# Glycolysis

The first stage of glucose breakdown. It happens in the cytoplasm, needs no oxygen and splits one glucose (6 carbons) into two pyruvate (3 carbons each).

Biology (university) · Part 03: Respiration and photosynthesis · Card 2 of 8 · Last updated 2026-10-19

## Analogy

Chopping a log in half so the pieces can go into the furnace.

## Example

**Glycolysis tally**

1 glucose in.
2 pyruvate out.
2 ATP net.
2 NADH made.

## In practice

Net gain per glucose is 2 ATP and 2 NADH.

## Practice

### Flashcards

- **Where does glycolysis occur and does it need oxygen?** In the cytoplasm. It needs no oxygen.
- **What is the net ATP yield of glycolysis per glucose?** 2 ATP (4 made, 2 used).

### Interview questions

**Outline the stages of aerobic respiration and say where each occurs and what it produces.**

- Glycolysis in cytoplasm: 2 ATP, 2 NADH
- Citric acid cycle in matrix: NADH, FADH2, CO2
- Electron transport and chemiosmosis in inner membrane: most ATP
- Oxygen is final electron acceptor

## Questions people ask

### Why does glycolysis use 2 ATP before making 4?

The first steps add phosphate groups to glucose using 2 ATP. This traps the sugar inside the cell and makes it unstable enough to split. The later steps then make 4 ATP. The net gain is 2 ATP, a small profit that pays for itself without needing oxygen.

### Is glycolysis found in all organisms?

Almost all. It happens in bacteria, archaea, plants, fungi and animals. This suggests it evolved very early, before oxygen filled the atmosphere. It needs no organelles, so even prokaryotes can run it. That makes it one of the oldest metabolic pathways we know.

## Related concepts

- [Citric acid cycle](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-3)
- [Electron transport chain](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-4)
- [Chemiosmosis](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-5)
- [Fermentation](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-6)

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