Membrane Transport, Diffusion & Osmosis — Print · Grade 7 Science

Membrane Transport, Diffusion & Osmosis

Learning goals

• I can explain membrane transport, diffusion & osmosis.
• I can use evidence, a model, and stated assumptions to support a biological explanation.

Drop a tea bag into still water and do not stir it. The colour spreads anyway, slowly and in every direction, until the whole cup is the same shade. Nothing pushed it and nothing pulled it. Every molecule in that cup is in constant random motion, bumping and rebounding millions of times a second, and that aimless jostling is enough on its own to spread the tea through the water. This is diffusion: the net movement of particles from where they are crowded to where they are sparse, produced by random motion and nothing else. The word that matters is net. Individual particles go both ways the whole time; there are simply more of them starting on the crowded side, so more happen to cross outward than inward, and the imbalance shows up as spreading.

Concentration is how crowded the particles are — how much of a substance there is in a given volume. It is always a comparison, so "a high concentration" means nothing until you say high compared with what. When two regions differ, we speak of a concentration gradient between them, and diffusion runs down that gradient. It stops being one-sided when the crowding evens out, and that state is equilibrium. Equilibrium is not stillness: the particles keep moving and keep crossing in both directions, but the two flows now match, so nothing changes overall. A cell exploits this constantly. Oxygen is used up inside a respiring cell, so its concentration inside stays low, so oxygen keeps diffusing in without the cell spending anything to fetch it.

Cells are wrapped in a selectively permeable membrane, which lets some substances through and blocks others, and that turns a simple spreading process into something a cell can live on. Water crosses freely; many dissolved substances do not. When water moves across such a membrane from the side with more water to the side with less — that is, from the weaker solution to the stronger one — the movement has its own name, osmosis. It is just diffusion of water, but it is worth a separate name because the consequences for a cell are dramatic, from a wilting lettuce leaf to a red blood cell bursting in pure water. Diffusion and osmosis cost the cell nothing, which is why they are called passive. Moving a substance the other way, against its gradient, requires energy, and that is active transport.

Build your understanding

Where this fits

The last two units named the membrane as the cell's selective boundary and left the mechanism unexplained. How does anything actually cross it, and why does a plant cell go firm in water while an animal cell bursts? This unit answers both with one idea — random motion — and separates what a cell gets for free from what it has to pay for.

Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.

Part 1 · Concentration only means something as a comparison

Concentration is an amount of substance in an amount of space: grams of salt per litre of water, particles per unit volume, molecules per cubic micrometre. Written on its own, a figure tells you very little. Saying "the solution is concentrated" invites the question: compared with what? A fluid that is concentrated relative to the inside of a cell may be dilute relative to seawater, and the cell's behaviour depends entirely on which comparison you mean.

So always name both sides. Inside this cell there are ten particles per unit volume; outside there are five. That sentence supports a prediction, and "the outside is dilute" does not. The difference between two concentrations is a concentration gradient, and the size of the gradient affects how fast diffusion goes: a steep difference spreads faster than a shallow one, which is why the first minute of the tea bag changes the water more than the tenth.

One more distinction worth fixing now. A concentration can change because the amount of substance changed, or because the volume changed. If water leaves a cell, the substances left behind become more concentrated without a single extra particle arriving. When you read a rising concentration, ask which of the two happened, because the explanations are different.

Pause and explain

Why must a concentration always be stated as a comparison?

Check your reasoning after answering

Because concentration is an amount in a volume, and whether that counts as high or low depends on what it is being compared with. The same fluid can be concentrated relative to a cell's interior and dilute relative to seawater. Only a stated comparison — ten particles per unit volume inside against five outside — supports a prediction about which way material moves.

Part 2 · Net movement happens without anything intending it

It is tempting to say particles "want" to spread out, or that they "move towards" the empty space. They do neither. Each particle moves randomly, changing direction every time it collides, with no information about where the crowding is. Put a hundred particles on the left of a container and ten on the right, and in the next instant each one sets off in a random direction. More set off from the left simply because there are more of them there, so more arrive on the right than return. The one-way appearance is a consequence of the numbers, not of any tendency.

That is why the correct phrase is net movement. Traffic crosses in both directions throughout, and the net is the difference between the two flows. When the numbers on each side even out, both flows continue at the same rate and the net becomes zero. Nothing has stopped. If you could watch individual particles at equilibrium, you would see them crossing constantly in both directions.

Thinking this way prevents two errors. The first is expecting diffusion to stop once something arrives — it does not stop, it balances. The second is imagining that a cell has to do something to make diffusion happen. It does not: oxygen enters a respiring cell for free, because respiration keeps the inside concentration low and the gradient does the rest.

Pause and explain

At equilibrium, have the particles stopped moving?

Check your reasoning after answering

No. They keep moving randomly and keep crossing in both directions at the same rate, so the net movement is zero while the motion continues. Equilibrium describes a balance between two flows, not stillness, and warming the system would speed both flows without changing the balance.

Part 3 · Water across a selective membrane, and what it does to cells

Now make the barrier selective: water can cross it, a dissolved substance cannot. Water still diffuses in both directions, but the side holding more dissolved particles has proportionally less water, so the net flow of water is towards the stronger solution. That is osmosis. Note what it is not: the dissolved substance is not pulling the water, and the substance is not moving at all. Only the water moves, and only because its own concentration differs across the barrier.

The consequences for cells are immediate. An animal cell in pure water takes in water with nothing to resist the swelling, and can burst; a red blood cell does exactly that, which is why a drip must be matched to blood. The same cell in strong salt solution loses water and shrivels. A plant cell in pure water also takes in water, but its cellulose wall pushes back and the cell becomes firm instead — the turgor from the last unit. In strong salt solution the plant cell loses water, and the membrane pulls away from the wall.

A sealed model makes the logic clear. Two compartments of equal volume, separated by a membrane water can cross and solute cannot: compartment A holds 5 solute particles per unit volume, compartment B holds 10. Water moves net from A to B, because A is the weaker solution. B gains volume, which dilutes it, while A loses volume, which concentrates it, and the movement slows as the two approach each other.

Pause and explain

In the model above, which way does water move, and what actually crosses the membrane?

Check your reasoning after answering

Water moves net from compartment A, with 5 particles per unit volume, into compartment B, with 10, because A is the weaker solution and so holds proportionally more water. Only water crosses; the solute cannot pass the membrane. The flow slows as B is diluted and A is concentrated by the volume change.

Part 4 · When a cell has to pay: active transport

Diffusion and osmosis are passive: they run down a gradient and cost the cell nothing. But cells constantly need the opposite. A root hair cell takes up mineral ions from soil water where those ions are scarcer than inside the root. A nerve cell holds sodium out and potassium in against both gradients, and resets that difference after every signal. Neither can happen by diffusion, because both run uphill.

Active transport is the uphill case. A protein pump in the membrane binds the substance and moves it across using energy, which the cell supplies from respiration. Three predictions follow and all are testable: active transport stops when a cell is starved of oxygen or poisoned, while diffusion carries on; it is specific, because each pump handles particular substances; and it can go on raising a concentration difference long after diffusion would have evened things out.

The practical way to tell them apart is to ask which way the substance is going relative to its gradient, and whether stopping the cell's energy supply stops the movement. Down the gradient and unaffected by energy means passive. Up the gradient and dependent on energy means active. A cell uses both at once, all the time, and the fact that it never has to spend anything on taking up oxygen is part of why it can afford the pumps.

Pause and explain

How would you test whether a cell is taking up a substance by diffusion or by active transport?

Check your reasoning after answering

Compare the concentrations inside and outside, and remove the cell's energy supply. If the substance moves from high concentration to low and continues when respiration is blocked, it is diffusion. If it moves from low concentration to high and stops when oxygen is withheld or respiration is poisoned, it is active transport, because the pumps have nothing to spend.

Apply the whole unit · Read a selective-membrane record

Illustrative model, not a measured experiment: two compartments of equal volume at equal pressure are separated by a membrane that water can cross and solute cannot. Compartment A holds 5 solute particles per unit volume; compartment B holds 10.

Your task

Predict the initial net movement of water, state exactly what crosses the membrane, and explain why reaching equilibrium does not mean the movement has stopped.

What your response should show

• Explain the relevant biological process accurately.
• Use the supplied evidence and show calculation steps with units where needed.
• Connect a labeled model or mechanism with the claim.
• State an assumption, evidence limitation, or useful next observation.

Response guidance — after your attempt

1. Compare the two solutions: B at 10 particles per unit volume is twice as concentrated as A at 5, so A is the weaker solution and holds proportionally more water.

2. Predict the net flow: water moves net from A into B, down its own concentration gradient, while the solute stays where it is because the membrane will not pass it.

3. Follow the consequence: B gains volume and is diluted, A loses volume and is concentrated, so the gap narrows from both ends and the net flow slows.

4. Describe equilibrium correctly: water keeps crossing in both directions at matched rates, so the net is zero while the motion continues.

Water moves net from A to B. B holds 10 solute particles per unit volume against A's 5, so B is the stronger solution and therefore the one with proportionally less water; water diffuses down its own gradient towards it. Only water crosses — the solute cannot pass this membrane, so its particles stay in the compartments they started in, and their number never changes. What changes is volume: B swells and is diluted, A shrinks and is concentrated, so the difference closes from both directions and the net flow slows as it does. At equilibrium the two concentrations match and the net movement is zero, but nothing has stopped: water molecules continue to cross in both directions at the same rate, and the balance between the two flows is what makes the net zero. Two assumptions are worth stating. The model holds the pressure equal on both sides, so nothing opposes the volume change the way a plant cell wall would. And it assumes the membrane passes water freely and the solute not at all, which is an idealisation — a real membrane is selective by degree, not absolutely.

Scientific references for this unit

• Topic reference or educational context; core explanation is supplied above

Parent / teacher teaching notes

Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.

Worked examples

Avoid these mistakes

• Mistake: particles want to spread out. Each particle moves at random with no information about where the crowding is; net movement is just the result of more particles starting on the crowded side.
• Mistake: at equilibrium everything stops. Particles keep crossing in both directions at matched rates, so the net is zero while the motion continues.
• Mistake: in osmosis the solute pulls the water across. The solute does not move at all; water diffuses down its own concentration gradient towards the stronger solution.
• Mistake: all membrane transport is free. Diffusion and osmosis cost nothing, but moving a substance against its gradient needs pumps and energy from respiration, which is why it stops when oxygen is withheld.

Words to know

diffusion

The net movement of particles from a crowded region to a sparse one, produced by random motion alone and costing the cell nothing.

concentration

The amount of a substance in a given volume; it only supports a prediction when stated as a comparison between two places.

osmosis

The diffusion of water across a selectively permeable membrane, from the weaker solution towards the stronger one.

selectively permeable

Letting some substances cross while holding others back, which is what turns simple diffusion into something a cell can live on.

equilibrium

The state where two opposite flows match, so the net movement is zero; the particles are still moving and still crossing.

Extend your scientific explanation

Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.

In the real world

Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.

Exit check

Is movement of a dissolved particle always osmosis?