PSII is the membrane protein complex found in oxygenic photosynthetic organisms (higher plants, green algae and cyanobacteria), which harnesses light energy to split H2O into O2, protons and electrons. It drives one of the most oxidising reactions known to occur in nature and is responsible for the production of atmospheric oxygen, essential for aerobic life on this planet. In addition, by catalysing the first step of the photosynthetic electron transport chain, PSII is also involved in the production of a substantial proportion of the global biomass.
Photosystem II, the Evolution of Non-cyclic Photosynthesis
Photosynthesis first evolved as an anoxygenic process in bacteria that were similar to the current green sulphur bacteria, where the transmission of an electron from the photosystem is accompanied by the extraction of a proton from hydrogen sulphide (H2S), producing sulphur as a by-product. In order to rejuvenate the pigment for further use, the electron takes a circular path from the original excitation of the pigment, through the photosynthetic electron transport system, back to the pigment molecule. This type of photosynthesis involved just one photosystem (P700) and culminated in the synthesis of ATP, an unstable energy source.
The advent of oxygenic photosynthesis provided an organism not only with ATP, but also with a stable energy source in the form of organic compounds that can be stored for later use. The transition from anoxygenic to oxygenic photosynthesis involved an extension of the existing system, whereby new reactions were added on to existing ones. This was achieved through a remarkable increase in protein complexity with the development of a second photosystem, photosystem II (PSII). The incorporation of hydrogen atoms into carbon-containing compounds required a source of reducing power, which came from the oxidation of water. However, it takes significantly more energy to split a hydrogen atom from water than it does from H2S. PSII contains chlorophyll a, first developed amongst cyanobacteria 2.5 billion years ago, which absorbs a shorter wavelength of light (680nm) with a higher energy level, and which is referred to as P680.
Plants, algae and some bacteria use two photosystems, PSI with P700 and PSII with P680. Using light energy, PSII acts first to channel an electron through a series of acceptors that drive a proton pump to generate ATP, before passing the electron on to PSI. Once the electron reaches PSI it has used most of its energy in producing ATP, but a second photon of light captured by P700 provides the required energy to channel the electron to ferredoxin, generating reducing power in the form of NADPH. The ATP and NADPH produced by PSII and PSI, respectively, are used in the light-independent reactions for the formation of organic compounds. This process is non-cyclic, because the electron from PSII is lost and is only replenished through the oxidation of water. Hence, there is a constant flow of electrons and associated hydrogens from water for the formation of organic compounds. It is this stripping of hydrogens from water that produces the oxygen we breathe.