KS3 · Biology

Diffusion as moving particles

Someone sprays deodorant across the room, and soon you can smell it. The particles have no idea where you're sitting. So how did they find you?

Watch deodorant spread across a room

1Just sprayed234560in the far half (of 20)

Stage 1 of 6: Just sprayed. in the far half (of 20): 0. paused

Just sprayed · 1/6drag to spread it →

The deodorant starts packed into one corner: lots of particles squashed into a small space. The far side of the room has none yet.

Drag along the path, or press Play. Keep an eye on the number: when does it stop changing, and are the particles still moving then?

Zoom in: predict, then check

Every particle moves at random, so why does the deodorant spread one way? Zoom right in on two tiny boxes of air.

Two neighbouring boxes of air are exactly the same size. Box A holds 12 deodorant particles; box B, next door, holds 3. Every particle is moving at random. What happens at the line between the boxes?

Solids, liquids and gases

Where can diffusion happen?

For each state, tick every property that is true. Then check your grid against the answer.

Solid
Liquid
Gas

Check your thinking

Why does the tea spread?

A teabag is dropped into a mug of cold water and left alone. Nothing heats the water. Even so, tea particles spread out from the teabag through the water.

Which of these is closest to what you think is going on?
How sure are you?

Apply it

Diffusion or not?

Will diffusion take place here?

Still to sort

Diffusion takes place (0)

Particles in a liquid or gas, with a concentration gradient.

No diffusion (0)

One of the two conditions is missing.

Where the line is: Moving particles aren't enough on their own. Diffusion also needs a concentration gradient.

4 of 4 still to sort.

Sort each situation, then read why it belongs there.

Diffusion in a cell

How sugar and oxygen reach the mitochondria

A schematic slice through a cell's cytoplasm. The line is a picture, not a measurement: the higher it sits, the more sugar and oxygen particles there are in that part of the cytoplasm. Tap each zone.

Zone 1 of 4

Just inside the membrane

Sugar and oxygen particles are at a higher concentration here, close to the cell membrane.

From the membrane to the centre, the concentration of sugar and oxygen falls, so their net movement through the cytoplasm is towards the centre. That's how diffusion makes sure the parts of a cell get the substances they need.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • All substances are made of particles arranged as a solid, a liquid or a gas, and that includes the cytoplasm of a cell and the substances in it.
  • Particles of liquids and gases are constantly moving in random directions, colliding and changing direction.
  • Concentration is the number of particles in a volume. A difference in concentration between two areas is a concentration gradient.
  • Diffusion is the net movement of a substance from an area of higher concentration to an area of lower concentration.
  • Diffusion needs a liquid or a gas and a concentration gradient. It is passive: no additional energy has to be added.

The big picture

Particles in liquids and gases are always moving in random directions. Where there are more of them (a higher concentration), more move away than come back, so there is a net movement from higher to lower concentration. That's diffusion. It needs a liquid or gas and a concentration gradient, it's passive (no extra energy needed), and the net movement stops once the particles are evenly spread, even though they keep moving. It's how substances like sugar and oxygen move through a cell's cytoplasm.

Key points

1Particles cross both ways, but more cross from the crowded side, so the net movement is down the concentration gradient.
2Diffusion can't happen in a solid, because its particles are in fixed positions.
3Particles aren't alive and don't choose to spread out: random motion does it.
4Evenly spread particles keep moving, but with no gradient there's no net movement, so diffusion stops.
5In a cell, sugar and oxygen diffuse through the cytoplasm from near the membrane towards the centre, reaching the mitochondria for cellular respiration.

Worked example

Problem

A drop of dye is added to a beaker of still water and left alone. After a while, the colour has spread evenly through all the water. Explain how the dye spread out, and say what the dye particles are doing at the end.

⚠ Watch out

Saying the particles stop moving when diffusion is over. They keep moving randomly. What stops is the net movement, because there's no concentration gradient left.

🧠

Memory hook

Crowded to spread out, by chance not by choice. When the gradient's gone, the net movement stops. The particles don't.

✓

Check yourself

Box X holds 50 gas particles; box Y, the same size, holds 20. Which way is the net movement? Do any particles cross the other way? And what happens once both hold 35?

Flashcards

(14)
How are the particles arranged in a solid?
In a regular arrangement, very close together and touching. They can't move around.
How do the particles behave in a liquid?
They have a little more space than in a solid, and can move around and over each other.
How do the particles behave in a gas?
They are very far apart and can move very freely.
How do the particles of liquids and gases move?
Constantly, in random directions. They collide all the time, and a collision can change their direction.
What is concentration?
The number of particles in a volume of liquid or gas. More particles in the same volume means a higher concentration.
What is a concentration gradient?
A difference in concentration from one area to another, so there's an area of higher concentration and an area of lower concentration.
What does 'net movement' mean?
The overall movement of particles from one area to another, once you've counted the particles moving in both directions.
Define diffusion.
The net movement of a substance from an area of higher concentration to an area of lower concentration.
Why can't diffusion happen in a solid?
The particles are in fixed positions. They can't move around, so they can't spread out.
What is a passive process?
One that doesn't need any additional energy (no heat, light or electricity). Diffusion is passive because the particles already have energy for their random movement.
What two things does diffusion need?
Particles in a liquid or a gas, and a concentration gradient. It doesn't need extra energy or a living organism.
Do particles choose to spread out?
No. Particles aren't alive. They spread out only because of their random motion.
Once particles are evenly mixed, what are they doing?
Still moving randomly. But there's no concentration gradient, so there's no net movement and diffusion no longer takes place.
How do sugar and oxygen reach a cell's mitochondria?
By diffusing through the cytoplasm, from a higher concentration near the cell membrane to a lower concentration towards the centre. The mitochondria use them in cellular respiration.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 29 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.