Metabolism
Glycolysis explained: steps, ATP yield and control
Glycolysis is the entry point for carbohydrate metabolism and one of the most reliably examined pathways in any biochemistry course.
The overview equation
Glycolysis splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules in the cytosol. It requires no oxygen, which is why it functions in both aerobic and anaerobic conditions.
Net reaction
Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O
Two phases
The pathway divides cleanly into an energy investment phase and an energy payoff phase — a distinction worth memorizing, because it explains why the gross and net ATP figures differ.
| Phase | Steps | ATP | Key point |
|---|---|---|---|
| Investment | 1–5 | −2 ATP | Glucose is phosphorylated twice, then split into two 3-carbon molecules |
| Payoff | 6–10 | +4 ATP, +2 NADH | Everything happens twice, once per 3-carbon molecule |
Gross yield is 4 ATP; net yield is 2 ATP because two were spent up front.
The three regulated steps
Exam questions frequently target regulation. Three steps are effectively irreversible and therefore serve as control points:
- Hexokinase (step 1) — traps glucose in the cell as glucose-6-phosphate; inhibited by its own product.
- Phosphofructokinase-1 (PFK-1) (step 3) — the rate-limiting step and the main control point. Inhibited by ATP and citrate (signals of energy abundance), activated by AMP and fructose-2,6-bisphosphate (signals of energy demand).
- Pyruvate kinase (step 10) — the final ATP-generating step, inhibited by ATP.
Common exam trap
If asked for the rate-limiting enzyme of glycolysis, the answer is PFK-1, not hexokinase. Hexokinase is the first step, which is not the same thing.
What happens to pyruvate
- With oxygen — pyruvate enters the mitochondrion, is converted to acetyl-CoA, and feeds the Krebs cycle.
- Without sufficient oxygen — pyruvate is reduced to lactate, regenerating the NAD+ that glycolysis needs to keep running. This is why glycolysis can continue briefly during intense exercise.
That NAD+ regeneration is the entire point of lactate production — a detail often missed when lactate is framed simply as a "waste product." See the three energy systems for how this plays out during exercise.
Frequently asked questions
What is the net ATP yield of glycolysis?
Glycolysis produces a gross yield of 4 ATP but consumes 2 ATP in the investment phase, giving a net yield of 2 ATP per glucose molecule, along with 2 NADH and 2 pyruvate.
What is the rate-limiting enzyme of glycolysis?
Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme. It is inhibited by ATP and citrate, and activated by AMP and fructose-2,6-bisphosphate.
Does glycolysis require oxygen?
No. Glycolysis occurs in the cytosol and does not require oxygen, which is why it functions under both aerobic and anaerobic conditions. Oxygen availability determines what happens to pyruvate afterwards.
Keep reading: The Krebs cycle explained · The three energy systems · What is glycogen?