Question

Clearly explain how Photosystem II collects energy for photosynthesis, the products of PS II, and how PSII is related to PSI in the photosynthesis pathway.

Answer 1

The photochemical reaction center chlorophyll a of PSII is known as P680 because it most effectively absorbs light of wavelength 680nm. In higher plants, chlorophyll molecules in the reaction center are associated with two membrane proteins termed as D1 and D2. PSII has located predominantly in the appressed membranes of grana thylakoid.

Noncyclic electron flow begins with the absorption of a proton by PSII. Upon absorption of a photon, P680 is excited to P680*, which rapidly transfers an electron to a nearby pheophytin and changes into P680+. Pheophytin is a chlorophyll in which magnesium atom has been replacing by two hydrogen atoms.

The positively charged P680+, a strong oxidizing agent, attracts an electron from an electron donor to regenerate the original P680. In oxygenic photosynthetic organisms, H20 acts as an electron donor. Splitting of water molecules yields four electrons, four protons and molecular oxygen.

The splitting of water is catalyzed by a protein complex, the oxygen-evolving complex, located on the luminal surface of the thylakoid membrane. The oxygen-evolving complex cycles through a series of oxidation states while abstracting protons and electrons from the water molecules and finally releases oxygen into inner thylakoid space.

Through Z scheme the PSII and PSI are connected. The excited PSII reaction center chlorophyll, P680*, transfers electrons to pheophytin. P680 oxidized by light is re-reduced by electrons from oxidation of water. Pheophytin transfers electrons to the plastoquinones QA and QB. The cytochrome b6f complex transfers electrons to plastocyanin which in turn reduces P700+ (oxidized 700). The acceptor of electrons from P700 is Ao. The next acceptor A1 is a quinone. A series of membrane-bound iron-sulfur proteins (FeSx, FeSA and FeSB) transfers electrons to soluble ferredoxin. The soluble flavoprotein ferredoxin-NADH reductase (FNR) reduces NADP+ to NADPH.