# Redox reaction

A reaction where one substance loses electrons (oxidation) and another gains them (reduction). The two always happen together. Cells use carriers such as NAD+ to move electrons.

Biology (university) · Part 03: Respiration and photosynthesis · Card 1 of 8 · Last updated 2026-10-19

## Analogy

A relay race: the baton (electron) passes from runner to runner.

## In practice

In respiration glucose is oxidised and oxygen is reduced to water.

## Practice

### Flashcards

- **What is oxidation?** The loss of electrons. Reduction is the gain of electrons.
- **What is the final electron acceptor in respiration?** Oxygen, which becomes water.

## Questions people ask

### What does OIL RIG mean?

It is a memory aid: Oxidation Is Loss, Reduction Is Gain, of electrons. If a molecule loses electrons it is oxidised, and if it gains them it is reduced. The two halves always come as a pair, because electrons lost by one substance must be gained by another.

### What is NAD+ and why is it important?

NAD+ is an electron carrier made from a vitamin. When it picks up electrons and a proton, it becomes NADH. It carries electrons from glucose breakdown to the electron transport chain. Without enough NAD+, glycolysis stops, which is why fermentation exists to recycle it.

## Related concepts

- [Electron transport chain](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-4)
- [Metabolism](https://studyvisually.com/learn/science/biology-university/biology-university-part-2/biology-university-part-2-concept-1)
- [ATP](https://studyvisually.com/learn/science/biology-university/biology-university-part-2/biology-university-part-2-concept-2)
- [Free energy](https://studyvisually.com/learn/science/biology-university/biology-university-part-2/biology-university-part-2-concept-3)

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