We have studied the mechanisms of two proteins that account for the majority of human energy expenditure: myosin in muscle cells, and the Sodium/Potassium Pump in nerve cells. Look carefully at those mechanisms again, or "play the movie" in your mind as you consider these single frames from the movie:
For each of these, we see a series of shape-changes. Some are caused by having other molecules bind to the protein. For example,
Some of the shape-changes include the breakage of ATP into ADP and phosphate:
Now, having studied these dynamic systems, see if you can answer this simple question: where do you see energy being used?
Energy in these Reactions
Actually, every shape-change requires energy. Most of the time, the energy is provided by the wiggling and bumping of the surrounding molecules, which we often refer to as "heat." Additional energy is provided by the release-of-energy associated with breakdown of ATP to ADP and phosphate...but this is never visible. There's no mystical ball of white light that flies out of ATP and makes the protein dance. In fact, the step in which myosin breaks ATP to ADP + P is not the step that requires the most energy. That step, or shape-change, is the "power stroke" in which myosin pulls itself forward along the actin filament. The power stroke is triggered by the simple act of having the phosphate fall off of the surface of the protein.
The mechanism of energy use in these reactions is remarkably mundane. It's just part of the mechanical shape-changes of the proteins. All of the molecules that these proteins interact with trigger shape-changes; they all act as "mechanical switches" that toggle the protein's shape from one form to another.
In principle, any toggle between "shape A" and "shape B" should be able to go from A to B, or from B to A. For example, if sodium ions can bind to the pump protein and flip it to a "closed" form, then it should be perfectly easy for the wiggling an bumping of thermal energy to open the pump protein and let the sodium ions swim back into the interior of the cell. This probably happens fairly often inside cells. So...why don't muscles "un-contract" and the Na/K Pump shove sodium into cells? This is where ATP comes in. It's easy to break ATP into ADP + P. It's hard to put it back together again. For one thing, the ADP and phosphate molecules swim away from each other once they've been separated. This is probably the easiest way to "see" what's happening. Once they have diffused away from each other, then the shape-changes are pretty much stuck, and can't go the other way.
In chemical terms, we'd describe this (partly because the ADP and P diffuse apart) as ATP molecules having a higher "inherent energy level" than do the combination of ADP and P. That is, breaking ATP to ADP and phosphate releases energy. Some of the energy is kinetic energy (the swimming apart aspect) and is referred to as "entropy;" part of the energy is referred to as "enthalpy." Together, these make up the total "free energy," or what I've called "inherent" energy in the molecules.
We can't see the energy (especially the "inherent" energy level of a molecule), but we can intuitively understand the impact of ADP and phosphate swimming off in different directions. This makes these reactions pretty much irreversible. From this, we can derive a kind of "rule of thumb:"