Overview
The overall reaction of glycolysis is:
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+ 2 + + 2 + 2 i | 2 | + 2 + 2 H+ + 2 + 2 H2O |
The use of symbols in this equation makes it appear unbalanced with respect to oxygen atoms, hydrogen atoms, and charges. Atom balance is maintained by the two phosphate (Pi) groups:
- each exists in the form of a hydrogen phosphate anion (HPO42-), dissociating to contribute 2 H+ overall
- each liberates an oxygen atom when it binds to an ADP (adenosine diphosphate) molecule, contributing 2 O overall
Charges are balanced by the difference between ADP and ATP. In the cellular environment, all three hydroxy groups of ADP dissociate into -O- and H+, giving ADP3-, and this ion tends to exist in an ionic bond with Mg2+, giving ADPMg-. ATP behaves identically except that it has four hydroxy groups, giving ATPMg2-. When these differences along with the true charges on the two phosphate groups are considered together, the net charges of -4 on each side are balanced.
For simple fermentations, the metabolism of one molecule of glucose to two molecules of pyruvate has a net yield of two molecules of ATP. Most cells will then carry out further reactions to 'repay' the used NAD+ and produce a final product of ethanol or lactic acid. Many bacteria use inorganic compounds as hydrogen acceptors to regenerate the NAD+.
Cells performing aerobic respiration synthesize much more ATP, but not as part of glycolysis. These further aerobic reactions use pyruvate and NADH + H+ from glycolysis. Eukaryotic aerobic respiration produces approximately 34 additional molecules of ATP for each glucose molecule, however most of these are produced by a vastly different mechanism to the substrate-level phosphorylation in glycolysis.
The lower-energy production, per glucose, of anaerobic respiration relative to aerobic respiration, results in greater flux through the pathway under hypoxic (low-oxygen) conditions, unless alternative sources of anaerobically oxidizable substrates, such as fatty acids, are found.
Metabolism of common monosaccharides, including glycolysis, gluconeogenesis, glycogenesis and glycogenolysis |
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Read more about this topic: Glycolysis