In eukaryotes, many copies of the electron transport chain molecules are found in the inner mitochondrial membrane. In prokaryotes, the electron transport chain components are found in the plasma membrane. As the electrons travel through the chain, they go from a higher to a lower energy level. Energy is released in these “downhill” electron transfers, and several of the protein complexes use the released energy to pump protons from the mitochondrial matrix to the intermembrane space, forming a proton gradient. You can see a representation of this change in energy level below.
All the components of the electron transport chain are embedded in or attached to the inner mitochondrial membrane. In the matrix, NADH deposits electrons at Complex I, turning into NAD+ and releasing a proton into the matrix. FADH2 in the matrix deposits electrons at Complex II, turning into FAD and releasing 2 H+. The electrons from Complexes I and II are passed to the small mobile carrier Q. Q transports the electrons to Complex III, which then passes them to Cytochrome C. Cytochrome C passes the electrons to Complex IV, which then passes them to oxygen in the matrix, forming water. It takes two electrons, 1/2 O2, and 2 H+ to form one water molecule. Complexes I, III, and IV use energy released as electrons move from a higher to a lower energy level to pump protons out of the matrix and into the intermembrane space, generating a proton gradient.
All of the electrons that enter the transport chain come from NADH and FADH2 molecules produced during earlier stages of cellular respiration: glycolysis, pyruvate oxidation, and the citric acid cycle.
NADH is very good at donating electrons in redox reactions (that is, its electrons are at a high energy level), so it can transfer its electrons directly to complex I, turning back into NAD+. As electrons move through complex I in a series of redox reactions, energy is released, and the complex uses this energy to pump protons from the matrix into the intermembrane space.
FADH2 not as good at donating electrons as NADH (that is, its electrons are at a lower energy level), so it cannot transfer its electrons to complex I. Instead, it feeds them into the transport chain through complex II, which does not pump protons across the membrane. Because of this, a FADH2 molecule causes fewer protons to be pumped (and contributes less to the proton gradient) than an NADH molecule.
Complex I. NADH transfers its electrons to complex I. Complex I is quite large, and the part of it that receives the electrons is a flavoprotein, meaning a protein with an attached organic molecule called flavin mononucleotide (FMN). FMN is a prosthetic group, a non-protein molecule tightly bound to a protein and required for its activity, and it’s FMN that actually accepts electrons from NADH. FMN passes the electrons to another protein inside complex I, one that has iron and sulfur bound to it (called an Fe-S protein), which in turns transfers the electrons to a small, mobile carrier called ubiquinone (Q in the diagram above).
Complex II. Like NADH, FADH2 deposits its electrons in the electron transport chain, but it does so via complex II, bypassing complex I entirely. As a matter of fact, FADH2 is a part of complex II, as is the enzyme that reduces it during the citric acid cycle; unlike the other enzymes of the cycle, it’s embedded in the inner mitochondrial membrane. FADH2 transfers its electrons to iron-sulfur proteins within complex II, which then pass the electrons to ubiquinone (Q), the same mobile carrier that collects electrons from complex I.
Beyond the first two complexes, electrons from NADH and FADH2 travel exactly the same route. Both complex I and complex II pass their electrons to a small, mobile electron carrier called ubiquinone (Q), which is reduced to form QH2 and travels through the membrane, delivering the electrons to complex III. As electrons move through complex III, more hydrogen ions are pumped across the membrane, and the electrons are ultimately delivered to another mobile carrier called cytochrome C (cyt C). Cyt C carries the electrons to complex IV, where a final batch of hydrogen ions is pumped across the membrane. Complex IV passes the electrons to O2, which splits into two oxygen atoms and accepts protons from the matrix to form water. Four electrons are required to reduce each molecule of O2, and two water molecules are formed in the process.
Complex III. Like complex I, complex III includes an iron-sulfur (Fe-S) protein, but it also contains two proteins of another type, known as cytochromes. Cytochromes are a family of related proteins that have heme prosthetic groups containing iron ions. In complex III, electrons are passed from one cytochrome to an iron-sulfur protein to a second cytochrome, then finally transferred out of the complex to a mobile electron carrier (cytochrome C). Like complex I, complex III pumps protons from the matrix into the intermembrane space, contributing to the hydrogen ion concentration gradient.
Complex IV. From complex III, cytochrome C delivers electrons to the last complex of the electron transport chain, complex IV. There, the electrons are passed through two more cytochromes, the second of which has a very interesting job: with the help of a nearby copper ion, it transfers electrons to O2 splitting oxygen to form two molecules of water. The protons used to form water come from the matrix, contributing to the hydrogen ion gradient, and complex IV also pumps protons from the matrix to the intermembrane space.
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• NADH and FADH2 pass their electrons to the electron transport chain, turning back into NAD+ and FAD. This is important because the oxidized forms of these electron carriers are used in glycolysis and the citric acid cycle and must be available to keep these processes running.
• The transport chain builds a proton gradient across the inner mitochondrial membrane, with a higher concentration of hydrogen ions in the intermembrane space and a lower concentration in the matrix. This gradient represents a stored form of energy and it can be used to make ATP.
• NADH is better at donating electrons in redox reactions than FADH2, meaning that FADH2 molecules cause fewer protons to be pumped than NADH molecules.
Cytochromes are a family of related proteins that have heme prosthetic groups containing iron ions. Electrons are passed from one cytochrome to an iron-sulfur protein to a second cytochrome, then finally transferred out of the complex to a mobile electron carrier cytochrome C, which can only carry one electron at a time.
Electron transport chain: A series of protein complexes embedded in the inner mitochondrial membrane that accept electrons from electron carriers in order to pump protons into the intermembrane space.
Electron carriers: Includes NADH and FADH2, which pass the electrons acquired during early stages of cellular respiration to the proteins in the electron transport chain.
Electrochemical gradient: The combination of a concentration gradient and electrical gradient due to an unequal distribution of ions across a membrane.