The synthesis of ATP by mitochondria is, of course, called oxidative phosphorylation. In an isolated system, the mitochondria return to state IV respiration when the ADP is used up. The state IV rate may be higher than before, due to an uncoupling effect by ADP itself. It must be emphasized that in living cells there is no such thing as state III or state IV respiration. Some of the energy in the gradient is lost as heat as protons escape via nonspecific mechanisms.
Some energy is used for symport processes as described elsewhere.
The degree to which each process takes place depends on cellular demand. Functions of mitochondria are subject to regulation just as are other cellular processes. Uncoupling of Isolated Mitochondria by ADP When conducting studies of state III respiration by isolated mitochondria a phenomenon is frequently observed in which the state IV rate that resumes after ADP is used up is greater than the initial state IV rate. Having a probable explanation for the observation can be reassuring. A probable explanation is an uncoupling effect by ADP itself.