The Electron Transport Chain

Glycolysis, the bridging reaction, and the Krebs cycle result in the synthesis of only four ATP molecules when one glucose is oxidized to six CO2 molecules. Most of the ATP that is generated comes from the oxidation of NADH and FADH2 in the electron transport chain.

The electron transport chain, which occurs in the mitochondria in eukaryotic cells and in the cytoplasm of prokaryotic cells, is composed of a series of electron carriers that transfer electrons from donor molecules, such as NADH and FADH2 to an acceptor atom like O2. The electrons move down an energy gradient, like water flowing down a series of waterfalls in rapids.

The difference in free energy that occurs between O2 and NADH releases large amounts of energy. The energy changes that occur at several points in the chain are very large and can provide the eventual production of large amounts of ATP. The free energy that electrons have entering the electron transport chain is greater in the beginning than at the end. It is this energy that enables the protons (H+) to be pumped out of the mitochondrial matrix.

When the electrons move through the chain they transfer this energy to the pumps within the plasma membrane. The electron transport chain will separate the energy that is released into smaller sections, or steps. The reactions of the electron chain take place in the inner membrane of the mitochondria in eukary-

otic cells or in the plasma membrane in prokaryotic cells. In the mitochondria this system is set up into four complexes of carriers.

Each of these carriers transports electrons part of the way to O2 (which is the final electron acceptor). The carriers, coenzyme Q and cytochrome C, connect these complexes. This process by which energy comes from the electron transport chain is provided by protons (H+) and are used to make ATP.

Three ATP molecules can be synthesized from ADP and Pi when two electrons pass from NADH to an atom of O2.

The electron transport chain used by bacteria and other prokaryotes can differ from the mitochondrial chain used in eukaryotic organisms. Bacteria, for example, vary in their electron carriers. Bacteria use cytochromes, heme proteins that carry electrons through the electron transport chain. (A heme is an organic compound, the center of which contains an iron atom surrounded by four nitrogen atoms.) Electrons can enter at several points and leave through terminal oxidases. Prokaryotic and eukaryotic electrons work using the same fundamental principles, although they differ in construction.

The electron transport chain in E. coli bacteria, for example, transports electrons from NADH to acceptors and moves protons across the plasma membrane. The E. coli electron transport chain is branched and contains different cyto-chromes. The two branches are cytochrome d and cytochrome o. Coenzyme Q donates electrons to both branches. These chains operate in different conditions. For example, the cytochrome d branch will function when O2 levels are low and does not actively pump protons, whereas the cytochrome o branch operates in higher O2 concentrations and is a proton pump.

During the aerobic metabolism of a single glucose molecule, ten pairs of electrons from NAD produce thirty ATP molecules, and two pairs of FAD produce four ATP molecules, making a total of 34 ATP molecules. Four substrate-level ATPs make a total of 38 molecules of ATP from one molecule of glucose.

The energy captured occurs through a process called chemiosmosis, formulated by British biochemist Peter Mitchell, who won the Nobel Prize in 1978. In chemiosmosis electrons flow down their electrochemical gradient across the inner mitochondrial membrane in eukaryotes and the cell membrane in prokary-otes through ATP syntase.

If the organism is in an aerobic environment, there are enzymes that can break down harmful chemicals. An example of such a chemical is hydrogen peroxide (H2O2). If the organism is in an anaerobic environment, they do not possess or cannot produce these aerobic enzymes and are susceptible to damage by O2. An example is the free radical superoxide. Organisms that follow this pathway produce less ATP. An example of these types of organisms is lactobacillus.

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