The electron transfer chain (also called the electron transport chain, or simply electron transport), is a series of protein complexers and lipid messengers spanning the inner mitochondrialmembrane that accepts electrons from electron donors such as NADH or succinate, shuttles these electrons from within the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space, creating an electrical and chemical gradient, and through the proton driven chemistry of the ATP synthase (aka the F0F1 particle), generates adenosine triphosphate (ATP).
There are five complexes normally associated with the electron transfer chain.
Complex V - ATP synthase, also known as the F0F1 particle.
All of these are proteolipid complexes, with the first four containing either flavins, iron-sulfur clusters, copper centers, or heme moieties. Complexes I, III, and IV are proton pumps. Complex II is part of the Krebs cycle and does not pump protons, and Complex V uses the electrochemical potential generated to create ATP. Complex IV is the terminus of the electron transfer chain, where oxygen from the lungs is reduced by electrons and hydrogen protons (provided by NADH and/or FADH2) to make water.
Cytochrome c is also an essential part of the electron transfer chain. It is a soluble protein loosely associated with the inner mitochondrial membrane, and transfers electrons between Complexes III and IV.
The chemiosmotic coupling hypothesis, as proposed by Nobel Prize in Chemistry winner Peter D. Mitchell explains that the electron transport chain and oxidative phosphorylation are coupled by a proton gradient across the inner mitochondrial membrane.
Note that electrons can enter the chain at three levels: at the level of a dehydrogenase, at the level of the quinone pool, or at the level of a mobile cytochromeelectron carrier.
Photosynthetic electron transport chains are discussed in greater detail in the articles Photophosphorylation, Photosynthesis, Photosynthetic reaction center and Light-dependent reaction.
In the course of the evolution of photosynthesis and the respiratorychain, proton gradients developed.
Among the similarities of photosynthesis and the respiratorychain is the use of the same proteins: ferredoxin, cytochromec, cytochrome b, and several others.
The respiratorychain is thus a variation of photosynthesis.
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