Step by Step
10
10 steps in the cytoplasm
Glycolysis occurs entirely in the cytoplasm and converts one glucose (6C) into two pyruvate (3C each). It requires no oxygen and works in both aerobic and anaerobic conditions — making it the universal energy pathway found in virtually every organism on Earth.
Inv
Investment phase (steps 1–5) — costs 2 ATP
Steps 1–5 consume 2 ATP to phosphorylate glucose (trapping it in the cell) and split it into two 3-carbon molecules. Key enzymes: hexokinase (step 1), phosphofructokinase-1/PFK-1 (step 3 — the committed, rate-limiting step), aldolase (step 4).
Memory trick: Investment phase = priming the pump. Spend 2 ATP to make the process work.
Pay
Payoff phase (steps 6–10) — produces 4 ATP + 2 NADH
Steps 6–10 generate 4 ATP (by substrate-level phosphorylation) and 2 NADH. Net gain: 4 − 2 = 2 ATP. Pyruvate kinase (step 10) is the final enzyme. PFK-1 is the key regulatory enzyme — inhibited by ATP and citrate (energy abundant), activated by AMP and fructose-2,6-bisphosphate.
Memory trick: Payoff phase = collecting the harvest. 4 ATP in, 2 already spent = net 2 ATP.
Fate
Fate of pyruvate
In aerobic conditions: pyruvate → acetyl-CoA (pyruvate dehydrogenase) → TCA cycle. In anaerobic conditions: pyruvate → lactate (animals/bacteria) or ethanol + CO₂ (yeast). Fermentation regenerates NAD⁺ so glycolysis can continue when oxygen is absent.
Applied Walkthrough
1
A red blood cell (no mitochondria) relies entirely on glycolysis for ATP — it must ferment pyruvate to lactate to regenerate NAD⁺ and keep glycolysis running.
2
Step 1: Hexokinase phosphorylates glucose → glucose-6-phosphate. This traps glucose in the cell (can't exit the membrane).
3
Step 3: PFK-1 commits fructose-6-phosphate to glycolysis (the point of no return). This is the key regulatory checkpoint.
4
Steps 6–10: 4 ATP produced by substrate-level phosphorylation. Net 2 ATP per glucose — enough to keep the cell alive when mitochondria can't function.
Exam Application
Exams test the net ATP yield (2), location (cytoplasm), the rate-limiting enzyme (PFK-1), regulators of PFK-1 (inhibited by ATP/citrate, activated by AMP/F-2,6-BP), and the fate of pyruvate under aerobic vs anaerobic conditions. The investment vs payoff phase concept and the committed step (PFK-1) are very high yield.
⚠ Common Trap
Students say glycolysis produces 4 ATP — it produces 4 but SPENDS 2 = NET 2. Also: PFK-1 is the rate-limiting step, not hexokinase (step 1). And glycolysis occurs in the CYTOPLASM, not the mitochondria.
✓ Quick Self-Check
1. Where does glycolysis occur?
In the cytoplasm — no mitochondria required.
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2. What is the net ATP yield from glycolysis?
2 ATP (4 produced minus 2 invested in the investment phase).
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3. What is the rate-limiting enzyme of glycolysis?
Phosphofructokinase-1 (PFK-1) — step 3, the committed step.
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4. What inhibits PFK-1?
High ATP and citrate (signals that energy is abundant — slow down glycolysis).
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5. What happens to pyruvate when oxygen is absent?
It is fermented to lactate (in animals) or ethanol + CO₂ (in yeast) to regenerate NAD⁺ and keep glycolysis running.
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