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Cellular Transport: Diffusion and Osmosis — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 2 of 16

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7Why cells need to move substances

Every cell must take in raw materials such as oxygen, glucose and water and get rid of waste such as carbon dioxide and urea. Some substances cross the cell membrane without the cell using energy; this is passive transport. Diffusion and osmosis are the two passive processes you must know for OCR Gateway GCSE Biology A.

The cell surface membrane is partially permeable: it lets small molecules such as water, oxygen and carbon dioxide through easily, but controls the movement of larger or charged particles.

Part 2 of 7Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Diffusion

It happens because particles in liquids and gases move randomly and constantly.

Particles move in both directions, but more move down the gradient than up it, so the net movement is from high to low.

Diffusion stops being a net process when the concentration is equal everywhere (dynamic equilibrium).

Examples in living things:

Oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses the other way.

Oxygen diffuses from the blood into respiring muscle cells.

Urea diffuses from liver cells into the blood plasma.

Carbon dioxide diffuses into a leaf through the stomata for photosynthesis.

Part 3 of 7Factors affecting the rate of diffusion

Factor	Effect on rate	Reason
Concentration gradient	Steeper gradient, faster diffusion	More particles move down the gradient than back up it
Temperature	Higher temperature, faster diffusion	Particles have more kinetic energy and move faster
Surface area	Larger area, faster diffusion	More particles can cross each second
Diffusion distance	Shorter distance, faster diffusion	Particles have less far to travel, for example through thin alveolus walls

Gas exchange surfaces are adapted with all of these in mind: large area, thin walls and a good blood supply that keeps the gradient steep by removing oxygen as fast as it arrives.

Part 4 of 7Osmosis

Osmosis is the diffusion of water from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution) through a partially permeable membrane. Water moves toward the more concentrated solution, so the solution with more dissolved solute gains water.

Effect on animal cells (which have no cell wall):

Osmosis

In a dilute solution, water enters by osmosis. The cell swells and may burst.

In a concentrated solution, water leaves. The cell shrinks and shrivels.

In an isotonic solution there is no net movement and the cell stays the same size.

Plant cells have a cell wall. In dilute surroundings water enters and the vacuole swells, pushing the membrane against the wall; the cell becomes firm (turgid), which supports soft plant tissue. In concentrated surroundings the cell loses water, becomes flaccid, and the plant wilts. In extreme cases the membrane pulls away from the wall (plasmolysis).

Part 5 of 7Required practical: osmosis in potato cylinders

Method points:

Required practical: osmosis in potato cylinders

Use a cork borer to cut identical cylinders of potato and trim to the same length; remove the skin.

Measure the initial mass of each cylinder (or length).

Place one cylinder in each of a series of sucrose solutions of different concentrations (including pure water) for a fixed time, for example 30 minutes, at the same temperature.

Remove, blot dry gently, and measure the final mass.

Percentage change in mass

Calculate percentage change in mass = (final mass - initial mass) / initial mass x 100.

Percentages are used so that cylinders of slightly different starting masses can be compared fairly.

Plot percentage change against sucrose concentration. Where the line crosses zero, there is no net water movement: the sucrose concentration there matches the concentration of the potato cell contents. Above zero means water gained; below zero means water lost. Repeating each concentration and calculating a mean improves reliability.

Part 6 of 7Linking diffusion and osmosis to exam contexts

Questions often ask you to apply these ideas to unfamiliar situations.

First decide what is moving: if it is water across a membrane, it is osmosis; if it is any other particle from high to low concentration, it is diffusion.

Then say which region has the higher concentration and describe the net direction.

Examples you should be able to explain are the uptake of water by root hair cells, the absorption of digested food in the small intestine, the loss of water from salted slugs,

and why vegetables crisp up in cold water.

Always use the phrase partially permeable membrane for osmosis and state the effect on the cell, not just the movement of the water.

Where graph data are given, describe the trend first, quote figures, then explain using particle movement.

Remember that osmosis explains why a person should not drink seawater: the salty fluid in the gut draws water out of the body cells, making dehydration worse.

Part 7 of 7Exam-style questions

Explain why the rate of diffusion of oxygen into a muscle cell increases when the person starts to run. [3]3 marks
A potato cylinder has mass 4.0 g at the start and 4.6 g after 30 minutes in a sucrose solution. Calculate the percentage change in mass and explain what happened. [3]3 marks
Describe what would happen to a red blood cell placed in pure water and explain why. [3]3 marks

Key terms

Term	Meaning
Diffusion	Net movement of particles from higher to lower concentration down a concentration gradient.
Osmosis	Net movement of water across a partially permeable membrane from a dilute to a more concentrated solution.
Partially permeable membrane	A membrane that allows some substances through but not others.
Concentration gradient	The difference in concentration between two regions.
Turgid	A plant cell that is firm because it is full of water and pushing against its wall.
Flaccid	A plant cell that has lost water and is soft and limp.
Passive transport	Movement of substances without the use of energy from respiration.

Avoid these mistakes

Saying water moves from "weak" to "strong" without referring to water concentration or solute concentration; always state which solution has more water.

Writing that particles "stop moving" at equilibrium; they keep moving but there is no net movement.

Thinking osmosis applies to any substance; it is only the movement of water across a partially permeable membrane.

Forgetting to calculate percentage change in the potato practical, so results from different-sized cylinders are not comparable.

Describing plant cells as bursting; the cell wall stops this and the cell becomes turgid instead.

Last updated: 20 October 2026