All living things run on energy. If the organism is a plant or autotrophic microbe, the energy comes from sunlight. For all other forms of life, energy is extracted from nutrients through the reactions of metabolism-cellular respiration.
* Cellular Respiration and the Electron Transport Chain *
Regardless of whether the original form of energy is sunlight or food, it must ultimately be converted to the cellular energy currency of adenosine triphosphate (ATP). For most organisms, this conversion is accomplished though cellular respiration, a series of pathways in which glucose (sugar) is broken down and the energy extracted is converted to ATP. The pathways of cellular respiration include glycolysis, conversion of acetyl-CoA, Kreb’s cycle and electron transport. Electron transport is the most complex and productive pathway of cellular respiration, producing 34 molecules of ATP for every molecule of glucose.
* Where the Electron Transport Chain Is Located *
Electron transport requires a membrane in order to work. In prokaryotic cells, those of bacteria and bacteria-like Achaeans, electron transport takes place in the cell’s plasma membrane. In eukaryotic cells, the more evolutionarily advanced and complex cells of animals, plants and fungi, electron transport takes place in cellular organelles known as mitochondria—the eukaryotic cell’s tiny power factories.
* How Electron Transport Works *
Most of the ATP made in cellular respiration comes from the stepwise release of energy, through a series of oxidation-reduction (redox) reactions between molecules embedded in the plasma membrane (prokaryotes) or mitochondria (eukaryotes).
It is easiest to understand how electron transport works to divide it into three main events:
1. Oxidation Reduction Reactions
During glycolysis, synthesis of acetyl-CoA and Kreb’s cycles the electron carriers NAD+ and FADH are reduced to form NADH and FADH2 respectively. These molecules are like little rechargeable batteries, and when NAD+ and FADH are reduced, this means that they accept and carry electrons and hydrogen ions (H+)—potential energy that can be used later in cellular respiration.
In the electron transport chain, these electron carriers are oxidized, transferring their electrons to the carrier molecules embedded in the ETC membrane. The electrons are then passed from one carrier molecule to another in a series of oxidation-reduction reactions, and finally, in aerobic respiration, to the final electron acceptor, oxygen (O2).
2. Creation of Hydrogen Ion Gradient
The energy from each electron being passed down the chain is used to pump a proton (H+) through each carrier molecule, from one side of the membrane to the other. This creates a proton gradient, a type of ion gradient (difference in ion concentration between two sides of a membrane), and gradients are potential energy available for cellular work.
3. Phosphorylation of ADP
The H+ on the side of the membrane in which they are most concentrated will eventually flow back across the membrane, down the electrochemical proton gradient through protein channels called ATP synthase. As each H+ moves back across the membrane, ATP synthase phosphorylates adenosine diphosphate (ADP) to make the high energy molecule ATP, which can be used for many different energy-requiring reactions throughout the cell.
This process is much easier to understand when students see it depicted in action. For a great demonstration, go to the Virtual Cell Animation Collection for an animation of the electron transport chain.
* Source *
Campbell & Reece (2001) Biology, 6th Edition. Benjamin Cummings.