Фотосинтез реакциялық орталығы: Жарық энергиясын сіңіретін молекулалық бірлік

Molecular unit responsible for absorbing light in photosynthesis

A photosynthetic reaction center is a complex of several proteins, pigments, and other co factors that together 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 the path of a series of protein bound co factors. These co factors are light absorbing molecules (also named chromophores or pigments) such as chlorophyll and pheophytin, as well as quinones. The energy of the photon is used to excite an electron of a pigment. The free energy created is then used, via a chain of nearby electron acceptors, for a transfer of hydrogen atoms (as protons and electrons) from H2O or hydrogen sulfide towards carbon dioxide, eventually producing glucose. These electron transfer steps ultimately result in the conversion of the energy of photons to chemical energy.

Reaction centers are present in all green plants, algae, and many bacteria. A variety in light harvesting complexes exist across the photosynthetic species. Green plants and algae have two different types of reaction centers that are part of larger supercomplexes known as P700 in Photosystem I and P680 in Photosystem II. 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. 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 electrons into an electron transport chain and pass energy to a hydrogen donor such as H2O to extract electrons and protons from it. In green plants, the electron transport chain has many electron acceptors including pheophytin, quinone, plastoquinone, cytochrome bf, and ferredoxin, which result finally in the reduced molecule NADPH, while the energy used to split water results in the release of oxygen. The passage of the electron through the electron transport chain also results in the pumping of protons (hydrogen ions) from the chloroplast's stroma and into the lumen, resulting in a proton gradient across the thylakoid membrane that can be used to synthesize ATP using the ATP synthase molecule. Both the ATP and NADPH are used in the Calvin cycle to fix carbon dioxide into triose sugars.

The process starts when light is absorbed by two BChl molecules that lie near the periplasmic side of the membrane. This pair of chlorophyll molecules, often called the "special pair", absorbs photons at 870 nm or 960 nm, depending on the species and, thus, is called P870 (for Rhodobacter sphaeroides) or P960 (for Blastochloris viridis), with P standing for "pigment"). Once P absorbs a photon, it ejects an electron, which is transferred through another molecule of Bchl to the BPh in the L subunit. This initial charge separation yields a positive charge on P and a negative charge on the BPh. This process takes place in 10 picoseconds (10−11 seconds). This reaction occurs at the reaction center of Photosystem II and takes place in cyanobacteria, algae and green plants.