An organism that lives in the presence of oxygen can extract a great deal of energy from glucose by running it through two main metabolic pathways: glycolysis and cellular respiration. By the end of these pathways, glucose has been completely oxidized and the cell has gained 32 molecules of ATP—a versatile energy carrier that fuels most kinds of cellular work.
In glycolysis, enzymes in the cytosol split glucose into two molecules of pyruvate. Pyruvate then enters a mitochondrion, where cellular respiration occurs. Cellular respiration occurs in three main phases: pyruvate oxidation, the citric acid cycle, and the respiratory chain. In the accompanying animation, we focus on the respiratory chain, the final phase of cellular respiration, and the phase in which the cell makes the bulk of its ATP.
In glycolysis and cellular respiration, a cell breaks down a molecule of glucose and uses its energy to form 32 molecules of ATP, an important cellular energy source. Most of this ATP is generated in mitochondria during the final phase of cellular respiration, during which electron transport and ATP synthesis take place.
A mitochondrion has two membranes—an inner and an outer membrane—that divide the organelle into separate compartments. The inner membrane contains the components required for electron transport (called the respiratory chain) and ATP synthesis.
NADH and FADH2 fuel the respiratory chain. They carry high-energy electrons that were removed from glucose or other food molecules during glycolysis and the citric acid cycle. The respiratory chain consists of a series of electron carriers, each of which holds donated electrons tighter and with greater affinity than the previous carrier in the chain.
NADH donates electrons to the first carrier, called NADH-Q reductase, which drives this carrier to pump protons across the membrane.
NADH-Q reductase derives its name from a redox reaction that it, as well as the other carriers in the chain, participate in. In redox reactions, molecules that gain electrons are said to be reduced, while those that lose electrons are said to be oxidized.
NADH-Q reductase then passes the electrons to and thereby reduces a mobile molecule called ubiquinone. In turn, ubiquinone passes the electrons to cytochrome c reductase, a transfer that is coupled to the pumping of protons out of the matrix and into the intermembrane space.
Cytochrome c reductase reduces the next carrier, cytochrome c, by donating electrons to it. The next carrier, called cytochrome c oxidase, oxidizes cytochrome c by taking electrons from it.
The electrons occupy a relatively low energy state in cytochrome c oxidase, but can occupy an even lower state by joining with oxygen. Oxygen is electron greedy and serves as the final resting place for the electrons originally donated by NADH. Without oxygen, the complexes in the electron transport chain would be stuck with their extra electrons, and the chain would be blocked, unable to transport electrons from new NADH molecules.
What about FADH2? This electron carrier starts a little further down the chain than NADH. Complex II is called succinate dehydrogenase, and it operates in both the citric acid cycle and the respiratory chain. Succinate dehydrogenase removes electrons from succinate in the citric acid cycle and gives them to FAD to form FADH2, and then transfers the electrons to ubiquinone.
The electrons continue down the chain in the same way, but because electrons from FADH2 enter the chain at a later stage than those from NADH, they will result in the production of less ATP. In the end, a total of four electrons and four protons combine to reduce oxygen, generating two molecules of water.
During the electron transfer, cytochrome c oxidase also pumps protons across the membrane.
Notice that the respiratory chain regenerates NAD+ and FAD, which can now be used again in earlier stages of glycolysis and cellular respiration, allowing these metabolic pathways to continue to operate.
The process of transferring electrons along the chain releases a great deal of energy, which has been used to pump protons out of the matrix and into the intermembrane space. This active transport of protons sets up an imbalance of both concentration and charge across the membrane. The imbalance represents potential energy.
The protons have a tendency to flow down their concentration and charge gradients. However, they can only flow by passing through a channel in a protein complex called ATP synthase. As protons flow through this channel they drive the ATP synthase to force together ADP and inorganic phosphate. ADP and inorganic phosphate are both negatively charged and it takes energy, such as that embodied in the proton flow, to bring them together into a single molecule of ATP.
During the early phases of glycolysis and cellular respiration, glucose is completely broken down. CO2 is liberated into the atmosphere, and the hydrogen atoms (H+ + e–) from glucose are donated to the energy carriers NAD+ and FAD to form NADH + H+ and FADH2. In order for glycolysis and cellular respiration to continue to operate on additional glucose molecules, these energy carriers must be recycled.
The work of the respiratory chain is, in part, to recycle these carriers. The carriers donate their extra hydrogen atoms to the respiratory chain and thereby convert back into NAD+ and FAD. NADH donates electrons to the first complex in the chain. FADH2 donates electrons to the second complex.
The other work of the respiratory chain is to transform the chemical energy of the hydrogen atoms (specifically, their electrons) into potential energy. In a series of redox reactions, electrons jump from one complex to another and, in the process, release energy. The chain uses the released energy to pump protons across the membrane, from a region of low concentration inside the mitochondrion to a region of high concentration within the intermembrane space. This concentration gradient represents potential energy.
The cell taps the potential energy of the gradient when protons flow back across the membrane through a pore in the ATP synthase complex. As the protons flow, they release energy, which the complex uses to convert ADP and inorganic phosphate to ATP. The production of ATP from energy derived from the flow of electrons through the respiratory chain is referred to as oxidative phosphorylation. Chemiosmosis is another term for ATP synthesis, referring to the use of a proton gradient to fuel the production of ATP.