Glycolysis [take a quiz on this topic]

Chapter Four: Cell Nutrition and Respiration Endocytosis and exocytosis Energy, exergonic reactions, and endergonic reactions How cells store energy: ATP Glycolysis Anaerobic processes The Krebs cycle and electron transport chain Chemiosmosis Energy yield for aerobic respiration Overview of photosynthesis Noncyclic electron flow and the Calvin cycle

As mentioned in the previous section, glycolysis is the first stage in the release of energy from the glucose molecule. It occurs in the cytoplasm with the help of a great many enzymes, and both aerobic and anaerobic organisms use glycolysis to break down glucose initially; only after this stage do aerobic organisms utilize oxygen to obtain additional energy.

Glucose is a very common sugar. Its molecular formula is C₆H₁₂O₆; that is, it contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. The previous section mentioned that glycolysis involves the breaking down of glucose into two smaller molecules or pyruvic acid. As you might expect, each pyruvic acid molecule has three carbon atoms, half of those in the original glucose molecule.

For glycolysis to occur, an initial "investment" of two ATP molecules is necessary. The first ATP molecule releases a phosphate group which then joins to the glucose molecule to form glucose phosphate. Then, the second ATP molecule contributes a phosphate group, forming a molecule called fructose diphosphate. The fructose diphosphate molecule soon splits into two molecules of PGAL (an abbreviation for a three-carbon molecule called glyceraldehyde phosphate). Each PGAL molecule releases electrons to a coenzyme called NAD+ (nicotinamide adenine dinucleotide) and phosphate groups and energy to ADP. As a result, two NAD+ molecules become NADH, and four molecules of ADP become ATP. In addition, the two molecules of PGAL have now become molecules of pyruvic acid, which has a molecular formula of C₃H₄O₃.

Remember that glycolysis required an "investment" of two ATP molecules before it could begin. Since four ATP molecules were formed as products of the reaction, there was a net gain of two ATP molecules.

At this point, pyruvic acid in anaerobic organisms undergoes one of two processes in order to obtain additional energy. These processes (discussed in the next section), however, are significantly less efficient than the processes which aerobes utilize: the Krebs cycle and the electron transport chain.