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.