Why does a slug shrivel up when you put salt on it? It's the same reason you can digest your lunch, and a fish can breathe underwater.
Diffusion: crowded to spread out
Spray perfume in one corner of a room, and soon the whole room smells of it. The particles spread out, from where they're crowded to where they're not. That's diffusion. Scientists call how crowded something is its concentration.
The slug
A slug's skin works like a membrane: a thin wall that lets water through, but not salt. Salt makes a really salty layer on the outside, with less water than inside the slug.
So water moves out, from where there's more water to where there's less. That's osmosis: diffusion, but for water, through a membrane. The slug dries out. (Please never try this.)
Your lunch
After you eat, your small intestine is crowded with digested food, like sugar, and your blood has less. So the sugar diffuses into your blood.
That's why your gut is lined with millions of tiny fingers called villi. They give it a huge surface to diffuse through. (At GCSE you'll also learn that active transport takes in the rest once the gut has less sugar than the blood.)
The fish
The water flowing over a fish's gills has more oxygen than its blood. So oxygen diffuses in.
What makes diffusion faster
- A bigger surface (like villi and gills).
- A thinner wall to cross.
- A bigger difference in concentration.
Why does a raisin swell up when you leave it in water?
Show the answer
The inside of the raisin is very sugary, so it has less water than the water around it. Water moves in through its skin by osmosis.
Save this for your next test
- Diffusion: particles spread from crowded to less crowded.
- Osmosis: diffusion of water through a membrane.
- Bigger surface, thinner wall and bigger difference make it faster.
- One rule explains the slug, your gut and fish gills.
Read the video transcript
- Why does a slug shrivel up when you put salt on it?
- It's the same reason you can digest your lunch, and a fish can breathe underwater.
- It's called diffusion. Spray perfume in one corner of a room, and soon the whole room smells of it.
- The particles spread out, from where they're crowded to where they're not. Scientists call how crowded something is its concentration.
- Now the slug. Its skin works like a membrane: a thin wall that lets water through, but not salt.
- The salt makes a really salty layer on the outside, with less water than inside the slug.
- So water moves out, from where there's more water to where there's less. That's osmosis: diffusion, but for water, through a membrane.
- And the slug dries out. So please, never try this.
- Now your lunch. After you eat, your small intestine is crowded with digested food, like sugar. Your blood has less.
- So the sugar diffuses into your blood.
- That's why your gut is lined with millions of tiny fingers called villi. They give it a huge surface to diffuse through.
- And the fish? The water flowing over its gills has more oxygen than its blood. So oxygen diffuses in.
- One rule, three jobs. Crowded to spread out. Water through a membrane? That's osmosis.
- Save this for your next test.
- Now you: why does a raisin swell up when you leave it in water? Tell me in the comments.
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