Cells, like all things, need energy to operate. CD players use batteries for
energy, cells use adenosine triphosphate (ATP) for energy.
players, cells can produce their own energy. People don't eat ATP, though. Their
consists mostly of proteins, lipids, and
polysaccharides (sugars) which need to be broken
down into ATP. There are three major stages in converting food into ATP: food
breakdown, glycolysis, and the Kreb Cycle.