Movement Into and Out of Cells
Everything a cell needs has to get in, and everything it doesn’t need has to get out. Cambridge IGCSE Biology (0610) covers exactly three ways this happens:
- Diffusion
- Osmosis
- Active transport
If a question describes something moving across a cell membrane, the answer is one of these three. Nothing else. Getting confident at telling them apart is most of this topic.
1. The cell membrane
Every one of these processes happens across the cell membrane, so it’s worth understanding what the membrane actually is before anything else.
The membrane is a bilayer — two layers of lipid (fat) molecules, with their water-hating tails facing inwards toward each other and their water-loving heads facing outwards. There is water in the cytoplasm inside the cell and water in the fluid outside, so the membrane arranges itself so that the fatty middle is shielded from both.
Embedded in the bilayer are proteins. These act as passages for substances that cannot get through the fatty middle on their own.
Permeability — three words that get confused
| Term | Meaning |
|---|---|
| Freely permeable | Everything can pass through |
| Partially permeable (also called selectively permeable) | Only some substances can pass through |
| Impermeable | Nothing can pass through |
The cell membrane is partially permeable. This single fact drives the whole topic — it’s the reason osmosis happens at all.
What can cross easily?
- Small molecules — oxygen, carbon dioxide, water — cross easily
- Fat-soluble substances cross easily, because the middle of the membrane is fatty
- Large molecules (starch, proteins) cannot cross
- Charged particles (ions such as Na⁺, mineral ions) do not cross the fatty layer freely — they need protein channels, and often energy
Worth remembering: starch cannot leave a cell because the molecule is simply too large to pass through the membrane. Glucose and iodine molecules are small enough. This is the whole basis of the classic visking tubing experiment.
2. Diffusion
Definition (full marks version): Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement.
Three parts of that definition earn marks, and students routinely drop the last two.
Diffusion does not go on forever
A common exam error is writing that particles move “until they have all moved across”. They don’t.
Picture a container split in two, with 1000 oxygen molecules on one side and none on the other. Put a hole in the divider. One crosses — 999 left. Another crosses — 998 left. This continues, but it stops being a net movement once the concentration is equal on both sides, at roughly 500 and 500.
Particles are still moving after that. They’re just moving equally in both directions, so there is no net movement.
That phrase — net movement — is a mark-scheme keyword. Leave it out and the answer can be marked wrong.
Everyday examples
- A tea bag in hot water. The coloured compounds are highly concentrated inside the bag and absent in the water, so they diffuse outwards until the whole cup is coloured.
- Perfume in a room. Concentration is high at the bottle and low across the room, so the scent spreads. Note this happens with no membrane at all — diffusion does not require one.
- A drop of ink in water. Spreads until the colour is even.
In the human body
- Lungs — oxygen diffuses from alveoli into blood; carbon dioxide diffuses the other way
- Body cells — oxygen diffuses in for respiration, carbon dioxide diffuses out
- Small intestine — some digested food products diffuse into the blood
The four factors that affect the rate of diffusion
The syllabus expects four, and expects you to explain each rather than just name it.
| Factor | Effect | Why |
|---|---|---|
| Temperature | Higher temperature → faster diffusion | Particles gain more kinetic energy, so they move faster and collide more frequently |
| Concentration gradient | Steeper gradient → faster diffusion | A bigger difference between the two regions means more net movement |
| Distance | Greater distance → slower diffusion | Particles have further to travel |
| Surface area (surface area : volume ratio) | Larger surface area → faster diffusion | More area is available for particles to cross at once |
Two things students commonly get wrong here:
- pH does not affect diffusion. It affects enzymes; that’s a different topic.
- Molecular size does affect the rate (lighter particles diffuse faster), but at IGCSE Biology this is treated as chemistry. Stick to the four factors above unless the question specifically points at molecule size.
3. Osmosis
Osmosis is often described as “a special kind of diffusion”, and that’s a fair starting point — but two things make it different, and both are needed for the marks.
Definition (full marks version): Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
The two things that make osmosis different from diffusion
1. It is only ever about water. If the question is about glucose, oxygen, mineral ions, sucrose or anything that isn’t water, the process is diffusion or active transport — never osmosis.
2. It requires a partially permeable membrane. Diffusion happens perfectly well with no membrane (the perfume example). Osmosis cannot happen without one. If there’s no membrane, water is just diffusing.
Say “water potential”, not “water concentration”
This is probably the single most repeated correction in this topic.
When you talk about osmosis, use water potential. Do not write “water concentration” or “concentration of water”. They describe the same idea, but the mark scheme wants the correct term and examiners will not always be generous.
- Pure/distilled water has the highest water potential
- A concentrated solution (lots of dissolved solute) has a low water potential
- Water always moves from high water potential → low water potential
Solutions, solutes and solvents
You need this vocabulary to answer osmosis questions properly.
solute + solvent = solution
- Solvent — the liquid, present in the larger amount (usually water)
- Solute — what’s dissolved in it, present in the smaller amount (salt, sugar, sucrose)
- Concentrated means a lot of solute, and therefore not much water — so a low water potential
- Dilute means little solute, and therefore a lot of water — so a high water potential
That last link is the one to lock in: more solute means less water means lower water potential.
4. Hypertonic, hypotonic and isotonic
These three terms describe the solution outside the cell, compared with the inside.
| Solution outside | Solute concentration outside | Water moves | Effect on cell |
|---|---|---|---|
| Hypertonic | Higher than inside | Out of the cell | Cell shrinks |
| Hypotonic | Lower than inside | In to the cell | Cell swells |
| Isotonic | Same as inside | No net movement | No change in size |
A memory hook that works: hyper = too much (solute), hypo = too little (solute), iso = the same.
What this looks like in a plant cell
Plant cells have a cell wall, so they behave differently from animal cells:
| Solution | What happens | Term |
|---|---|---|
| Hypotonic | Water enters, the cell swells and pushes against the cell wall | Turgid |
| Isotonic | No net movement; the cell contents just touch the wall | Flaccid |
| Hypertonic | Water leaves, the contents shrink and pull away from the cell wall | Plasmolysed (the process is plasmolysis) |
Real examples worth quoting
- Making pickles. Raw mango pieces are placed in brine — a concentrated salt solution. The solution outside is hypertonic, so water leaves the mango cells by osmosis and the pieces shrink and shrivel.
- Market vegetable stalls. Sellers sprinkle water over vegetables to keep them looking fresh. The water outside is hypotonic, so water moves into the cells, keeping them turgid rather than limp.
How to write the explanation in an exam
A question showing plant tissue in a concentrated solution wants a structured answer, not just “it shrinks”. Include:
- What happened — the tissue lost water / decreased in mass
- The process — by osmosis
- The reason, using water potential — the tissue had a higher water potential than the solution
- The membrane — through a partially permeable membrane
- The solution — name it as a hypertonic / concentrated solution
Example of a full-mark answer:
The cells lost water by osmosis, because the cells had a higher water potential than the surrounding hypertonic solution, so water moved out through the partially permeable membrane. This caused the tissue to decrease in mass.
5. Active transport
Definition (full marks version): Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration — that is, against a concentration gradient — using energy released by respiration.
Down vs against the gradient
This pair of phrases is worth learning precisely, because questions use them directly:
- Down / along the concentration gradient = high → low = passive, no energy needed
- Against the concentration gradient = low → high = active, energy needed
The standard analogy: rolling down a hill needs no effort and happens by itself; cycling up the hill needs energy. Diffusion and osmosis are the roll down. Active transport is the climb.
Where does the energy come from?
From respiration, which happens in the mitochondria. Cells that carry out a lot of active transport tend to have many mitochondria.
(You may see the energy-carrying molecule named as ATP — adenosine triphosphate. That’s useful background, but 0610 only requires “energy from respiration”.)
How it actually works: carrier proteins
Active transport uses carrier proteins embedded in the cell membrane.
- The substance binds to a carrier protein
- The protein changes shape — this is the step that uses energy
- The substance is released on the other side of the membrane
Think of a porter carrying luggage: the protein picks the particle up on one side and puts it down on the other, and the effort is in the movement.
Examples in the syllabus
- Root hair cells absorbing mineral ions from the soil
- Villi in the small intestine absorbing glucose into the blood
- Kidney tubules (in the nephron) reabsorbing glucose
6. Root hair cells — the example that ties it all together
Root hair cells are worth studying closely because they use two of the three processes, and exam questions love them.
The problem: soil water is dilute. It contains very few mineral ions, but plenty of water. So how does a plant absorb the minerals it needs, when they’re moving in the wrong direction?
The solution:
- The root hair cell uses active transport to pump mineral ions in from the soil, against the concentration gradient, using energy from respiration.
- This makes the inside of the root hair cell highly concentrated in solutes — which gives it a low water potential.
- Soil water now has a higher water potential than the cell.
- So water floods in by osmosis — passively, with no further energy cost.
The plant spends a small amount of energy on the ions and gets its water movement for free.
Why root hairs are the shape they are
Root hairs are long, thin extensions. So are villi in the small intestine, and so is the grape-bunch structure of alveoli in the lungs.
They all share one adaptation: increased surface area. Wherever efficient absorption or exchange happens in biology, look for increased surface area — it’s a reliable mark almost anywhere in the syllabus.
Two further points on root hair cells: they have no chloroplasts (they’re underground, so photosynthesis is pointless) and they absorb water by osmosis but mineral ions by active transport.
7. All three side by side
| Diffusion | Osmosis | Active transport | |
|---|---|---|---|
| What moves | Any particle | Water only | Any particle (often ions) |
| Direction | High → low | High → low water potential | Low → high |
| Concentration gradient | Down / along | Down / along | Against |
| Energy needed? | No | No | Yes (from respiration) |
| Membrane required? | No | Yes — partially permeable | Yes |
| Passive or active? | Passive | Passive | Active |
| Example | Gas exchange in alveoli | Water uptake by roots | Mineral ion uptake by root hairs |
8. Mistakes that cost marks
These come up again and again.
Writing “water concentration” instead of “water potential.” Use water potential whenever the question is about osmosis.
Leaving out “partially permeable membrane.” It’s part of the definition of osmosis, and it’s usually a mark on its own.
Leaving out “net” movement. In an isotonic solution, water is still moving both ways. The correct statement is that there is no net movement, not that there’s no movement.
Saying osmosis applies to solutes. It never does. Osmosis is water and only water.
Saying pH affects diffusion. It doesn’t. pH affects enzymes.
Half a definition. “Diffusion is the spreading of substances” earns very little. You need direction (high to low), the gradient, and where it stops.
Saying diffusion needs a membrane. It doesn’t. Perfume spreading across a room is diffusion with no membrane involved.
Mixing up glycogen and glucagon. Not this topic, but it appears constantly in the same papers. Glycogen is stored carbohydrate; glucagon is a hormone.
9. Practical work (Paper 6)
This topic supplies a large share of Paper 6 experiments, so the practical side is worth revising alongside the theory.
Osmosis in potato or radish tissue
The standard investigation: the effect of the concentration of a solution on osmosis in plant tissue.
- Independent variable: concentration of the solution (e.g. several different sodium chloride or sucrose concentrations)
- Dependent variable: change in mass of the plant tissue — not “osmosis”, which is too vague to score
- Control variables: same type of plant tissue, same dimensions/thickness, same volume of solution, same time, same temperature
- Method outline: cut plant tissue into cylinders of equal size → measure initial mass → place in each solution for a set time → remove, surface dry with a paper towel, measure final mass
- Safety: cutting on a flat surface, cutting away from the hand
A note on the word “cylinders”: mark schemes for this experiment often say to cut the tissue into cylinders. This means cutting a potato or radish into cylinder shapes — not using a measuring cylinder. It’s a genuinely common misreading.
Percentage change in mass
Because different pieces of tissue start at different masses, comparing raw mass changes isn’t fair. Percentage change allows a valid comparison.
percentage change in mass = ((final mass − initial mass) / initial mass) × 100A positive result means the tissue gained water. A negative result means it lost water.
If asked why percentage change is used rather than change in mass: because the starting masses are not the same, so percentage change allows a fair comparison.
If asked to find the concentration with the same water potential as the tissue: read off the point where the line crosses zero percentage change — no net movement means the water potentials are equal.
Diffusion in agar jelly
Agar containing an indicator is exposed to acid of different concentrations, and the distance the colour change travels is measured.
Conclusion: as the concentration of acid increases, the rate of diffusion increases.
Cutting the agar into cubes of different sizes demonstrates surface area : volume ratio — smaller cubes change colour fastest, because the diffusion distance to the centre is shorter.
Visking (dialysis) tubing
Starch solution inside the tubing, iodine solution outside. After some time the contents turn blue-black.
The explanation: iodine molecules are small enough to diffuse into the tubing, where they react with the starch. Starch does not diffuse out, because its molecules are too large to pass through the membrane.
Note this is diffusion, not osmosis — the substance that moved was iodine, not water.
Answer-writing points that recur
- Give the dependent variable as the thing you actually measure (change in mass, time taken, distance moved) — never as the process itself
- Repeat the experiment at least twice more, and say so explicitly. “Repeat” alone often scores nothing
- The reason for repeating is to identify anomalous results — a stronger answer than “to make it more accurate”
- An anomalous result is one that does not fit the pattern. It should be excluded when calculating the mean
- Never give “human error” as a source of error. Sources of error must relate to the method itself
- A control experiment for these investigations means a setup with distilled water in place of the solution, for comparison
Frequently asked questions
Is osmosis a type of diffusion? Yes. It’s diffusion of water specifically, through a partially permeable membrane. But in an exam, only use the word osmosis when water is what’s moving.
Does osmosis need energy? No. Osmosis and diffusion are both passive. Only active transport requires energy from respiration.
What is the difference between diffusion and active transport? Direction and energy. Diffusion goes down the concentration gradient with no energy cost. Active transport goes against the gradient and requires energy from respiration.
Why does a plant cell not burst in pure water, when an animal cell does? The plant cell wall is strong and inelastic. It resists the inward pressure of water, so the cell becomes turgid and stops. An animal cell has no wall and can burst.
What is water potential in simple terms? How much free water is available. Pure water has the highest water potential; adding solute lowers it. Water always moves from higher to lower water potential.
Do root hair cells absorb water by active transport? No — water enters by osmosis. Active transport is used for mineral ions. Mixing these up is a frequent error.
Quick revision checklist
- I can define diffusion, osmosis and active transport in full, including the phrases that earn marks
- I always write water potential, never “water concentration”
- I always include partially permeable membrane in the definition of osmosis
- I can name and explain the four factors affecting the rate of diffusion
- I know that osmosis applies to water and nothing else
- I can use hypertonic, hypotonic and isotonic correctly
- I can link them to turgid, flaccid and plasmolysed in plant cells
- I understand why “no net movement” matters
- I can explain how root hair cells use active transport and osmosis together
- I can calculate percentage change in mass and read a zero-crossing off a graph
- I can state a dependent variable as a measurable quantity, not as a process
These notes cover topic 3 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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