Glycolysis is a series of chemical reactions in the cytoplasm of a cell in which glucose is essentially split in half. In the process, a small amount of energy is released and captured in ATP. For many organisms, glycolysis is just the first of three steps—together called cellular respiration—in which glucose is completely broken down and used to produce a much larger amount of ATP.
Glycolysis has two distinct phases: an "uphill" preparatory phase and a "downhill" payoff phase. One way to understand the two phases of glycolysis is to compare the process to a downhill bike ride. Glucose has a lot of potential energy. A cyclist at an uphill starting spot has a lot of potential energy, too. With little effort, the rider taps the potential energy and coasts to the downhill spot.
But sometimes, to reach a downhill spot, the cyclist must first go uphill—investing energy to get to the top of the hill. But on the way down, the cyclist gets back most of that energy investment, along with the reward of a longer ride.
Glycolysis is similar. Although glucose is packed with energy, the energy is not readily released unless energy from ATP is first added. When ATP is added to the reaction, the resulting sugar molecule now has two phosphate groups, which makes it less stable than before—and ripe for chemical breakdown.
In the payoff phase, three reactions now yield energy. In one reaction, each of the halves from glucose is attached to a high-energy phosphate group, and each donates high-energy electrons, along with protons (also called hydrogen ions), to electron carriers called NAD+.
The resulting NADH molecules are high-energy electron carriers that are used later in cellular respiration for an even greater energy payoff. The essential difference between NAD+ and NADH is that NADH carries one additional proton and two additional high-energy electrons.
As glycolysis continues and more bonds from the two halves of glucose are broken, the energy released is quickly recaptured by attaching phosphate groups to molecules of ADP, creating energy-rich ATP molecules.
Two molecules of water are also produced during glycolysis.
Near the end of glycolysis, two ADP molecules take the remaining phosphate groups, forming two additional ATP molecules. By the end of glycolysis, a molecule of glucose has been broken down into two molecules of pyruvate.
Glycolysis results in the production of 2 NADH and 4 ATP molecules. However, two ATP molecules were originally invested (represented by the two remaining ADP molecules), so glycolysis nets 2 ATP and 2 NADH molecules per molecule of glucose. However, much more energy still remains in the pyruvate molecules. Cells get much more "energy bang" for their "food buck" if they now perform the Krebs cycle and the electron transport chain.