Glycolysis Study Pack

Location and Universality

• •Glycolysis takes place entirely in the cytoplasm (cytosol), not in the mitochondria, which means it is available to both prokaryotic and eukaryotic cells.
• •Because it does not require oxygen, glycolysis can proceed under aerobic and anaerobic conditions alike, making it evolutionarily ancient and metabolically fundamental.

Relationship to Downstream Pathways

• •In the presence of oxygen, pyruvate produced by glycolysis enters the mitochondrial matrix, where it is converted to acetyl-CoA by the pyruvate dehydrogenase complex and fed into the citric acid cycle.
• •When oxygen is unavailable, cells regenerate NAD⁺ from NADH through fermentation (either lactic acid fermentation in animal muscle cells or alcoholic fermentation in yeast), allowing glycolysis to continue.
• •NADH generated during glycolysis delivers electrons to the electron transport chain under aerobic conditions, ultimately enabling the synthesis of approximately 2–3 additional ATP per NADH molecule through oxidative phosphorylation.

• •Step 1 — Glucose Phosphorylation by Hexokinase
• •Hexokinase transfers a phosphate group from ATP to carbon 6 of glucose, producing glucose-6-phosphate (G6P) and ADP.
• •Phosphorylating glucose traps it inside the cell (charged molecules cannot cross the plasma membrane) and prevents it from being used in other pathways without first passing through glycolysis.
• •Step 2 — Isomerization to Fructose-6-Phosphate
• •Phosphoglucose isomerase converts glucose-6-phosphate into fructose-6-phosphate (F6P), shifting the carbonyl group from carbon 1 to carbon 2.
• •This rearrangement is necessary to position the molecule correctly for the second phosphorylation.
• •Step 3 — Second Phosphorylation by Phosphofructokinase
• •Phosphofructokinase (PFK) adds a second phosphate group (again from ATP) to carbon 1 of fructose-6-phosphate, forming fructose-1,6-bisphosphate.
• •PFK is the primary regulatory control point of glycolysis: high levels of ATP inhibit PFK, while high levels of AMP and ADP activate it, matching glycolytic rate to the cell's energy demand.
• •Steps 4–5 — Cleavage and Interconversion
• •Aldolase cleaves fructose-1,6-bisphosphate into two 3-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
• •Triose phosphate isomerase rapidly converts DHAP into G3P, so the net result is two molecules of G3P entering the payoff phase — effectively doubling every subsequent step.