Glucose, by nature, is a small molecule with the ability to diffuse in and out of the cell. By phosphorylating glucose (adding a phosphoryl group in order to create a negatively charged phosphate group), glucose is converted to glucose-6-phosphate and trapped within the cell as the cell membrane is negatively charged. This reaction occurs due to the enzyme hexokinase, an enzyme that helps phosphorylate many six-membered ring structures. Glucose-6-phosphate cannot travel through the cell membrane and is therefore coerced to stay inside the cell. Phosphorylation takes place in step 3, where fructose-6-phosphate is converted to fructose-1,6-bisphosphate. This reaction is catalyzed by phosphofructokinase.