Electron Transport Chain (ETC): Definition, Location & Importance

Most living cells produce energy from nutrients through cellular respiration that involves the taking up of oxygen to release energy. The electron transport chain or ETC is the third and final stage of this process, the other two being glycolysis and the citric acid cycle.

The energy produced is stored in the form of ATP or adenosine triphosphate, which is a nucleotide found throughout living organisms.

The ATP molecules store energy in their phosphate bonds. The ETC is the most important stage of cellular respiration from an energy point of view because it produces the most ATP. In a series of redox reactions, energy is liberated and used to attach a third phosphate group to adenosine diphosphate to create ATP with three phosphate groups.

When a cell needs energy, it breaks the third phosphate group bond and uses the resulting energy.

Many of the chemical reactions of cell respiration are redox reactions. These are interactions between cellular substances that involve reduction and oxidation (or redox) at the same time. As electrons are transferred between molecules, one set of chemicals is oxidized while another set is reduced.

A series of redox reactions make up the electron transport chain.

The chemicals that are oxidized are reducing agents. They accept electrons and reduce the other substances by taking their electrons. These other chemicals are oxidizing agents. They donate electrons and oxidize the other parties in the redox chemical reaction.

When there are a series of redox chemical reactions taking place, electrons can be passed on through multiple stages until they end up combined with the final reducing agent.

The cells of advanced organisms or eukaryotes have a **nucleus** and are called eukaryotic cells. These higher level cells also have small membrane-bound structures called mitochondria that produce energy for the cell. Mitochondria are like small factories that generate energy in the form of ATP molecules. Electron transport chain reactions take place inside the mitochondria.

Depending on the work the cell does, cells may have more or fewer mitochondria. Muscle cells sometimes have thousands because they need a lot of energy. Plant cells have mitochondria as well; they produce glucose via photosynthesis, and then that is used in cellular respiration and, eventually, the electron transport chain in the mitochondria.

The ETC reactions take place on and across the inner membrane of the mitochondria. Another cell respiration process, the **citric acid cycle, takes place inside the mitochondria and delivers some of the chemicals needed by the ETC reactions. The ETC uses the characteristics of the inner mitochondrial membrane** to synthesize ATP molecules.

A mitochondrion is tiny and much smaller than a cell. To see it properly and study its structure, an electron microscope with a magnification of several thousand times is required. Images from the electron microscope show that the mitochondrion has a smooth, elongated outer membrane and a heavily folded inner membrane.

The inner membrane folds are shaped like fingers and reach deep into the interior of the mitochondrion. The inside of the inner membrane contains a fluid called the matrix, and between the inner and outer membranes is a viscous fluid-filled region called the intermembrane space.

The citric acid cycle takes place in the matrix, and it produces some of the compounds used by the ETC. The ETC takes electrons from these compounds and returns the products back to the citric acid cycle. The folds of the inner membrane give it a large surface area with lots of room for electron transport chain reactions.

Most single cell organisms are prokaryotes, which means the cells lack a nucleus. These prokaryotic cells have a simple structure with a cell wall and cell membranes surrounding the cell and controlling what goes into and out of the cell. Prokaryotic cells lack mitochondria and other membrane-bound organelles. Instead, cell energy production takes place throughout the cell.

Some prokaryotic cells such as green algae can produce glucose from photosynthesis, while others ingest substances that contain glucose. The glucose is then used as food for cell energy production via cell respiration.

Because these cells don't have mitochondria, the ETC reaction at the end of cell respiration has to take place on and across the cell membranes located just inside the cell wall.

The ETC uses high energy electrons from chemicals produced by the citric acid cycle and takes them through four steps to a low energy level. The energy from these chemical reactions is used to pump protons across a membrane. These protons then diffuse back through the membrane.

For prokaryotic cells, proteins are pumped across the cell membranes surrounding the cell. For eukaryotic cells with mitochondria, the protons are pumped across the inner mitochondrial membrane from the matrix into the intermembrane space.

Chemical electron donors include NADH and FADH while the final electron acceptor is oxygen. The chemicals NAD and FAD are given back to the citric acid cycle while the oxygen combines with hydrogen to form water.

The protons pumped across the membranes create a proton gradient. The gradient produces a proton-motive force that allows the protons to move back through the membranes. This proton movement activates ATP synthase and creates ATP molecules from ADP. The overall chemical process is called oxidative phosphorylation.

Four chemical complexes make up the electron transport chain. They have the following functions:

• Complex I takes electron donor NADH from the matrix and sends electrons down the chain while using the energy to pump protons across the membranes.
• Complex II uses FADH as an electron donor to supply additional electrons to the chain.
• Complex III passes the electrons to an intermediate chemical called cytochrome and pumps more protons across the membranes.
• Complex IV receives the electrons from the cytochrome and passes them on to half of an oxygen molecule that combines with two hydrogen atoms and forms a water molecule.

At the end of this process, the proton gradient is produced by each complex pumping protons across the membranes. The resulting proton-motive force draws the protons through the membranes via the ATP synthase molecules.

As they cross into the mitochondrial matrix or the interior of the prokaryotic cell, the action of the protons allows the ATP synthase molecule to add a phosphate group to an ADP or adenosine diphosphate molecule. ADP becomes ATP or adenosine triphosphate, and energy is stored in the extra phosphate bond.

Each of the three cellular respiration phases incorporates important cell processes, but the ETC produces by far the most ATP. Since energy production is one of the key functions of cell respiration, ATP is the most important phase from that point of view.

Where the ETC produces up to 34 molecules of ATP from the products of one glucose molecule, the citric acid cycle produces two, and glycolysis produces four ATP molecules but uses up two of them.

The other key function of the ETC is to produce NAD and FAD from NADH and FADH in the first two chemical complexes. The products of the reactions in ETC complex I and complex II are the NAD and FAD molecules that are required in the citric acid cycle.

As a result, the citric acid cycle is dependent on the ETC. Since the ETC can only take place in the presence of oxygen, which acts as the final electron acceptor, the cell respiration cycle can only operate fully when the organism takes in oxygen.

All advanced organisms need oxygen to survive. Some animals breathe in oxygen from the air while aquatic animals may have gills or absorb oxygen through their skins.

In higher animals, the red blood cells absorb oxygen in the lungs and carry it out into the body. Arteries and then tiny capillaries distribute the oxygen throughout the body's tissues.

As mitochondria use up oxygen to form water, oxygen diffuses out of the red blood cells. Oxygen molecules travel across cell membranes and into the cell interior. As existing oxygen molecules are used up, new molecules take their place.

As long as there is enough oxygen present, the mitochondria can supply all the energy the cell needs.

Glucose is a carbohydrate that, when oxidized, produces carbon dioxide and water. During this process, electrons are fed into the electron transport chain.

The flow of electrons is used by protein complexes in the mitochondrial or cell membranes to transport hydrogen ions, H+ , across the membranes. The presence of more hydrogen ions outside a membrane than inside creates a pH imbalance with a more acidic solution outside the membrane.

To balance the pH, the hydrogen ions flow back across the membrane through the ATP synthase protein complex, driving the formation of ATP molecules. The chemical energy harvested from the electrons is changed to an electrochemical form of energy stored in the hydrogen ion gradient.

When the electrochemical energy is released through the flow of the hydrogen ions or protons through the ATP synthase complex, it is changed to biochemical energy in the form of ATP.

The ETC reactions are a highly efficient way to produce and store energy for the cell to use in its movement, reproduction and survival. When one of the series of reactions is blocked, the ETC no longer functions, and cells that rely on it die.

Some prokaryotes have alternate ways of producing energy by using substances other than oxygen as the final electron acceptor, but eukaryotic cells depend on oxidative phosphorylation and the electron transport chain for their energy needs.

Substances that can inhibit ETC action can block redox reactions, inhibit proton transfer or modify key enzymes. If a redox step is blocked, the transfer of electrons stops and oxidation proceeds to high levels on the oxygen end while further reduction takes place at the beginning of the chain.

When protons can't be transferred across the membranes or enzymes such as ATP synthase are degraded, the production of ATP stops.

In either case, cell functions break down and the cell dies.

Plant-based substances such as rotenone, compounds such as cyanide and antibiotics such as antimycin can be used to inhibit the ETC reaction and bring about targeted cell death.

For example, rotenone is used as an insecticide, and antibiotics are used to kill bacteria. When there is a need to control organism proliferation and growth, the ETC can be seen as a valuable point of attack. Disrupting its function deprives the cell of the energy it needs to live.