Now what exactly does the process of phosphorylation actually entail? Well in this process that functional group that we transfer from some molecule onto that target enzyme of protein is the phosphoryl group. Now the question is what is the source of this phosphoryl group? What is that molecule that exists inside our cells that we can actually detach the phosphoryl group and attach it onto that protein? Well, inside our body, inside our cells we have a very high abundance of high energy ATP molecules adenosine triphosphates which are produced by the mitochondria of the cell. And so because we have this abundancy of high energy ATP molecules that contain the phosphoryl groups, these are the molecules that are the source of this phosphoryl group.