Cellular Respiration and Fermentation

Cellular respiration breaks down glucose to produce ATP in three connected stages. Glycolysis happens first, in the cytoplasm outside the mitochondria, splitting one glucose molecule into two molecules of pyruvate and producing a small net gain of ATP directly -- notably, glycolysis does not require oxygen. If oxygen is available, pyruvate then enters the mitochondrial matrix for the Krebs cycle (also called the citric acid cycle), which finishes breaking the carbon skeleton down completely, releasing carbon dioxide as a byproduct (the same carbon dioxide you exhale) and loading up electron carrier molecules. Those loaded electron carriers feed the electron transport chain, embedded in the mitochondria's folded inner membrane (the cristae, whose folding dramatically increases surface area for this final stage), where oxygen finally accepts the spent electrons at the very end of the chain and the great majority of the cell's ATP is produced.

Because the electron transport chain requires oxygen as the final electron acceptor, this full three-stage pathway is called aerobic respiration and yields roughly 36 to 38 ATP molecules per glucose. When oxygen is not available, a cell can still run glycolysis on its own, but must recycle its electron carriers by a different, simpler process called fermentation in order to keep glycolysis running at all. In human muscle cells during intense exercise, this is lactic acid fermentation, which produces the familiar burning sensation in overworked muscles; in yeast, it is alcoholic fermentation, producing ethanol and carbon dioxide gas, put to direct use in both bread-making (the carbon dioxide creates the rise) and brewing. Fermentation produces far less ATP than aerobic respiration, since it stops after glycolysis rather than continuing on to the Krebs cycle and electron transport chain.