Photosynthesis

•The thylakoid membranes of chloroplasts have two different kinds of photosystems, PSI and PSII, each with its own photochemical reaction center and set of antenna molecules.

•Pho-tosystem II (PSII) is a pheophytin-quinone type osystem (like the single photosystem of purple bacteria) and contains roughly equal amounts of chlorophylls a and b

•Excitation of its reaction-center P680 drives electrons through the cytochrome b61f complex with movement of protons across the thylakoid membrane.

•Photosystem I (PSI) is structurally and functionally related to the type I reaction center of green sulfur bacteria. It has a reaction center designated P700 and a high ratio of chlorophyll n to chlorophyll b.

•Excited P700 passes electrons to the Fe-S protein ferredoxin, then to NADP-, producing

•NADPH.

•Electrons are carried between the two photosystems by the soluble protein plastocyanin, a one-electron carrier functionally simiiar to cytochrome c of mitochondria.

When each photosystem absorbs one photon each, an electron is transferredFor evolution of oxygen four electrons are to be released hence 4x2 =8 photons to be absorbed, 4 by each photosystem

Structure and functioning of photosystem II

PQB receives two electrons in two transfers and also two protons from water to become PQBH2

From PQBH2 electrons pass to cytochrome b6f complex and then to plastocyanin

The binding site for plastoquinone is the point of action of many commercial herbicides that kill plants by blocking electron transfer through the cytochrome b6f complex and preventing photosynthetic ATP production.

Complex I and associated proteins

Or A1 a phyloquinone

Fe-S protein

Fe-S protein

Fe-S protein

From Ferredoxin electrons go to NADP+

The electron-carrying cofactors of PSI and the light-harvesting complexes are part of a supramolecular complex, the protein consists of three identical complexes each with 11 proteins. . In this structure the many antenna chlorophyll and carotenoid molecules are precisely arrayed around the reaction center. . In contrast to the single path of electrons in PSII, the electron flow initiated by absorption of a photon is be-lieved to occur through both branches of carriers in PSL

Electrons temporarily stored in plastoquinol as a result of the excitation of P680 in PSII are carried to P700 of PSI via the cytochrome b6f complex and the soluble proteinplastocyanin. Like Complex III of mitochondria, the cytochrome b6f complexcontains a b-type cytochrome with two heme groups (designated bH and bL), a Rieske iron-sulfur protein, and cy'tochrome f (named for the Latin frons, "leaf ').

The path of electron flow is

PQBH2 Cytochrome f Plastocyanin P700

Plastoquinol (PQHr) formed in PSll is oxidized by the cytochrome b6f complex in a series of steps like those of the Q cycle in the cytochrome bc1 complex (Complex lll) of mitochondria . One electron from PQH2 passes to the Fe-S center of the Rieske protein, the other to heme b1 of cytochrome b6. The net effect is passage of electrons from PQH2 to the soluble protein plastocyanin, which carries them to PS I.

Localization of PSI and PSll in thylakoid membrane