cellular respiration is when a cell converts from oxygen and carbohydrates into carbon dioxide, water, and energy & living things require respiration to use energy ->  living things must convert oxygen into carbon dioxide to use energy  living things must convert oxygen into carbon dioxide to use energy & breathing is when animals convert oxygen into carbon dioxide ->  living things must breathe to use energy  breathing out is when animals exhale carbon dioxide from their lungs into the air & animals are a source of carbon dioxide by exhaling ->  animals are a source of carbon dioxide by breathing  animals are a source of carbon dioxide by breathing & a source of something produces that something ->  animals produce carbon dioxide by breathing  living things must breathe to use energy & animals produce carbon dioxide by breathing -> animals produce carbon dioxide by using energy