ADP as a substrate for phosphorylation

In most aerobic cells the level of ATP exceeds that of ADP by 4- to 10-fold.

It is convenient to think of respiratory control as a dependence of respiration on ADP as a substrate for phosphorylation.

If the energy demands on a cell cause ATP to be consumed at high rates, the resultant accumulation of ADP will stimulate respiration, with concomitant activation of ATP resynthesis.

Conversely, in a relaxed and well-nourished cell, ATP accumulates at the expense of ADP, and the depletion of ADP limits the rate of both electron transport and its own phosphorylation to ATP.

Thus, the energy-generating capacity of the cell is closely attuned to its energy demands.