Cellular respiration, as an important method of ATP generation, represents one of the most basic metabolic processes of organisms. The physics and chemistry of cellular respiration help define such things as where organisms live, ecosystem-level interactions between species, and the morphology of multicellularity. We define cellular respiration as ATP-generating pathways in which electron transport systems (ETSs; Figure 1) are coupled to the generation of ATP through a process known as che-miosmosis (Figure 2). In contrast, ATP generation can also involve substrate-level phosphorylation (SLP), a process that uses energy within substrates, such as carbohydrates, to couple phosphates onto ADP (Figure 3). This latter process is exemplified by several steps in the glycolytic pathway (Figure 4), which may also be linked to cellular respiration by supplying reduced substrate to ETSs. Cellular respiration may be considered ecologically in terms of energy movement within microbe-based ecosystems; in terms of its evolution and relative benefits as compared to, for example, SLP; and in terms ofthe tradeoffs involved in the resulting challenges of gas exchange (O2 and CO2), especially in larger organisms. In the next four sections of this article, we employ the terms 'cellular respiration' and 'respiration' synonymously. The final section considers respiration exclusively as gas exchange in the whole organism.

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