Photosystem II and Electron Transport Chain

Photosystem II (PS II) is a complex of proteins and pigments that capture and convert light energy into chemical energy in photosynthesis. It is located in the thylakoid membrane of chloroplasts and is the first step in the light-dependent reactions of photosynthesis.

PS II contains chlorophyll a molecules that absorb light energy and transfer it to other pigments and eventually to a reaction center, where an electron is excited to a higher energy level. This electron is then transferred to an acceptor molecule, which starts the electron transport chain (ETC) in PS II.

Acceptors of Electrons in PS II

There are several acceptor molecules in the ETC of PS II that accept the excited electron from the reaction center. The first acceptor of electrons from an excited chlorophyll molecule of photosystem II is a quinone molecule.

Quinone molecules are lipid-soluble molecules that can move freely in the thylakoid membrane. When they accept an electron from PS II, they become reduced and form a quinol molecule. This quinol molecule can then move to the cytochrome b6f complex, which is the next step in the ETC of PS II.

The cytochrome b6f complex accepts the quinol molecule and uses the electron to pump protons across the thylakoid membrane from the stroma to the lumen. This creates a proton gradient that is used to generate ATP by ATP synthase.

After the electron is transferred to the cytochrome b6f complex, it is eventually passed to photosystem I (PS I) via a series of carriers, including plastocyanin. In PS I, another photon of light is absorbed and used to excite another electron to a higher energy level, which is then transferred to an acceptor molecule and used to reduce NADP+ to NADPH.

Conclusion

In summary, the first acceptor of electrons from an excited chlorophyll molecule of photosystem II is a quinone molecule. This quinone molecule is then used to generate a proton gradient that is used to generate ATP, and the electron is eventually passed to photosystem I to reduce NADP+ to NADPH.

A quinone is a class of organic compounds, that are formally derived from aromatic compounds by conversion of an even number of -CH= groups into -C(=O)- groups with any necessary rearrangement of double bonds resulting in a fully conjugated cyclic structure. It is the first acceptor molecule from photosystem II.