Glycolysis can be defined simply as the lysis, or splitting, of sugar. More particularly, it is the controlled breakdown of glucose, a 6-carbon carbohydrate, into pyruvate, a 3-carbon carbohydrate. Organisms frequently store complex carbohydrates, such as glycogen or starch, and break these down into glucose units which can then enter into glycolysis.
Two features of glycolysis suggest that it has an ancient evolutionary origin. First, the same series of reactions occur in virtually all cells, including bacteria, plants, fungi, and animals. Second, glycolysis does not require oxygen, making it appropriate for primeval cells which had to live in a world with very little atmospheric oxygen.
Glycolysis has several important features:
- It breaks down one molecule of glucose, a 6-carbon molecule, into two molecules of pyruvate, a 3-carbon molecule, in a controlled manner by ten or more enzymatic reactions. The oxidation of glucose is controlled so that the energy in this molecule can be used to manufacture other high energy compounds (see 2 and 3 below).
- It makes a small amount of ATP, a process known as substrate-level phosphorylation. For each glucose molecule that is broken down by glycolysis, there is a net gain of two molecules of ATP.
- It makes NADH (reduced nicotinamide adenine dinucleotide), a high energy molecule which can be used to make ATP in the electron transfer chain (see below). For each glucose molecule that is broken down by glycolysis, there is a net gain of two molecules of NADH.
- It makes compounds which can be used to synthesize fatty acids. In particular, some of the carbohydrate intermediates of glycolysis are used by other enzymatic reactions to synthesize fatty acids, the major constituents of lipids, important energy storage molecules.