"The properties of the transition metals are also uniquely fit for their participation in the electron transport chain, which is crucial to the process of cellular respiration.
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Briefly, the electron transport chain involves the flow of electrons through a respiratory chain.
Electrons pass through three protein complexes that are embedded in the inner mitochondrial membrane:
NADH-Q oxidoreductase (Complex I);
Q-cytochrome c oxidoreductase (Complex III);
and cytochrome c oxidase (Complex IV).
Complex I, a large multi-subunit protein, is the enzyme that catalyzes the transfer of electrons from the reducing agent (electron donor) NADH to coenzyme Q.
The electrons are relayed to cytochrome c at Complex III, and Complex IV transfers the electrons to oxygen, which is thus reduced to water.
Complexes I, III, and IV serve as proton pumps, using the energy from electron transfer to transport protons from the matrix into the intermembrane space.
The complexes utilize the energy given up by the flow of electrons. The inner mitochondrial membrane is impermeable to protons, leading to their accumulation in the intermembrane space.
Like water behind a dam, this build-up of protons stores potential energy. A chemical turbine called ATP synthase then facilitates the flow of protons down their concentration gradient from the inner membrane space to the matrix, using the energy released in the process to create ATP.
Essential to this process is a unique property of the transition metal atoms, namely, their possessing different redox potentials — that is, their ability to accommodate varying numbers of electrons in their outermost shells.
The extent to which the outer shell is full of electrons will determine the atom’s affinity for electrons (with a less full outer shell having a stronger affinity for electrons than one that is fuller). Furthermore, the redox potential (that is, the affinity for electrons) of the transition metals “can be fine-tuned by appropriate choice of ligands to encompass almost the entire biologically significant range of redox potentials.”
This makes it possible to organize a chain of transition metal atoms, each with an increasing redox potential, in order for electrons to be drawn from one metal atom to the next in a series of discrete ordered steps.
No other atoms, besides the transition metal atoms, have the properties needed to undertake this task. It is also noteworthy that no alternative mechanism has ever been employed in any known lifeform to generate the large quantities of ATP needed to sustain life."
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