So the red term phosphoryl group has been transferred onto this oxygen. Now because inside our cells, we have this abundancy of ATP molecules. It's inside the cells that the proteins basically undergo this process. And those proteins found outside the cells do not actually undergo this process of phosphorylation because outside the cells, ATP is not abundant. So reversible phosphorylation occurs inside the cells where the concentration of ATP is high and abundant. But proteins found outside the cells are not regulated in this method of phosphorylation because outside the cells we don't have an abundant supply of ATP like we have inside the cells.