In glycolysis, glucose is partially oxidized, forming two smaller molecules called pyruvate

Glycolysis, the fi rst of the three stages in cell respiration, is where glucose oxidation begins (see Figure 7.9). By the end of glycolysis, glucose has been split into two molecules of pyruvate. These pyruvates will then be the starting material for the Krebs cycle. A small amount of ATP is generated in glycolysis, and some of the electrons originally with the glucose are transferred to NAD+, forming NADH. We will now consider these events in more detail.

Glycolysis is common to prokaryotic and eukaryotic cells alike. Glycolysis literally means “the splitting of glucose,” which accurately describes what happens. The important steps of glycolysis are presented in Figure 7.10. Observe that a molecule of glucose (a 6-carbon molecule) is broken down in a series of steps, into two molecules of pyruvate (a 3-carbon molecule). Along the way, several notable events occur. First, early in glycolysis there are energy-requiring (endergonic) steps, coupled to ATP hydrolysis, which provides the energy necessary for them to occur. Although the goal of cell respiration is to make ATP, a small amount of ATP must be consumed in these early steps.