GCSE Biology — Cell Transport: Diffusion, Osmosis & Active Transport

Taught through the skin of a White's tree frog

Approx. 60 minutes · KS4 · Teaching-led

Before you start

A White's tree frog can drown in water it is sitting in, and it can dehydrate to death in a tank that has water in it. Both of those are true, and both come down to a single idea from GCSE Biology.

This lesson is mostly explanation. There are a few questions at the end, but the point of it is to understand the mechanism well enough that the questions become obvious.

Nothing is timed.

What you'll be able to do by the end

• Explain diffusion, osmosis and active transport, and say precisely how they differ.
• Use the words concentration gradient, partially permeable and dilute correctly.
• Predict which way water will move in a given situation, and explain why.
• Explain why active transport needs energy when the other two do not.
• Apply all three to a real living animal.

Part 1 — Why anything moves at all

Start with the thing underneath all three processes.

Particles in a liquid or a gas are constantly moving. They are not moving towards anything — the movement is random, in all directions, all the time. Nothing is steering.

Now picture a drop of ink in a glass of water. It spreads out. Every school has done this.

The thing to be careful about is why it spreads. It is tempting to say the ink "moves to where there is less ink," as though it wants to spread out. It does not want anything. Here is what actually happens.

At the start there is a lot of ink on the left and none on the right. The ink particles are moving randomly, so some drift right and some drift left. But there are far more ink particles on the left than the right, so far more of them drift rightward than leftward. Not because they prefer it — just because there are more of them there to begin with.

Over time this evens out. Once ink is spread evenly, particles are still moving in both directions, but now the numbers going each way are equal, so nothing appears to change.

That is the whole idea, and it is worth sitting with, because every process in this lesson is built on it:

Random movement plus an uneven starting distribution produces net movement from high concentration to low concentration.

That difference in concentration between two places has a name: the concentration gradient. Movement down a gradient — high to low — requires no energy input, because the random movement was happening anyway.

Part 2 — Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient, as a result of random movement.

Read that definition again and notice what is in it. "Net" — because particles go both ways and we are describing the overall result. "Random" — because nothing is steering. No mention of energy, because none is required.

Diffusion in the frog

A White's tree frog does something a mammal cannot. It absorbs oxygen through its skin, directly into the blood vessels underneath.

Follow the gradient. There is more oxygen in the air and water outside than in the blood beneath the skin, because the blood is constantly using oxygen up. So oxygen diffuses inward, down its gradient. Meanwhile carbon dioxide is being produced by respiration, so there is more of it inside than out, and it diffuses outward.

Two gases, two gradients, opposite directions, same process. Neither costs the frog anything, which is precisely why this works.

What makes diffusion faster

Three factors, and each one makes sense if you go back to Part 1.

A steeper concentration gradient. A bigger difference between the two sides means a bigger imbalance in the random drift, so faster net movement. This is why moving air over a surface speeds up gas exchange — it removes what has arrived and keeps the difference large.

A larger surface area. More area means more places for particles to cross at once. The frog's skin is a large surface relative to its body, and it is folded and moist, which increases it further.

A shorter distance to travel. Thin surfaces are crossed faster. Frog skin is extremely thin where exchange happens, and it lies directly over blood vessels, so the distance from outside air to bloodstream is tiny.

Every gas exchange surface in biology has these three features. Lungs, gills, leaves, frog skin — all of them are large, thin, and kept next to a supply that maintains the gradient. When you meet a new one in an exam, you already know what to look for.

The cost of that adaptation

The frog's skin is thin and permeable because that is what makes gas exchange work. But permeable means permeable to everything, not selectively to oxygen. Chlorine, cleaning residue, soap, hand cream — all of it crosses too, by exactly the same process, in exactly the same direction, down exactly the same kind of gradient.

There is no mechanism by which the skin could let oxygen in and keep a detergent out. Diffusion does not select. This is why hands are wetted and rinsed clean before an amphibian is touched, and why the water is treated before it goes in the tank.

The adaptation and the vulnerability are the same feature.

Part 3 — Osmosis

Osmosis is diffusion with two extra conditions. Get those two conditions clear and the topic stops being confusing.

Osmosis is the movement of water molecules from a dilute solution to a concentrated solution, through a partially permeable membrane.

Condition one: it is about water

Diffusion can be about anything — oxygen, carbon dioxide, a dye, a smell. Osmosis is only ever about water.

Condition two: there is a membrane

A partially permeable membrane has holes small enough to let water molecules through but too small for the dissolved solute particles, which are bigger.

Now think about what that produces. The solute cannot move to even itself out, because it cannot fit through. But the water can. So the only way for the two sides to become more similar is for water to move.

And it moves in the direction that dilutes the concentrated side.

The wording trap

This is where marks are lost, so slow down here.

A dilute solution has lots of water and little solute. A concentrated solution has little water and lots of solute.

Water moves from dilute to concentrated. In water's own terms, that is still high concentration to low concentration — there is more water on the dilute side. It is the same rule as diffusion, described from the water's point of view.

The confusion arises entirely because we usually describe a solution by how much solute it has, while osmosis is about the water. When you read a question, translate it first: "concentrated salt solution" means "not much water." Then the direction is obvious.

Osmosis in the frog

A frog does not drink. It absorbs water through its skin, from the surface it is sitting on. It has a patch on its underside adapted for exactly this.

In clean fresh water the frog's body fluids are more concentrated than the water outside, so water moves in by osmosis. That is how the animal hydrates, and it is why a shallow dish and damp surfaces matter more than any water bowl.

Now change the water. Put the frog in water with a high concentration of dissolved salts, so the outside is more concentrated than the frog's body fluids. The gradient reverses. Water now moves out of the frog, across the same skin, by the same process. The animal dehydrates while sitting in water.

Nothing has gone wrong with the frog. Osmosis is doing exactly what it always does. The direction changed because the gradient changed.

Turgid, flaccid, plasmolysed

Plant cells behave differently from animal cells because they have a cell wall, and the vocabulary shows up in exams.

• Turgid — a plant cell in dilute solution takes in water, swells, and presses against its cell wall. The wall prevents bursting. This pressure is what holds a non-woody plant upright.
• Flaccid — the cell has lost water and is no longer pressing outward. The plant wilts.
• Plasmolysed — severe water loss has pulled the cell membrane away from the cell wall.

An animal cell has no wall. In a very dilute solution it takes in water and can burst — lysis. In a very concentrated solution it loses water and shrivels — crenation.

This is why the concentration of blood plasma is controlled so tightly. There is no wall to protect the cells, so the surrounding fluid has to be kept right.

Part 4 — Active transport

Diffusion and osmosis both move substances down a gradient. Neither needs energy, because random movement does the work.

But organisms frequently need to move a substance the wrong way — from where there is little of it to where there is already a lot. Nothing in Part 1 will do that. Random movement will never, on average, pile particles up against a gradient.

So it has to be forced, and forcing costs energy.

Active transport is the movement of substances against a concentration gradient, from a lower to a higher concentration, using energy from respiration.

How it works

Carrier proteins sit in the cell membrane. Each binds a specific substance, changes shape using energy supplied by respiration, and releases the substance on the other side — regardless of which side already has more.

Three consequences follow, and all three are examinable:

It requires respiration. Stop respiration and active transport stops. This is why cells doing a lot of it contain large numbers of mitochondria — root hair cells, and the cells lining the small intestine, are the standard examples.

It is selective. Carrier proteins are specific. Unlike diffusion, which will carry anything small enough, active transport moves only what there is a carrier for.

It can be saturated. There are a limited number of carriers. Once all are working, adding more substance does not increase the rate.

Active transport in an amphibian

An amphibian living in fresh water has a real problem. Its body fluids are more concentrated than the water around it, so water constantly enters by osmosis and dissolved ions constantly leak out, down their gradients. Both processes are relentless and neither can be switched off.

The animal deals with this by actively transporting ions back in through specialised cells in the skin, against the gradient, using energy from respiration. It also produces large volumes of dilute urine to get rid of the excess water.

So all three processes are running at once in the same animal, across the same skin:

• Diffusion — oxygen in, carbon dioxide out. No energy cost.
• Osmosis — water in from the environment. No energy cost.
• Active transport — ions pulled back in against their gradient. Energy cost, continuously.

That is the whole topic operating in one animal, and it is why a frog cannot simply be put in any water and left.

Part 5 — Putting the three side by side

The one question that identifies the process: which way is it moving relative to the gradient?

Down the gradient, and it is water through a membrane — osmosis. Down the gradient, anything else — diffusion. Against the gradient — active transport, and energy is involved.

Almost every exam question on this topic can be answered by asking that one question first.

Part 6 — Questions

1. Define diffusion. (2 marks)

2. Explain two ways in which osmosis differs from diffusion. (2 marks)

3. A frog is placed in water containing a high concentration of dissolved salts. Explain what happens to the water in the frog's body and why. (3 marks)

4. Explain why active transport requires energy but diffusion does not. (3 marks)

5. Explain why the same feature that allows a frog to absorb oxygen through its skin also makes it vulnerable to contaminated water. (3 marks)

6. Cells lining the small intestine contain large numbers of mitochondria. Explain why. (3 marks)

If you want to go further

• Work out which of the three processes is happening in each of: a smell crossing a room; a wilting plant; a root hair cell taking up minerals; a red blood cell in pure water.
• Related lessons: Adaptation · Cells & Cell Structure · Homeostasis.

Notes for the adult

Curriculum: GCSE Biology — diffusion, osmosis and active transport; factors affecting the rate of diffusion; exchange surfaces; and the role of mitochondria in active transport.

Deliberately teaching-heavy. Parts 1 to 5 are explanation, with only six questions at the end. This topic fails for most students not because they cannot answer questions but because they never understood why particles move at all, so Part 1 spends time on that before naming anything.

The two misconceptions targeted. That particles "want" to spread out, addressed in Part 1; and the dilute-to-concentrated wording of osmosis, addressed in Part 3 with the translation step. The second is the most common cause of lost marks on this topic.

Why the frog rather than a potato. The standard teaching example is a potato in sugar solution, which students can complete without believing it describes anything. Running all three processes through one real animal shows them operating simultaneously, which is what actually happens in life and is rarely shown.

Access: read-aloud, dyslexia-friendly font, enlarged text, high contrast and calm mode. The explanation is broken into short paragraphs with frequent subheadings so a reader using read-aloud can stop and restart at a sensible point.

No timing, no scoring.

Ready for more? Open the full unit →