GCSE · Biology · AQA · Spec 8461

Diffusion

Oxygen is slipping into your blood right now, and nothing is pushing it. A single-celled organism gets what it needs straight through its surface. So why do you need lungs?

Grow a cube and watch its surface fall behind

Side length of the cube (cm): 5. Surface area ÷ volume: 1.2

Drag the point along the curve. Side 1 → ratio 6. Side 2 → ratio 3. Double the side, halve the ratio.

Exam line: Single cell, large SA:V: diffusion across its surface can meet its needs. Large organism, small SA:V: its outer surface alone can't.

Watch out: Read the height as surface per unit of volume. At side 3 the reading is 2: that's 2 cm² of surface for every 1 cm³ inside, a ratio of 2 : 1. The bigger cube still has MORE surface in total; it just has far more volume to serve.

What is actually crossing the surface?

Why does oxygen end up in your blood?

You breathe in. Oxygen moves from the air in your lungs into your blood, while carbon dioxide moves from your blood into that air. Meanwhile, all over your body, the waste product urea moves out of your cells into the blood plasma, to be removed by your kidneys.

Why does the oxygen end up moving into your blood? Pick the idea closest to what you think right now.

The oxygen particles 'know' your blood needs them, so they head towards it.Your lungs squeeze the oxygen, pushing it through into the blood.The particles move about randomly. There are more of them in the air in my lungs than in my blood, so more cross into the blood than come back.Oxygen moves in until both sides have the same amount, and then the particles stop moving.

How sure are you?

I'm sureI think soI'm guessing

The oxygen particles 'know' your blood needs them, so they head towards it.

This is a common misconception

Particles don't know anything and don't aim anywhere. Each one just moves about at random, bumping into other particles and changing direction. The trick is that there are far more oxygen particles in the air in your lungs than in your blood, so more of them happen to wander into the blood than wander back out. Nothing is chosen. It's just numbers.

Your lungs squeeze the oxygen, pushing it through into the blood.

This is a common misconception

Breathing moves air in and out of your lungs, but nothing forces particles through the membrane. They cross it on their own, moving randomly. What makes more go one way than the other is the difference in concentration on the two sides.

The particles move about randomly. There are more of them in the air in my lungs than in my blood, so more cross into the blood than come back.

This matches the evidence

Exactly right, and that's diffusion: particles spreading out, giving a NET movement from a higher concentration to a lower one. Run the same idea on the other two. Carbon dioxide is more concentrated in the blood than in the air, so its net movement is out. Urea is more concentrated in your cells than in the plasma, so its net movement is into the plasma.

Oxygen moves in until both sides have the same amount, and then the particles stop moving.

This is a common misconception

Half right. When the concentrations are equal, the NET movement stops, but the particles never do. They keep crossing both ways, just in equal numbers, so neither side gains overall. Diffusion is a difference in traffic, not particles switching off.

Particles move randomly in all directions. Diffusion is the NET movement that results, from where they're more concentrated to where they're less. No wanting, no pushing.

Predict, then check

Oxygen is diffusing into a cell from the water around it. You change ONE thing at a time.

Which ONE change would make oxygen diffuse into the cell more slowly?

Put numbers on it

Calculate the ratio yourself

Two model cells are cubes: one has sides of 1 cm, the other has sides of 4 cm. Calculate the surface area to volume ratio of each, and decide which cube's surface can handle exchange more effectively for the volume inside it.

Small cube: 6 faces, each 1 × 1 = 1 cm², so surface area = 6 cm². Volume = 1 × 1 × 1 = 1 cm³.Worked for you. Now do the big cube the same way.
Small cube: SA:V = 6 ÷ 1, so the ratio is 6 : 1Divide surface area by volume.
missing step

Which line is step 3?

Big cube: one face = 4 × 2 = 8 cm²Big cube: one face = 4 × 4 = 16 cm²Big cube: one face = 4 × 4 × 4 = 64 cm²

Exam line: Surface area first, then volume, then divide: SA:V = surface area ÷ volume, written as a ratio to 1.

Why size matters

Reason it through

Why does a large multicellular organism need exchange surfaces and a transport system, when a single cell manages with its surface alone?

Link 1 of 4

First link · your turn

What happens to the surface area to volume ratio as an organism gets bigger?

Locked — reveal the link above first

Sort the adaptations

The four tricks exchange surfaces use

Which trick is each feature using to make exchange faster?

Still to sort

Large surface area (0)

More surface, so more particles can cross at once.

Where the line is: It's about how much surface there is, not how thick it is.

Thin membrane: short diffusion path (0)

Particles don't have far to go.

Where the line is: It's about the distance across, not the amount of surface.

Efficient blood supply (animals) (0)

Blood keeps carrying substances to or away from the surface.

Where the line is: Blood keeps the gradient steep on the blood side; ventilation does it on the air or water side.

Ventilated (animals, gas exchange) (0)

Fresh air or water keeps arriving at the surface.

Where the line is: Ventilation moves air or water past the surface; the blood supply moves blood.

10 of 10 still to sort.

Small intestine, lungs, gills, roots and leaves look nothing alike. Sort their features and you'll see the same few tricks again and again.

Exam line: In a written answer, give the feature AND what it does: 'the villus wall is one cell thick, so the diffusion path is short.' The feature alone is only half the point.

Watch out: A blood supply and ventilation are animal tricks. Plants don't have either, so no plant feature belongs in those two columns.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How substances slip in and out of cells by diffusion, what speeds it up, and why getting bigger forces living things to build exchange surfaces.

What you need to know

Substances move into and out of cells across the cell membrane by diffusion: oxygen and carbon dioxide in gas exchange, and urea from cells into the blood plasma to be removed by the kidneys.
Diffusion is the spreading out of the particles of a gas, or of a substance in solution, giving a net movement from a higher concentration to a lower concentration.
The rate of diffusion depends on the concentration gradient, the temperature and the surface area of the membrane.
Small organisms have a large surface area to volume ratio. Large organisms have a small one, so they need exchange surfaces and a transport system.
An exchange surface works better with a large surface area, a thin membrane (short diffusion path), an efficient blood supply (animals) and ventilation (animals, for gas exchange).

The big picture

Diffusion is the spreading out of particles, giving a net movement from a higher concentration to a lower one. It's how substances such as oxygen, carbon dioxide and urea move into and out of cells across their membranes. It's faster with a steeper concentration gradient, a higher temperature and a larger membrane surface area. As an organism gets bigger, its surface area to volume ratio falls, so a large organism can't rely on its outer surface. It needs specialised exchange surfaces, with a large surface area, a short diffusion path and, in animals, a good blood supply and ventilation, plus a transport system to reach every cell.

Key points

1Diffusion: the net movement of particles from a higher to a lower concentration, down a concentration gradient.

2Particles move randomly in all directions. The NET movement is from where there are more of them to where there are fewer, and it stops when concentrations are equal, even though the particles keep moving.

3Diffusion is faster with a steeper concentration gradient, a higher temperature and a larger membrane surface area.

4Surface area to volume ratio = surface area ÷ volume. For a cube with sides L: surface area = 6 × L × L, volume = L × L × L.

5As size increases, the surface area to volume ratio decreases, because volume grows faster than surface area.

6A single-celled organism's large ratio lets diffusion across its surface meet its needs. Multicellular organisms need specialised exchange surfaces and a transport system.

7Exchange surfaces: small intestine and lungs in mammals, gills in fish, roots and leaves in plants.

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Worked example

Problem

A cube-shaped model cell measures 2 cm × 2 cm × 2 cm. A long, thin model cell measures 1 cm × 1 cm × 8 cm. Both have the same volume. Which has the larger surface area to volume ratio?

⚠ Watch out

Saying a large organism has a smaller surface area. In total it has MORE surface area. What's smaller is its surface area compared with its volume, so always say 'a smaller surface area to volume ratio'.

🧠

Memory hook

Get bigger and your surface falls behind your volume. Exchange surfaces fight back: more surface, a shorter path, and (in animals) blood flow and breathing to keep the gradient steep.

✓

Check yourself

Cover the page. Carbon dioxide is more concentrated inside a cell than outside: which way is its net movement? Name the three rate factors. What happens to a cube's SA:V when its side doubles?

Flashcards

(14)

What is diffusion?

The spreading out of particles of a gas, or of a substance in solution, giving a net movement from a higher concentration to a lower concentration.

Name three substances that move into or out of cells by diffusion.

Oxygen and carbon dioxide (in gas exchange), and urea (from cells into the blood plasma, to be excreted by the kidneys).

Do particles stop moving when concentrations on both sides are equal?

No. They keep moving randomly both ways in equal numbers, so the NET movement is zero.

What three factors affect the rate of diffusion?

The concentration gradient, the temperature and the surface area of the membrane.

Why does a steeper concentration gradient speed up diffusion?

With a bigger difference in concentration, many more particles cross from the crowded side than cross back, so the net movement is faster.

Why does a higher temperature speed up diffusion?

The particles move faster, so more of them cross the membrane each second.

Why does a larger membrane surface area speed up diffusion?

There's more membrane for particles to pass through, so more of them can cross at the same time.

How do you calculate the surface area to volume ratio of a cube?

Surface area = 6 × side × side. Volume = side × side × side. Divide surface area by volume and write it as a ratio to 1.

What happens to the surface area to volume ratio as an organism gets bigger?

It gets smaller, because volume increases faster than surface area.

Why can a single-celled organism rely on diffusion across its surface?

It has a relatively large surface area to volume ratio, so enough molecules can cross its surface to meet its needs.

Why do multicellular organisms need exchange surfaces and a transport system?

Their small surface area to volume ratio means the outer surface can't exchange enough. Exchange surfaces get enough in and out; the transport system carries it to and from every cell.

What four things make an exchange surface more effective?

A large surface area; a thin membrane (short diffusion path); an efficient blood supply (animals); ventilation (animals, for gas exchange).

How does an efficient blood supply help an exchange surface?

Blood keeps carrying substances away from (or to) the surface, which keeps the concentration gradient steep.

Name the five exchange surfaces you should be able to explain.

The small intestine and lungs in mammals, gills in fish, and roots and leaves in plants.

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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Grow a cube and watch its surface fall behind

Why does oxygen end up in your blood?

Calculate the ratio yourself

The four tricks exchange surfaces use

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