Diffusion, osmosis and why surfaces are folded
Every exchange surface in biology — lung, gut, root, gill — solves the same problem: move particles fast enough across a boundary. The design answers are strikingly similar.
Lesson overview
What you'll learn in this lesson
Understand diffusion in terms of the particle model, and the concepts of solutions, solvents, solutes and solubility
Key learning points
- • Diffusion is a statistical result
- • Osmosis is a special case
- • Rate depends on three things
- • Folded, thin, moist and supplied
This lesson at a glance
- 30 minutes
- 25 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 25
Watch first
Diffusion, osmosis and why surfaces are folded
Watch how a gradient, a thin wall and a big surface make exchange fast.
In a nutshell
Understand diffusion in terms of the particle model, and the concepts of solutions, solvents, solutes and solubility
What you already know
Part 2 of 25
Before we start
What you already know
You should already know that particles in liquids and gases move randomly, and that cells need oxygen and glucose.
Key words
Part 3 of 25
Key words
Words you'll need today
- Diffusion
- Net movement of particles from high to low concentration.
- Osmosis
- Diffusion of water across a partially permeable membrane.
- Concentration gradient
- The difference in concentration that drives diffusion.
- Surface area to volume ratio
- How much surface a shape has for each unit of volume.
- Partially permeable
- A membrane that lets some particles through but not others.
Reset break
Part 4 of 25
Pause
That's sitting 1 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 5 of 25
Learning cycle 1 of 2
What drives movement
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 6 of 25
Learn
Diffusion is a statistical result
No particle knows where the crowd is. Particles move randomly, but where there are more of them on one side, more happen to cross outwards than inwards. The net movement continues until concentrations are equal, and then random motion carries on with no net change.
Explore the idea
Part 7 of 25
Learn
Osmosis is a special case
When a membrane blocks the solute but not water, water moves from the dilute solution towards the concentrated one. This is why plant cells become turgid in pure water and plasmolysed in strong sugar solution, and why drinking seawater dehydrates you.
Reset break
Part 8 of 25
Pause
That's sitting 2 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 9 of 25
Quick check
Part 10 of 25
Learning cycle
Part 11 of 25
Learning cycle 2 of 2
Why exchange surfaces look the way they do
A short piece of teaching, then a check to make sure it has landed.
Reset break
Part 12 of 25
Pause
That's sitting 3 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Explore the idea
Part 13 of 25
Learn
Rate depends on three things
A steeper concentration gradient, a larger surface area and a shorter diffusion distance all speed diffusion up. Blood flow and ventilation matter because they keep the gradient steep, constantly removing what has just crossed.
Explore the idea
Part 14 of 25
Learn
Folded, thin, moist and supplied
Alveoli, villi and root hairs all show the same features: huge surface area, walls one cell thick, a moist surface and a transport system alongside. Learn the pattern once and every exchange surface at GCSE becomes recognisable.
Quick check
Part 15 of 25
Reset break
Part 16 of 25
Pause
That's sitting 4 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 17 of 25
Common mix-ups
Part 18 of 25
Common mix-ups
Things people often get wrong
- Diffusion does not stop at equilibrium — the net movement stops, the motion does not.
- Osmosis moves water, not solute.
- A big organism has a smaller surface area to volume ratio, which is why it needs a transport system.
Challenge round
Part 19 of 25
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Reset break
Part 20 of 25
Pause
That's sitting 5 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Challenge round
Part 21 of 25
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
No ____ knows where the crowd is.
A steeper concentration gradient, a larger surface area and a shorter ____ distance all speed diffusion up.
Challenge round
Part 22 of 25
Game · Recall cards
Diffusion happens because particles...
Card 1 of 4
Exit quiz
Part 23 of 25
Exit quiz
Show what you've learned
6 questions, marked together at the end. Nothing is timed.
1. Net diffusion stops when...
2. Partially permeable means...
3. A plant cell in pure water becomes...
4. Large organisms need transport systems because...
5. Diffusion distance should be...
6. Blood flow keeps the gradient...
Mastery quiz
Part 24 of 25
Marked quiz
End of lesson quiz: Diffusion, osmosis and why surfaces are folded
4 questions, marked with the reasoning shown. No timer.
1. Diffusion happens because particles...
2. Osmosis moves water towards the...
3. Which change speeds up diffusion?
4. Alveoli and villi are similar because both are...
Lesson round-up
Part 25 of 25
Lesson round-up
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Ask LunaPart 1 of 25 · Watch & discover