ATP is nearly ubiquitous, and is generally regarded as a sort of universal currency in cellular energetics. ATP is an unstable molecule, and large amounts can not be accumulated in anticipation of cellular activity. Hence, ATP is often generated during activity, especially energy-intensive activities such as insect flight. ATP is part of the adenylate system. These three molecules, AMP, ADP and ATP, store and transduce most of the energy used in cellular metabolism.
Cells generally maintain a homeostasis of energy charge.
When the energy charge declines below a set point appropriate to any given cell, there follows a shift toward ATP-generating pathways. On the other hand, when the energy charge is high, cells favor ATP-consuming pathways. Just to give you a number, the energy charge of most cells is about 0.85, perhaps something higher than we might expect. We gain a bit more understanding by regarding the biological significance of the adenylate system. The adenylate system influences cellular metabolism in two ways.