Transforming Light Energy Into Charge Separation
Reaction centres are present in all green plants, algae, and many bacteria. Although these species are separated by billions of years of evolution, the reaction centres are homologous for all photosynthetic species. In contrast, a large variety in light-harvesting complexes exist between the photosynthetic species. Green plants and algae have two different types of reaction centres that are part of larger supercomplexes known as photosystem I P700 and photosystem II P680. The structures of these supercomplexes are large, involving multiple light-harvesting complexes. The reaction centre found in Rhodopseudomonas bacteria is currently best understood, since it was the first reaction centre of known structure and has fewer polypeptide chains than the examples in green plants.
A reaction centre is laid out in such a way that it captures the energy of a photon using pigment molecules and turns it into a usable form. Once the light energy has been absorbed directly by the pigment molecules, or passed to them by resonance transfer from a surrounding light-harvesting complex, they release two electrons into an electron transport chain.
Light is made up of small bundles of energy called photons. If a photon with the right amount of energy hits an electron, it will raise the electron to a higher energy level. Electrons are most stable at their lowest energy level, what is also called its ground state. In this state, the electron is in the orbit that has the least amount of energy. Electrons in higher energy levels can return to ground state in a manner analogous to a ball falling down a staircase. In doing so, the electrons release energy. This is the process that is exploited by a photosynthetic reaction centre.
When an electron rises to a higher energy level, decrease in the reduction potential of the molecule in which the electron resides occurs. This means that the molecule has a greater tendency to donate electrons, the key to the conversion of light energy to chemical energy. In green plants, the electron transport chain that follows has many electron acceptors including phaeophytin, quinone, plastoquinone, cytochrome bf, and ferredoxin, which result in the reduced molecule NADPH. The passage of the electron through the electron transport chain also results in the pumping of protons (hydrogen ions) from the chloroplast's stroma into the lumen, resulting in a proton gradient across the thylakoid membrane that can be used to synthesise ATP using ATP synthase. Both the ATP and NADPH are used in the Calvin cycle to fix carbon dioxide into triose sugars.
Read more about this topic: Photosynthetic Reaction Centre
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