Glucose is used to produce high-energy ATP in a cell.
The three phosphate groups in an ATP molecule carry negative charges that strongly repel each other and give ATP a large amount of potential energy.
Fermentation is an alternate pathway used when oxygen levels are low.
Glycolysis can take place in the absence of oxygen; its product, pyruvate, enters the cellular respiration pathway or undergoes fermentation depending on the availability of oxygen.
All of the carbon atoms in glucose are incorporated into carbon dioxide: Two molecules are formed as pyruvate is converted to acetyl CoA, and four molecules are formed during the Krebs cycle.
The electrons lose energy as they move down the chain, and this energy is used to create a proton gradient that drives the synthesis of ATP.
Electron transport and chemiosmosis (oxidative phosphorylation) can yield around 26 molecules of ATP.
Glycolysis takes place in the cytoplasm, whereas the Krebs cycle and the electron transport chain, which generate the largest amounts of ATP during cellular respiration, take place in the mitochondria.