Glycolysis: Preparatory Phase

Glucose is the primary source of energy in most cells of the human body. It is catabolized inside the cell cytosol by a series of enzymes to yield two molecules of pyruvate - a process called glycolysis.

Glycolysis is divided into two phases, based on whether ATP is catabolized or synthesized.

In the first phase of glycolysis, hexokinase phosphorylates glucose to produce glucose-6-phosphate, consuming one ATP.

Next, phosphoglucose isomerase converts glucose-6-phosphate into fructose-6-phosphate, which is further phosphorylated by phosphofructokinase into fructose-1,6-bisphosphate consuming another ATP.

Aldolase then cleaves fructose-1,6-bisphosphate to yield two 3-carbon molecules, glyceraldehyde-3-phosphate or G3P and dihydroxyacetone phosphate or DHAP.

Finally, the triosephosphate isomerase converts DHAP to G3P in a reversible reaction. So, in the first phase, glucose is converted into two G3P molecules by expending two ATP.