Final answer:
Hexokinase catalyzes the phosphorylation of glucose using ATP as the source of phosphate. This prevents the glucose molecule from leaving the cell.
Step-by-step explanation:
The enzyme hexokinase catalyzes the phosphorylation of glucose, using ATP as the source of phosphate, producing glucose-6-phosphate. This reaction prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins and it cannot leave the cell because the negatively charged phosphate will not allow it to cross the hydrophobic interior of the plasma membrane.