What happens to the uterine lining between day 27 and day 1? what is this process called?

It is called the Human Menstruation cycle.

A. radioactive dating

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.