Cells get energy through a' process called cellular respiration。 During this process cells convert sugar( called giudosé) and oxygen into water and carbon dioxide、 Carbon dioxide is the gas we breathe out。 This whole process releases energy for the cell to use The energy is stored as ATP。 The cell keeps ATP in storage、 like" back up power。 " It can be taken out to be used as needed。 By storing ATP、 the cell always has the energy、 it needs .