Glycolysis is the universal, ten‑step pathway that extracts energy from glucose, producing ATP for every cell in the body. In aerobic organisms, the full cellular respiration chain can generate 36–38 ATP per glucose, while glycolysis alone yields 2 ATP.

Overview of Glycolysis

Glucose enters the cytoplasm, is phosphorylated, split into two 3‑carbon fragments, and ultimately converted into pyruvate. The process nets two ATP and produces NADH, which fuels downstream respiration or fermentation.

Essential Reactants and Enzymes

• Glucose – the sole starting substrate.
• Two ATP molecules – used in the investment phase to phosphorylate glucose and fructose.
• NAD⁺ – required to oxidize glyceraldehyde‑3‑phosphate and regenerate NAD⁺ for the reaction to proceed.
• Specific enzymes – 10 protein catalysts that control the pace and fidelity of each step.

Oxygen is not necessary for glycolysis; fermentation can sustain the pathway by converting pyruvate to lactate, regenerating NAD⁺.

The Investment Phase

Glucose is first phosphorylated to glucose‑6‑phosphate, then rearranged to fructose‑6‑phosphate and phosphorylated again to fructose‑1,6‑bisphosphate. This costly two‑ATP “investment” is essential for the subsequent energy‑producing steps.

The Return Phase

Fructose‑1,6‑bisphosphate splits into two molecules of glyceraldehyde‑3‑phosphate. Through a series of rearrangements and phosphorylations, each is converted to pyruvate, generating four ATP and two NADH in the process. Net gain: two ATP.

End Products

The overall balanced equation for glycolysis is:

C6H12O6 + 2 ATP + 2 NAD+ → 2 C3H4O3 + 4 ATP + 2 NADH + 2 H+

Pyruvate then enters the mitochondria for aerobic respiration when oxygen is abundant, or is converted to lactate in the cytoplasm under hypoxic conditions.

Learn more about glycolysis on Wikipedia.