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

This study guide teaches Topic 3, Movement into and out of cells, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028, and covers every learning outcome in sections 3.1 (Diffusion and osmosis) and 3.2 (Active transport). The 5090 syllabus is not tiered: every candidate studies the same outcomes, and this content can appear on Paper 1, Paper 2 and the practical papers.

Use it with the revision notes for this topic and the practice questions. The course hub is at Cambridge O Level Biology, and the printable checklist lets you tick off each outcome. The cell membrane, cell wall and root hair cell all come from Cell structure and organisation, so revise that first if those structures are hazy.

What this topic covers

Syllabus ref	What you must be able to do
3.1.1	Describe water as a solvent in digestion, excretion and transport
3.1.2	Say that the energy for diffusion and osmosis is kinetic energy of random movement
3.1.3	Define diffusion
3.1.4	Investigate surface area, temperature, concentration gradient and distance as factors in diffusion
3.1.5	Define osmosis in terms of water potential
3.1.6	Explain how water pressure inside cells supports plants
3.1.7	Describe osmosis in plant and animal tissues; explain water potential gradients in water uptake and loss
3.1.8	Investigate plant tissue in different solutions, using turgid, turgor pressure, plasmolysis and flaccid
3.1.9	Investigate osmosis with dialysis tubing
3.2.1	Define active transport
3.2.2	Explain why active transport matters for ion uptake by root hair cells

3.1.1 Water as a solvent

Many substances dissolve in water, so they can move, react and be carried.

• Digestion. Enzymes work on food dissolved or suspended in water. The products of digestion, such as glucose and amino acids, are soluble and dissolve in water so they can be absorbed into the blood.
• Excretion. Urea and excess salts are removed from the body dissolved in water, as urine.
• Transport. Blood plasma is mostly water and carries dissolved glucose, amino acids, ions, urea and carbon dioxide. In plants, xylem water carries dissolved mineral ions and phloem sap carries dissolved sugars.

3.1.2 and 3.1.3 Diffusion

Molecules and ions in liquids and gases move at random because they have kinetic energy. This is the only energy diffusion and osmosis use; no energy from respiration is needed, so both are passive.

Diffusion is 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.

The word net matters. Particles move both ways, but more move from high to low concentration because there are more of them there. At equal concentrations particles still move, but there is no net movement. Oxygen diffusing from the alveoli into the blood is a typical example.

3.1.4 Factors that affect the rate of diffusion

The syllabus limits you to four factors.

Factor	Change that speeds diffusion up	Why
Surface area	Larger surface area	More particles can cross at the same time
Temperature	Higher temperature	Particles have more kinetic energy and move faster
Concentration gradient	Steeper gradient (bigger difference)	Far more particles moving down the gradient than up it
Distance	Shorter distance	Particles have less far to travel

Surface area and the SA:V ratio

As an object gets bigger, its volume grows faster than its surface area, so its surface area to volume ratio (SA:V) falls.

Worked example 1. Cubes of pink agar (containing an indicator and alkali) are cut with sides of 1 cm, 2 cm and 4 cm and dropped into dilute acid. The pink colour disappears as acid diffuses in. Calculate SA:V for each cube and predict which goes colourless first.

Surface area of a cube = 6 × side²     Volume = side³

1 cm cube: SA = 6 × 1² = 6 cm²    V = 1³ = 1 cm³     SA:V = 6 : 1
2 cm cube: SA = 6 × 2² = 24 cm²   V = 2³ = 8 cm³     SA:V = 24 : 8 = 3 : 1
4 cm cube: SA = 6 × 4² = 96 cm²   V = 4³ = 64 cm³    SA:V = 96 : 64 = 1.5 : 1

The 1 cm cube has the largest SA:V and its centre is only 0.5 cm from the surface, so it goes colourless first. The 4 cm cube is last: its centre is 2 cm in.

Investigating the other three factors

• Temperature. Same-sized cubes in the same acid, in water baths at different temperatures. Higher temperature, faster loss of colour.
• Concentration gradient. Same cubes and temperature, acid at several concentrations. Stronger acid, faster loss of colour.
• Distance. Blocks of different thickness, or how far a dye spreads into agar in a fixed time.

Change one factor, keep the others constant, and repeat each condition to get a mean.

3.1.5 Osmosis and water potential

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.

• Pure water has the highest water potential. Dissolving solutes lowers it, so a concentrated solution has a lower water potential than a dilute one.
• A partially permeable membrane, such as the cell membrane, lets water molecules through but not some larger solute molecules, such as sucrose.

Like diffusion, osmosis needs no energy from respiration.

3.1.6 and 3.1.7 Osmosis in plant and animal tissues

Plant cells

A plant cell has a strong cellulose cell wall outside the membrane and a large vacuole full of cell sap.

• In a solution of higher water potential than the cell sap (for example pure water), water enters by osmosis. The vacuole swells and pushes the cytoplasm and membrane against the cell wall. The wall resists, so pressure builds up. This outward pressure of water on the wall is turgor pressure, and the cell is turgid. The wall stops the cell bursting.
• In a solution of lower water potential, water leaves by osmosis. The vacuole shrinks and the cell loses turgor pressure, becoming flaccid. If enough water leaves, the membrane and cytoplasm pull away from the cell wall. This is plasmolysis.

Support. Non-woody plants, such as young stems and leaves, are held up by turgid cells pressing against each other and against their walls. When a plant loses more water than it takes in, its cells become flaccid and it wilts.

Animal cells

Animal cells have no cell wall, so nothing resists the pressure as water enters.

• In pure water, red blood cells take in water by osmosis, swell and burst.
• In a solution of lower water potential, they lose water by osmosis and shrink.
• In a solution of equal water potential there is no net movement of water, so the cells keep their shape.

Uptake and loss of water: why the gradient matters

Water moves only if there is a water potential gradient, and always towards the lower water potential.

• Uptake. Soil water usually has a higher water potential than root hair cell sap, so water enters by osmosis, then moves across the root down a water potential gradient.
• Loss. If salt or too much fertiliser makes soil water more concentrated, the gradient can reverse. Water leaves the roots and the plant wilts.
• The steeper the gradient, the faster the net movement of water.

3.1.8 Investigating plant tissue in different solutions

Method. Cut potato cylinders with a cork borer to the same length, blot and weigh. Put one in each sucrose solution for the same time at the same temperature, then blot and reweigh. Use percentage change in mass, because starting masses differ.

Worked example 2. Use the results below to find the percentage change in mass and estimate the sucrose concentration at which the potato would neither gain nor lose mass.

Sucrose / mol per dm³	Start mass / g	End mass / g
0.0	2.50	2.80
0.2	2.40	2.58
0.4	2.60	2.65
0.6	2.50	2.35
0.8	2.40	2.16
1.0	2.50	2.20

% change = (end mass − start mass) / start mass × 100

0.0: (2.80 − 2.50) / 2.50 × 100 = +12.0 %
0.2: (2.58 − 2.40) / 2.40 × 100 = +7.5 %
0.4: (2.65 − 2.60) / 2.60 × 100 = +1.9 %
0.6: (2.35 − 2.50) / 2.50 × 100 = −6.0 %
0.8: (2.16 − 2.40) / 2.40 × 100 = −10.0 %
1.0: (2.20 − 2.50) / 2.50 × 100 = −12.0 %

The change goes from positive to negative between 0.4 and 0.6 mol per dm³. Plot percentage change against concentration, draw the line, and read off where it crosses zero: about 0.45 mol per dm³. At that concentration the solution and the potato cells have the same water potential.

Explaining the results.

• At 0.0 to 0.4 mol per dm³ the solution has a higher water potential than the cell sap. Water enters by osmosis and the cells become turgid: the cylinders gain mass and feel firm.
• At 0.6 to 1.0 mol per dm³ the solution has a lower water potential. Water leaves by osmosis and the cells become flaccid, with many plasmolysed in the strongest solutions: the cylinders lose mass and bend easily.

3.1.9 Investigating osmosis with dialysis tubing

Dialysis (Visking) tubing is a partially permeable membrane. Water crosses it much faster than sucrose does (some sucrose can leak out, but more slowly), so over a short experiment it can model a cell.

Set-up. Fill a tied bag of tubing with sucrose solution (or fit it to a capillary tube) and stand it in distilled water. Water enters by osmosis, down the water potential gradient, so the bag gains mass or the liquid rises up the tube. A control bag of distilled water in distilled water should not change.

Worked example 3. A bag of sucrose solution has a mass of 12.5 g. After 30 minutes in distilled water its mass is 14.0 g. Calculate the percentage increase and the mean rate of mass gain.

% increase = (14.0 − 12.5) / 12.5 × 100 = 1.5 / 12.5 × 100 = 12.0 %
mean rate  = 1.5 g / 30 min = 0.05 g per min

3.2 Active transport

Active transport is 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.

Ion uptake by root hair cells

Plants need mineral ions such as nitrate (for amino acids and proteins) and magnesium (for chlorophyll). Their concentration in soil water is usually lower than inside the root hair cell, so diffusion would move them out, not in. Root hair cells take ions in by active transport, using energy released by respiration in their mitochondria.

So anything that stops respiration, such as lack of oxygen in waterlogged soil, cuts ion uptake sharply even though the soil still contains ions.

Common errors

• Leaving net and down a concentration gradient out of the diffusion definition.
• Defining osmosis without the partially permeable membrane.
• Saying plant cells burst in water. The wall stops them; they become turgid.
• Saying the cell wall shrinks in plasmolysis. The membrane and cytoplasm pull away from it.
• Comparing raw mass changes when starting masses differ.
• Saying active transport uses “energy” without saying it comes from respiration.

Where to go next

Check your recall with the revision notes, then try the practice questions. Biological molecules is the next topic, and the free diagnostics give a quick check across topics.

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.