Electron transport chain- Definition, Components, Steps, FAQs

The electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. The ETC is the final stage of cellular respiration, where most of the energy (ATP) is produced from the oxidation of NADH and FADH2, which are the products of previous cycles such as the citric acid cycle, fatty acid oxidation, and amino acid metabolism. The ETC creates an electrochemical gradient of protons across the membrane, which drives the synthesis of ATP by a process called oxidative phosphorylation. The enzyme that catalyzes this process is called ATP synthase, which uses the energy of the proton gradient to convert ADP and phosphate into ATP. The ETC also consumes molecular oxygen (O2) as the final electron acceptor, which is reduced to water (H2O) in the process. This is why aerobic respiration requires oxygen to produce energy. In anaerobic respiration, other molecules such as sulfate or nitrate can act as the final electron acceptors. In eukaryotes, the ETC is located in the inner mitochondrial membrane, which is folded into cristae to increase the surface area for electron transfer. In prokaryotes, the ETC is located in the plasma membrane or in some cases, in specialized membranes called thylakoids. The ETC consists of four major complexes (I-IV) and two mobile electron carriers (coenzyme Q and cytochrome c). Each complex contains multiple protein subunits and cofactors that facilitate the transfer of electrons. The complexes are named according to their order of function in the chain. The ETC can be summarized by the following equation:

NADH + 1/2 O2 + H+ + ADP + Pi → NAD+ + ATP + H2O

Source: Jain JL, Jain S, and Jain N (2005). Fundamentals of Biochemistry. S. Chand and Company. Nelson DL and Cox MM. Lehninger Principles of Biochemistry. Fourth Edition.

The electron transport chain is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions and couple this electron transfer with the transfer of protons across a membrane. The electron transport chain is the final stage of cellular respiration, where most of the ATP or energy is produced from glucose.

The location of the electron transport chain depends on the type of organism and the mode of respiration. In eukaryotic organisms, such as animals, plants, fungi, and protists, the electron transport chain is located within the mitochondria, which are membrane-bound organelles that produce energy for the cell. The proteins of the electron transport chain span the inner mitochondrial membrane, which separates the matrix and the intermembrane space of the mitochondria. The inner mitochondrial membrane is impermeable to various ions and contains uncharged molecules, electron transport chain complexes, and ATP synthase enzymes. The electron transport chain creates an electrochemical gradient of protons across the inner mitochondrial membrane, which drives the synthesis of ATP by ATP synthase.

In photosynthetic eukaryotes, such as plants and algae, there is another electron transport chain that is located on the thylakoid membrane of the chloroplasts. The chloroplasts are membrane-bound organelles that capture light energy and convert it into chemical energy. The thylakoid membrane contains photosystems, which are complexes of proteins and pigments that absorb light and transfer electrons to electron carriers. The electron transport chain in the thylakoid membrane uses light energy to pump protons across the membrane, creating a proton gradient that drives the synthesis of ATP by ATP synthase.

In prokaryotic organisms, such as bacteria and archaea, the electron transport chain is located on the plasma membrane, which is the outermost layer of the cell that regulates the movement of substances in and out of the cell. The plasma membrane contains various proteins and lipids that are involved in different functions, such as transport, signaling, and energy production. The electron transport chain in prokaryotes transfers electrons from different donors, such as organic compounds or inorganic molecules, to different acceptors, such as oxygen or other molecules. The electron transport chain creates a proton gradient across the plasma membrane, which drives the synthesis of ATP by ATP synthase.

The location of the electron transport chain is important because it determines how efficiently the cell can produce energy from different sources and how it can adapt to different environmental conditions. The location also affects how the cell interacts with other cells and molecules in its surroundings.

Electrons in the chain are transferred from substrate to oxygen through a series of electron carriers. There are about 15 different chemical groups that accept or transfer electrons through the electron chain.

The electron transport chain consists of four multisubunit protein complexes located in the inner mitochondrial membrane. The proteins in each complex oxidize NADH and/or FADH2 and carry the electrons to the next acceptors.

The four complexes are:

• Complex I (NADH-Coenzyme Q Oxidoreductase): It contains flavin mononucleotide (FMN), which accepts two electrons and a proton from NADH to become FMNH2; also contains iron-sulfur clusters (Fe-S), which assist in the transfer of the electrons to coenzyme Q (CoQ). The transfer of electrons is catalyzed by NADH dehydrogenase. The complex also pumps four protons across the membrane, creating a proton gradient.
• Complex II (Succinate-Coenzyme Q Oxidoreductase): It contains succinate dehydrogenase, flavin adenine dinucleotide (FAD), and Fe-S clusters. The enzyme complex catalyzes the transfer of electrons from succinate to CoQ via FAD and Fe-S clusters. This complex runs parallel to Complex I, but does not pump any protons across the membrane.
• Complex III (Cytochrome bc1 Oxidoreductase): It contains cytochrome b, cytochrome c1, and a specific Fe-S cluster. The enzyme complex catalyzes the transfer of two electrons from reduced CoQH2 to two molecules of cytochrome c. Meanwhile, the protons from CoQH2 are released across the membrane, creating a proton gradient. Cytochromes are red or brown colored proteins that contain a heme group that carries the electrons. Each cytochrome transfers one electron at a time.
• Complex IV (Cytochrome c Oxidase): It contains cytochrome a and cytochrome a3, which form a binuclear center that transfers electrons to molecular oxygen (O2), forming water. This is the final step of the electron transport chain, where oxygen is the final electron acceptor. The complex also pumps four protons across the membrane, creating a proton gradient.

In addition to these complexes, there are two mobile electron carriers that shuttle electrons between them:

• Ubiquinone (Coenzyme Q or CoQ): It is a lipid-soluble molecule that can move within the inner mitochondrial membrane. It accepts electrons from Complex I and II and transfers them to Complex III. It can carry two electrons at a time, forming ubiquinol (CoQH2) when reduced.
• Cytochrome c: It is a water-soluble protein that can move in the intermembrane space. It accepts electrons from Complex III and transfers them to Complex IV. It can carry one electron at a time, changing its iron center from Fe3+ to Fe2+ when reduced.

The following table summarizes the components and electron carriers of the electron transport chain:

The electron transport chain consists of a series of oxidation-reduction reactions that lead to the release of energy. A summary of the reactions in the electron transport chain is:

NADH + 1/2O2 + H+ + ADP + Pi → NAD+ + ATP + H2O

This equation shows that NADH is oxidized to NAD+ by transferring two electrons and one proton to oxygen, which is reduced to water. The energy released by this reaction is used to synthesize ATP from ADP and inorganic phosphate (Pi).

Similarly, FADH2 is another electron donor that is oxidized to FAD by transferring two electrons and two protons to oxygen, forming water and ATP:

FADH2 + 1/2O2 + 2H+ + ADP + Pi → FAD + ATP + 2H2O

The electron transport chain can also be written in terms of the complexes and electron carriers involved:

Complex I: NADH + H+ + CoQ → NAD+ + CoQH2

Complex II: Succinate + FAD + CoQ → Fumarate + FADH2 + CoQH2

Complex III: CoQH2 + 2cyt c (Fe3+) → CoQ + 2cyt c (Fe2+) + 4H+

Complex IV: 4cyt c (Fe2+) + O2 → 4cyt c (Fe3+) + 2H2O

These equations show how electrons are transferred from NADH and FADH2 to coenzyme Q (CoQ), then to cytochrome c (cyt c), and finally to oxygen. Protons are also pumped across the inner mitochondrial membrane, creating a proton gradient that drives ATP synthesis.

The electron transport chain consists of four large protein complexes that are embedded in the inner mitochondrial membrane, together called the respiratory chain or the electron-transport chain. These complexes are named as Complex I, II, III, and IV and they catalyze the transfer of electrons from electron donors (NADH and FADH2) to electron acceptors (O2) via redox reactions.

• Complex I (Mitochondrial complex I) is composed of NADH dehydrogenase and iron-sulfur centers that catalyze the transfer of two electrons from NADH to ubiquinone (UQ), a mobile electron carrier. At the same time, the complex pumps four protons (H+) across the membrane, creating a proton gradient that drives ATP synthesis.
• Complex II (Mitochondrial complex II) is composed of succinate dehydrogenase, flavin adenine dinucleotide (FAD), and iron-sulfur centers. The complex catalyzes the transfer of electrons from succinate, a product of the citric acid cycle, to ubiquinone via FAD and iron-sulfur centers. This complex does not pump protons across the membrane and thus does not contribute to the proton gradient.
• Complex III (Mitochondrial complex III) is composed of cytochrome b, cytochrome c1, and a specific iron-sulfur center. The complex catalyzes the transfer of two electrons from reduced ubiquinone (UQH2) to two molecules of cytochrome c, another mobile electron carrier. Cytochrome c is a protein that contains a heme group that can accept and donate one electron at a time. The complex also pumps four protons from the matrix to the intermembrane space for each pair of electrons transferred.
• Complex IV (Mitochondrial complex IV) is composed of cytochrome a and cytochrome a3, which together form cytochrome oxidase. This is the final complex of the chain and it catalyzes the transfer of four electrons from four molecules of cytochrome c to molecular oxygen (O2), forming water as a byproduct. The complex also pumps two protons from the matrix to the intermembrane space for each pair of electrons transferred.

The electron transport chain is coupled with oxidative phosphorylation, which is the process of synthesizing ATP from ADP and inorganic phosphate (Pi) using the energy of the proton gradient. The proton gradient is maintained by the action of the complexes that pump protons from the mitochondrial matrix to the intermembrane space. The protons then flow back to the matrix through a channel in a large enzyme called ATP synthase, which uses the energy of the proton flow to catalyze the formation of ATP.

The electron transport chain and oxidative phosphorylation are responsible for most of the ATP production in aerobic respiration. The theoretical maximum yield of ATP per glucose molecule is 38 ATPs, but in reality, it is lower due to leakages and inefficiencies in the system. The actual yield varies depending on the type of cells and organisms.

The electron transport chain involves a series of redox reactions that transfer electrons from donors such as NADH and FADH2 to acceptors such as oxygen, while releasing energy that is used to synthesize ATP. The electron transport chain can be divided into four main steps:

Step 1: Transfer of electrons from NADH to Ubiquinone (UQ)

NADH is produced in various metabolic pathways such as glycolysis, the citric acid cycle, and fatty acid oxidation. NADH transfers its electrons to a flavin mononucleotide (FMN) molecule, which is part of complex I (NADH dehydrogenase) in the inner mitochondrial membrane. FMN then passes the electrons to a series of iron-sulfur (Fe-S) centers, which have different affinities for electrons. The Fe-S centers transfer the electrons to a lipid-soluble molecule called ubiquinone (UQ) or coenzyme Q, which can diffuse freely in the membrane. As a result of this electron transfer, NADH is oxidized to NAD+ and UQ is reduced to UQH2. Complex I also pumps four protons (H+) from the mitochondrial matrix to the intermembrane space, creating a proton gradient.

Step 2: Transfer of electrons from FADH2 to UQ

FADH2 is another electron donor that is produced in the citric acid cycle and fatty acid oxidation. FADH2 transfers its electrons to a flavin adenine dinucleotide (FAD) molecule, which is part of complex II (succinate dehydrogenase) in the inner mitochondrial membrane. FAD then passes the electrons to another series of Fe-S centers, which eventually transfer them to UQ, forming UQH2. Unlike complex I, complex II does not pump any protons across the membrane.

Step 3: Transfer of electrons from UQH2 to cytochrome c

UQH2 is a mobile electron carrier that can move between complexes in the membrane. UQH2 transfers its electrons to complex III (cytochrome bc1 complex), which consists of cytochrome b, cytochrome c1, and an Fe-S center. Complex III uses a mechanism called the Q cycle to transfer electrons from UQH2 to another mobile electron carrier called cytochrome c, which is a small protein attached to the outer surface of the inner mitochondrial membrane. Cytochrome c contains a heme group that can accept and donate one electron at a time. For each UQH2 oxidized, two cytochrome c molecules are reduced. Complex III also pumps four protons from the matrix to the intermembrane space for each pair of electrons transferred.

Step 4: Transfer of electrons from cytochrome c to oxygen

Cytochrome c carries its electrons to complex IV (cytochrome c oxidase), which consists of cytochrome a, cytochrome a3, and two copper centers. Complex IV catalyzes the final step of the electron transport chain, in which electrons are transferred from cytochrome c to molecular oxygen (O2), forming water (H2O). For each pair of electrons transferred, two molecules of water are produced. Complex IV also pumps two protons from the matrix to the intermembrane space for each pair of electrons transferred.

The overall result of these steps is that electrons are passed from high-energy donors (NADH and FADH2) to low-energy acceptors (O2 and H2O), releasing energy that is used to create a proton gradient across the inner mitochondrial membrane. The proton gradient drives the synthesis of ATP by a process called chemiosmosis, which involves the movement of protons back into the matrix through a protein complex called ATP synthase. ATP synthase uses the energy of the proton gradient to phosphorylate ADP into ATP, which is then used by the cell for various metabolic processes.

The electron transport chain produces water and ATP as end products. Other intermediate compounds of the citric acid cycle can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. The electron transport chain also produces NAD+, FAD, and protons, which end up outside the mitochondrial matrix. The protons are pumped across the cristal membrane using the free energy of electron transport. The mechanism by which ATP is formed in the electron transport chain is called chemiosmotic phosphorylation.

The number of ATP molecules produced by the electron transport chain depends on the type of electron donor and the mode of transport. For each NADH molecule that enters the chain, about 2.5 ATP molecules are generated. For each FADH2 molecule that enters the chain, about 1.5 ATP molecules are generated. This is because FADH2 bypasses complex I and enters at complex II, which does not pump protons across the membrane. Therefore, less proton gradient is created and less ATP is synthesized by FADH2 oxidation.

The total number of ATP molecules produced by the electron transport chain per glucose molecule oxidized by aerobic respiration is about 30-32 ATPs. This accounts for most of the ATP generated by cellular respiration. However, this number may vary depending on the efficiency of transport systems and the use of proton gradient for other purposes besides ATP synthesis.

The end products of electron transport are summarized in the table below:

Here are some common questions and answers about the electron transport chain that might help you understand the topic better.

Q: What is the purpose of the electron transport chain?

A: The purpose of the electron transport chain is to produce ATP, which is the main energy currency of the cell. The electron transport chain converts the energy stored in NADH and FADH2, which are generated from other metabolic pathways, into a proton gradient across the inner mitochondrial membrane. This proton gradient drives the synthesis of ATP by a protein complex called ATP synthase.

Q: What are the four complexes of the electron transport chain?

A: The four complexes of the electron transport chain are:

• Complex I: This complex transfers electrons from NADH to ubiquinone (CoQ) and pumps four protons across the membrane.
• Complex II: This complex transfers electrons from succinate or other donors to CoQ but does not pump protons.
• Complex III: This complex transfers electrons from CoQH2 to cytochrome c and pumps four protons across the membrane.
• Complex IV: This complex transfers electrons from cytochrome c to oxygen, forming water, and pumps two protons across the membrane.

Q: What is the final electron acceptor of the electron transport chain?

A: The final electron acceptor of the electron transport chain is oxygen. Oxygen is reduced to water by accepting four electrons and four protons at complex IV. Oxygen is essential for aerobic respiration, which produces more ATP than anaerobic respiration.

Q: How many ATPs are produced by the electron transport chain?

A: The exact number of ATPs produced by the electron transport chain depends on several factors, such as the type of shuttle system used to transport NADH from the cytosol to the mitochondria and the efficiency of coupling between electron transport and ATP synthesis. However, a rough estimate is that each NADH produces about 2.5 ATPs, and each FADH2 produces about 1.5 ATPs. Therefore, for each glucose molecule oxidized by glycolysis, the citric acid cycle, and the electron transport chain, about 30-32 ATPs are produced.

Q: What are some inhibitors of the electron transport chain?

A: Some inhibitors of the electron transport chain are substances that block or interfere with the flow of electrons or protons through the complexes. For example:

• Rotenone and amytal are inhibitors of complex I, which prevent NADH from donating electrons to CoQ.
• Antimycin A is an inhibitor of complex III, which prevents CoQH2 from donating electrons to cytochrome c.
• Cyanide and carbon monoxide are inhibitors of complex IV, which prevent cytochrome c from donating electrons to oxygen.
• Oligomycin is an inhibitor of ATP synthase, which prevents protons from flowing back to the matrix and generating ATP.

These inhibitors can affect cellular respiration and energy production and can be lethal in high doses.

We are Compiling this Section. Thanks for your understanding.