We live in a midrange of energy as well as size. A living organism must have enough energy to drive it, but not enough to destroy it. Nuclear fission and fusion reactions are useless to a living cell because they create so much energy that the molecules and arrangements of molecules that the cell needs to function would be completely shattered. The main problem today in commercial adaptation of controlled nuclear fusion as an energy source is that we know of no material that can withstand the temperatures developed by fusion, and that can be used to make a container for the reaction. For stars this problem is solved by simple physical separation in space.

Living chemical systems tap a milder energy source, the energy involved in the making and breaking of chemical bonds between atoms, rather than the energy involved in altering atomic nuclei. Such chemical reactions have nothing to do with nuclear forces, but involve only the shifting about of electrons outside the nucleus. Energies involved in covalent bonds are typically one ten-millionth the energies of nuclear reactions, yet these relatively weak interatomic bonds are strong enough to tie atoms together, and to build structures of a complexity unmatched at the stellar or nuclear levels. When we turn our backs on the harsh conditions of stellar interiors and nuclear reactions, we enter the world of the molecule. Large and complex molecules are not common in our universe, but they are essential for the occurrence of life.