Breaking down of glucose involves two processes. In the first step, it is broken into three-carbon molecules called pyruvate. The pyruvate is further broken down into energy in following different ways in various organisms.
1) Aerobic respiration : In this case, pyruvate is broken down into water and carbon dioxide along with release of energy. It commonly occurs in mitochondria of cells.
2) Anaerobic respiration: In aerobic respiration break down of pyruvate takes place in presence of oxygen to give rise 3 molecules of carbon dioxide and water. And pyruvate is converted into ethanol and carbon dioxide.