A more general way to describe the last reaction in electron transport is:

X_oxidized + e^– → X_reduced

Part of the amazing success of bacteria and archaea is that they have evolved biochemical pathways that allow them to exist in environments where O₂ is scarce or absent. As you will see in the next section, for most animals and plants, the anaerobic (no O₂) catabolism of glucose generally yields much less energy than aerobic catabolism. However, many bacteria and archaea exploit their environments to use *alternative electron acceptors. This allows them to complete the electron transport chain and produce ATP even in the absence of O₂. Table 9.2 summarizes some of these pathways, which are referred to as anaerobic respiration. Note that some of these microbes use ions as electron acceptors while others use small molecules.