What protects the brain and the spinal cord?

   Membranes called meninges cover and protect the brain and spinal cord. The skull and spinal column also protect the brain and spinal cord, respectively. The spinal cord is made up by individual vertebrae.

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.