Hi Tara! This is a small topic that behaves like a big one, because examiners come back to it in almost every paper — in gas exchange, in transport in plants, in digestion, in kidneys. There are only three ideas here. Diffusion: particles spread out on their own because they are already moving. Osmosis: the same thing, but for water, across a membrane that lets water through and holds solutes back. Active transport: the cell paying energy to move something the wrong way up a gradient. What makes Topic 3 the biggest mark-loser in the whole first half of the syllabus is not the science — it is three words. Diffusion is net movement, not just movement. Membranes are partially permeable, never “semi-permeable”. And water moves down a water potential gradient, not “towards the solute”. Get those three right and this topic becomes free marks. Let’s build it properly.
The Big Idea: Nothing Is Ever Still
Put a drop of ink in a glass of perfectly still water and come back an hour later. The whole glass is pale blue. Nobody stirred it. No pump moved it. So what did?
Every particle in a liquid or a gas is constantly moving in a random direction, because it has kinetic energy. It travels a tiny distance, collides with something, bounces off in a new direction, and does it again, millions of times a second. That is true of the ink particles and of the water particles alike. Nothing about that motion knows or cares where the ink “should” go.
Now think about the boundary between the crowded ink drop and the clear water beside it. Particles cross that boundary in both directions all the time. But there are far more ink particles on the ink side, so more of them happen to wander out than happen to wander back in. The result, added up over billions of particles, is a steady drift of ink into the water. That drift is diffusion.
Write “diffusion is the movement of particles from high to low concentration” and you have described a one-way stream of particles marching in formation. That is not what happens, and examiners refuse it. Particles move in every direction at once; only the overall balance runs downhill. One word — net — converts a wrong sentence into a right one. Write it every single time.
The same word saves you again at equilibrium. When the ink is evenly spread, particles have not stopped moving — equal numbers now cross in each direction, so the net movement is zero. “The particles stop moving” is a guaranteed lost mark.
The Four Factors — and Only These Four
The syllabus is explicit about which factors you must be able to investigate: surface area, temperature, concentration gradient and distance. Learn them as a set of four, because “state two factors affecting the rate of diffusion” is a routine question and anything outside this list earns nothing.
| Factor | Increase it and the rate… | Why | Where it shows up in the exam |
|---|---|---|---|
| Surface area | increases | more places for particles to cross at the same time | villi and microvilli, alveoli, root hairs, gill filaments, folded membranes |
| Temperature | increases | particles have more kinetic energy, so they move faster | why diffusion is quicker at 37 °C than in a cold pond |
| Concentration gradient | increases | a bigger difference means a bigger imbalance in random crossings | ventilation, blood flow, respiration using oxygen up |
| Distance | decreases | a random walk over a long path takes disproportionately longer | alveolus wall one cell thick, thin leaves, flat worms |
Three of the four factors speed diffusion up when you increase them — surface area, temperature, gradient. Only distance is the odd one out and slows it down. If you can remember “three up, one down”, you will never write “a thicker membrane increases the rate” in a hurry.
Surface Area to Volume Ratio — Why Size Is the Enemy
A cell is supplied across its surface but has to keep its whole volume alive. So what matters is not how much surface it has, but how much surface it has per unit of volume — the surface area to volume ratio.
Take a cube of side l. Surface area = 6l². Volume = l³. So the ratio = 6l² ÷ l³ = 6 ÷ l. Every time the object gets bigger, the ratio gets smaller. That single line of algebra explains why Amoeba needs no lungs and an elephant does.
“Which cube has the greatest surface area?” — the biggest one, obviously. “Which decolourises first?” — the smallest one. Both are true and they are not contradictory: the big cube has more total surface but vastly more volume to supply, so its ratio is worse. Whenever a question compares sizes, it is asking about the ratio, not the raw area.
Why Diffusion Never Runs Out
Here is a question that catches people out. Oxygen diffuses into a muscle cell from the blood. Why doesn’t the cell just fill up with oxygen until the concentrations match and diffusion stops?
Because respiration keeps using the oxygen up. The concentration inside never gets a chance to rise, so the gradient is permanently maintained and oxygen keeps flowing in. The mirror image works for carbon dioxide: respiration keeps making it, so its concentration inside stays high and it keeps diffusing out.
This is one of the most reusable two-mark answers in the whole syllabus. “Respiration continually uses up the oxygen, so its concentration inside the cell stays low and the concentration gradient is maintained.” It works for photosynthesis and carbon dioxide in a leaf, for oxygen at a gill, for glucose absorption in the gut — anywhere a gradient somehow refuses to disappear.
Importance of Diffusion in Living Things
Some substances move into and out of cells by diffusion through the cell membrane. Oxygen diffuses into a respiring cell and carbon dioxide diffuses out, both across the cell membrane. In a palisade cell in the light, carbon dioxide diffuses in and oxygen diffuses out. The cell wall of a plant cell is fully permeable, so it is the membrane that the particles must cross.
| What diffuses | Where | Why the gradient exists |
|---|---|---|
| Oxygen | alveolus → blood; blood → respiring cell; water → gill | respiration uses oxygen up inside cells |
| Carbon dioxide | respiring cell → blood → alveolus; air → palisade cell in light | respiration makes it; photosynthesis uses it up |
| Digested food (glucose, amino acids) | small intestine → blood | blood carries the absorbed food away, keeping its concentration low |
| Urea | liver cell → blood | the liver keeps making it; the kidney keeps removing it |
| Water vapour | leaf air spaces → outside air (transpiration) | the air outside is usually drier |
10 mm: 600 mm² ÷ 1000 mm³ = 0.6 : 1
20 mm: 2400 mm² ÷ 8000 mm³ = 0.3 : 1
30 mm: 5400 mm² ÷ 27 000 mm³ = 0.2 : 1
Planning a Diffusion Investigation
Basic method. Make agar containing an indicator, for example phenolphthalein with a little sodium hydroxide, so the agar is pink. Cut cubes with a sharp knife and a ruler. Put each cube into a beaker of dilute hydrochloric acid and time how long it takes for the whole cube to go colourless. The acid diffuses in and the colour disappears.
- Independent variable: the factor you change. Dependent variable: the time taken to go colourless (rate = 1 ÷ time). Controlled variables: keep the other factors the same.
- Surface area: cubes of side 1 cm, 2 cm and 3 cm, in the same concentration, volume and temperature of acid.
- Temperature: identical 1 cm cubes in acid held in water baths at 20, 30, 40 and 50 °C, with the same concentration and volume of acid.
- Concentration gradient: identical 1 cm cubes in acid of 0.25, 0.50 and 1.00 mol dm⁻³, with the same volume and temperature.
- Distance: time how long the colour takes to disappear from the surface to the centre of cubes of different sizes, or measure how far the colour change has moved into a long block of agar every minute.
- Repeat each condition three times and calculate a mean. Use a large volume of acid, so that its concentration hardly falls during the experiment.
Water Is the Solvent Everything Else Depends On
Before osmosis, why water matters at all. Water is the solvent of life: substances have to be dissolved before they can be moved, reacted or removed. Digestion: enzymes work in solution, so food molecules must be dissolved in water for enzymes to act on them, and the small, soluble products (glucose, amino acids) stay dissolved so they can be absorbed into the blood. Excretion: waste products such as urea, and excess ions, dissolve in water and are removed from the body in urine (and some in sweat). Transport: substances are carried dissolved in water: glucose, amino acids, ions, urea and carbon dioxide in blood plasma; mineral ions in xylem; sucrose in phloem. A substance that is not dissolved cannot be carried in these fluids. Nothing about a cell works dry.
And because water is such a good solvent, cells are surrounded by solutions of different concentrations — which is where the trouble starts.
Osmosis Is Just Diffusion, With Two Extra Conditions
Osmosis is not a new mechanism. It is diffusion — random movement producing a net drift — with two restrictions bolted on:
- the substance that moves is water, and only water;
- the barrier is a partially permeable membrane, which lets water through but holds larger solute molecules back.
Say it out loud until it is automatic: more solute, less free water, lower water potential. Half the mistakes in this topic come from having the scale upside down. Once you have it the right way up, every direction question answers itself — water moves towards the lower water potential, which is the more concentrated side.
1. “Semi-permeable”. This is the old term. Cambridge mark schemes want partially permeable. It is a free mark, and it is lost every year by students who understood the biology perfectly.
2. “Water moves to where there is more solute.” This gives the right prediction with the wrong mechanism, so it survives for years before failing you. Water molecules are not attracted by solute. They move at random; a concentrated solution simply has fewer free water molecules, which is what “lower water potential” means. Say down the water potential gradient and the mark is yours.
Plant Cells: Turgid, Flaccid, Plasmolysed
A plant cell is a bag of concentrated cell sap inside a partially permeable membrane, wrapped in a strong, inelastic cellulose cell wall. That wall changes everything.
| State | Solution outside | Water movement | What you would see |
|---|---|---|---|
| Turgid | more dilute than the cell sap (higher water potential) | net movement in | vacuole full, contents pressed hard against the wall; tissue firm |
| Flaccid | slightly more concentrated | net movement out | turgor pressure zero, membrane still against the wall; tissue soft |
| Plasmolysed | much more concentrated (lower water potential) | net movement out | membrane separated from the wall, contents shrunken into the middle |
Turgor pressure, and why plants stand up
As water enters a plant cell, the vacuole swells and pushes the cytoplasm outwards against the cell wall. The wall is strong and barely stretches, so it pushes back. That outward push of the contents on the wall is turgor pressure, and it rises until it exactly balances the tendency of water to enter — at which point net entry stops and the cell is fully turgid.
The syllabus wording is worth quoting exactly: plants are supported by the pressure of water inside the cells pressing outwards on the cell wall. Thousands of turgid cells pressed against each other make a stem rigid without any skeleton at all. Lose the water and the cells go flaccid, the pressure falls and the plant wilts. Water it and it stands up again within the hour — because nothing was broken, only depressurised.
Plasmolysis is also reversible in a living cell. Put a plasmolysed onion cell back in water and the protoplast expands and presses on the wall again. That recovery is the standard proof that the cell was alive all along.
Animal Cells: No Wall, No Mercy
An animal cell has a cell surface membrane and nothing else outside it. So when water floods in, there is nothing to push back — the cell simply swells until the membrane fails and it bursts. Red blood cells in distilled water do exactly this, and the mixture turns transparent red as their contents escape.
Turgid, turgor pressure, flaccid, plasmolysis all require a cell wall. Never use them for an animal cell. An animal cell in a dilute solution bursts; in a concentrated solution it shrinks and crinkles. Writing “the red blood cell was plasmolysed” is an instant lost mark even when the underlying reasoning is perfect.
The Potato Practical — and the Graph You Must Be Able to Read
This is the single most examined experiment in Topic 3, so learn the method as a story you could tell.
- Cut identical cylinders from one potato with a cork borer (different potatoes have different sap concentrations).
- Blot each one gently, weigh it, record the initial mass.
- Put one in each sucrose concentration for a fixed time.
- Remove, blot in exactly the same way, reweigh — the final mass.
- Calculate the percentage change in mass and plot it against concentration.
Trap 1: quoting the tested concentration that gave the smallest change instead of reading where the line crosses zero. Interpolate — the answer usually lies between two data points.
Trap 2: saying “osmosis has stopped”. It has not. Water molecules still cross the membrane both ways; the net movement is zero.
Trap 3: saying “the concentrations are equal” and stopping there. Say the water potentials are equal — that is the language of the supplement and it is what the mark scheme prints.
The Dialysis Tubing Practical
Method. Soak a 10 cm length of dialysis (Visking) tubing in water, knot one end, add 10 cm³ of 1.0 mol dm⁻³ sucrose solution, knot the other end, rinse, blot dry and weigh it. Place it in a beaker of distilled water. After 30 minutes remove it, blot it dry in the same way and reweigh it. Calculate the percentage change in mass.
Control. A second bag filled with distilled water and placed in distilled water. Its mass should not change, which shows that the change in the test bag is caused by the sucrose solution.
Why it works. The tubing is partially permeable: water molecules pass through, but sucrose molecules are too large. The sucrose solution has a lower water potential than the distilled water, so water enters the bag by osmosis and the bag gains mass.
Variation. Attach a capillary tube to the bag. The liquid rises up the tube as water enters; record the height every 5 minutes.
Water Potential Decides Where Water Goes
Uptake by roots. The cell sap of a root hair cell contains dissolved sugars and ions, so it has a lower water potential than the soil water. Water therefore enters the root hair cell by osmosis through its partially permeable cell membrane. That cell now has a higher water potential than the cell next to it, so water passes from cell to cell across the root towards the xylem, always down the water potential gradient.
Loss. If the soil water has a lower water potential than the cell sap (after salt water floods a field, or after far too much fertiliser) water leaves the root by osmosis and the plant wilts even though the soil is wet.
Animals. Red blood cells sit in plasma that has the same water potential as their cytoplasm, so there is no net movement of water. If the plasma became more dilute, water would enter and the cells could burst; if it became more concentrated, water would leave and the cells would shrink. This is why the water potential of the blood must be kept constant.
The Big Idea: Sometimes a Cell Has to Push Uphill
Diffusion and osmosis are free, but they will only ever take you downhill. A root hair cell in dilute soil water already contains far more nitrate than the soil around it. Diffusion would carry nitrate out. If the plant relied on diffusion it would slowly leak away the very ions it needs.
So the cell cheats. It spends energy to move ions against their concentration gradient — from where they are scarce to where they are already plentiful. That is active transport, and it is the only one of the three processes that costs the cell anything.
How a Protein Carrier Works
A carrier protein spans the whole thickness of the membrane. An ion binds to it on the outside; the carrier uses energy from respiration to change shape; the ion is released on the inside. The carrier then returns to its original shape, ready for the next one. Nothing is broken down and nothing is rebuilt — the nitrate that binds is the nitrate that arrives.
“Why do root hair cells contain many mitochondria?” The wrong answer — and it is very common — is “for osmosis”. Osmosis is passive and costs the cell nothing at all, so mitochondria have nothing to do with water uptake. The right answer is: mitochondria are the site of aerobic respiration, which releases the energy needed for the active transport of mineral ions against a concentration gradient. Three links, often three marks.
How to Prove a Movement Is Active
Because active transport is the only one of the three that depends on respiration, you can switch it off. Take away the oxygen, add a respiratory inhibitor such as cyanide, or cool the tissue to a few degrees above freezing — and:
| Treatment | Diffusion / osmosis | Active transport |
|---|---|---|
| Remove oxygen (nitrogen atmosphere, waterlogged soil) | continues almost unchanged | falls dramatically |
| Respiratory inhibitor (e.g. cyanide) | continues | stops |
| Cool to 5 °C | slows a little | almost stops |
| Block the carrier proteins | water still crosses freely | stops |
So a data table showing that a movement stops without oxygen and runs from low to high concentration is a two-line proof of active transport. That is why experiments of exactly this shape turn up in Paper 4 every year.
Why active transport matters
Ion uptake by root hairs. Soil water is extremely dilute in nitrate, phosphate, potassium and magnesium, yet plants need all of them — nitrogen for amino acids and proteins, magnesium for chlorophyll. Without active transport a plant simply could not obtain them, however wet the soil.
Absorption in the small intestine. Glucose and amino acids are absorbed until virtually none is left in the gut — long after the gradient has reversed. Only active transport can finish the job, which is why gut lining cells are packed with mitochondria and covered in microvilli.
Reabsorption in the kidney. Glucose is reabsorbed from the kidney tubule back into the blood until the urine contains none at all — again, working against the gradient.
Carrier saturation. Because there is a fixed number of carriers, the rate of active transport reaches a plateau at high external concentrations: every carrier is already working flat out. Simple diffusion, by contrast, keeps rising in a straight line. That difference in graph shape is itself evidence for which process is operating.
Why This Topic Loses More Marks Than It Should
Topic 3 is not hard. Almost everyone who loses marks here understands the biology perfectly well. What they lose marks for is wording — and because the same three or four phrases appear in every question, the same marks are lost over and over again. This section is a repair kit.
| What students write | Why it fails | Write this instead |
|---|---|---|
| “particles move from high to low concentration” | describes a one-way stream; particles actually move in all directions | “the net movement of particles from a region of higher to lower concentration” |
| “semi-permeable membrane” | the older term; not on the mark scheme | “partially permeable membrane” |
| “water moves to where there is more solute” | right prediction, wrong mechanism — solutes do not attract water | “water moves down the water potential gradient, from higher to lower water potential” |
| “osmosis stops” / “the particles stop moving” | movement never stops; only the balance changes | “there is no net movement” |
| “the red blood cell was plasmolysed” | plasmolysis needs a cell wall | “the red blood cell burst” or “shrank and crinkled” |
| “the plant cell burst” | the cell wall prevents this | “the plant cell became turgid” |
| “water is actively transported into the root” | there is no such thing; water is never actively transported | “water enters by osmosis” |
| “the mitochondria provide energy for osmosis” | osmosis is passive and costs the cell nothing | “energy from respiration is needed for active transport of ions” |
| “it is a fair test” | names no variable | “the concentration gradient was kept the same so only the size varied” |
| “the salt sucks the water out” | not a mechanism | “the salt solution has a lower water potential, so water leaves by osmosis” |
Turgid, turgor pressure, flaccid, plasmolysed — plant cells only, because all four depend on the cell wall.
Burst, shrink, crinkle — animal cells only, because they have no wall.
Getting these the wrong way round is the fastest way to lose a mark you had already earned.
The Three Sentence Templates
Almost every explanation in this topic is one of three sentences with the nouns changed. Learn them as templates and you will never be stuck for the first line of an answer.
Command Words — What Each One Is Actually Asking
| Command | What to do | Typical Topic 3 example |
|---|---|---|
| State / Name | one short answer, no reason needed | “Name the process by which water enters a root hair cell.” |
| Describe | say what happens, with figures if there is data | “Describe how uptake changes as oxygen concentration increases.” |
| Explain | say why — every “because” is a potential mark | “Explain why the plant cell does not burst.” |
| Suggest | the answer is not in your notes; reason from principles | “Suggest why uptake is not zero without oxygen.” |
| Calculate | show working, give the unit and the sign | “Calculate the percentage change in mass.” |
| Predict | state the outcome and justify it from the pattern | “Predict the time taken for a 25 mm cube.” |
| Compare | use comparative words, and use a matched pair of figures | “Compare uptake with and without oxygen.” |
A three-mark “explain” wants three separate ideas, not one idea said three ways. In this topic the three are almost always: the direction of the gradient, the process and how it crosses the membrane, and the consequence for the cell or tissue. Before you write, count the marks and plan that many distinct points.
Data and Graph Technique
| Skill | How to earn the mark | How it is thrown away |
|---|---|---|
| Reading an isotonic point | read where the line crosses zero, interpolating between points | quoting the tested concentration nearest to zero instead |
| Describing a trend | state the direction and quote figures with units from both ends | “it goes up” with no data |
| Comparing two data sets | use a matched pair from the same row or column | picking two numbers at random from different conditions |
| Explaining a plateau | name what has become limiting — usually the number of carrier proteins | “the cell is full” |
| Drawing a conclusion | stay inside the range tested | extrapolating: “so it keeps rising forever” |
| Evaluating | name a specific weakness and its effect on the result | “there could have been errors” |
Every single time: 1. change = final − initial (keep the sign). 2. divide by the initial mass, never the final. 3. multiply by 100. 4. write the sign in the answer. 5. sanity check — gained mass means water entered, which means the solution was more dilute than the cell sap.
The Decision Tree for “Which Process?”
The idea that water leaves is credited once. But “sucks” is not a mechanism and the question demanded water potential; “semi-permeable” is the wrong term; “floppy” is not flaccid or plasmolysed; and the cells are not dead — plasmolysis is reversible.
Student answer: “Diffusion is when particles move from a high concentration to a low concentration until they are evenly spread and then they stop.”
Student answer: “Because they need lots of energy to take in water by osmosis from the soil.”
Student answer: “At 0.42 the potato and the solution are the same, so osmosis has stopped.”