Cells are simply the basic units from which living things are made of, some are single cell organisms while others could be made of thousands, millions or billions of cells.
Metabolism refers to the sum total of chemical reactions that take place usually, but not by any means always, inside cells e.g they could also occur in the blood plasma or in inter-cellular spaces. They are indispensable for sustaining life.
The reason why the cells have more ADP than ATP molecules is that the cells have the tendency to use more ATP, rather than restore ADP molecules.
When the body produces energy, in order to be able for it to be released in the body and to be used by the body, the ATP molecules break up. During that break up of the ATP molecule it actually loses one phosphate group, thus becoming an ADP molecule. Since this process is going on constantly, the ATP molecules are constantly breaking up thus resulting in constant new ADP molecules, so they are easily becoming outnumbered.
