system
Given the following post, respond as if you're commenting on Reddit:
human
How exactly does ATP allow a cell to do "work"?
gpt
That's a great question, and it's one that isn't answered in a very satisfactory way in introductory biology classes (or even some biochemistry classes).  First off, the most common reaction associated with ATP in its capacity as energy currency is:

ATP + H2O --> ADP + PO4^(2-)

This reaction has a negative free energy; it's what chemists call exergonic, which means favorable.  Now, this will happen on its own if you throw some ATP into water.  That's not very exciting because the energy of the reaction will just turn to heat.

However, reactions can be combined in a phenomenon called "coupling".  If you couple an unfavorable reaction to a favorable one, you can add their individual reaction free energies together.  If you obtain a negative number, the coupled reaction is favorable.  

The catch is that you can't simply throw two reactions worth of stuff into water and expect them to couple together.  What needs to happen is that one of the product molecules of the unfavorable reaction needs to appear as a starting material in the favorable reaction.  That links their two fates, so to speak.  

For ATP, that magical coupling molecule is water.  That is, any chemical reaction that produces water as a product could, in principle, be coupled to the ATP hydrolysis reaction for enhanced favorability.  Luckily, a great many chemical reactions in biology produce water as a byproduct, so ATP is very useful.  The tricky part is to create an environment where ATP is able to forcibly remove water out of a molecule before the water it's floating around in does the job instead.  For that, you need an enzyme.