 Glycolysis is the first step of cellular respiration. Glycolysis takes place in the cytoplasm, represented here by the yard. Glycolysis means to break down sugar. Our cells break down glucose from our food. First, two ATP molecules give a phosphate group each to one glucose molecule. When the two ATP lose a phosphate group, they become ADP. Next, the glucose splits into two 3-carbon molecules. Two electron carrier molecules, called NAD+, then donate another phosphate group each to the three-carbon molecules. The NAD+, molecules also pick up an electron and become NADH. NADH goes straight to the third part of cellular respiration, the electron transport chain. Now each of the three-carbon molecules gives up their phosphate groups to ADP molecules, making four ATP molecules. Since two ATP were put into the system, a net of two ATP are produced in glycolysis per glucose molecule. The two three-carbon molecules are now molecules of pyruvate.