photosynthesis

1. 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.

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

3. Structure and functioning of photosystem II

5. 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.

6. Or A1 a phyloquinone
Fe-S protein
Fe-S protein
Fe-S protein
From Ferredoxin
electrons go to NADP+

7. 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

8. 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 protein
plastocyanin. Like Complex III of
mitochondria, the cytochrome b6f complex
contains 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

9. 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.

10. Localization of PSI and PSll in thylakoid membrane