The overall job cellular respiration does
Every cell needs a steady supply of usable energy to function, and that energy comes packaged as a molecule called ATP (adenosine triphosphate). Cellular respiration is the process that breaks down glucose and uses the energy released to build ATP. The overall summary equation is simple to state:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP)
Glucose plus oxygen produces carbon dioxide, water, and usable energy. The complexity — and the reason this takes three distinct stages rather than one step — is in how that energy gets captured efficiently instead of just being released as heat.
Stage one: glycolysis
Glycolysis happens in the cytoplasm, outside any specialized organelle, and doesn't require oxygen to occur. One molecule of glucose (a six-carbon sugar) is split into two molecules of pyruvate (a three-carbon compound), producing a net gain of 2 ATP and 2 NADH (a molecule that carries captured energy forward to later stages, similar in role to ATP but used differently).
Glycolysis is a comparatively small energy payoff on its own — most of the usable energy in glucose is still locked in the pyruvate produced here. What glycolysis mainly accomplishes is preparing the fuel: breaking a large molecule into smaller pieces that the next stages can process further, whether or not oxygen is available.
Stage two: the citric acid cycle (Krebs cycle)
If oxygen is available, pyruvate moves into the mitochondria and is converted into a molecule called acetyl-CoA, which enters the citric acid cycle. This cycle runs twice per original glucose molecule (once for each pyruvate produced in glycolysis), and each turn releases carbon dioxide as waste while producing a small amount of ATP directly, along with a larger yield of NADH and a related carrier molecule called FADH2.
The citric acid cycle's main output isn't ATP directly — it's these carrier molecules, loaded with captured energy, headed toward the third stage where most of the actual ATP gets made.
Stage three: the electron transport chain
The electron transport chain, embedded in the inner mitochondrial membrane, is where the carrier molecules from the earlier stages (NADH and FADH2) finally pay off. They deposit their captured energy into a chain of proteins that uses it to pump hydrogen ions across the membrane, building up a concentration gradient. That gradient then drives a molecular machine called ATP synthase, which uses the flow of hydrogen ions back across the membrane to physically spin and produce large quantities of ATP — this single stage accounts for the large majority of the ATP produced across the entire process.
This is also the stage that requires oxygen directly: oxygen is the final electron acceptor at the end of the chain, combining with electrons and hydrogen ions to form water. Without oxygen available, the electron transport chain backs up and stops functioning, which is why this stage — and the citric acid cycle before it — only proceed under aerobic (oxygen-present) conditions.
Why three stages instead of one
Releasing all of glucose's stored energy in a single uncontrolled step would mostly produce heat, which cells can't use to do biochemical work. Splitting the process into stages lets the cell capture energy incrementally, in the smaller, controlled packets that carrier molecules and ATP represent, rather than losing most of it as an uncapturable burst. Each stage hands off partially-processed fuel or captured energy to the next, and the electron transport chain — the final, most productive stage — depends entirely on the carrier molecules the earlier two stages built up.
What happens without oxygen
When oxygen isn't available, cells fall back on fermentation, which allows glycolysis to keep running (producing a small, steady trickle of ATP) but skips the citric acid cycle and electron transport chain entirely — this is why fermentation yields dramatically less ATP per glucose molecule than full aerobic respiration. Muscle cells during intense exercise, when oxygen delivery can't keep pace with demand, temporarily rely on this less efficient fermentation pathway, which is part of why sustained hard exercise eventually requires slowing down to let oxygen-dependent respiration catch back up.