<P> Starting with glucose, 1 ATP is used to donate a phosphate to glucose to produce glucose 6 - phosphate . Glycogen can be converted into glucose 6 - phosphate as well with the help of glycogen phosphorylase . During energy metabolism, glucose 6 - phosphate becomes fructose 6 - phosphate . An additional ATP is used to phosphorylate fructose 6 - phosphate into fructose 1, 6 - disphosphate by the help of phosphofructokinase . Fructose 1, 6 - diphosphate then splits into two phosphorylated molecules with three carbon chains which later degrades into pyruvate . </P> <P> Glycolysis can be literally translated as "sugar splitting". </P> <P> Pyruvate is oxidized to acetyl - CoA and CO by the pyruvate dehydrogenase complex (PDC). The PDC contains multiple copies of three enzymes and is located in the mitochondria of eukaryotic cells and in the cytosol of prokaryotes . In the conversion of pyruvate to acetyl - CoA, one molecule of NADH and one molecule of CO is formed . </P> <P> This is also called the Krebs cycle or the tricarboxylic acid cycle . When oxygen is present, acetyl - CoA is produced from the pyruvate molecules created from glycolysis . Once acetyl - CoA is formed, aerobic or anaerobic respiration can occur . When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle . However, if oxygen is not present, fermentation of the pyruvate molecule will occur . In the presence of oxygen, when acetyl - CoA is produced, the molecule then enters the citric acid cycle (Krebs cycle) inside the mitochondrial matrix, and is oxidized to CO while at the same time reducing NAD to NADH . NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation . To fully oxidize the equivalent of one glucose molecule, two acetyl - CoA must be metabolized by the Krebs cycle . Two waste products, H O and CO, are created during this cycle . </P>

When is oxygen used during cell respiration and what is formed from the oxygen