Glycolysis: The Big Picture

Main Ideas

• Glycolysis is a widespread metabolic pathway that processes glucose through 10 steps, ending in two pyruvate molecules.
• Glucose can be oxidized through glycolysis, stored as glycogen, used to build structural materials, or routed through the pentose phosphate pathway to produce ribose-5-phosphate for nucleotide synthesis.
• Other carbohydrate sources—including starch, glycogen, galactose, and lactose—can be converted into glucose or intermediates that feed into glycolysis.

The Two Phases of Glycolysis

Energy Investment Phase (Steps 1–5)

• Phosphate groups are added to glucose, helping trap and destabilize it.
• The six-carbon sugar is split into two three-carbon molecules.
• The pathway consumes 2 ATP.

Energy Payoff Phase (Steps 6–10)

• Each three-carbon molecule is processed, so the reactions in this phase occur twice per original glucose.
• The molecules are oxidized to pyruvate.
• The pathway produces 4 ATP and 2 NADH.

Net Result per Glucose

Glycolysis produces 2 pyruvate, 2 ATP net, and 2 NADH. ATP is made by substrate-level phosphorylation, in which a phosphate is transferred directly from a pathway intermediate to ADP.

NADH carries electrons to the electron transport chain. Pyruvate also retains substantial energy that can be extracted through further oxidation beyond glycolysis.

How to Analyze Metabolic Pathways

When studying glycolysis and other pathways, track the following:

• Reaction free energies: A strongly negative ΔG indicates a reaction that is highly favorable in the forward direction.
• Regulatory steps: Reactions with strongly negative ΔG are often effectively one-way and can serve as control points. In glycolysis, the presenter identifies steps 1, 3, and 10 as regulatory steps.
• Cellular conditions: Standard free-energy values may differ from actual cellular ΔG, which depends on regulated reactant and product concentrations. A reaction that appears favorable under standard conditions may be close to equilibrium and reversible in cells.
• Energy-currency molecules: Track where ATP and NADH are consumed and produced.
• Oxidation and carbon fate: Energy-producing pathways oxidize carbon atoms. Following individual carbons—for example, with carbon-13 labeling—can help reveal where they go and how reactions work.
• Enzymes and reaction types: Identify each enzyme and the chemical transformation it catalyzes.

Speakers and Sources

• One unnamed presenter or instructor introduces glycolysis and explains its pathway overview, energy accounting, and principles for analyzing metabolic reactions.