It is a very simple explanation. When the chemical bonds are broken inside our body, the energy that gets released in our body is used to perform several important functions. This is the main relation between chemical bonds and energy in the body. I hope that this is the answer that has come to your great help.
The 3d sublevel is not filled until after the 4s sublevel, because the 3d sublevel has more energy than the 4s sublevel, and less energy than the 4p sublevel. (:
