With the general strategy of glucose metabolism in mind, we now can look more closely at the first and oldest part of the process, glycolysis. Ten steps are involved in the breakdown of glucose to pyruvate, as outlined on the next page, and each step is controlled by its own enzyme. This process takes place in solution in the cytoplasm, or fluid, of the cell.The first five steps are only pump-priming operations designed to convert one molecule of glucose into two molecules of glyceraldehyde-3-phosphate (G3P):

The free energy that is brought to this reaction by ATP is stored in the phosphate bonds of G3P. These two molecules of G3P produced from a glucose molecule stand poised at the top of the energy hill in the graph on page 9, ready to tumble down to the level of pyruvate and ultimately to C0₂ and H₂0, releasing energy in the process. To make G3P from glucose, the glucose first is phosphorylated with ATP and rearranged to fructose-6-phosphate, and a second phosphate group is added from another ATP. This molecule then is broken into two fragments, and one fragment is rearranged so that both of them end as G3P.

In the next five steps, from G3P to pyruvate, the energy in the G3P molecule is "cashed in" by using it to make ATP from ADP, and NADH from NAD⁺:
The largest single free energy drop occurs between G3P and diphosphoglycerate (DPG), with the storage of energy in NADH. The large free energy yield occurs because this really is a disguised oxidation step, converting an aldehyde into a phosphate ester on the same oxidation level as a carboxylic acid. (Can you see this in the molecular diagrams on page 13?) Four smaller free energy steps then take DPG to pyruvate, with the production of four molecules of ATP. Two of these ATP make up for the two that were used to get the process started, and the other two remain as energy "profit" from the reactions. The entire process from glucose to pyruvate shows a net gain of two ATP and two NADH, which eventually will yield six more ATP, for a total of eight moles of ATP per mole of glucose consumed. Pyruvate is not the end of the energy road by any means, as the free energy graph on page 9 indicates, but the yield obtained from glycolysis is respectable.