The general scheme for finding electrons for CO2 fixationis to open up Photosystem I and remove the electrons, eventuallygettingthem to NADP which can donate them to the dark reaction. In bacterialphotosynthesisthe process may be quite complex. The electrons are removed fromPhotosystemI at the level of a cytochrome, then moved through an energy-consumingreverseelectron transport system to an iron-sulfur protein, ferredoxin,which reduces NADP to NADPH2. The electrons that replenishPhotosystemI come from the oxidation of an external photosynthetic electrondonor,which may be H2S, other sulfur compounds, H2, orcertain organic compounds.
All phototrophic bacteria are capable of performing cyclicphotophosphorylationas described above and in Figure 16 and below in Figure 18. Thisuniversalmechanism of cyclic photophosphorylation is referred to as PhotosystemI. Bacterial photosynthesis uses only Photosystem I (PSI), but themore evolved cyanobacteria, as well as algae and plants, have anadditionallight-harvesting system called Photosystem II (PSII). Photosystem IIis used to reduce Photosystem I when electrons are withdrawn from PSIforCO2 fixation. PSII transfers electrons from H2Oandproduces O2, as shown in Figure 20.
Photosynthesis is the conversion of light energy intochemicalenergy that can be used in the formation of cellular material from CO2.Photosynthesis is a type of metabolism separable into a catabolic andanaboliccomponent. The catabolic component of photosynthesis is the lightreaction,wherein light energy is transformed into electrical energy, thenchemicalenergy. The anabolic component involves the fixation of CO2and its use as a carbon source for growth, usually called the darkreaction.In photosynthetic procaryotes there are two types of photosynthesis andtwo types of CO2 fixation.
The Light Reactions depend upon the presence ofchlorophyll,the primary light-harvesting pigment in the membrane ofphotosyntheticorganisms. Absorption of a quantum of light by a chlorophyll moleculecausesthe displacement of an electron at the reaction center. The displacedelectronis an energy source that is moved through a membrane photosyntheticelectrontransport system, being successively passed from an iron-sulfur protein(X ) to a quinone to a cytochrome and back to chlorophyll (Figure 16below).As the electron is transported, a proton motive force is established onthe membrane, and ATP is synthesized by an ATPase enzyme. This mannerofconverting light energy into chemical energy is called cyclicphotophosphorylation.
T1 - FTIR characterization of the primary electron donor in double mutants combining the heterodimer HL(M202) with the LH(L131), HF(L168), FH(M197), or LH(M160) mutations
Diagrammatic arrangement of protein complexes and diffusible components in relation to a thylakoid. PSII, photosystem II; D1/D2, the two main subunits of the reaction centre of photosystem II; O, subunit PsbO, which protects the manganese cluster of the OEC; , , cytochromes 6 and , respectively, of the cytochrome 6 complex; IV, subunit IV of the cytochrome 6 complex; PC, plastocyanin; 6, cytochrome 6; PSI, photosystem I; A/B, the two main subunits of photosystem I (PsaA, PsaB); C, D, E, F, subunits of photosysem I; Fd, ferredoxin; FNR, ferredoxin–NADP oxidoreductase; F0, F1, intrinsic and extrinsic components of ATP synthase; III, multiple copies of subunit III involved in proton translocation; β, γ, subunits of F1.
In plants and cyanobacteria the ‘light reactions’ of photosynthesis use light energy to generate reducing power in the form of nicotinamide–adenine dinucleotide phosphate (NADPH) and metabolic energy as adenosine triphosphate (ATP), which are subsequently used in the ’dark reactions’ to drive the reduction of carbon dioxide and synthesis of carbohydrate. The light reactions, as conventionally defined, are actually a complex series of processes which include not only reactions in which light participates directly, but also closely associated reactions that indirectly depend on light.
In plant photosynthesis, the photosynthetic electron donor is H2O,which is lysed by photosystem II, resulting in the production of O2.Electrons removed from H2O travel through Photosystem II toPhotosystem I as described in Figure 20 above. Electrons removed fromPhotosystemI reduce ferredoxin directly. Ferredoxin, in turn, passes the electronsto NADP.
While photosynthesis is highly-evolved in the procaryotes, itapparentlyoriginated in the Bacteria and did not spread or evolve in Archaea.But the Archaea, in keeping with their unique ways, are not withoutrepresentativeswhich can conduct a type of light-driven photophosphorylation. The extremehalophiles, archaea that live in natural environments such as theDeadSea and the Great Salt Lake at very high salt concentration (as high as25 percent NaCl) adapt to the high-salt environment by the developmentof "purple membrane", actually patches of light-harvestingpigmentin the plasma membrane. The pigment is a type of rhodopsin called bacteriorhodopsinwhich reacts with light in a way that forms a proton gradient on themembraneallowing the synthesis of ATP. This is the only example in nature of nonphotosynthetic photophosphorylation. These organisms areheterotrophsthat normally respire by aerobic means. The high concentration of NaClin their environment limits the availability of O2 forrespirationso they are able to supplement their ATP-producing capacity byconvertinglight energy into ATP using bacteriorhodopsin.