4.13: The first step of cellular respiration: glycolysis is the universal energy-releasing pathway.

To generate energy, fuels such as glucose and other carbohydrates, as well as proteins and fats, are broken down in three steps: (1) glycolysis, (2) the Krebs cycle, and (3) the electron transport chain. Glycolysis means the splitting (lysis) of sugar (glyco-), and it is the first step that all organisms on the planet take in breaking down food molecules; for many single-celled organisms, this one step is sufficient to provide all of the energy they need (FIGURE 4-29).

As FIGURE 4-30 illustrates, glycolysis is a sequence of chemical reactions (there are 10 in all) through which glucose is broken down, resulting in two molecules of a substance called pyruvate. Glycolysis has two distinct phases: an “uphill” preparatory phase and a “downhill” payoff phase.

Just as you sometimes have to spend money to make money, before any energy can be extracted from glucose, some energy must be added to the molecule. This addition occurs during the “uphill” phase. The additional energy (which comes from ATP) destabilizes the glucose molecule, making it ripe for chemical breakdown. Once the glucose can be broken down chemically, the “payoff phase” begins as energy stored in its bonds can be harnessed as the bonds are broken.

Three of the 10 steps in glycolysis yield energy. In two of these three steps, as bonds in the sugar are broken and other, lower-energy bonds are formed, the energy released is quickly harnessed by the attachment of phosphate groups to molecules of ADP to create energy-rich ATP molecules. In the third energy-yielding step of glycolysis, electrons originally from the glucose are transferred to NAD⁺ to become the high-energy electron carrier NADH. Later (in an electron transport chain in the mitochondria), this energy will be converted to ATP. The net result of glycolysis is that each glucose molecule is broken down into two molecules of pyruvate. During this breakdown, some of the released energy is captured in the production of energy-rich ATP molecules and molecules of the high-energy electron carrier NADH. Two molecules of water are also produced during glycolysis.

Glycolysis can proceed regardless of whether any oxygen is present. When oxygen is present, glycolysis isn’t the end of the story. But in the absence of oxygen and in many yeasts and bacteria, glycolysis is the only game in town for fueling activity. Because single-celled organisms have much lower energy needs, they can function solely on the yields of glycolysis. For many organisms (including humans), however, glycolysis is a springboard to further energy extraction. The additional energy payoffs come from the Krebs cycle and the electron transport chain.