Diffusion: particles spread out

The smell of food

You are in your room. Someone cooks in the kitchen. After a short time, you smell the food. How did the smell reach you?

The smell is made of tiny particles. They move by themselves, in all directions, and they spread through the air. After some time, they are everywhere in the house.

Diffusion

Diffusion is the movement of particles from a place where there are many of them to a place where there are fewer, until they are spread out evenly. We say that the particles move from a higher concentration to a lower concentration.

Put a drop of dye in a glass of still water. At first, all the dye is in one place. After some time, it is spread through all the water. Nobody stirred. The particles moved by chance, and the spread came out of that.

• Particles move all the time and in all directions. There is no push from the cell, so diffusion does not need energy from the cell.
• The net movement is from the higher concentration to the lower. Some particles move the other way too, but more go from the crowded side.
• When the particles are even, they still move, but there is no net movement any more.

You should find that after four times as long the dye has spread only twice as far. Warmth makes the particles move faster, so the curve flattens sooner.

Diffusion in living things

• Oxygen enters a cell. The cell uses oxygen, so the oxygen inside is low and the outside is higher. So oxygen diffuses in.
• Carbon dioxide is made in the cell, so it is higher inside, and it diffuses out.
• In a leaf, carbon dioxide diffuses in through small holes.

What makes diffusion faster?

• A higher temperature. The particles move faster.
• A bigger difference in concentration. More particles move from the crowded side.
• A shorter distance. This is the most important one for cells.

The time that diffusion needs grows very quickly with the distance. Twice the distance takes 4 times as long, and 10 times the distance takes 100 times as long. This is another reason why cells are small.

Try it in Python

This program shows diffusion by chance: 200 particles in a row of 30 places, all starting at the left end. In every step, each particle moves one place left or right. The graph counts the particles in the right half.

Practise with new numbers

Common mistakes

• Thinking diffusion needs energy. The particles move by themselves.
• Thinking diffusion stops when things are even. The particles still move; only the net movement stops.
• Thinking double the distance doubles the time. It takes 4 times as long.
• Forgetting that it is the net movement that goes from more to fewer.

Remember these

• Diffusion is the net movement of particles from a higher concentration to a lower concentration.
• It needs no energy from the cell. It is faster when it is warmer and when the distance is short.
• Oxygen enters cells and carbon dioxide leaves cells by diffusion.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why does diffusion not need energy from the cell?

2. A drop of ink is put in warm water and another in cold water. In which does the ink spread faster?

3. A molecule needs 2 s to diffuse across a thin cell. About how long does it need for a cell 3 times as wide?

4. Which way does carbon dioxide diffuse in a working cell, and why?