Phases of Glycolysis and Net ATP Production
Introduction
Glycolysis is the central metabolic pathway where one glucose molecule is broken down into two molecules of pyruvate. It occurs in the cytoplasm and functions in both aerobic and anaerobic organisms. The pathway consists of ten enzymatic reactions that can be divided into two distinct phases: the energy investment phase and the energy payoff phase.
1. Energy Investment Phase (Steps 1–5)
- Step 1: Glucose is phosphorylated to glucose-6-phosphate by hexokinase/glucokinase, consuming one ATP.
- Step 2: Glucose-6-phosphate is isomerized to fructose-6-phosphate by phosphoglucose isomerase.
- Step 3: Fructose-6-phosphate is converted to fructose-1,6-bisphosphate using one more ATP via phosphofructokinase-1 (PFK-1).
- Step 4: Aldolase splits fructose-1,6-bisphosphate into two 3-carbon molecules—glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
- Step 5: DHAP is converted into G3P, resulting in two G3P molecules from one glucose.
2. Energy Payoff Phase (Steps 6–10)
- Step 6: G3P is oxidized to 1,3-bisphosphoglycerate, producing NADH.
- Step 7: 1,3-Bisphosphoglycerate donates a phosphate to ADP forming ATP.
- Step 8: 3-Phosphoglycerate is rearranged to 2-phosphoglycerate.
- Step 9: Enolase converts 2-phosphoglycerate to phosphoenolpyruvate (PEP).
- Step 10: PEP donates a phosphate to ADP via pyruvate kinase, producing another ATP and forming pyruvate.
Net ATP and Final Equation
The glycolysis pathway consumes 2 ATP (in steps 1 and 3) and produces 4 ATP (steps 7 and 10, two times each), resulting in a net gain of 2 ATP molecules per glucose. Additionally, 2 NADH molecules are formed.
Final Equation:
Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 ATP + 2 H₂O + 2 H⁺
Conclusion
Glycolysis is a highly efficient and ancient metabolic pathway that breaks down glucose to provide energy and metabolic intermediates. It produces a net gain of 2 ATP and 2 NADH per glucose molecule, making it vital for cellular respiration.