The cell membrane lets some materials through. Many of them cross without the cell spending any energy, simply because particles are always moving.
Key idea
Diffusion: particles spread from where they are more crowded to where they are less crowded. Osmosis: the diffusion of water across a membrane, toward the side with more dissolved material (less water).
Why it works
Particles move at random. Where they are crowded, more of them happen to wander away than wander in, so they spread out. Once they are even, they keep moving, but as many cross each way as the other: a balance called equilibrium. Water does the same. A salty side has fewer free water molecules, so more water crosses into it than out of it.
Worked examples
Worked example: Oxygen in the lungs
The air sacs in your lungs have more oxygen than the blood flowing past them. Which way does oxygen move?
- Particles diffuse from more crowded to less crowded.
Answer: From the air sacs into the blood.
Worked example: A cell in salt water
A cell with 1% salt inside sits in 5% salt water. The membrane lets water through but not salt. What happens?
- Water moves toward the side with more dissolved salt.
- The outside is saltier.
- Water leaves the cell, and it shrinks.
Answer: Water moves out; the cell shrinks.
Worked example: Limp celery
Why does limp celery get crisp in plain water?
- The celery cells have more dissolved material than plain water.
- Water moves into the cells, which swell and press against their walls.
Answer: Water enters the cells by osmosis.
Common mistake
Thinking water moves toward the side with more water, or that particles stop once they are spread evenly. Water moves toward the saltier side. At equilibrium the particles keep moving, in both directions at the same rate.