22.3 Electron Transport Chains

You now know that ATP is generated by using the energy released when hydrogen ions flow down their gradient. How is the hydrogen ion gradient established and maintained? The answer is by an electron transport chain, in which the energy from electrons is used to pump hydrogen ions against their gradient, establishing a region of high concentration. This gradient is a source of potential energy, which is then used to produce ATP.

Redox Reactions

Electron transport chains rely on the transfer of electrons. Electron transfer involves oxidation and reduction reactions, which occur at the same time. An oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to another compound is a reduction reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation reduction reactions, or redox reactions.

 

Electron transfer followed by ionic bonding. In this example of a redox reaction, sodium is oxidized because it loses an electron, and fluorine is reduced because it gains an electron. (NaF by Wdcf is used under a Creative Commons Attribution-ShareAlike license).

Many molecules can exist in a reduced form or an oxidized form. The reduced form has more free energy than the oxidized form.

 

Oxidized and reduced forms of N A D H
The electron carrier nicotinamide adenine dinucleotide (NAD) in its oxidized and reduced forms. The reduced form has more energy. NADH is critically important in aerobic respiration. It shuttles electrons from glucose and other food molecules to the protein complexes of the electron transport chain. (NAD by Melissa Hardy is in the public domain).

Electrons and Energy

The removal of an electron from a molecule (oxidizing it), results in a decrease in potential energy in the oxidized compound. The electron is shifted to a second compound, reducing the second compound. The shift of an electron from one compound to another removes some potential energy from the first compound (the compound being oxidized) and increases the potential energy of the second compound (the compound being reduced). The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of high-energy electrons allows the cell to transfer and use energy in an incremental fashion—in small packages rather than in a single, destructive burst.

Electron Transport Chains

Electron transport chains involve electrons being passed from one molecule to another. The major components are protein complexes embedded in a membrane (inner mitochondrial membrane, chloroplast thylakoid membrane, or prokaryotic plasma membrane). Electron transport is a series of redox reactions that resembles a relay race or bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain. The flow of electrons is exergonic. Some of the energy released can be used to do work, specifically pumping hydrogen ions against their gradient. This is what maintains the gradient that drives ATP production.

 

General illustration of an electron transport chain
As electrons are passed from protein complex to protein complex, energy is released. This exergonic process is coupled to the endergonic process of actively transporting hydrogen ions across a membrane. The gradient of hydrogen ions is a source of potential energy that can be used to do work. The exergonic process of hydrogen ions diffusing down their gradient is coupled to the endergonic process of ATP synthesis. (Electron Transport Chain by Melissa Hardy is used under a Creative Commons Attribution-NonCommercial license. Created with BioRender.com).
Text adapted from OpenStax Biology 2e and used under a Creative Commons Attribution License 4.0.
Access for free at https://openstax.org/books/biology-2e/pages/1-introduction
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