Considerably more energy is available to the cell from respiration, a process in which the electrons from molecules of sugar are transferred not to another organic molecule but to an inorganic molecule. The most familiar respiratory process (aerobic respiration) uses oxygen as the final electron acceptor. The sugar is completely broken down to carbon dioxide and water, yielding a maximum of 38 molecules of ATP per molecule of glucose. Electrons are transferred to oxygen using the electron transport chain, a system of enzymes and cofactors located in the cell membrane and arranged so that the passage of electrons down the chain is coupled with the movement of protons (hydrogen ions) across the membrane and out of the cell. Electron transport induces the movement of positively charged hydrogen ions to the outside of the cell and negatively charged ions to its interior. This ion gradient results in the acidifi cation of the external medium and an energized plasma membrane with an electrical charge of 150 to 200 millivolts. The generation of ion gradients, including this protonmotive force (gradient of protons), is a common aspect of energy generation and storage in all living organisms. The gradient of protons is used directly by the cell for many processes, including the active transport of nutrients and the rotation of flagella. The protons also can move from the exterior of the cell into the cytoplasm by passing through a membrane enzyme called the F1F0-proton-translocating ATPase, which couples this proton movement to ATP synthesis in a process identical to that which occurs in the mitochondria of eukaryotic cells. Bacteria that are able to use respiration produce far more energy per sugar molecule than do fermentative cells, because the complete oxidation (breakdown) of the energy source allows complete extraction of all of the energy available as shown by the substantially greater yield of ATP for respiring organisms than for fermenting bacteria. Respiring organisms achieve a greater yield of cell material using a given amount of nutrient; they also generate fewer toxic end products. The solubility of oxygen in water is limited, however, and the growth and survival of populations of aerobic bacteria are directly proportional to the available supply of oxygen. Continuous supplies of oxygen are available only to bacteria that come into contact with air, as occurs when bacteria are able to float on a surface that exposes them to air or when the medium in which the bacteria live is stirred vigorously
Respiration can also occur under anaerobic conditions by processes called anaerobic respiration, in which the final electron acceptor is an inorganic molecule, such as nitrate (NO3
−), nitrite (NO2 −), sulfate (SO4 2−), or carbon dioxide (CO2). The energy yields available to the cell using these acceptors are lower than in respiration with oxygen—much lower in the case of sulfate and carbon dioxide—but they are still substantially higher than the energy yields available from fermentation. The ability of some bacteria to use inorganic molecules in anaerobic respiration can have environmental significance. E. coli can use oxygen, nitrate, or nitrite as an electron acceptor, and Pseudomonas stutzeri is of major global importance for its activity in denitrification, the conversion of nitrate to nitrite and dinitrogen gas (N2). Desulfovibrio and Desulfuromonas reduce sulfate and elemental sulfur (S), respectively, yielding sulfide (S2−), and the bacterium Acetobacterium woodii and methanogenic archaea, such as Methanobacterium thermautotrophicum, reduce carbon dioxide to acetate and methane, respectively. The Archaea typically use hydrogen as an electron donor with carbon dioxide as an electron acceptor to yield methane or with
sulfate as an electron acceptor to yield sulfide.