Phosphorylated sugar molecules do not readily penetrate cell mem- branes, because there are no specific transmembrane carriers for these compounds, and because they are too polar to diffuse through the lipid core of membranes. The irreversible phosphorylation of glu- cose (Figure 8.12), therefore, effectively traps the sugar as cytosolic glucose 6-phosphate, thus committing it to further metabolism in the cell. Mammals have several isozymes of the enzyme hexokinase that catalyze the phosphorylation of glucose to glucose 6-phosphate