Anatomy and Physiology 12 · Biology · Lesson 5.1
Releasing ATP: glycolysis
- Level
- Grade 12
- Reading time
- 13 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Splitting glucose to start ATP production.
What is glycolysis?
Short answer
Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm, where one six-carbon glucose molecule is split into two three-carbon pyruvate molecules. It produces a net gain of two ATP and two NADH per glucose and does not require oxygen. The pyruvate then enters aerobic respiration or fermentation depending on oxygen availability.
Concept preview
What you will be able to do
- 01Describe how a single glucose molecule is split into two pyruvate molecules in the cytoplasm.
- 02Distinguish the ATP investment phase from the ATP payoff phase of glycolysis.
- 03State the net yield of glycolysis as 2 ATP and 2 NADH per glucose.
- 04Explain what substrate-level phosphorylation is and why glycolysis needs no oxygen.
Every time your muscles contract, your neurons fire, or a yeast cell buds, the energy comes from breaking apart a fuel molecule — and for almost every organism on Earth, that first act of fuel-breaking is glycolysis. The word means, quite literally, 'sugar-splitting', and that is exactly what happens: a six-carbon glucose molecule is torn in half to make two three-carbon molecules of pyruvate. What makes glycolysis so remarkable is that it happens not inside any organelle but out in the open cytoplasm of the cell, using a chain of ten enzymes that nearly all living things share. Its universality is a clue to its ancient origin, dating back to a time before oxygen was common in the air.
School Plan
Releasing ATP: glycolysis
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Related topics
Frequently asked questions
- Where does glycolysis take place?
- In the cytoplasm of the cell, not inside the mitochondria.
- Does glycolysis need oxygen?
- No. Glycolysis is anaerobic; oxygen is only required by the later stages of aerobic respiration.