Glycolysis literally means “splitting sugars. ” In glycolysis, glucose (a six carbon sugar) is split into two molecules of a three–carbon sugar. Glycolysis yields two molecules of ATP (free energy containing molecule), two molecules of pyruvic acid and two “high energy” electron carrying molecules of NADH. Glycolysis can occur with or without oxygen. Glycolysis (glucose + –lysis) is the metabolic pathway that converts glucose into pyruvate.
The free energy released in this process is used to form the high–energy compounds ATP (adenosine triphosphate) and NADH (Nicotinamide adenine dinucleotide). Glycolysis is a purely anaerobic reaction. While it can happen in the presence of oxygen, oxygen is never involved in the reaction, nor does it alter it.
Ingested carbohydrates are first broken down by our saliva, and then by enzymes in our stomach. Glucose, from our food, is then taken into our cells, and during glycolysis, glucose is split into two 3–carbon molecules of pyruvate. Glucose is not the only sugar used by our body; other sugars are either converted to glucose or introduced at other points in the glycolytic pathway. Likewise, fats and proteins can be used as energy sources. These foods are catabolized and the metabolic breakdown products enter the cellular respiratory pathway at various points. We generally take glucose as an example because, it makes use of all the metabolic steps, and hence illustrates the complete process of cellular respiration. Glycolysis is initiated by the addition of a phosphate (P), from ATP, to a molecule of glucose; this destabilizes the glucose molecule and the bonds are then easily broken to release energy.
Glycolysis is a ten–step process. It takes two molecules of ATP to break down one molecule of glucose to pyruvate. Therefore, there is a net gain of two ATPs and two NADHs from one molecule of glucose from glycolysis alone. Most of the energy remains in the pyruvate molecule.