Good Learning Co.
SavannahYear 8
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ScienceBiology foundations for GCSE (AQA 8461)30 min★★★ difficulty

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
  • 19 parts to scroll through
  • 4 quick checks
  • Marked quiz at the end
  • Gentle pace: short sittings with pauses

Words to know

diffusionparticleconceptssolutionssolvents

Scroll down — the lesson carries on below

Part 1 of 19 · Discover5%
1

Watch & discover

Part 1 of 19

Watch first

Diffusion, osmosis and why surfaces are folded

An English teaching video on this topic from DrBiology. Watch it, then scroll down for the lesson.

In a nutshell

Understand diffusion in terms of the particle model, and the concepts of solutions, solvents, solutes and solubility

2

What you already know

Part 2 of 19

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.

3

Key words

Part 3 of 19

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.
4

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

What drives movement

A short piece of teaching, then a check to make sure it has landed.

5

Explore the idea

Part 5 of 19

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.

6

Explore the idea

Part 6 of 19

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.

7

Quick check

Part 7 of 19

Quick check

Diffusion happens because particles...

8

Quick check

Part 8 of 19

Quick check

Osmosis moves water towards the...

9

Reset break

Part 9 of 19

Pause

That's sitting 1 of 2 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.

Stop here for now
10

Learning cycle

Part 10 of 19

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.

11

Explore the idea

Part 11 of 19

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.

12

Explore the idea

Part 12 of 19

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.

13

Quick check

Part 13 of 19

Quick check

Which change speeds up diffusion?

14

Quick check

Part 14 of 19

Quick check

Alveoli and villi are similar because both are...

15

Common mix-ups

Part 15 of 19

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.
16

Challenge round

Part 16 of 19

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

17

Challenge round

Part 17 of 19

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.

18

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: Diffusion, osmosis and why surfaces are folded

6 questions, marked with the reasoning shown. No timer.

  1. 1. Net diffusion stops when...

  2. 2. Partially permeable means...

  3. 3. A plant cell in pure water becomes...

  4. 4. Large organisms need transport systems because...

  5. 5. Diffusion distance should be...

  6. 6. Blood flow keeps the gradient...

19

Lesson round-up

Part 19 of 19

Lesson round-up

Ready when you are

Quiz score

Not sat

Games

Not played

Points this lesson

0

Best run

0 in a row

Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Case study

Why the school greenhouse failed

A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.

The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.

The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.

Written for Good Learning Co.

Reading questions

  1. Name the variable the club changed the second time, and one variable they kept the same.
  2. Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
  3. Write a conclusion for the club's report, saying how far the evidence supports it.

  • Method and results

    Writing

    Write the aim, what you changed, what you measured and what you kept the same, then record your results in a table.

    1 page + results table

  • Conclusion

    Writing

    Explain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.

    6–8 sentences

  • Labelled diagram

    Diagram

    Draw the apparatus or the process in pencil, label it fully, and photograph it.

    1 labelled diagram

  • Reading and notes

    Reading

    Read the lesson text again and write the key ideas in your own words.

    Half a page of notes

Saved as you type.

Part 1 of 19 · Watch & discover

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