sentences: a source of something produces that something & cellular respiration is a source of energy for cell activities ->  cellular respiration produces energy for cell activites  cellular respiration produces energy for cell activites & cellular respiration is when cells extract energy from food to produce energy ->  cellular repiration produces energy by extracting energy from food  cellular repiration produces energy by extracting energy from food & all cells perform cellular respiration -> all cells produce energy by extracting energy from food