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Glycolysis is the metabolic process by which glucose is converted to pyruvate (also known as pyruvic acid). It is the first of the three stages of [carbohydrate metabolism]. The process of glycolysis is important in producing energy for the cell, particularly in anaerobic conditions. It occurs in the cytosol of the cell. The word glycolysis is of Greek origin, where 'glykos' means sweet, and 'lysis' means splitting. Hence, Glycolysis literally means "sugar splitting" or "sugar breaking"; this accurately describes the process of glycolysis, in which a 6-carbon sugar molecule is broken down into two 3-carbon molecules. There are three stages in glycolysis which are Investment stage, Splitting stage and Energy Yielding stage. Glycolysis consists of ten separate reactions, each catalysed by a different enzyme. Glycolysis is regulated by three different control enzymes. The first of these enzymes is hexokinase which phosphorylates glucose, turning it into glucose-6-phosphate. The phosphorylated form of glucose is incapable of leaving the cell through the GLUT uniport transporter proteins in the cell membrane. The second is phosphofructokinase. This enzyme allows the production fructose-1,6-bisphosphate and is the rate-limiting step. The final control enzyme is pyruvate kinase which controls the rate of production of pyruvate, which is the final product of glycolysis. For each molecule of glucose that goes through the process of glycolysis, there is a net gain of 2 ATP molecules, 2 NADH molecules and 2 water molecules The pyruvate converted from glucose in Glycolysis then enters into the Citrate Acid Cycle which takes place in the Mitochondria[1][2].

Under anaerobic conditions, glycolysis is the only source of ATP. ATP is generated from ADP and an inorganic phosphate due to enough free energy being generated from the oxidation of glyceraldehyde-3-phosphate to 3-phosphoglycerate (an aldehyde to a carboxylic acid)[3]. The concentration of NAD+ in the cytosol is not high, and it must therefore be regenerated from NADH in order for glycolysis to continue. Under aerobic conditions, the hydrogen is transferred from NADH to one of several carriers that deliver it to the respiratory chain in the mitochondria, and ultimately to oxygen. Under anaerobic conditions, this is impossible; therefore, other means for hydrogen disposal are required. Pyruvate therefore acts as a hydrogen acceptor and is reduced to lactate by lactate dehydrogenase. The lactate is released into the bloodstream, where it accumulates; it is removed and recycled after the restoration of oxygen supply. The muscle pain caused by lactate accumulation forces us to discontinue anaerobic exercise after a short while. In plants and some other microorganisms such as yeast, instead of lactate, carbon dioxide and ethanol are produced to continue the regeneration of NAD+ and thus still supplying minimal ATP.


  1. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York, W. H. Freeman
  2. Alberts et al., 2008, Molecular Biology of the Cell, 5th edition, pg 88-91, Garland Science, New York.
  3. Alberts et al., 2015, Molecular Biology of the Cell, 6th edition, pg 76-77, Garland Science, New York.
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