thumb|400px|right|Light-dependent reactions of photosynthesis at the thylakoid membrane

thumb|Location of the psa genes in the [[chloroplast genome of Arabidopsis thaliana. The 21 protein-coding genes involved in photosynthesis are displayed as green boxes.]]

Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses light energy to catalyze the transfer of electrons across the thylakoid membrane from plastocyanin to ferredoxin. Ultimately, the electrons that are transferred by Photosystem I are used to produce the moderate-energy hydrogen carrier NADPH. The photon energy absorbed by Photosystem I also produces a proton-motive force that is used to generate ATP. PSI is composed of more than 110 cofactors, significantly more than Photosystem II.

History

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. 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. downstream of the cysteines and could contribute to dimerisation of PsaA/PsaB. The terminal electron acceptors F and F, also [4Fe-4S] iron-sulfur clusters, are located in a 9-kDa protein called PsaC that binds to the PsaA/PsaB core near F.

{| class="wikitable"

|+ Components of PSI (protein subunits, lipids, pigments, coenzymes, and cofactors).

|-

! Protein subunits

! Description

|-

| row 2, cell 1| PsaA

| rowspan=2| Related large transmembrane proteins involved in the binding of P700, A0, A1, and Fx. Part of the photosynthetic reaction centre protein family.

|-

| row 3, cell 1|PsaB

|-

| row 4, cell 1|PsaC

| row 4, cell 2|Iron-sulfur center; apoprotein for F<sub>a</sub> and F<sub>b</sub>

|-

| row 5, cell 1|PsaD

| row 5, cell 2|Required for assembly, helps bind ferredoxin.

|-

| row 6, cell 1|PsaE

| row 6, cell 2|

|-

| row 7, cell 1|PsaI

| row 7, cell 2|May stabilize PsaL. Stabilizes light-harvesting complex II binding.

|-

| row 8, cell 1|PsaJ

| row 8, cell 2|

|-

| row 9, cell 1|PsaK

| row 9, cell 2|

|-

| row 10, cell 1|PsaL

| row 10, cell 2|

|-

| row 11, cell 1|PsaM

| row 11, cell 2|

|-

| row 12, cell 1|PsaX

| row 12, cell 2|

|-

| row 13, cell 1|cytochrome b<sub>6</sub>f complex

| row 13, cell 2|Soluble protein

|-

| row 14, cell 1|F<sub>a</sub>

| row 14, cell 2|From PsaC; In electron transport chain (ETC)

|-

| row 15, cell 1|F<sub>b</sub>

| row 15, cell 2|From PsaC; In ETC

|-

| row 16, cell 1|F<sub>x</sub>

| row 16, cell 2|From PsaAB; In ETC

|-

| row 17, cell 1|Ferredoxin

| row 17, cell 2|Electron carrier in ETC

|-

| row 18, cell 1|Plastocyanin

| row 18, cell 2|Soluble protein

|-

! Lipids

! Description

|-

| row 19, cell 1|MGDG II

| row 19, cell 2|Monogalactosyldiglyceride lipid

|-

| row 20, cell 1|PG I

| row 20, cell 2|Phosphatidylglycerol phospholipid

|-

| row 21, cell 1|PG III

| row 21, cell 2|Phosphatidylglycerol phospholipid

|-

| row 22, cell 1|PG IV

| row 22, cell 2|Phosphatidylglycerol phospholipid

|-

! Pigments

! Description

|-

| row 23, cell 1|Chlorophyll a

| row 23, cell 2|90 pigment molecules in antenna system

|-

| row 24, cell 1|Chlorophyll a

| row 24, cell 2|5 pigment molecules in ETC

|-

| row 25, cell 1|Chlorophyll a<sub>0</sub>

| row 25, cell 2|Early electron acceptor of modified chlorophyll in ETC

|-

| row 26, cell 1|Chlorophyll a′

| row 26, cell 2|1 pigment molecule in ETC

|-

| row 27, cell 1|β-Carotene

| row 27, cell 2|22 carotenoid pigment molecules

|-

! Coenzymes and cofactors

! Description

|-

| row 28, cell 1|Q<sub>K</sub>-A

| row 28, cell 2|Early electron acceptor vitamin K<sub>1</sub> phylloquinone in ETC

|-

| row 29, cell 1|Q<sub>K</sub>-B

| row 29, cell 2|Early electron acceptor vitamin K<sub>1</sub> phylloquinone in ETC

|-

| row 30, cell 1|FNR

| row 30, cell 2|Ferredoxin- oxidoreductase enzyme

|-

| row 31, cell 1|

| row 31, cell 2|Calcium ion

|-

| row 32, cell 1|

| row 32, cell 2|Magnesium ion

|}

Photon

Photoexcitation of the pigment molecules in the antenna complex induces electron and energy transfer. 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. 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.

P700 reaction center

The P700 reaction center is composed of modified chlorophyll a that best absorbs light at a wavelength of 700&nbsp;nm. P700 receives energy from antenna molecules and uses the energy from each photon to raise an electron to a higher energy level (P700*). These electrons are moved in pairs in an oxidation/reduction process from P700* to electron acceptors, leaving behind P700. The pair of P700* - P700 has an electric potential of about −1.2 volts. The reaction center is made of two chlorophyll molecules and is therefore referred to as a dimer.

Phylloquinone

A phylloquinone, sometimes called vitamin K, is the next early electron acceptor in PSI. It oxidizes A in order to receive the electron and in turn is re-oxidized by F, from which the electron is passed to F and F. The reduction of F<sub>x</sub> appears to be the rate-limiting step. In one model, F passes an electron to F, which passes it on to F to reach the ferredoxin. Fd moves to carry an electron either to a lone thylakoid or to an enzyme that reduces . FNR may also accept an electron from NADPH by binding to it.

Ycf4 protein domain

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

Evolution

Molecular data show that PSI likely evolved from the photosystems of green sulfur bacteria. The photosystems of green sulfur bacteria and those of cyanobacteria, algae, and higher plants are not the same, but there are many analogous functions and similar structures. Three main features are similar between the different photosystems. First, redox potential is negative enough to reduce ferredoxin.