Energy-requiring Steps of Glycolysis

The energy-requiring phase is the first phase of glycolysis. As glucose enters the cell, the enzyme hexokinase transfers a phosphate group from a molecule of ATP to the 6-carbon sugar to produce glucose 6-phosphate, which becomes trapped inside the cell due to its negative charge.

Next, the enzyme phosphoglucose isomerase catalyzes the conversion of the phosphoglucose into one of its isomers, fructose 6-phosphate.

The phosphofructose can now be phosphorylated by a rate-limiting enzyme, phosphofructokinase, to produce fructose 1,6-bisphosphate.

Finally, with two phosphate groups attached, the sugar molecule is cleaved by aldolase into two 3-carbon isomers, glyceraldehyde 3-phosphate or G3P, and dihydroxyacetone phosphate, DHAP. Another enzyme, triosephosphate isomerase, converts the DHAP into G3P to yield two molecules.

This way, during the energy investment phase, a net total of two ATPs are used to split one initial glucose molecule into two smaller sugars.