Respiration has two meanings in biology: the physical process of gas exchange where organisms take in oxygen and release carbon dioxide, also called breathing or ventilation. The chemical process in cells where fuels like sugars and fats are broken down to release energy, also called cellular or biochemical respiration. Cellular respiration occurs through aerobic respiration which uses oxygen to break down glucose and produce carbon dioxide, water and ATP energy, or anaerobic respiration which breaks down glucose without oxygen to produce lactic acid or ethanol.