Movement of substances into and out of cells — Edexcel International GCSE Biology
Diffusion, osmosis and active transport, what affects their rate, and practicals using living and non-living systems.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. It is a passive process, so it does not need energy from respiration. Oxygen, carbon dioxide and dissolved substances such as glucose diffuse into and out of cells through the cell membrane.
Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane. It is a passive process that does not need energy from respiration. Cell membranes are partially permeable, so cells gain or lose water by osmosis when the surrounding solution is more dilute or more concentrated than the cytoplasm.
Active transport is the movement of substances from a more dilute solution to a more concentrated solution, against a concentration gradient, using energy from respiration. It lets cells take in substances that are in short supply outside them. For example, root hair cells absorb mineral ions from the soil even when the concentration inside the cell is higher.
Four factors affect how fast substances move into and out of cells: surface area to volume ratio, distance, temperature and concentration gradient. A larger surface area to volume ratio, a shorter distance, a higher temperature and a steeper concentration gradient all increase the rate. As an organism gets bigger, its surface area to volume ratio gets smaller.
Practical: diffusion and osmosis
PracticalDiffusion and osmosis can be investigated with living systems such as potato cylinders and non-living systems such as Visking tubing and agar cubes. Potato cylinders gain mass in dilute solutions and lose mass in concentrated ones because water moves by osmosis. Glucose diffuses through Visking tubing but starch cannot, and small agar cubes change colour fastest.
Exam questions
7 exam-style questions (22 marks), each with its mark scheme.
Answer the questionsFlashcards
12 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of structure and functions in living organisms on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
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Quick check questions
Define diffusion.
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The net movement of particles from an area of higher concentration to an area of lower concentration.
What is osmosis?
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The diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
Which of diffusion, osmosis and active transport needs energy from respiration?
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Active transport.
What happens to the surface area to volume ratio as an organism gets bigger?
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It gets smaller.
Why does a potato cylinder lose mass in a concentrated sugar solution?
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Water moves out of the potato cells into the more concentrated solution by osmosis.
Frequently asked questions
How does osmosis differ from diffusion?
Osmosis is a special type of diffusion that involves only water moving through a partially permeable membrane. Diffusion is the net movement of any particles, such as oxygen or carbon dioxide, from a higher to a lower concentration. In osmosis, water moves from a dilute solution to a more concentrated solution. Neither process needs energy from respiration.
Why does osmosis not require energy?
Osmosis does not require energy because it is a passive process. Water simply diffuses from a dilute solution to a more concentrated solution through a partially permeable membrane, so no energy from respiration is used. Only active transport, which moves substances against a concentration gradient, needs energy from respiration.
Why is active transport needed in plant roots?
Active transport is needed in plant roots because root hair cells often contain a higher concentration of mineral ions than the soil. Diffusion would move ions out of the cell, so the cells must use energy from respiration to move ions in against the concentration gradient. This lets plants take in ions that are in short supply.
Why does the surface area to volume ratio limit cell size?
As a cell or organism gets bigger, its surface area to volume ratio gets smaller, so there is less surface for each unit of volume. Cells deep inside are a long way from the surface, so diffusion is too slow to supply them. A single-celled organism has a large ratio and short distances, so diffusion is fast enough.
Why does a potato lose mass in salt solution?
A potato cylinder loses mass in a concentrated salt or sugar solution because the solution is more concentrated than the potato cell contents. Water moves out of the cells by osmosis through the partially permeable cell membranes. In pure water the opposite happens: water moves into the cells and the mass increases.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026