So what we ultimately want to do in this process is we want to attach a phosphoryl group onto the GDP, the guanitine diphosphate, to form the guanosine triphosphate GTP. The problem with carrying out this step is it requires an input of energy. So the process by which we attach the pi onto the GDP to form the GTP is an endergonic process. And so we actually have to undergo some other process that is exergonic to basically couple this endergonic process. Now remember one important fact about succincto enzyme A, and generally speaking, in the citric acid cycle, whenever we see a thyl ester bond between the carbon and the sulfur of the coenzyme a molecule, that bond showed in red is a very unstable high end energy bond. In fact, when we cleave this bond, that will release a certain amount of free energy.