🎓 Prepared by students from Boğaziçi University

What is Glycolysis?

Glycolysis is the metabolic pathway that breaks down one glucose molecule into two pyruvate molecules in the cytoplasm, occurring in nearly all living cells. It's the first stage of cellular respiration and works with or without oxygen.

Short answer

Glycolysis is a 10-step enzymatic pathway that splits one glucose molecule into two pyruvate molecules, yielding a net gain of 2 ATP and 2 NADH per glucose.

The Two Phases of Glycolysis

1
Energy Investment Phase
Glucose is phosphorylated and split into two 3-carbon molecules, consuming 2 ATP.
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2
Payoff Phase
Each 3-carbon molecule is oxidized to pyruvate, producing 4 ATP and 2 NADH total.
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3
Net Result
2 ATP, 2 NADH, and 2 pyruvate molecules are generated per glucose.

Try it: interactive calculator

Net ATP yield

2ATP

= 4-2

ATP produced (payoff phase)4 ATPATP invested (investment phase)2 ATP

Step-by-step worked examples

In glycolysis, a cell invests 2 ATP in the investment phase and produces 4 ATP in the payoff phase. What is the net ATP yield per glucose?

ATP produced = 4
ATP invested = 2
Net ATP = ATP_produced − ATP_invested = 4 − 2 = 2 ATP

If a cell runs glycolysis on 5 glucose molecules, how many net ATP and NADH molecules are produced in total?

Net ATP per glucose = 2, NADH per glucose = 2
Total net ATP = 5 × 2 = 10 ATP
Total NADH = 5 × 2 = 10 NADH

A muscle cell processes 3 glucose molecules through glycolysis. How many pyruvate molecules result, and how many total ATP are consumed in the investment phase?

Pyruvate per glucose = 2
Total pyruvate = 3 × 2 = 6 pyruvate molecules
ATP invested per glucose = 2
Total ATP invested = 3 × 2 = 6 ATP

Flashcards

Quick quiz

Q1.What is the net ATP yield from one glucose molecule in glycolysis?

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

Q2.Where in the cell does glycolysis take place?

Correct answer: C. Glycolysis occurs in the cytoplasm and doesn't need mitochondria or oxygen.

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

Correct answer: B. Glucose (6 carbons) splits into two 3-carbon pyruvate molecules.

Q4.How many NADH molecules are produced per glucose in glycolysis?

Correct answer: B. The payoff phase produces 2 NADH molecules per glucose.

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Common mistakes

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

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Glycolysis requires oxygen. — Correct: Glycolysis is anaerobic—it happens with or without oxygen, in the cytoplasm.

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Glycolysis fully breaks glucose down to CO2 and water. — Correct: Glycolysis only produces pyruvate; complete breakdown needs the Krebs cycle and oxidative phosphorylation.

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Glycolysis occurs in the mitochondria. — Correct: Glycolysis occurs in the cytoplasm; only later stages of respiration occur in mitochondria.

FAQ

What is glycolysis?

It's the metabolic pathway that breaks down glucose into two pyruvate molecules in the cytoplasm, the first stage of cellular respiration.

What is the glycolysis formula for net ATP?

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

What are examples of glycolysis products?

Each glucose molecule yields 2 net ATP, 2 NADH, and 2 pyruvate molecules.

How do you calculate the net ATP yield in glycolysis?

Subtract the 2 ATP invested in the early steps from the 4 ATP produced in the payoff phase, giving a net gain of 2 ATP.