Glycolysis: The Energy Investment Phase

Glycolysis is the first step of cellular respiration in which glucose, a six-carbon sugar, is split into two different three-carbon sugars. The three-carbon sugars are then oxidized and rearranged to form two molecules of pyruvate (an ionized form of pyruvic acid).

There are two phases in glycolysis: the energy investment phase and the energy payoff phase. The energy investment phase requires an input of two molecules of ATP. In the energy payoff phase, four molecules of ATP are produced for a net total of two ATP during glycolysis.

The steps of the energy investment phase are explained below. Notice that all of the steps are catalyzed by enzymes.

The enzyme hexokinase catalyzes the transfer of a phosphate group from ATP to glucose, releasing ADP in the process. Glucose becomes glucose 6-phosphate, which is more chemically reactive than glucose. It is trapped in the cell because its phosphate group carries a negative charge.
The enzyme phosphoglucoisomerase converts glucose 6-phosphate to fructose 6-phosphate.
The enzyme phosphofructokinase catalyzes the transfer of a phosphate group from ATP to the opposite end of fructose 6-phosphate, which invests a second ATP molecule and releases ADP. This forms fructose 1,6-biphosphate. This is a key regulatory step in glycolysis because, once this step occurs, the substrate is committed irreversibly to glycolysis. Phosphofructokinase is inhibited by ATP; as ATP accumulates, glycolysis slows down.
The enzyme aldolase cleaves fructose 1,6-biphosphate into two different three-carbon sugars: glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
The enzyme isomerase converts DHAP to G3P so that G3P can be used in the next phase.

The next phase of glycolysis is the energy payoff phase, which you can read about here.

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