[enuru99]

Like I know that ∆H of ATP->ADP is negative, thus is exothermic. So supposedly we use the hydrolysis of ATP to do useful things but I cannot quiet understand how can we use the "released energy". What's more, does this "released energy" everybody talks about even mean heat?

Edit: In few words what I'm asking is how the hydrolysis of ATP leads to "work" being done which seems like a fairly normal question but there are not many sites that cover it.

[Corribus]

It is not so much that ATP "releases energy". It's that ATP is a "high potential energy molecule", which means it is capable of being involved in a lot of thermodynamically favorable processes.

Think of it this way: suppose you are standing at the top of a mountain with a sled. You have a lot of potential energy, which means you have the capability of going a lot of different places. You can go to the east, to the west, to the north, all you need is a push in the right direction. And even if there are a number of smaller peaks and valleys in the way, it doesn't matter because your potential energy is so large you can cruise right over them. When you fly off in whatever direction, that potential energy will be transformed into a useful product: kinetic energy (speed), which gets you where you want to go. And some not-so-useful byproducts (heat and sound - due to friction).

ATP is at the top of the chemical hill. It has a lot of useful chemical potential energy. The energy that is stored isn't "released into the environment" like an explosion. Rather, it is used to drive biochemically useful reactions. The bond energy within ATP is used to form and break bonds in other biochemical substrates, typically with the help of enzymes to make the reactions more efficient. And as usual there are byproducts such as heat, which has its own value to most organisms.

[Babcock_Hall]

There are several ways in which cleavage of a phosphoanhydride bond in ATP can do work.  One, the enzyme Na⁺ K⁺ ATPase follows coupling rules.  ATP may only be hydrolyzed when 3 sodium ions and 2 potassium ions are transported in opposite directions.  The sodium ions are moved uphill thermodynamically.  Two, myosin/actin ATPase hydrolyzes ATP to drive a conformational change that causes filaments to slide past each other, leading to muscle contraction.

Other enzymes do not hydrolyze ATP; instead, they use other nucleophiles.  Consider the synthesis of glucose 6-phosphate, the first step in glycolysis:

Glucose + H₂O glucose 6-phosphate (moderately uphill)
ATP + H₂O ADP + phosphate (very downhill)
Hexokinase is an enzyme that effectively combines these two reactions into a single one:
Glucose + ATP glucose 6-phosphate + ADP (moderately downhill)

It is preferable to think in terms of ΔG, as opposed to ΔH.