Question
List all 10 steps of glycolysis with their enzymes. Identify the energy investment phase and the energy payoff phase. What is the net ATP and NADH yield from one molecule of glucose?
(NCERT Class 11 — high-weightage for NEET)
Solution — Step by Step
Glycolysis occurs in the cytoplasm and does not require oxygen.
| Step | Reaction | Enzyme | Energy |
|---|---|---|---|
| 1 | Glucose → Glucose-6-phosphate | Hexokinase | -1 ATP |
| 2 | G6P → Fructose-6-phosphate | Phosphoglucose isomerase | — |
| 3 | F6P → Fructose-1,6-bisphosphate | Phosphofructokinase (PFK) | -1 ATP |
| 4 | F-1,6-BP → DHAP + G3P | Aldolase | — |
| 5 | DHAP → G3P | Triose phosphate isomerase | — |
After step 5, we have 2 molecules of G3P (glyceraldehyde-3-phosphate). Everything from here is doubled.
| Step | Reaction | Enzyme | Energy (per G3P) |
|---|---|---|---|
| 6 | G3P → 1,3-BPG | G3P dehydrogenase | +1 NADH |
| 7 | 1,3-BPG → 3-PGA | Phosphoglycerate kinase | +1 ATP |
| 8 | 3-PGA → 2-PGA | Phosphoglycerate mutase | — |
| 9 | 2-PGA → PEP | Enolase | — |
| 10 | PEP → Pyruvate | Pyruvate kinase | +1 ATP |
Since each glucose gives 2 G3P molecules, multiply everything by 2: 4 ATP + 2 NADH produced.
- ATP invested: 2 (steps 1 and 3)
- ATP produced: 4 (steps 7 and 10, each doubled)
- Net ATP = 4 - 2 = 2 ATP
- Net NADH = 2 NADH (step 6, doubled)
One glucose () → 2 pyruvate () + 2 ATP + 2 NADH
Why This Works
Glycolysis is the universal energy-harvesting pathway — found in virtually every living cell. It works without oxygen because it was one of the earliest metabolic pathways to evolve (before atmospheric oxygen existed).
The investment phase uses ATP to phosphorylate glucose, making it reactive and trapping it inside the cell (phosphorylated sugars can’t cross the cell membrane). The payoff phase recovers more ATP than was invested, plus captures high-energy electrons in NADH.
PFK (step 3) is the rate-limiting enzyme and the main regulatory point. High ATP inhibits PFK (cell has enough energy), while high AMP activates it (cell needs energy). This feedback ensures glycolysis runs only when needed.
Alternative Method — Key Enzymes to Memorise
For NEET, you don’t need all 10 enzymes. Focus on the three irreversible steps (the regulatory ones):
- Hexokinase (step 1) — commits glucose to glycolysis
- PFK (step 3) — the key regulatory enzyme, rate-limiting step
- Pyruvate kinase (step 10) — the final ATP-generating step
NEET frequently asks: “Which step of glycolysis is irreversible and rate-limiting?” The answer is step 3 (PFK). Hexokinase is also irreversible, but PFK is the committed step — it’s the point of no return for glucose entering glycolysis. Remember: PFK = Pace-Finding Kinase (the one that sets the pace).
Common Mistake
The most frequent NEET error: students say glycolysis produces 4 ATP. It produces 4 ATP by substrate-level phosphorylation, but the net yield is only 2 ATP because 2 ATP were consumed in the investment phase. Always report the net value unless specifically asked for gross production. Also, don’t forget the 2 NADH — they’re worth 5 ATP (or 3 ATP, depending on shuttle) when oxidised in the ETC later.