Glycolysis in Yeast and in Humans

Glycolysis is the process of breaking down the six-carbon sugar glucose into two molecules of pyruvate, a 3-carbon compound, via oxidation (Ross, Tronson & Ritchie, 2008).Pyruvate then enters either of three pathways – the Krebs Cycle, alcohol fermentation or homolactic fermentation. Glycolysis in yeast differs from that in humans because of the nature of this organism’s metabolism.Glycolysis is also a process of harvesting energy out of glucose. This is accomplished through substrate-level phosphorylation or the transfer of a phosphate group directly from a phosphorylated substrate (Nester, Anderson, Roberts, Pearsall & Nester, 2001). In the process, adenosine diphosphate (ADP) is converted to adenosine triphosphate (ATP) as the form of energy in the cell.

In the synthesis of pyruvate, glycolysis is composed of 10 reactions involving substrates and their specific enzymes. It is divided into the preparatory and energy-yielding phases. The preparatory phase, involving the first four steps, requires the expenditure of two molecules of ATP so that glucose can be converted into glucose 6-phosphate, fructose 6-phosphate and further to glyceraldehyde-3 phosphate (Voet, Voet & Pratt, 2008).In the energy-yielding phase, involving the rest of the steps, glyceraldehyde-3 phosphate becomes 1,3 biphosphoglycerate then 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate and finally pyruvate (Voet, Voet & Pratt, 2008). The substrates 1,3 biphosphoglycerate and phosphoenolpyruvate undergo phosphorylation, donating their phosphates to ADP to create ATP.Glycolysis in yeast varies from humans mainly with respect to the fate of pyruvate. A eukaryotic unicellular fungus, yeast is a facultative anaerobe.

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This means that it can switch from aerobic to anaerobic respiration depending on the presence of oxygen. In aerobic conditions, pyruvate is directed to the Krebs Cycle while in anaerobic conditions, pyruvate undergoes alcohol fermentation. In humans, pyruvate may enter the Kreb’s Cycle or homolactic fermentation.When oxygen is present, oxidation causes a carbon atom to detach from pyruvate in the form of carbon dioxide leaving behind two acetyl groups. These combine with Coenzyme A to form Acetyl Coenzyme A. Binding with Coenzyme A allows diffusion into the matrix of mitochondria (Ross, Tronson & Ritchie, 2008). What follows is the Krebs Cycle.Also called the citric acid cycle, Krebs Cycles is a series of eight reactions, again catalyzed by enzymes, which converts Acetyl CoA into oxaloacetate (Nester, Anderson, Roberts, Pearsall & Nester, 2001).

The condensation of oxaloacetate back into Acetyl CoA permits the endless repetition of this cycle in order for the final steps representing complete glucose metabolism following glycolysis to occur.Hydrogen and electrons are also transferred to nicotimanide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) making them NADH+ and FADH2 respectively. These compounds serve as electron carriers in the final metabolism of glucose. Water and carbon dioxide are also created as waste products.In the absence of oxygen in yeast, pyruvate undergoes decarboxylation in the alcohol fermentation pathway. Pyruvate is reduced by NADH into carbon dioxide and ethanol producing energy in the process (Nester, Anderson, Roberts, Pearsall & Nester, 2001). In humans, strenuous physical activity exhausts the oxygen in body cells.

The lack of oxygen in muscle cells then forces pyruvate to proceed to homolactic fermentation where it is similarly reduced by NADH to produce lactic acid.Four ATP molecules are created during substrate-level phosphorylation. However, two ATP molecules were also expended. Hence, for every glucose molecule undergoing glycolysis, the net energy produced is two ATP molecules. The Krebs Cycle and fermentation also produce two ATP molecules each.List of ReferencesNester, N.

W., Anderson, D.G.

, Roberts, C.E., Pearsall, N.N.

and Nester, M.T. (2001). Microbiology: A Human Perspective. New York: McGraw Hill, Co.

Ross, P.M., Tronson, D.A.

and Ritchie, R.J. (2008). “Increasing Conceptual Understanding of Glycolysis & the Krebs Cycle Using Role-Play”. The American Biology Teacher 70(3), pp. 163+. Retrieved March 26, 2009 from http://www.

questia.com.Voet, D.J, Voet, J.G.

and Pratt, C.W. (2008). Fundamentals of Biochemistry: Life at the Molecular Level (3rd ed). New Yersey: John Wiley and Sons, Inc.