Glycolysis

Glycolysis is a series of biochemical reactions by which a molecule of glucose (Glc) is oxidized to two molecules of pyruvic acid (Pyr).

The word glycolysis is from Greek glyk (meaning sweet) and lysis (meaning dissolving). It is the initial process of many pathways of carbohydrate catabolism, and serves two principal functions: generation of high-energy molecules (ATP and NADH), and production of a variety of six- or three-carbon intermediate metabolites, which may be removed at various steps in the process for other intracellular purposes (such as nucleotide biosynthesis).

Glycolysis is one of the most universal metabolic processes known, and occurs (with variations) in many types of cells in nearly all types of organisms. Glycolysis alone produces less energy per glucose molecule than complete aerobic oxidation, and so flux through the pathway is greater in anaerobic conditions (i.e., in the absence of oxygen).

The most common and well-known type of glycolysis is the Embden-Meyerhof pathway, initially elucidated by Gustav Embden and Otto Meyerhof. The term can be taken to include alternative pathways, such as the Entner-Doudoroff Pathway. However, glycolysis will be used here as a synonym for the Embden-Meyerhof pathway.

Overview

The overall reaction of glycolysis is:

So, for simple fermentations, the metabolism of 1 molecule of glucose has a net yield of 2 molecules of ATP. Cells performing respiration synthesize much more ATP, but this is not considered part of glycolysis proper, although these aerobic reactions do use the product of glycolysis. Eukaryotic aerobic respiration produces an additional 34 molecules (approximately) of ATP for each glucose molecule oxidized. Unlike most of the molecules of ATP produced via aerobic respiration, those of glycolysis are produced by substrate-level phosphorylation.

In eukaryotes, glycolysis takes place within the cytosol of the cell. Some of the glycolytic reactions are conserved in the Calvin cycle that functions inside the chloroplast. This is consistent with the fact that glycolysis is highly conserved in evolution, being common to nearly all living organisms. This suggests great antiquity; it may have originated with the first prokaryotes, 3.5 billion years ago or more.

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