If you have just eaten, your body secretes to uptake glucose; if you have not eaten all day, your body secretes to promote glycogen breakdown.

If you have just eaten, your body will secrete insulin. Insulin is the hormone that allows glucose to enter cells. That way they will have the proper energy for their normal functions.
If you have not eaten for a while, your body is running out of glucose to feed the cells so it needs to secrete another hormone called glucagon. This hormone is the complete opposite of insulin because it breaks down glycogen, proteins and fats into glucose.

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.