# Osmosis

The movement of water across a selectively permeable membrane from lower solute concentration to higher solute concentration. Water moves toward the saltier side.

Biology (university) · Part 01: Cell structure and membranes · Card 8 of 9 · Last updated 2026-10-19

## Analogy

Water rushing toward the crowded party to even out the crowd.

## Example

**Salt bath**

Cell inside: 0.9% salt.
Outside: 3% salt.
Water leaves the cell.
The cell shrinks.

## In practice

A red blood cell in pure water swells and can burst. In salty water it shrivels.

## Practice

### Flashcards

- **In which direction does water move in osmosis?** From the side with fewer dissolved particles to the side with more.

### 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

### Is osmosis just a type of diffusion?

Diffusion is the spread of any particles from high to low concentration. Osmosis is the special case of water moving across a membrane that blocks some solutes. In osmosis the water moves toward the side with more dissolved solute, which is the same as moving from its own higher to lower concentration.

### What is turgor pressure in plant cells?

Plant cells have a rigid cell wall. When water enters, the cell swells until the wall pushes back with turgor pressure and no more water can enter. This keeps the cell firm, which is why a well-watered plant stands upright and a dry one wilts.

## Related concepts

- [Fluid mosaic model](https://studyvisually.com/learn/science/biology-university/biology-university-part-1/biology-university-part-1-concept-7)
- [Active transport](https://studyvisually.com/learn/science/biology-university/biology-university-part-1/biology-university-part-1-concept-9)

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