Both photosystem II (PSII) and photosystem I (PSI) contain an electron transport chain, which extracts energy when an excited electron loses its energy of excitation in a stepwise fashion. The photosystem carries out a series of oxidation/reduction reactions similar to those in electron transport. Events in the photosystem are as follows:
1. Absorption of a photon by the light harvesting system. The photon is funneled to a reaction center chlorophyll designated P680 in Figure 17.12.
2. Excitation of P680 raises the molecule from the ground state to an excited state at -0.8 volt.
3. The excited P680 is able to quickly transfer an electron from P680 to a lower-energy primary electron acceptor, such as pheophytin a (Ph), shown in Figure 17.12b.
4. The electron is transferred to a series of plastoquinone molecules (Plastoquinone QA and QB) associated with PSII proteins.
5. Two protons are picked up by QB and the reduced plastoquinone, QH2 (plastoquinol), is released into the lipid portion of the thylakoid membrane. The overall reduction of plastoquinone is shown here.
6. Plastoquinol interacts with the membrane-bound cytochrome b6f complex which contains cytochromes and iron-sulfur proteins.
7. The b6f complex donates electrons to a copper protein called plastocyanin (PC). The oxidation of plastoquinone results in release of two protons into the thylakoid lumen.
At this point, the electrons are transferred from PSII to a P700 reaction center in PSI. Plastocyanin is a mobile protein in the thylakoid lumen.
Note that b6f plays a role analogous to the cytochrome reductase complex in mitochondria and resembles it closely.
Note also that the P680 has been left in an oxidized state (P680+), which is a strong oxidant. P680 recovers electrons from water, which is split in the presence of an electron acceptor, releasing oxygen.
The electron acceptor is a protein containing a cluster of four oxygen-bridged manganese atoms (Figure 17.13). The process "dismembers" two water molecules, releasing oxygen, which diffuses out of the chloroplast. The four protons that are produced from the two water molecules are released into the thylakoid lumen, creating a pH gradient between the lumen and stroma.
The reaction summary for PSII is shown here.