ATP and "High-Energy Phosphate Bonds"

The energy difference between ATP and ADP + phosphate is far less than the energy difference between glucose and its complete metabolic products, CO2 and H2O.

If all of the calories of glucose were used to build ATP molecules, we'd get about 97 ATP molecules for each glucose that cells metabolize. We don't get that many. We get something closer to 38. The process of transferring energy from food molecules to ATP molecules is not 100% efficient. Nothing is. 39% efficiency is really very good. The rest of the energy is lost through things like all of those enzyme shape changes, during which the enzymes bump water molecules and other molecules, and transfer kinetic energy to them. As these molecules move around a bit faster, bumping into other molecules, the kinetic energy is dissipated as thermal energy. This is part of how metabolism helps keep you warm.

97 ATPs per glucose? Yikes! Glucose must have a lot more energy in it than ATP does -- about 100 times more. So, why do we always see textbooks showing ATP with a little explosion diagram around it? Why do we see ATP's phosphate-phosphate bond referred to as a high energy phosphate bond? Mostly, it's traditional; the textbooks have been this way for decades, and textbooks typically repeat the ideas presented in the earlier editions.

The last phosphate bond of ATP, between the 2nd and 3rd phosphates, is more of a Goldilocks Bond. It has a high enough energy that breaking it can give a useful nudge to the enzyme that breaks it. [The nudge shows up, of course, as a shape-change in the enzyme.] At the same time, however, the bond has a low enough energy that enzymes can break it relatively easily, and so that cellular metabolism can re-build it relatively easily. A really strong bond would pack a wallop when broken, but it would be hard to rebuild. A really wimpy bond would be easy to rebuild, but wouldn't have much effect on enzymes when it's broken. The relevant bond of ATP is just right, which makes ATP a convenient molecule to use for moving energy around in cells.