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The Electron Transport Chain of Cellular Respiration

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The electron transport chain of cellular respiration is a series of molecules embedded within the inner mitochondrial membrane of a eukaryotic cell's mitochondria. Most of the molecules (electron carriers) are proteins with a tightly bound prosthetic group (a nonprotein component essential for enzymatic function). The molecules exist in multiprotein complexes numbered I to IV. The electron transport chain carries out a series of redox reactions that creates an electrochemical gradient, which then powers the production of ATP by chemiosmosis. A redox reaction occurs as the electron carriers transition from oxidized to reduced states by accepting electrons and then donating them to their neighbors. It is important to note that electronegativity (the measure of a molecule's attraction to electrons) increases as electrons progress through the chain. In other words, the first electron carrier is the least electronegative, and the last electron carrier is the most electronegative.


The chain begins at complex I, when the molecule NADH (nicotinamide adenine dinucleotide) donates it electrons to the first molecule of the chain, FMN. NADH transitions from its reduced state to its oxidized state, NAD+. FMN is a flavoprotein, named so because of its prosthetic group known as flavin mononucleotide. By accepting electrons from NADH, FMN become reduced. When it donates its electrons to the next electron carrier, FMN becomes oxidized. This a redox reaction.

The second electron carrier is an iron-sulfur protein (FeS) of complex I. FeS is a protein with an iron atom and a sulfur atom tightly bound to it. The iron sulfur protein passes its electrons to the third electron carrier, ubiquinone, also known as coenzyme Q. Ubiquinone is unique in that is the only carrier molecule that is not a protein; it is a small, hydrophobic molecule. It isn't part of a complex; CoQ is mobile. Ubiquinone then passes its electrons to complex III (there is a complex II and it will be discussed later). The first molecule of complex III is cytochrome b (cyt b). The rest of the carriers are primarily cytochromes, proteins with a bound heme group (an iron atom that accepts and donates electrons). 

After cyt b is cyt c1, and then cyt c. Cytochrome c, similar to CoQ, is not part of a complex; it is a mobile electron carrier. When released from the mitochondria, cyt c is a cell death factor in apoptosis. Cytochrome c then transfers its electrons to complex IV, consisting of cyt a and cyt a3. Cytochrome a3 is the last cytochrome in the chain. It transfers its electrons to oxygen (O2), the final electron acceptor. Oxygen is the most electronegative molecule in the transport chain. Once it is reduced, it picks up a pair of hydrogen ions from the surrounding solution and forms water (H2O).

An alternative route for FADH2 involves complex II rather than complex I. FADH2 is the other product of the citric acid cycle. It donates its electrons at a lower energy level than NADH does. When FADH2 donates its electrons to complex II (which consists of an iron-sulfur protein), it becomes NAD. From there, the electrons continue down the same chain as NADH: ubiquinone, complex II, cyt c, complex IV and O2.

Now, an electrochemical gradient has been created. There is a difference in the H+ concentration between opposite sides of the inner membrane. This gradient drives the next process in cellular respiration: chemiosmosis.

                                                                                                                    ________

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(1) NeuroExplorer
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