called NADox and phosphorylated; both these actions are catalyzed by a single enzyme. The strongly exergonic oxidation reaction supplies the energy to create high-energy bonds between each G3P and its new phosphate group and produces two molecules of NADre. Thus, in the seventh reaction, by breaking this high-energy bond, each of the two molecules of the diphosphorylated three-carbon compound can supply enough energy to phosphorylate a molecule of ADP into ATP. Next, during the eighth reaction, the remaining phosphate group of each three-carbon compound is transferred to another location on the same molecule; the resulting molecule is dehydrated in the ninth reaction. In the tenth reaction, the phosphate bond of each of the three carbon molecules is broken to produce a molecule of ATP and a molecule of pyruvic acid, a three-carbon compound with no phosphate groups attached. Although a net gain of two ATP molecules has resulted (recall that two three-carbon sugars reacted), the energy stored in these molecules represents only two percent of the total present in the original molecule of glucose. The two pyruvic acid molecules must be broken apart to release more of the energy. Where oxygen is available, most organisms have evolved aerobic processes for degrading the pyruvic acid in order to release more of the
original glucose molecule's energy. But where no oxygen is available, each pyruvic acid molecule is subjected to a reduction reaction by a molecule of NADre to produce two molecules of either ethanol or lactic acid and two of NADox. This process is called fermentation, and, although it does reoxidize a cell's typically limited supply
