Cambridge O Level Biology 5090: Movement into and out of cells (5090) – Revision Notes

For full explanations and worked examples, read the study guide for this topic first.

These notes cover Topic 3, Movement into and out of cells, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028: sections 3.1 (Diffusion and osmosis) and 3.2 (Active transport). The syllabus is not tiered, so every outcome here applies to every candidate, on Papers 1 and 2 and on the practical papers (diffusion and osmosis are listed experimental contexts). When you have been through them, test yourself with the practice questions. The course hub lists every 5090 topic, and the printable checklist lets you track the outcomes.

Definitions to learn word for word

Term	Definition
Diffusion (3.1.3)	The net movement of molecules or ions 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
Osmosis (3.1.5)	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
Active transport (3.2.1)	The movement of molecules or ions into or out of a cell through the cell membrane, from a region of their lower concentration to a region of their higher concentration (against a concentration gradient), using energy released during respiration
Turgor pressure (3.1.8)	The pressure of water inside a plant cell pushing outwards on the cell wall
Turgid	A plant cell that has taken in water and is firm, with the membrane pressed against the wall
Flaccid	A plant cell that has lost water and turgor pressure and is soft
Plasmolysis	The membrane and cytoplasm pulling away from the cell wall as a plant cell loses water

3.1.1 Water as a solvent

Process	What water does
Digestion	Enzymes act on food in water; soluble products (glucose, amino acids) dissolve so they can be absorbed
Excretion	Urea and excess salts leave the body dissolved in water as urine
Transport	Plasma carries dissolved glucose, amino acids, ions, urea and carbon dioxide; xylem water carries dissolved mineral ions

3.1.2 Where the energy comes from

• Diffusion and osmosis use the kinetic energy of random movement of molecules and ions.
• No energy from respiration is used. They are passive.
• Active transport is different: it uses energy released during respiration.

3.1.4 Four factors that affect diffusion

Factor	Faster diffusion when…
Surface area	it is larger
Temperature	it is higher (more kinetic energy)
Concentration gradient	it is steeper
Distance	it is shorter

SA:V for a cube of side s: surface area = 6s², volume = s³, so SA:V = 6/s. Halving the side doubles the ratio.

Method in steps: agar block investigation

1. Cut blocks of indicator-stained agar to the sizes you need.
2. Put them into the same volume and concentration of acid at the same temperature.
3. Time how long each takes to change colour completely.
4. Repeat for each size and calculate a mean time.
5. Change only one factor at a time: size (surface area), temperature, acid concentration (gradient) or block thickness (distance).

3.1.5 to 3.1.7 Osmosis and water potential

• Pure water has the highest water potential.
• More solute dissolved means lower water potential.
• Water always moves by osmosis towards the lower water potential.
• No water potential gradient means no net movement of water.

What happens to cells

Outside solution compared with cell	Plant cell	Animal cell (e.g. red blood cell)
Higher water potential (e.g. pure water)	Gains water, becomes turgid; wall stops it bursting	Gains water, swells and bursts
Same water potential	No net change	No net change
Lower water potential (concentrated)	Loses water, becomes flaccid, then plasmolysed	Loses water and shrinks

Support in plants (3.1.6): turgid cells press outwards on their walls and on each other, holding up stems and leaves. Flaccid cells cannot do this, so the plant wilts.

Uptake and loss of water (3.1.7): soil water normally has a higher water potential than root hair cell sap, so water enters by osmosis. Salty or over-fertilised soil can have a lower water potential than the cell sap, so water leaves the roots instead.

3.1.8 Plant tissue in solutions

Method in steps

1. Cut same-size cylinders of potato (or similar tissue) with a cork borer.
2. Blot and weigh each.
3. Put each in a different sucrose or salt solution, same volume, same time, same temperature.
4. Remove, blot gently, reweigh.
5. Calculate percentage change in mass = (final − initial) / initial × 100.
6. Plot percentage change against concentration. Where the line crosses zero, the solution has the same water potential as the cells.

Worked reminder. A chip goes from 4.0 g to 3.4 g. Change = (3.4 − 4.0) / 4.0 × 100 = −15 %. The solution had a lower water potential than the cells; water left by osmosis and the cells became flaccid.

3.1.9 Dialysis tubing

• Visking (dialysis) tubing is partially permeable: water passes through quickly, sucrose more slowly and starch not at all.
• Bag of sugar solution in water: water enters, the bag gains mass and becomes firm, or the level rises in an attached capillary tube.
• Bag of water in concentrated sugar solution: water leaves, the bag loses mass and goes limp.
• Control: bag of water in water, no change.

3.2 Active transport

• Moves ions against a concentration gradient (low to high).
• Needs energy from respiration, so needs living, respiring cells.
• Root hair cells take up mineral ions, such as nitrate and magnesium, from soil water where the ion concentration is lower than in the cell. Diffusion alone could not do this.
• Lack of oxygen (for example waterlogged soil) or a respiratory poison reduces respiration and so reduces ion uptake.

Must-know distinctions

Diffusion	Osmosis	Active transport
Any molecules or ions	Water molecules only	Molecules or ions
Down a concentration gradient	Higher to lower water potential	Against a concentration gradient
Kinetic energy only	Kinetic energy only	Energy from respiration
Membrane not required	Partially permeable membrane required	Through the cell membrane

• Turgid vs flaccid: turgid cells are full and firm; flaccid cells have lost turgor pressure.
• Flaccid vs plasmolysed: flaccid is soft; plasmolysed is more extreme, with the membrane pulled off the wall.
• Plant vs animal cells in water: plant cells become turgid; animal cells burst.

Quick self-test

1. What is the source of energy for diffusion?
2. Give the full definition of osmosis.
3. State the four factors the syllabus lists as affecting the rate of diffusion.
4. Calculate the SA:V ratio of a 3 cm agar cube.
5. Explain why a 3 cm agar cube takes longer to change colour than a 1 cm cube.
6. A potato chip changes from 5.0 g to 5.6 g in a sucrose solution. Calculate the percentage change in mass.
7. Explain the result in question 6.
8. Why does a red blood cell burst in distilled water when an onion cell does not?
9. Describe what plasmolysis looks like under the microscope.
10. Explain why a plant wilts on a hot, dry day.
11. Explain why a root hair cell cannot take up nitrate ions by diffusion.
12. Suggest why adding a respiratory poison to a root reduces ion uptake but not water uptake at first.

Answers

1. The kinetic energy of the random movement of molecules and ions.
2. 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.
3. Surface area, temperature, concentration gradient and distance.
4. Surface area = 6 × 3² = 54 cm²; volume = 3³ = 27 cm³; SA:V = 54 : 27 = 2 : 1.
5. It has a smaller SA:V (2 : 1 against 6 : 1), so less surface per unit volume for acid to enter, and the distance to its centre is greater (1.5 cm against 0.5 cm).
6. (5.6 − 5.0) / 5.0 × 100 = +12 %.
7. The solution had a higher water potential than the potato cells, so water entered the cells by osmosis and they became turgid.
8. The red blood cell has no cell wall, so it swells until it bursts. The onion cell’s cellulose wall resists the pressure, so it becomes turgid instead.
9. The cytoplasm and membrane have shrunk and pulled away from the cell wall, leaving a gap between them; the vacuole is small.
10. It loses more water than it takes in, so its cells lose water, lose turgor pressure and become flaccid; they no longer support the stems and leaves.
11. The nitrate concentration in soil water is lower than in the cell, so diffusion would move nitrate out of the cell. Uptake has to be against the gradient, by active transport.
12. Ion uptake is active transport, which needs energy from respiration. Water uptake is by osmosis, which uses only kinetic energy, so it does not depend on respiration directly.

Where marks are usually lost

• Missing net or random movement from the diffusion definition.
• Writing “from high to low concentration” for osmosis instead of using water potential.
• Forgetting partially permeable membrane in the osmosis definition.
• Saying active transport uses energy “from the cell” instead of energy released during respiration.
• Saying that plant cells burst, or that the cell wall shrinks in plasmolysis.
• Using raw mass change instead of percentage change when starting masses differ.
• Getting the sign of the percentage change wrong for a loss in mass.
• Explaining a temperature effect without mentioning kinetic energy.
• Saying “the cell gains water because the solution is weak” without comparing water potentials inside and outside.

Next steps

Work through the practice questions, and use the free diagnostics to spot weak topics. The cell membrane and root hair cell are revised in Cell structure and organisation revision notes.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 3: Movement into and out of cells, published by Cambridge Assessment International Education.