The last step in cellular respiration, called the electron transport chain, takes place in mitochondria. This step is essential for producing enough ATP for animals and many other organisms to survive. The first two steps in cellular respiration—glycolysis and the Krebs cycle—produced a small amount of ATP from the breakdown of glucose. These steps also produced NADH and FADH2 molecules. Through the electron transport chain, the cell can now use the energy in NADH and FADH2 molecules to make many more molecules of ATP.
The mitochondrion has two membranes, an outer membrane and an inner membrane, and is essentially a bag within a bag. The proteins of the electron transport chain reside in the inner membrane. Both NADH and FADH2 donate electrons to the chain. We will use NADH as an example.
As electrons move from one complex to another in the chain, they transfer some of their energy to proteins that pump protons, also known as hydrogen ions, across the membrane. With each transfer, the electrons lose energy, which is used to pump more protons. The process of electron transfer results in a difference in the concentration of protons on the two sides of the membrane. This concentration difference—called a gradient—is a form of potential energy.
The oxygen that we breathe is essential for electron transport. Oxygen is exceptionally electron greedy and snatches electrons from the end of the electron transport chain. After grabbing electrons, it combines with protons to form water.
From the operation of the electron transport chain, a concentration gradient of protons forms across the membrane. Like water held behind a dam, given the opportunity, the protons will tend to flow across the membrane barrier. A complex in the membrane provides a passageway for the protons, and uses the energy from the proton flow to power the production of ATP. The protons will tend to flow in this direction until the concentrations are the same on both sides of the membrane—that is, until the gradient disappears. If the electron transport chain stops for any reason, such as a lack of oxygen to capture electrons at the end of the chain, the gradient will quickly disappear, turning off ATP production.
The NAD+ molecules left over from the electron transport chain are later recycled back to glycolysis and the Krebs cycle, where they will capture more high-energy electrons from food molecules.
ATP, produced in plenty during this last stage of cellular respiration, contains energy that had its origin in a molecule of glucose. Along the way, the energy took many forms, including the energy held by electron carriers, the energy released by transporting electrons in the electron transport chain, and the energy stored in a concentration gradient across a membrane. In the form of ATP, the energy originally in the bonds of glucose can be used to fuel cellular work.
Why is cyanide such a deadly poison?
Cyanide is a deadly poison because it cripples the electron transport chain. Cyanide binds to the last complex in the chain and, in so doing, blocks oxygen from binding.
Recall that during the operation of the electron transport chain, NADH donates electrons to the beginning of the chain. The process of electron transport fuels the pumping of protons across the membrane. However, with cyanide already bound, oxygen cannot grab the electrons at the end of the chain. Therefore, no new electrons from NADH can be added at the beginning.
Under these conditions, electron transport and proton pumping grind to a halt. Without the active pumping of protons to one side of the membrane, the proton concentrations on both sides soon equalize.
No longer do protons rush through the ATP-producing complex, and no more ATP is produced. Cells can't live long on the small quantity of ATP made by glycolysis, so without the ATP that comes from the electron transport chain, cells die.