Cellular respiration begins with glucose. Glucose is the primary fuel used in glycolysis, the first stage of cellular respiration. This molecule is found in the gel-like substance called cytoplasm that fills the cell. In glycolysis, glucose undergoes phosphorylation by ATP. The ATP is converted back to ADP. The glucose molecule is rearranged and undergoes a second phosphorylation by ATP. The second ATP is also converted back to ADP. Glucose, a six-carbon molecule, is split into 2 three-carbon molecules of PGAL. As oxidation takes place, NAD picks up a hydrogen atom from each molecule to form 2NADH. Phosphorylation takes place again. For each molecule, 2ADP combine with two phosphates to form a total of 4 ATP, and pyruvic acid molecules are formed. A net gain of 2 ATP have been formed. As the pyruvic acid molecules make their way toward the mitochondria, two more molecules of NADH are formed. A carbon atom also breaks off and combines with two oxygen atoms to produce the waste product carbon dioxide. As a result of these reactions, each pyruvic acid molecule is transformed into a two-carbon compound called an acetyl group. The two acetyl groups combine with a molecule of coenzyme A each to form two acetyl coenzyme A molecules. These molecules are processed in a complex pathway called the Krebs cycle. As the Krebs cycle progresses through both pyruvic acid molecules, six molecules of NADH, two molecules of ATP, and additional carbon dioxide are all formed. FAD picks up a hydrogen atom from each pyruvic acid molecule, and a total of 2 FAD2 are formed. NADH and FAD2 transport their hydrogen atoms and electrons to the electron transport chain. The electrons are passed down the chain of attracting molecules until they reach oxygen. Joined with hydrogen, they combine with the oxygen to form water. As the electrons flow down the electron chain, they release energy that is used to make more ATP. Eight of the ten NADH form three ATP each, while the other two form two ATP. Each of the FADH2 form two ATP. The number of ATP produced here in the electron transport chain is 32. The total number of ATP is 38, but the use of two ATP in glycolysis gives a net gain of 36 ATP.