Respiration is different than breathing. Breathing is the physical act of inhaling and exhaling. Respiration, however, is one big term that encompasses several metabolic processes.
Breathing is the mechanism that land (terrestrial) animals use to bring oxygen into the bodies and to remove carbon dioxide from their bodies. Respiration is the overall process for producing energy in the form of adenosine triphosphate (ATP) from the fuel and oxygen that an organism acquires.
Aerobic respiration occurs when oxygen is available. In humans, this is the first choice for metabolism of nutrients and production of ATP.
Aerobic respiration has three steps: glycolysis, the Krebs cycle, and oxidative phosphorylation.
Glycolysis is the process that breaks down glucose and converts it to pyruvate.
The Krebs cycle takes the pyruvate and puts it through conversions that result in the production of coenzymes, as well as a molecule of ATP and some carbon dioxide.
Oxidative phosphorylation is the process that takes the coenzymes and passes them through an electron transport chain until ATP is produced.
At the end of the entire process of respiration — that is, after all three steps have been completed — the result is 36 molecules of ATP for you to expend any way you wish. The more you use, the more you make, though, so don’t be stingy with your energy.
Anaerobic respiration — metabolism without oxygen — is used as a back-up system during times that there is not enough oxygen available. The metabolic pathways of anaerobic respiration are entered only to keep cells from dying.
Without aerobic respiration producing molecules that accept electrons, all respiratory cycles come to a grinding halt. When that happens, ATP is not produced; if ATP is not produced, a cell soon dies. Every cell needs a steady supply of ATP to keep cycling through its respiratory cycles.