After high-energy electrons are created during the citric acid cycle, they are shuttled to an electron transport chain where ATP can be generated for the cell. Oxidative phosphorylation is the production of ATP through chemiosmosis in the mitochondria through an electron transport chain. (Chemiosmosis is the movement of ions across a semipermeable membrane down their electrochemical gradient, which results in a charge that can be used as a source of energy.) It involves two distinct processes, electron transport with the help of the electron transport chain and phosphorylation of ADP to ATP. This entire process not only requires oxygen, or is aerobic, but it also takes place in the mitochondria of the cell.
The first step that occurs is the oxidation of NADH and FADH2, which is a process that is coupled to the electron transport chain. As these high energy molecules, NADH and FADH2, pass through this chain, both lose electrons after being oxidized to NADH+ and FADH+, respectively. During this process of oxidation, protons are pumped across their proton gradient and electrons are shuttled through the protein complexes, with the help of electron carriers, until they reach the final electron acceptor called oxygen. Oxygen receives electrons that are in the last protein complex and combines with protons inside the mitochondrial cell to make water.
This combination drives the second step of oxidative phosphorylation which is the synthesis of ATP. When NADH and FADH2 are oxidized the intermembrane space becomes positively charged because protons were pumped there. But the mitochondrial matrix becomes negatively charged because there is a loss of protons and high concentration of electrons in this space. This difference drives the diffusion of protons back inside the mitochondrial matrix through a protein complex called ATP synthase. When this happens ATP synthase takes energy produced from this proton shuttling process and uses it to convert ADP to ATP.
Electron Transport Chain
When glycolysis, oxidation of pyruvate, and the citric acid cycle occur, glucose is catabolized (broken down into smaller units that are oxidized to release energy or used in other anabolic reactions), carbon dioxide is produced, and energy is released. There are two remaining molecules, part of the original glucose-metabolism reaction, still unaccounted for: oxygen (O2) and water (H2O). A series of redox reactions using NAD and FAD occur to produce these molecules and additional energy for the cell. This is done through the electron transport chain.The electron transport chain is a series of electron transporters that move electrons from NADH and FADH2 to oxygen molecules; protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water. It is an aerobic process. This pathway occurs after the citric acid cycle and is the only pathway in cellular respiration that requires atmospheric oxygen. Atmospheric oxygen comes from the external environment.
The Four Complexes of the Electron Transport Chain
Four large protein complexes are the components of the electron transport chain; these are referred to as complex I through complex IV, or proton pumps.
- Complex I: A large protein, called NADH dehydrogenase, oxidizes NADH and transfers two electrons. With the help of a prosthetic group, an electron carrier called ubiquinone connects complex I to complex II. Prosthetic groups are non-protein molecules requiring another protein to be active. As a result, four hydrogen ions are pumped out of the mitochondrial matrix (where the citric acid cycle takes place) to the intermembrane space.
- Complex II and Ubiquinone: FADH2 does not pass through complex I. Rather, it is received directly by complex II. In complex II, the enzyme succinate dehydrogenase oxidizes FADH2. It also transfers two electrons through proteins which helps reduce ubiquinone. Reduced ubiquinone moves through a hydrophobic region (an area that repels water molecules) of the electron transport chain, where it is oxidized by complex III.
- Complex III: This complex is also called cytochrome c reductase. This enzyme oxidizes ubiquinone and transfers one electron at a time through proteins with the help of special compounds and a prosthetic group. This causes the oxidized form of cytochrome c to be reduced. In the end, four hydrogen ions are transported out of the mitochondrial matrix and into the intermembrane space. A total of eight hydrogen ions have now moved into the intermembrane space.
- Complex IV: This is also called cytochrome c oxidase. It functions to oxidize cytochrome c and transfer each electron generated through proteins that a specific prosthetic group. The result of doing this is to reduce oxygen gas, which in turn takes two hydrogen ions from the matrix. Both hydrogen ions combine with the oxygen to make water. Two more additional hydrogen ions are pumped out of the matrix into the intermembrane space. As hydrogen ions are produced, they move from an area of higher concentration to an area of lower concentration and back into the mitochondrial matrix.
Establishing a Proton Gradient and ATP Synthase
Ninety percent of the ATP made during aerobic glucose breakdown comes from chemiosmosis. When ATP is produced using chemiosmosis in the mitochondria, it is called oxidative phosphorylation. At the conclusion of glucose breakdown, the number of ATP molecules produced varies. This is because the number of protons moved through the membrane to produce ATP via the electron transport chain varies among species. It is estimated that when a cell utilizes all of these pathways during the breakdown of glucose, about 34% of the energy contained in glucose is extracted for use and storage in the cell. In general, oxidative phosphorylation (electron transport and chemiosmosis) produces 32 to 34 ATP molecules.
In the absence of oxygen, pyruvate can be broken down via fermentation. Fermentation produces two ATP molecules without oxygen. As a result of the fermentation reaction, NAD+ is regenerated and can return to the glycolytic pathway, in which glucose is broken down into pyruvate in the cytoplasm, to pick up more electrons during the energy-harvesting phase. Less energy is generated during fermentation than aerobic metabolic pathways, but enough energy is generated for the cell to continue working under anaerobic (absence of oxygen) conditions. There are two common fermentation pathways: lactic acid fermentation and alcohol fermentation (also called ethanol fermentation).
The advantage of fermentation is that when oxygen is limited, the cell is unable to keep storing the pyruvate produced during glycolysis. Thus, it can rely on fermentation to process pyruvate and convert it into lactic acid (a compound that forms when a body breaks down carbohydrates to use for energy when oxygen levels are low). Typically in cells such as muscle cells, fermentation is important when oxygen is used up by excessive exercise. In this case, the muscle cells can rely on fermentation to convert pyruvate into lactic acid. Although this produces two molecules of ATP, which is standard with glycolysis, the end product of lactic acid correlates with muscle cell fatigue because of an insufficient rate of ATP generation.