Cellular Respiration 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 molecu