Glycolysis

Glycolysis is the process by which cells break down glucose, a type of sugar, to produce energy. It occurs in the cytoplasm and doesn’t require oxygen. During glycolysis, glucose is converted into smaller molecules called pyruvate, while generating a small amount of energy in the form of ATP. This process is a fundamental step in cellular respiration and provides energy for various cellular activities.

Process of Glycolysis

  1. Glucose Activation:
    • Glycolysis begins with the activation of glucose, a six-carbon sugar molecule, which requires two ATP molecules.
    • The ATP molecules are hydrolyzed to ADP, releasing energy and phosphorylating glucose, forming glucose-6-phosphate.
  2. Glucose Conversion:
    • Glucose-6-phosphate is converted into its isomer, fructose-6-phosphate, through an enzymatic rearrangement.
  3. Fructose-6-Phosphate Activation:
    • Fructose-6-phosphate is phosphorylated by ATP, forming fructose-1,6-bisphosphate.
    • This step is catalyzed by the enzyme phosphofructokinase (PFK-1) and is a crucial regulatory step in glycolysis.
  4. Cleavage and Formation of Triose Phosphates:
    • Fructose-1,6-bisphosphate is split into two three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
  5. Isomerization:
    • DHAP is converted into G3P through the enzyme triose phosphate isomerase. Both molecules are now in the form of G3P.
  6. Energy Production:
    • G3P is further converted into 1,3-bisphosphoglycerate (1,3-BPG) through a series of reactions.
    • During these reactions, two molecules of NAD+ are reduced to NADH, and one molecule of ATP is synthesized through substrate-level phosphorylation.
  7. ATP Generation:
    • 1,3-BPG transfers its phosphate group to ADP, forming ATP and 3-phosphoglycerate.
  8. Rearrangement and Phosphorylation:
    • 3-phosphoglycerate is converted into 2-phosphoglycerate through an enzymatic rearrangement.
  9. Dehydration:
    • 2-phosphoglycerate loses a water molecule, forming phosphoenolpyruvate (PEP).
  10. ATP Production and Pyruvate Formation:
    • PEP donates its phosphate group to ADP, generating ATP and forming pyruvate.
    • This is the second ATP synthesis through substrate-level phosphorylation in glycolysis.

Function of Glycolysis

  • Energy Production: Glycolysis generates a small amount of ATP (adenosine triphosphate), which is the primary energy currency of cells. The production of ATP provides the cell with readily available energy for various cellular processes.
  • NADH Generation: Glycolysis also produces NADH (nicotinamide adenine dinucleotide), a coenzyme that carries high-energy electrons. NADH plays a vital role in later stages of cellular respiration, where it donates its electrons to the electron transport chain to generate more ATP.
  • Precursor for Other Metabolic Pathways: The intermediate molecules formed during glycolysis, such as pyruvate, can serve as precursors for other metabolic pathways. Pyruvate, for example, can be further processed in aerobic conditions to produce more ATP through the citric acid cycle and oxidative phosphorylation.
  • Anaerobic Energy Production: In the absence of oxygen, glycolysis can proceed to fermentation, where pyruvate is converted into lactate (lactic acid) or ethanol, allowing the regeneration of NAD+ for continued ATP production in the absence of oxygen.