The head of a phospholipid interacts with water because it is

Hydrophilic

A phospholipid is comprised of a phosphate hydrophilic head, which means that it is "water-loving," and a fatty acid hydrophobic tail, which is "water-hating." The head and the tail are joined together by a glycerol molecule. 

The phosphate head is attracted to water because it is charged (i.e. negatively). Water is a polar molecule, which means that there is an uneven distribution of charges within its molecular structure with the oxygen side being "more negative" than the rest of the atom (which is "more positive" near the hydrogen). Thus, the negatively-charged nature of the phosphate head and the parts of the water molecule which are positively charged enable the two to form an "attraction" towards one another. 

On the other hand, the hydrophobic tail is nonpolar, which means that it does not have a "more positive" or "more negative" side or part in its molecular structure. These differences in structure with water make the hydrophobic tail unattracted to water molecules and more attracted to other uncharged, nonpolar molecules (such as fats and oils).  

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.