#which pump the protons into the intermembrane space

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Adenosine triphosphate

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Electron Transport Chain

Electron Transport Chain Definition

The electron transport chain is a crucial step in oxidative phosphorylation in which electrons are transferred from electron carriers, into the proteins of the electron transport chain which then deposit the electrons onto oxygen atoms and consequently transport protons across the mitochondrial membrane. This excess of protons drives the protein complex ATP synthase, which is the final step in oxidative phosphorylation and creates ATP.

Electron Transport Chain Location

The electron transport chain is located within mitochondria, and the proteins of the electron transport chain span the inner mitochondrial membrane. This can be seen in the image below.

The electron transport chain consists of 4 main protein complexes. Each complex has a different role in the chain, some accepting electrons from carriers and some which serve to transfer electrons between the different complexes. The basic function of the electron transport chain is to move protons into the intermembrane space.

ATP synthase, which is not part of the process, is also located on the mitochondrial inner membrane. This complex will use the electrochemical gradient of the protons to essentially extract energy from the pressure of the protons wanting to cross the membrane to the mitochondrial matrix. This energy is then used to add a phosphate group to an ADP molecule, forming ATP. The electron transport chain must first extract the energy it needs to pump the hydrogen ions from electron carriers.

Electron Transport Chain Steps

Step One: Electron Carriers

Electron carriers get their energy (and electrons) from reactions during glycolysis and the Krebs cycle. These reactions release energy from molecules like glucose by breaking the molecules in smaller pieces and storing the excess energy in the bonds of the recyclable electron carriers.

Step Two: Hydrogen Ion Pump

These carriers are then transported to the inner mitochondrial membrane, where they can interact with the proteins of the electron transport chain. These carriers dump their electrons and stored energy in complexes I and II. These protein units relieve the electron carriers of excess hydrogen atoms. The electrons stay with the proteins, while the hydrogen atoms are left in the matrix. The electrons from these bonds pass through complexes I and II, through coenzyme Q. This specialized protein functions solely in passing electrons from these complexes to complex III.

Complex III serves as a hydrogen ion pump. It actively takes the energy from the electrons and uses it to pump the hydrogen ions against their natural gradient. Because the ions cannot easily travel through the membrane, they build up in the intermembrane space between the inner membrane and the outer membrane. This allows for the establishment of a proton-motive force, which will later be used by ATP synthase to store energy in molecules which can be used by other proteins as a source of energy.

Step Three: Disposing of the Electrons

The final step of the electron transport chain is to remove the electrons with lower energy out of the system. This allows for new electrons to be added, part of the reason the process is called a chain. Cytochrome C is the complex which transfers the electrons to the final protein in the electron transport chain. Complex IV has a unique function both pumping hydrogen ions as well as depositing the electrons on a final electron acceptor.

In the case of aerobic organisms, this acceptor is oxygen. Found in the form of dissolved gas in the blood, complex IV donates the electrons to two free hydrogens and one oxygen atom. The complex catalyzes the reaction, creating water. This allows the electron transport chain to release the electrons, freeing up a new spot in complex IV. This spot is filled by electrons from complex III, and so on all the way back up the electron transport chain.

Electron Transport Chain Products

During the course of the electron transport chain, only two things are really created. First, water is created as the electron transport chain deposits spent electrons into new water molecules. These water molecules can be reabsorbed by the body for use elsewhere or can be dispelled in the urine. Second, while the electron transport chain does not create ATP it does create the proper conditions for ATP to be produced. This is called the proton-motive force and is a product of the electron transport chain transporting hydrogen ions to one side of the inner mitochondrial membrane.

Stopping the Electron Transport Chain

One of the best ways to understand the function and purpose is to understand what happens if the electron transport chain stops. This can happen from two basic scenarios. The electron transport chain can stop because it does not have a source of electrons, or it can stop because it can no longer pass electrons on.

The first scenario would be caused by something like starvation. Without a source of glucose or other energy-rich molecules, cells would not be able to collect electrons on electron carriers. Without anything to transfer, the chain would simply stop pumping hydrogen ions. In turn, ATP synthase would stop functioning and the entire cell would soon run out of energy and deteriorate.

The second scenario is somewhat more common and happens when cells run out of oxygen. Organisms which are facultative anaerobes are able to use different processes when there is no oxygen for oxidative phosphorylation. In some organisms the process of fermentation allows glycolysis to continue, producing only a small amount of ATP. Without the electron transport chain, the cell still needs to recycle electron carriers. In the case of alcohol fermentation, the electron carriers dump their electrons in a reaction which creates ethanol as a final product. This allows glycolysis to continue producing ATP, allowing the cells to live through periods of low oxygen content.

Quiz

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Electron Transport Chain and Oxidative Phosphorylation

Oxidative phosphorylation is a process involving a flow of electrons through the electron transport chain, a series of proteins and electron carriers within the mitochondrial membrane. This flow of electrons allows the electron transport chain to pump protons to one side of the mitochondrial membrane. As the protons build up, they create a proton-motive force, a type of electrochemical pressure. This pressure is relived through specialized protein complexes, which capture the energy of the protons as they flow to the other side of the membrane. The energy is then used to bond a phosphate group to the molecule adenosine diphosphate (ADP), creating adenosine triphosphate (ATP). This completes the process of oxidative phosphorylation.

Steps of Oxidative Phosphorylation

Before the Electron Transport Chain

For the electron transport chain to be able to pump protons to one side of the mitochondrial inner membrane, it must first have a source of those electrons and protons. There are several cellular processes which lead to the oxidation (“burning”) of various cellular food sources. These processes include glycolysis, the citric acid cycle, the fatty acid beta-oxidation metabolism, and the oxidation of amino acids.

All of these processes involve the transfer of electrons and protons to coenzymes. The most common coenzymes are nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). NAD can be reduced with electrons and a proton to become NADH, while FAD can take on two protons and four electrons to become FADH2. These coenzymes can bind to the proteins of the electron transport chain, and transfer their electrons and protons. This becomes the first stage in the electron transport chain.

Within the Electron Transport Chain

The electron transport chain consists of four protein complexes, simply named complex I, complex II, complex III, and complex IV. Each complex is designed to receive electrons from a coenzyme or one of the other complexes in the chain. The actions each complex takes can be seen in the image below.

Complex I is responsible for relieving NADH of its hydrogen and electrons. The energy received by taking the electrons allows complex I to pump the hydrogen atom through the inner mitochondrial membrane, which concentrates hydrogens in the intermembrane space. The electrons are then passed to coenzyme Q (CoQ). CoQ can take on hydrogens and electrons, and can be reduced to CoQH2. The coenzyme transfers the electrons to complex III.

Meanwhile, complex II is also receiving electrons and protons. These come from FADH2, from the citric acid cycle. Complex II relieves FADH2 of its electrons, and passes them to CoQ. The coenzyme passes them to complex III, which now receives electrons and their energy from two sources. This allows complex III to pump large amounts of hydrogen across the membrane. Cytochrome c (Cyt c) allows the electrons to be passed to complex IV, the final complex in the electron transport chain. This complex passes the electrons to oxygen molecules, where they bind with hydrogens to produce water. With the final bit of energy, another proton is passed through the membrane.

ATP Synthesis

At this point, the electron transport chain has built up a large number of hydrogen ions in the intermembrane space. It did this with the energy it received through passing electrons through a series of energy releasing reactions. The final step of oxidative phosphorylation is the production of ATP, or the process of phosphorylation.

This process takes place in a complex called ATP synthase. This large complex uses the proton-motive force to attach phosphate groups to ADP molecules. Because there are so many protons built up in the intermembrane space, they want to push their way to the other side. ATP synthase uses this energy to undergo a conformational change. In doing so, it forces the ATD and phosphate group together, and reduces the energy they need to bond. ATP can then go on to fuel reactions all over the cell, when it is exported from the mitochondria.

The Electron Transport Chain Within Oxidative Phosphorylation

Oxidative phosphorylation is part of a larger system, cellular respiration. The 4 steps of cellular respiration can be seen in the image below. The first step occurs outside of the mitochondria. This involves the breakdown of glucose, lipids, or amino acids. This step is symbolized here with “Glycolysis” only. Remember that there are other ways to generate pyruvate and intermediates the Krebs cycle (citric acid cycle).

The remaining steps take place within the mitochondria. The yellow lines in the image represent the generation of reduced coenzymes, or molecules which are carrying electrons. While some ATP is generated during glycolysis and the citric acid cycle, the majority is generated through oxidative phosphorylation. The electron transport chain is symbolized by the red staircase, representing the successive release of energy from the electrons. The orange arrows represent ATP synthase, which creates ATP through the proton-motive force.

Oxidative Phosphorylation within Cellular Respiration

Therefore, the electron transport chain is a part of oxidative phosphorylation, which itself is the last stage of cellular respiration. The truly interesting thing about these processes is that they are conserved across evolution. The electron transport chain can be observed in the most basic of organisms. Any eukaryote (cell with organelles), has mitochondria and therefore uses this exact same method to produce ATP. Even plants, which are often considered so different than animals, rely on the same process of oxidative phosphorylation.

Interestingly, the process of photophosphorylation is very similar to oxidative phosphorylation. This process is used in photosynthesis. However, instead of using oxygen to create water, it uses water to create oxygen. Basically the opposite of oxidative phosphorylation, photosynthesis uses an electron transport chain of its own to carry energy from sunlight into the bonds of sugar molecules. The plant can then use these molecules to feed other cells within its body. Just as an animal would, it breaks the glucose into pyruvate, and the pyruvate enters the mitochondria and eventually undergoes oxidative phosphorylation powered by the electron transport chain.

Quiz

1. Which of the following is a true statement? A. Oxidative phosphorylation and the electron transport chain are unrelated B. Oxidative phosphorylation drives the electron transport chain C. Oxidative phosphorylation relies on the electron transport chain

Answer to Question #1

C is correct. The process of ATP synthase attaching phosphate groups to ADP is the process of phosphorylation. The energy used for this process comes from the oxidation of various substances, and the electrons received from doing so. These electrons generate a proton gradient, which drives ATP synthase.

2. What would happen to a cell if there was no electron transport chain? A. The cell would have no energy B. The cell would fall apart C. The cell would have less energy

Answer to Question #2

C is correct. While oxidative phosphorylation does provide a huge supply of energy, there are other pathways cells can take to make energy. Remember that the electron transport chain needs oxygen. Without oxygen, it will stop working. This is when cells have to resort to less efficient methods of energy production such as fermentation.

3. As a scientist in your laboratory, you extract the mitochondria from your own cell, and from the cells of your favorite house plant. You put each mitochondria in a small dish, surrounded with pyruvate. You measure how much ATP each mitochondria makes. What do you expect? A. The animal mitochondria will make more ATP B. The plant mitochondria will make more ATP C. They will produce roughly the same amount of ATP

Answer to Question #3

C is correct. All mitochondria are thought to have arisen from the same bacterial ancestor billions of years ago. Thus, in plants and animals they operate essentially in the same way. A major different between plants and animals may come in the number of mitochondria per cell. An animal may pack their muscle cells with mitochondria to provide energy for contractions, where a plant cell may only need a handful of mitochondria in each cell to provide their energy needs.

References

Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology (6th ed.). New York: W.H. Freeman and Company.

Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.

Widmaier, E. P., Raff, H., & Strang, K. T. (2008). Vander’s Human Physiology: The Mechanisms of Body Function (11th ed.). Boston: McGraw-Hill Higher Education.