Final answer:
In glycolysis, the two phosphates added to glucose come from ATP molecules during the phosphorylation steps catalyzed by the enzymes hexokinase and phosphofructokinase.
Step-by-step explanation:
Early in glycolysis, two phosphates are added to the glucose molecule. These phosphates come from adenosine triphosphate (ATP) molecules. The first phosphate is added when glucose is converted into glucose-6-phosphate by the enzyme hexokinase. This reaction uses one ATP as the donor of the phosphate group. Subsequently, under the action of the enzyme phosphofructokinase, glucose-6-phosphate is converted into fructose-6-phosphate, and then another ATP molecule donates its phosphate group to form fructose-1,6-bisphosphate. Essentially, the two ATP molecules invested early in glycolysis result in the formation of fructose-1,6-bisphosphate, which is then split into two phosphorylated three-carbon molecules.