Cellular respiration is a process that eukaryotes and prokaryotes use to harvest energy. In humans this energy comes from the food we eat. Cellular respiration is a catabolic pathway used to make ATP which is then used to do work in the cells. There are three main stages to cellular respiration; glycolysis, the krebs cycle, and electron transport. Glycolysis means “splitting sugars”. In glycolysis a 6 carbon glucose molecule is split into two three carbon molecules, and releasing two ATP and two NADH. Glycolysis occurs with or without oxygen. When oxygen is present gylcolysis is the first stage of cellular respiration. Without oxygen glycolysis produces only small amounts of ATP in a process called fermentation.The krebs cycle occurs after the two pyruvic acid molecules are converted into acetyl CoA. This process produces two ATP and NADH and FADH. The krebs cycle occurs only in the presence of oxygen. The electron transport chain uses the NADH molecules and occurs in the mitochondrial membrane. The NADH is split into NAD and H ions, releasing two high energy electrons in the process. These electrons pass through many electron carriers pumping hydrogen ions into the intermembrane space in the process. This creates a hydrogen ion gradient used by ATP synthase to make ATP. At the end of the electron transport chain an oxygen molecule is combined with four hydrogen ions to form 2 water molecules. The maximum production of ATP in cellular respiration is between 36 and 38 molecules.