Cambridge IGCSE Biology 0610. Topic 3: Movement into and out of cells

Diffusion, Osmosis & Active Transport

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Diffusion moves particles down a concentration gradient, osmosis moves water through a partially permeable membrane, and active transport moves substances against a concentration gradient using energy from respiration. The direction and the membrane or energy requirement must be stated precisely.

How Cambridge tests this topic

This topic is a marks goldmine that students throw away on wording. Osmosis has a precise definition the mark scheme demands word-for-word, and examiners deliberately set questions where you must use 'water potential', 'partially permeable membrane' and 'net movement' correctly. The other classic loss is confusing osmosis (water only) with diffusion (any particle) and forgetting that active transport needs energy from respiration against the gradient.

Focused lessons

Break the topic into manageable skills

Use these guides when one concept, calculation or question pattern needs more attention than the full-topic overview.

Osmosis and Water Potential: Core vs Extended Wording

Core and Supplement · Cambridge 0610 Topic 3.2

Active Transport: Against the Gradient Using Energy

Core and Supplement · Cambridge 0610 Topic 3.3

Diffusion and the Concentration Gradient

Core and Supplement · 3.1

Factors Affecting Diffusion Rate

Core and Supplement · 3.1

Surface Area to Volume Ratio

Core and Supplement · 3.1

Diffusion: get the definition and the factors

Define diffusion as 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. The word net matters. Particles move both ways, but the overall movement is down the gradient. No energy from respiration is needed; diffusion is passive.

Examiners love asking what increases the rate of diffusion. Bank these four: higher temperature (particles move faster), steeper concentration gradient, shorter distance across the membrane, and larger surface area. (Extended only) You should also explain how surface-area-to-volume ratio affects diffusion in larger organisms. Bigger organisms have a smaller ratio, so they need specialised exchange surfaces. Real exam contexts include oxygen and carbon dioxide moving across gas exchange surfaces, linking to transport in humans.

Osmosis: the definition examiners want word-for-word

This is the single most marked-down definition in the topic. Learn it exactly: osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.

Three phrases must all appear for full marks: water molecules (not 'water' alone, and never solute), water potential (higher to lower. Extended candidates must use this term; Core candidates may use 'dilute to concentrated solution'), and partially permeable membrane. A frequent fail is writing 'from low to high concentration'. For water potential it is high to low, but for solute concentration it is the opposite, which is why examiners insist on the water-potential framing. Osmosis only moves water; if the particle is anything else, it is diffusion.

Turgor and plasmolysis in plant cells (Extended)

(Extended only) When a plant cell sits in a dilute solution, water enters by osmosis, the vacuole swells and pushes the cytoplasm against the cell wall. The cell becomes turgid. The cell wall stops it bursting, and turgor pressure supports the plant. The mark scheme rewards 'turgid' and 'the cell wall prevents the cell from bursting'.

In a concentrated solution, water leaves by osmosis, the cell becomes flaccid, and if enough water leaves the cell membrane pulls away from the cell wall. This is plasmolysis, and the cell is plasmolysed. Animal cells have no wall, so in a dilute solution they swell and burst (lyse), and in a concentrated solution they shrink/crenate. Wilting in plants is the visible result of cells losing turgor. Examiners often give a diagram and ask you to name the state. Learn turgid, flaccid and plasmolysed precisely.

Active transport: the one that needs energy

Define active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against the concentration gradient) using energy from respiration. The marks live in three ideas: against the gradient, energy from respiration, and the involvement of carrier/transport proteins in the membrane (Extended only for the protein detail).

Classic contexts: root hair cells absorbing mineral ions from soil where ion concentration is already low inside the cell would be the wrong way for diffusion, and the gut absorbing glucose after most has been absorbed. The high-difficulty area here is saying active transport 'uses energy' without saying from respiration: that link is what they reward. Never write that it 'creates energy'. If you want this drilled against past-paper questions, you can take a trial class with our team.

The potato osmosis practical (Paper 6 favourite)

The investigation almost always involves potato cylinders or chips left in sugar/salt solutions of different concentrations, measuring change in mass or length. You must be able to:

Identify the independent variable (concentration of solution) and dependent variable (change in mass).
Calculate percentage change in mass = (final − initial) ÷ initial × 100. Using percentage controls for different starting masses.
Explain results: cylinders gain mass in dilute solutions (water enters by osmosis) and lose mass in concentrated solutions (water leaves).
Identify the concentration where there is no change in mass: here the solution has the same water potential as the cell contents.

State control variables (temperature, time, volume of solution, size of cylinder) to score the 'fair test' marks. This practical is a near-guaranteed appearance on the alternative to practical paper.

Key terms to connect with this topic

Use these definitions inside explanations and questions rather than memorising them as isolated sentences.

Active transport: The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy from respiration.
Carrier protein: A membrane protein that binds to specific molecules or ions and transports them across the cell membrane. Carrier proteins are involved in active transport, using energy from respiration to move substances against the concentration gradient.
Concentration gradient: The difference in concentration of a substance between two regions. Diffusion occurs down a concentration gradient (from higher to lower concentration), while active transport moves substances against the concentration gradient (from lower to higher concentration).
Diffusion: The net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement.
Flaccid: The state of a plant cell that has lost some water so that the cytoplasm no longer pushes firmly against the cell wall. The cell is soft and limp but not plasmolysed. Flaccid cells cause wilting in plants.
Net movement: The overall direction in which particles move when diffusion or osmosis occurs. Individual molecules move randomly in all directions, but there is a net movement from the region of higher concentration to the region of lower concentration because more molecules move in that direction.
Osmosis: 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.
Partially permeable membrane: A membrane that allows some molecules to pass through but not others, depending on their size or other properties. The cell membrane is partially permeable, allowing small molecules like water to pass through while blocking larger solute molecules.
Plasmolysis: The shrinkage of a plant cell's cytoplasm away from the cell wall when the cell is placed in a concentrated solution. Water leaves the vacuole and cytoplasm by osmosis, causing the cell membrane to pull away from the cell wall.
Turgid: The state of a plant cell that has absorbed maximum water by osmosis so that the vacuole is swollen and the cytoplasm presses firmly against the cell wall. The cell wall prevents bursting. Turgid cells provide support for the plant.
Turgor: The state of a plant cell when it has absorbed water by osmosis and the cytoplasm pushes outwards against the cell wall. The cell wall prevents the cell from bursting and the cell is described as turgid. Turgor pressure helps support the plant and keeps stems and leaves rigid.
Water potential: A measure of the tendency of water molecules to move from one region to another. Pure water has the highest water potential. Adding solute to water lowers its water potential. Water always moves by osmosis from higher water potential to lower water potential.

Browse the full IGCSE Biology glossary when a related term needs checking.

Apply the topic in practical work

These practical guides show how the Biology appears in methods, variables, measurements, graphs and evaluation.

Variables and Fair Tests in IGCSE Biology: The independent variable is deliberately changed, the dependent variable is measured, and controlled variables are kept constant. A control experiment is a separate comparison that removes the factor being tested.
Graphs, Tables and Units for IGCSE Biology Practicals: Put the independent variable on the x-axis and dependent variable on the y-axis, label both with units, use a sensible scale covering most of the grid, plot small precise points and draw the appropriate best-fit line or curve.
Osmosis Investigation With Potato Cylinders: Place equal potato cylinders in a range of solution concentrations, measure initial and final mass, calculate percentage change, repeat and plot mean percentage change against concentration. Zero change estimates equal water potential.

Try a worked question (5 marks)

A student places identical potato cylinders into sucrose solutions of increasing concentration and records the percentage change in mass. The cylinder in distilled water increased in mass, while the cylinder in the most concentrated solution decreased in mass. (a) Explain, in terms of water potential, why the cylinder in distilled water increased in mass. (b) Explain why the cylinder in the most concentrated solution decreased in mass.

Show the answer and marking guidance

(a) distilled water has a higher water potential than the cell contents/cytoplasm; so water moves into the cells by osmosis; through the partially permeable (cell) membrane, increasing mass; (b) the concentrated solution has a lower water potential than the cell contents; so water moves out of the cells by osmosis (down the water potential gradient); causing the cylinder to lose mass;

Each semicolon separates a distinct answer point. This is original practice content, not an official Cambridge past-paper question.

Common mix-ups to avoid

Use these as a correction checklist after attempting a question.

Defining osmosis without 'partially permeable membrane'. The phrase is required for the mark.
Saying water moves 'from low to high concentration' in osmosis. It moves from high to low water potential; the framing must be water potential.
Calling the movement of glucose or oxygen 'osmosis'. Osmosis is water only; everything else is diffusion or active transport.
Saying active transport 'uses energy' without specifying energy from respiration.
Confusing turgid, flaccid and plasmolysed, or claiming an animal cell becomes 'plasmolysed' (no wall, so it bursts instead).
Using raw change in mass instead of percentage change, which ignores different starting masses.

How to explain the graph clearly

For a 6-mark 'explain the results of this osmosis investigation' answer, write parallel paragraphs for each solution using consistent water-potential language. Use stems like 'The solution had a higher/lower water potential than the cell contents, so water moved in/out of the cells by osmosis through the partially permeable membrane, causing the mass to increase/decrease.' Then name the point of no net change ('equal water potential, so no net movement'). Repeating the same precise vocabulary across each condition is exactly what the mark scheme tallies.

Review the full extended-response method →

Malaysia and syllabus context

The potato osmosis practical is run in nearly every Malaysian IGCSE lab, but timing and class sizes mean many students see it demonstrated rather than handled, so the Paper 6 version catches them out. Both May/June and Oct/Nov sittings reliably feature osmosis. The water-potential wording is more demanding than SPM Biology, where the term is treated more loosely. Extended students must drill the precise phrasing.

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