Glycolysis and Fermentation: Cellular Energy Pathways

Glycolysis: The Anaerobic Breakdown of Glucose

Glycolysis is a set of reactions that degrade glucose (6C) into two molecules of pyruvic acid (3C). Performed in the hyaloplasm, it is an anaerobic process. From each molecule of glucose, the cell obtains 2 ATP and 2 NADH + H⁺.

Ten Steps of the Glycolytic Pathway

1. Step 1: Phosphorylation of glucose into glucose-6-phosphate by ATP, catalyzed by hexokinase.
2. Step 2: Glucose-6-phosphate is isomerized to fructose-6-phosphate by isomerase.
3. Step 3: Phosphorylation by ATP to form fructose-1,6-diphosphate (6C), catalyzed by kinase.
4. Step 4: The molecule is divided to form two molecules: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
5. Step 5: Dihydroxyacetone phosphate is an isomer of the aldose glyceraldehyde-3-phosphate; through isomerization, it transforms into it, resulting in two molecules of triose phosphate.
6. Step 6: Oxidation catalyzed by glyceraldehyde-3-phosphate dehydrogenase; two glyceraldehyde molecules are oxidized to 1,3-diphosphoglycerate. An aldehyde is oxidized to a carboxylic acid, and the coenzyme NAD⁺ is reduced to NADH + H⁺.
7. Step 7: Transfer of a phosphate group to ADP, which is phosphorylated to ATP, yielding 3-phosphoglycerate via kinase.
8. Step 8: The phosphate group moves from position 3 to 2 to form 2-phosphoglycerate via mutase.
9. Step 9: The removal of water and dehydration gives phosphoenolpyruvate via enolase.
10. Step 10: A phosphate group is transferred to ADP to form ATP and pyruvate via kinase.

Overall Chemical Assessment

Glucose + 2 ADP + 2 P_i + 2 NAD⁺ → 2 Pyruvate + 2 ATP + 2 H₂O + 2 H⁺ + 2 NADH

Fermentation Processes

Fermentations allow the production of ATP under anaerobic conditions. In these cases, glycolysis is the main source of ATP, and pyruvate is converted into products excreted by the cell, such as lactate or ethanol and CO₂.

Lactic Fermentation

Glucose → 2 Pyruvate + 2 ATP + 2 NADH + 2 H⁺ → 2 Lactate (CH₃-CHOH-COO^-) + 2 NAD⁺

Alcoholic Fermentation

Glucose → 2 Pyruvate (CH₃-CO-COO^-) + 2 ATP + 2 NADH + 2 H⁺ → 2 CO₂ + 2 Acetaldehyde (CH₃-CHO) → 2 CO₂ + 2 Ethanol (CH₃-CH₂OH) + 2 NAD⁺

Metabolic Pathways of Pyruvate

Pyruvate can follow two main pathways:

• Under aerobic conditions: Pyruvate is oxidized to acetyl coenzyme A and CO₂ by pyruvate dehydrogenase. The reaction is a decarboxylation and oxidation, in which NAD⁺ captures electrons (e^-) and protons (p⁺) and transforms into NADH (Pyruvate + CoA + NAD⁺ + ADP + P_i → CO₂ + Acetyl-CoA + NADH + H⁺).
• Under anaerobic conditions: Pyruvate is reduced using the NADH formed by glycolysis in fermentation.