Electron transport chain associates electron carriers (such as NADH and FADH2) and mediating biochemical reactions that produce adenosine triphosphate (ATP), which is a major energy intermediate in living organisms. Only two sources of energy are available to biosynthesize organic molecules and maintain biochemical and kinetic processes in living organisms: oxidation-reduction (redox) reactions and some forms of radiation, such as sunlight (used for photosynthesis). Organisms that use redox reactions to produce ATP are called chemotrophs. Organisms that use sunlight are called phototrophs. Both chemotrophs and phototrophs use electron transport chains to convert energy into ATP. This is achieved through a three-step process:
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- Gradually sap energy from high-energy electrons in a series of individual steps.
- Use that energy to forcibly unbalance the proton concentration across the membrane, creating an electrochemical gradient.
- Use the energy released by the drive to re-balance the proton distribution as a means of producing ATP.
The Electron Transport Chain is also called the ETC. ATP is made by an enzyme called ATP Synthase. The structure of this enzyme and its underlying genetic code is remarkably conserved in all known forms of life.
ATP synthase is powered by a transmembrane electrochemical potential gradient, usually in the form of a proton gradient. The function of the electron transport chain is to produce this gradient. In all living organisms, a series of redox reactions is used to produce a transmembrane electrochemical potential gradient.
Redox reactions are chemical reactions in which electrons are transferred from a donor molecule to an acceptor molecule. The underlying force driving these reactions is the Gibbs free energy of the reactants and products. The Gibbs free energy is the energy available ("free") to do work. Any reaction that decreases the overall Gibbs free energy of a system will proceed spontaneously.
The transfer of electrons from a high-energy molecule (the donor) to a lower-energy molecule (the acceptor) can be spatially separated into a series of intermediate redox reactions. This is an electron transport chain.
The fact that a reaction is thermodynamically possible does not mean that it will actually occur; for example, a mixture of hydrogen gas and oxygen gas does not spontaneously ignite. It is necessary either to supply an activation energy or to lower the intrinsic activation energy of the system, in order to make most biochemical reactions proceed at a useful rate. Living systems use complex macromolecular structures (enzymes) to lower the activation energies of biochemical reactions.It is possible to couple a thermodynamically favorable reaction (a transition from a high-energy state to a lower-energy state) to a thermodynamically unfavorable reaction (such as a separation of charges, or the creation of an osmotic gradient), in such a way that the overall free energy of the system decreases (making it thermodynamically possible), while useful work is done at the same time. Biological macromolecules that catalyze a thermodynamically unfavorable reaction if and only if a thermodynamically favorable reaction occurs simultaneously underlie all known forms of life.
Electron transport chains capture energy in the form of a transmembrane electrochemical potential gradient. This energy can then be harnessed to do useful work. The gradient can be used to transport molecules across membranes. It can be used to do mechanical work, such as rotating bacterial flagella. It can be used to produce ATP high-energy molecules that are necessary for growth.
A small amount of ATP is available from substrate-level phosphorylation (for example, in glycolysis). Some organisms can obtain ATP exclusively by fermentation. In most organisms, however, the majority of ATP is generated by electron transport chains.