🎓 Prepared by students from Boğaziçi University

What is Glycolysis?

Glycolysis is the first stage of cellular respiration, breaking down one glucose molecule into two molecules of pyruvate in the cytoplasm. It works the same in nearly every living cell, whether oxygen is present or not, and sets up the reactions that follow in aerobic respiration.

Short answer

Glycolysis splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules, producing a net gain of 2 ATP and 2 NADH per glucose, without requiring oxygen.

The Three Phases of Glycolysis

1
Energy Investment Phase
Glucose is phosphorylated twice, using 2 ATP, to form fructose-1,6-bisphosphate.
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2
Cleavage
The 6-carbon sugar splits into two 3-carbon G3P (glyceraldehyde-3-phosphate) molecules.
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3
Energy Payoff Phase
Each G3P is oxidized, generating 2 ATP and 1 NADH — 4 ATP and 2 NADH total from both.
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4
Net Result
2 pyruvate, a net gain of 2 ATP, and 2 NADH are produced per glucose molecule.

Try it: interactive calculator

Net ATP produced

2ATP

= 1*2

Glucose molecules processed1 molecules

Step-by-step worked examples

One glucose molecule enters glycolysis. How many ATP are invested and how many are produced in the payoff phase?

Investment phase uses 2 ATP to phosphorylate glucose
Payoff phase produces 4 ATP (2 ATP per G3P × 2 G3P)
Net ATP = 4 − 2 = 2 ATP

If a cell processes 5 glucose molecules through glycolysis, how much net ATP and NADH result?

Net ATP per glucose = 2
Net ATP for 5 glucose = 5 × 2 = 10 ATP
NADH per glucose = 2, so NADH for 5 glucose = 5 × 2 = 10 NADH

How many pyruvate molecules are produced from 3 glucose molecules?

Each glucose yields 2 pyruvate
For 3 glucose: 3 × 2 = 6 pyruvate molecules

Flashcards

Quick quiz

Q1.What is the net ATP yield of glycolysis per glucose molecule?

Correct answer: B. 4 ATP are produced but 2 are invested, giving a net of 2 ATP.

Q2.Where in the cell does glycolysis take place?

Correct answer: B. Glycolysis occurs in the cytoplasm, outside the mitochondria.

Q3.How many pyruvate molecules result from one glucose molecule?

Correct answer: B. The 6-carbon glucose splits into two 3-carbon pyruvate molecules.

Q4.Does glycolysis require oxygen to proceed?

Correct answer: B. Glycolysis is anaerobic; oxygen is only needed for later stages of aerobic respiration.

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Common mistakes

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Glycolysis produces 4 ATP total. — Correct: 4 ATP are produced, but 2 were invested — the net yield is 2 ATP.

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Glycolysis happens inside the mitochondria. — Correct: Glycolysis takes place in the cytoplasm, before pyruvate ever enters the mitochondria.

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Glycolysis requires oxygen. — Correct: Glycolysis is an anaerobic pathway; oxygen is not needed for this stage.

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One glucose molecule makes only one pyruvate. — Correct: Glucose is a 6-carbon molecule that splits into two 3-carbon pyruvate molecules.

FAQ

What is glycolysis?

Glycolysis is the metabolic pathway that breaks down one glucose molecule into two pyruvate molecules in the cytoplasm, yielding a net 2 ATP and 2 NADH.

What is the formula for net ATP in glycolysis?

Net ATP = ATP produced − ATP invested = 4 − 2 = 2 ATP per glucose molecule.

What are examples of glycolysis calculations?

For 5 glucose molecules, net ATP = 5 × 2 = 10 ATP, and NADH = 5 × 2 = 10 NADH.

How is glycolysis different from the rest of cellular respiration?

Glycolysis is the only stage that doesn't need oxygen and happens outside the mitochondria; the Krebs cycle and electron transport chain follow it and require oxygen.