The process by which organisms convert food into energy is known as cellular respiration. This metabolic pathway involves breaking down glucose and other nutrients to produce adenosine triphosphate (ATP), which cells use for energy. In aerobic respiration, glucose is combined with oxygen to generate ATP, carbon dioxide, and water. This process occurs in the mitochondria of cells and consists of glycolysis, the Krebs cycle, and the electron transport chain. In anaerobic respiration, organisms undergo fermentation to produce energy without oxygen, resulting in byproducts such as lactic acid or alcohol.