Transport in cells — Edexcel GCSE Combined Science
Diffusion, osmosis and active transport, the core practical on osmosis in potatoes, and calculating percentage change in mass.
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 passive, so it needs no energy from respiration. Oxygen and carbon dioxide diffuse through cell membranes. The rate is higher with a steeper concentration gradient, a higher temperature and a larger surface area.
Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane. The membrane lets small molecules such as water through, but not larger molecules. Osmosis is a passive process, so it does not need energy from respiration. It only describes the movement of water, not of other particles.
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 the cell. For example, root hair cells take in mineral ions from the soil even when the concentration inside the cell is higher.
Core practical: osmosis in potatoes
PracticalIn this core practical, potato cylinders are weighed, left in different concentrations of sugar or salt solution, then blotted dry and weighed again. A gain in mass means water moved in by osmosis because the solution was more dilute than the cell contents; a loss means it was more concentrated. Where the graph crosses 0% change, the concentrations are equal.
Percentage change in mass lets you compare potato cylinders that started with different masses. It is calculated as (final mass − initial mass) ÷ initial mass × 100. A positive answer is a percentage gain and a negative answer is a percentage loss. For example, a cylinder that goes from 3.00 g to 2.55 g has a change of −15%.
Exam questions
5 exam-style questions (16 marks), each with its mark scheme.
Answer the questionsFlashcards
9 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of key concepts in biology 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 a region of higher concentration to a region of lower concentration.
Define osmosis.
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The diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
What does active transport need that diffusion does not?
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Energy from respiration.
A potato cylinder gains mass in a solution. What does this tell you about the solution?
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It is more dilute than the cell contents.
Write the equation for percentage change in mass.
Show answer
(final mass − initial mass) ÷ initial mass × 100.
Frequently asked questions
How does osmosis differ from diffusion?
Osmosis is the diffusion of water only, through a partially permeable membrane, from a dilute solution to a more concentrated solution. Diffusion is the net movement of any particles, such as oxygen or carbon dioxide, from a higher to a lower concentration. Both are passive processes, so neither needs energy from respiration.
Why does osmosis not require energy?
Osmosis does not require energy because it is a passive process. Water diffuses from a dilute solution to a more concentrated solution through a partially permeable membrane, so no energy from respiration is needed. Active transport is different: it moves substances against a concentration gradient, so it does need energy from respiration.
Why is active transport important in root hair cells?
Active transport lets root hair cells take in mineral ions from the soil even when the concentration of ions is higher inside the cell than in the soil. This movement is against the concentration gradient, so diffusion cannot do it. Active transport uses energy from respiration to move the ions into the cell.
What factors affect the rate of diffusion?
The rate of diffusion is higher when the concentration gradient is steeper, the temperature is higher and the surface area is larger. A steeper gradient means a bigger difference in concentration. A higher temperature gives particles more kinetic energy, so they move faster. A larger surface area lets more particles cross at once.
Why do potato cylinders lose mass in salt solution?
Potato cylinders lose mass when the salt solution is more concentrated than the potato cell contents. Water moves out of the cells by osmosis, from the more dilute cell contents to the more concentrated solution. In a solution more dilute than the cell contents, water moves in and the cylinders gain mass.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026