# Active transport

Moving a substance across a membrane against its concentration gradient, using energy (usually ATP). Carrier proteins called pumps do the work.

Biology (university) · Part 01: Cell structure and membranes · Card 9 of 9 · Last updated 2026-10-19

## Analogy

Pushing a trolley uphill: it needs effort.

## Example

**Pump count**

One cycle:
3 Na+ out
2 K+ in
1 ATP used

## In practice

The sodium-potassium pump moves 3 sodium ions out and 2 potassium ions in, using one ATP each cycle. Nerve cells depend on it.

## Practice

### Flashcards

- **How does active transport differ from diffusion?** Active transport goes against the gradient and uses energy. Diffusion goes down the gradient and needs none.

### Interview questions

**Explain how a cell moves water and ions across its membrane, including one example that needs energy.**

- Water follows solute by osmosis
- Small gradients allow diffusion through channels
- Active transport needs ATP
- Sodium-potassium pump as example

## Questions people ask

### What is the difference between primary and secondary active transport?

Primary active transport uses ATP directly, as the sodium-potassium pump does. Secondary active transport uses the gradient that a pump has already built. For example, glucose enters gut cells together with sodium ions, which flow back down their gradient and drag the glucose in.

### Why do cells spend so much energy on pumps?

Gradients store energy like a charged battery. Nerve signals, nutrient uptake and keeping cell volume stable all depend on them. In many animal cells, a large share of the ATP budget goes to the sodium-potassium pump alone, and in nerve cells it is especially high.

## Related concepts

- [Fluid mosaic model](https://studyvisually.com/learn/science/biology-university/biology-university-part-1/biology-university-part-1-concept-7)
- [Osmosis](https://studyvisually.com/learn/science/biology-university/biology-university-part-1/biology-university-part-1-concept-8)

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