|
A photosynthetic reaction centre is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from sunlight or transferred as excitation energy via light-harvesting antenna systems, give rise to electron transfer reactions along a series of protein-bound co-factors. These co-factors are light-absorbing molecules (also named chromophores or pigments) such as chlorophyll and phaeophytin, as well as quinones. The energy of the photon is used to promote an electron to a higher molecular energy level of a pigment. The free energy created is then used to reduce a chain of nearby electron acceptors, which have subsequently higher redox-potentials. These electron transfer steps are the initial phase of a series of energy conversion reactions, ultimately resulting in the production of chemical energy during photosynthesis. ==Transforming light energy into charge separation== Reaction centers are present in all green plants, algae, and many bacteria. Although these species are separated by billions of years of evolution, the reaction centers 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 centers 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 center found in ''Rhodopseudomonas'' bacteria is currently best understood, since it was the first reaction center of known structure and has fewer polypeptide chains than the examples in green plants.〔(Biochemistry:Fifth Edition ), Chapter 19.〕 A reaction center 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.〔(Understanding the atom ) (2000). Retrieved Feb 28, 2010.〕 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.〔Arie Uittenbogaard (2005). (Quantum mechanics ) Retrieved Feb 28, 2010.〕 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 center. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「photosynthetic reaction centre」の詳細全文を読む スポンサード リンク
|