Photosystem II

By Jennifer McDowall

Although only 1% of the light that reaches the earth is captured by photosynthesis, it is enough to provide the energy needed to drive all life on earth. By using sunlight to convert inorganic carbon dioxide and water (low potential energy) into organic glucose with high potential energy C-H bonds, photosynthesis provides a fuel source that can be stored for later use – by all organisms, photosynthetic or not. In addition, even though photosynthesis only produces molecular oxygen as a by-product, it was produced in sufficient quantities by early oxygenic photosynthetic organisms to change the earth’s atmosphere, and to continue to provide all the oxygen that we breathe.

The photosynthetic process that occurs in plants, algae and some bacteria can be separated into light-dependent and -independent reactions. A series of light-dependent reactions are responsible for the generation of ATP using light as an energy source, which are followed by a series of light-independent reactions that use this ATP to drive the formation of organic compounds from carbon dioxide. The light-dependent reactions occur on a photosynthetic membrane, which is the cell membrane in bacteria, and the thylakoid within the chloroplast in plants and algae. The light-capturing pigment is embedded within the photosynthetic membrane, where networks of pigment molecules work together within a photosystem to effectively channel energy. There are different pigments, each of which can absorb a characteristic range of photons, including carotenoids such as beta-carotene-derived retinol (the same pigment used by the human eye), and the more predominant chlorophyll. Chlorophyll absorbs a more narrow energy range than carotenoids, but with a much higher efficiency. Chlorophylls use a magnesium atom at the centre of a porphyrin ring to absorb light energy, which boosts an electron within the magnesium to a higher energy level. The excited electron is shuttled along a series of electron carrier molecules embedded in the thylakoid to a transmembrane protein pump, which transports it across the membrane to the chloroplast interior. The subsequent exit of a proton through an ATPase channel drives the chemiosmotic synthesis of ATP.

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.

Next: The Proteins of Photosystem II