antenna complex

The antenna complex is composed of molecules of chlorophyll and carotenoids mounted on two proteins.^[11] These pigment molecules transmit the resonance energy from photons when they become photoexcited. Antenna molecules can absorb all wavelengths of light within the visible spectrum.^[12] The number of these pigment molecules varies from organism to organism. For instance, the cyanobacterium Synechococcus elongatus (Thermosynechococcus elongatus) has about 100 chlorophylls and 20 carotenoids, whereas spinach chloroplasts have around 200 chlorophylls and 50 carotenoids.^[12][3] Located within the antenna complex of PSI are molecules of chlorophyll called P700 reaction centers. The energy passed around by antenna molecules is directed to the reaction center. There may be as many as 120 or as few as 25 chlorophyll molecules per P700.^[13]

P700 reaction center

The P700 reaction center is composed of modified

The two modified chlorophyll molecules are early electron acceptors in PSI. They are present one per PsaA/PsaB side, forming two branches electrons can take to reach F_x. A₀ accepts electrons from P700*, passes it to A₁ of the same side, which then passes the electron to the quinone on the same side. Different species seems to have different preferences for either A/B branch.[15]

Phylloquinone

A

Three proteinaceous iron–sulfur reaction centers are found in PSI. Labeled F_x, F_a, and F_b, they serve as electron relays.^[18] F_a and F_b are bound to protein subunits of the PSI complex and F_x is tied to the PSI complex.^[18] Various experiments have shown some disparity between theories of iron–sulfur cofactor orientation and operation order.^[18] In one model, F_x passes an electron to F_a, which passes it on to F_b to reach the ferredoxin.^[15]

Ferredoxin

Plastocyanin is an electron carrier that transfers the electron from cytochrome b6f to the P700 cofactor of PSI in its ionized state P700⁺.^[10][21]

Ycf4 protein domain

The Ycf4 protein domain found on the thylakoid membrane is vital to photosystem I. This thylakoid transmembrane protein helps assemble the components of photosystem I. Without it, photosynthesis would be inefficient.^[22]

Evolution

Molecular data show that PSI likely evolved from the photosystems of

• Biohybrid solar cell

References