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The coupling of metabolic and light energy to the performance of transmembrane work through the intermediary of electroosmotic gradients. Processes include synthesis of adenosine triphosphate (ATP) by oxidative phosphorylation or by photosynthesis, production of heat, accumulation of small molecules by active transport, movement of bacterial flagella, uptake of deoxyribonucleic acid (DNA) during bacterial conjugation, genetic transformation and bacteriophage infection, and insertion or secretion of proteins into or through membranes.
Mitochondria are the powerhouses of the eukaryotic cell and the site of synthesis of ATP by oxidative phosphorylation. In the oxidation portion of ATP synthesis, reductants, such as reduced nicotinamide adenine dinucleotide (NADH) and succinate, are generated during metabolism of carbohydrates, lipids, and protein. These compounds are oxidized through the series of redox reactions performed by membrane-bound complexes, called electron transport or respiratory chains. See Adenosine triphosphate (ATP), Mitochondria
Bacteria do not contain mitochondria, but many of the functions of the mitochondrial membrane are carried out by the bacterial cytoplasmic membrane. Many bacteria also use respiratory chains. This resemblance to mitochondria is more than chance. The evidence, although mostly circumstantial, suggests that mitochondria, chloroplasts, and perhaps other eukaryotic organelles were originally free-living bacteria. These bacteria and larger proto-eukaryotic cells became mutually symbiotic, so that neither was complete or viable without the other. The animal and plant kingdoms arose from these endosymbiotic events.
Photosynthesis is the conversion of light energy into chemical energy. Overall photosynthetic bacteria and the chloroplasts of eukaryotic plants capture sunlight or other light and use that energy to generate both ATP and a reductant for use in biosynthesis. The mechanism of photophosphorylation, that is, the use of light energy to drive the phosphorylation of adenosine diphosphate (ADP) to ATP, resembles that of oxidative phosphorylation. See Photosynthesis
Oxidative phosphorylation and photophosphorylation are but specialized examples of chemiosmotic energy coupling. Among the forms of useful energy are chemical energy, such as that derived from fossil fuels, and light energy in the case of solar cells. Electricity is transmitted to motors, which couple electrical energy to the performance of work. Bacterial cells, mitochondria, and chloroplasts have protonic generators and protonic motors. Respiratory and photosynthetic electron transport chains are generators of proton currents' proton motive forces, which then drive the various motors of the cell or organelle. When the H+-translocating ATPase is “plugged in,” the proton current drives phosphorylation. There are other motors present in the cell. Most membranes contain specific transport systems for small molecules, such as ions, sugars, and amino acids. Many of these transport systems are protonic; that is, they use the energy of the proton motive force to drive the accumulation or extrusion of their substrate.