Glycolysis

Glycolysis, the Embden-Meyerhof pathway, is a central metabolic pathway involved in glucose catabolism. It is highly conserved across most organisms, reflecting its fundamental role in cellular energy production. This process occurs in the cytoplasm and can function both in the presence and absence of oxygen, making it versatile for various organisms and environmental conditions.

Stages of Glycolysis

Glycolysis is a ten-step pathway that converts glucose into pyruvate, generating a net gain of two ATP molecules and two NADH molecules per glucose molecule. It consists of two phases: the preparatory phase, where glucose is phosphorylated and cleaved into two three-carbon intermediates: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP)using two ATP. DHAP is subsequently converted to G3P, ensuring both three-carbon molecules follow the same downstream reactions.

In the energy-conserving stage, G3P undergoes oxidation, reducing NAD+ to NADH. This stage involves substrate-level phosphorylation, where four ATP molecules are generated from the high-energy intermediates 1,3-bisphosphoglycerate and phosphoenolpyruvate. The final product of glycolysis is pyruvate, generating two ATP and two NADH molecules per glucose molecule.

Pyruvate can proceed to further metabolic pathways, such as the tricarboxylic acid cycle under aerobic conditions or fermentation under anaerobic conditions, to sustain energy production.

Fate of Pyruvate

The metabolic fate of pyruvate depends on the organism and environmental conditions. Under aerobic conditions, pyruvate undergoes oxidative decarboxylation to acetyl-CoA, entering the tricarboxylic acid (TCA) cycle. The electrons harvested during glycolysis and the TCA cycle are transferred to oxygen via the electron transport chain (ETC), enabling oxidative phosphorylation. This process generates a significantly higher yield of ATP.

In anaerobic conditions, alternative electron acceptors such as nitrate, sulfate, or fumarate replace oxygen in the ETC. However, many microorganisms rely solely on glycolysis for ATP production through fermentation. During fermentation, pyruvate is reduced to lactic acid or ethanol, regenerating NAD+ from NADH to maintain glycolytic flux.

Glycolysis provides the foundation for cellular respiration and fermentation, adapting to diverse environmental and physiological needs while maintaining energy homeostasis.