The process of cellular respiration can be described as a set of metabolic (catabolic) reactions that convert biochemical energy from nutrients into adenosine triphosphate (ATP). Nutrients or reactants of these reactions are carbohydrates, fats, and proteins, more precisely their basic components like glucose, free fatty acids amino acids. Aerobic cellular respiration requires oxygen (O2) in order to create energy (ATP). The first step is the breakdown of glucose to pyruvate which occurs in the cytosol (no O2 needed). Released pyruvate then enters the mitochondria in order to be fully oxidized by the Krebs cycle (citric acid cycle). The products of this process are carbon dioxide (CO2) and water (H2O) and ATP.