Light-harvesting complex
A light-harvesting complex consists of a number of chromophores which are complex subunit proteins that may be part of a larger super complex of a photosystem, the functional unit in photosynthesis. It is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. The light which is captured by the chromophores is capable of exciting molecules from their ground state to a higher energy state, known as the excited state. This excited state does not last very long and is known to be short-lived. Light-harvesting complexes are found in a wide variety among the different photosynthetic species, with no homology among the major groups. The complexes consist of proteins and photosynthetic pigments and surround a photosynthetic reaction center to focus energy, attained from photons absorbed by the pigment, toward the reaction center using Förster resonance energy transfer. Function Photosynthesis is a process where light is absorbed or harvested by pigment protein complexes which are able to turn sunlight into energy. Absorption of a photon by a molecule takes place when pigment protein complexes harvest sunlight leading to electronic excitation delivered to the reaction centre where the process of charge separation can take place. when the energy of the captured photon matches that of an electronic transition. The fate of such excitation can be a return to the ground state or another electronic state of the same molecule. When the excited molecule has a nearby neighbour molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another. This process is called resonance energy transfer, and the rate depends strongly on the distance between the energy donor and energy acceptor molecules. Before an excited photon can transition back to ground state, the energy needs to be harvested. This excitation is transferred among chromophores where it is delivered to the reaction centre. Light-harvesting complexes have their pigments specifically positioned to optimize these rates. In purple bacteria Purple bacteria is a type of photosynthetic organism with a light harvesting complex consisting of two pigment protein complexes referred to as LH1 and LH2. Within the photosynthetic membrane, these two complexes differ in terms of their arrangement. The LH1 complexes surrounds the reaction centre, while the LH2 complexes are arranged around the LH1 complexes and the reaction centre in a peripheral fashion. Purple bacteria use bacteriochlorophyll and carotenoids to gather light energy. These proteins are arranged in a ring-like fashion creating a cylinder that spans the membrane. In green bacteria The main light harvesting complex in Green bacteria is known as the chlorosome. The chlorosome is equipped with rod-like BChl c aggregates with protein embedded lipids surrounding it. Chlorosomes are found outside of the membrane which covers the reaction centre. Green sulphur bacteria and some Chloroflexia use ellipsoidal complexes known as the chlorosome to capture light. Their form of bacteriochlorophyll is green. In cyanobacteria and plants Chlorophylls and carotenoids are important in light-harvesting complexes present in plants. Chlorophyll