I am looking for examples in biology in which a metabolite that can diffuse freely across a cell membrane (through passive diffusion), once inside, gets modified to a form that cannot diffuse back anymore. The metabolite, in this way, gets effectively trapped inside and this can lead to a concentration gradient inside vs. outside.
One such example is the one of glucose. Glucose can penetrate cells through facilitated diffusion. Once inside, it gets phosphorylated to G6P which cannot diffuse out anymore. The phosphorylation is powered by ATP. At the end, thanks to this mechanism, one can have in principle [Gluc]_blood < [Gluc]_cell + [G6P]_cell. However, I didn't find references measuring higher glucose concentration in the cytoplasm compared to the one in the blood. Do you know any?
Any additional example in which the same happens would be appreciated.