Energy from glycolysis is used to make ATP by two different processes. During glycolysis the glucose molecules are each split into two smaller molecules. The initial glucose molecules contain six carbon atoms each. Each molecule of glucose produces two molecules of pyruvic acid, and each pyruvic acid molecule contains three carbon atoms. During glycolysis, energy is released from the bonds of glucose molecules and is used to join free phosphate ions (also called inorganic phosphate or Pi) with molecules of adenosine diphosphate (ADP) to make ATP. This type of ATP synthesis is called substratelevel phosphorylation.

As a by-product, however, electrons are also stripped from glucose. These electrons are immedi ately trapped and held by another very important molecule, the electron carrier nicotinamide adenine dinucleotide (NAD). By convention, the empty electron carrier is denoted as NAD+. When the molecule is carrying a pair of electrons, it is denoted as NADH, since the molecule also picks up a hydrogen nucleus, or proton. The electrons held by NADH represent potential energy. In the presence of oxygen, these electrons can be passed to the electron transport system to make ADP by oxidative phosphorylation, while at the same time regenerating NAD+, which is required to maintain glycolysis. This second process for making ATP results in about eight times as much ATP per glucose molecule than from substrate-level phosphorylation in glycolysis. Because fermentation is carried out in the absence of oxygen, this process cannot be used. Instead, the NADH must be relieved of its electrons by an alternative process. The NAD+ regeneration mechanism varies according to the type of organism.

Glucose molecules are relatively stable and do not split readily. For glucose molecules to split, they must be energized by the addition of two phosphate groups to each glucose molecule from two ATP molecules. The third phosphate from each ATP molecule is transferred, along with its high-energy bond. Therefore, the initial steps of glycolysis actually use ATP, depleting some of the cell's energy stores. Once glucose is energized, it readily splits under the influence of the appropriate enzyme. Each half of the glucose molecule then attaches another phosphate group from the cell's pool of Pi.Ina series of reactions, each half of the glucose molecule generates two ATP molecules by substrate-level phosphorylation. Therefore, glycolysis results in a net gain of two molecules of ATP per molecule of glucose. At the end of glycolysis there are two three-carbon molecules of pyruvate left over for each original glucose molecule.

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