system
Given the following post, respond as if you're commenting on Reddit:
human
What is energy?

I'm a kinesiology major so the energy I learn about comes from ATP. When the terminal phosphate breaks off from the rest of the phosphates, the bond breaks and energy is released. But what *is* that energy? Is it actually something, or is just a term used to describe something with no physical qualities?

Also, I suck at categorizing my posts so sorry if I should have chosen something else.
gpt
Energy is a conserved mathematical property in time-invariant systems. It lets us quantify how much work can be done. It is no more or less physical than any other measurement - say, height of a table.

So let's say a book sits on a shelf. We can say it has gravitational potential energy because once it falls off that shelf, it will gain kinetic energy. The _system_ of "book on shelf" therefore has energy - more energy than the system of "book on floor".

I will just add that the breaking of a bond does not release energy - it's a very common but incorrect short-hand taught, especially in biology. In all chemical systems, the breaking of a chemical bond _requires_ energy, since a chemical bond is by definition an energetically favourable configuration of electrons - think bottom of a valley if the valley is the energy landscape. Therefore to break the bonds requires input of energy.

When you say that the phosphoester bond in ATP is broken and energy is released, what's meant is that the bond is broken _and new, more stable bonds are formed_, and that total reaction releases energy. It is akin to pushing a ball out of the aforementioned valley, only to have to roll down to a even deeper valley.

So a chemical bond stores energy in the sense that there can be alternate configurations of atoms that are even more stable, thereby releasing energy to do work on something else.