Most of our energy comes from a tiny little energy factory in our cells. This tiny little factory was essentially a small bacterium called mitochondrion that our ancestral cells swallowed when they had a little microscopic stand-off. Eventually they got to know each other over the years and are now running the multibillion cell industry that is our body. In order to handle the beast, cells had to tame down their protein synthesis capability down to a strict minimum where the mitochondria produce energy but nothing more. A proof of that is the poor quality of mitochondria protein synthesis system compared to the regular cell’s one.