Glycolysis (EMP Pathway)
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Glycolysis is the partial oxidation of glucose into two molecules of pyruvic acid (C₃H₄O₃) in the absence of O₂. It occurs in the cytoplasm of all living organisms.
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Its scheme was developed by Gustav Embden, Otto Meyerhof, and J. Parnas, hence it is also called the EMP pathway.
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In anaerobes, glycolysis is the sole respiratory process.
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In plants, glucose comes from sucrose (an end product of photosynthesis) or storage carbohydrates. Sucrose is converted to glucose and fructose by the enzyme invertase. These monosaccharides enter the glycolytic pathway.
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Glucose and fructose are phosphorylated by hexokinase to form glucose-6-phosphate, which is isomerized to fructose-6-phosphate. Subsequent metabolism of glucose and fructose is identical.
Steps of Glycolysis:
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Glycolysis involves 10 steps controlled by various enzymes.
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ATP is used in two steps:
- Conversion of glucose to glucose-6-phosphate.
- Conversion of fructose-6-phosphate to fructose-1,6-diphosphate.
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Fructose-1,6-diphosphate splits into dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL).
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PGAL is oxidized, combining with inorganic phosphate to form 1,3-bisphosphoglycerate (BPGA). During this, two redox-equivalents (2 H-atoms) are transferred from PGAL to NAD⁺, forming NADH + H⁺.
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BPGA converts to 3-phosphoglyceric acid (PGA), releasing energy that forms ATP.
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Additional ATP is produced when phosphoenolpyruvate (PEP) converts to pyruvic acid.
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In glycolysis, 4 ATP molecules are directly synthesized from one glucose molecule.
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Pyruvic acid (pyruvate) is the key product of glycolysis. Its fate depends on cellular needs, following one of three pathways:
- Lactic acid fermentation.
- Alcoholic fermentation.
- Aerobic respiration (Krebs’ cycle).