Transport in cells — AQA GCSE Combined Science: Trilogy
Diffusion, surface area to volume ratio, exchange surfaces, osmosis and active transport, and how they differ.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Diffusion is the spreading out of particles of a gas or of a substance in solution, resulting in a net movement from an area of higher concentration to an area of lower concentration. Oxygen, carbon dioxide and urea move by diffusion. The rate is faster with a bigger concentration gradient, a higher temperature and a larger membrane surface area.
Surface area to volume ratio is the surface area divided by the volume. A single-celled organism has a large ratio, so diffusion across its surface meets its needs. As an organism gets bigger, its volume increases faster than its surface area, so the ratio falls. Multicellular organisms therefore need specialised exchange surfaces and a transport system.
An exchange surface is a surface specialised for moving materials into and out of an organism, such as the small intestine, lungs, gills, roots and leaves. Its effectiveness is increased by a large surface area and a thin membrane that gives a short diffusion path. In animals, an efficient blood supply and, for gas exchange, ventilation also help.
Osmosis
PracticalOsmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. In the required practical, plant tissue gains mass in dilute solutions as water moves in, and loses mass in concentrated solutions as water moves out. Where the mass does not change, the solution matches the concentration inside the cells.
Active transport moves substances from a more dilute solution to a more concentrated solution, against a concentration gradient, using energy from respiration. It lets root hair cells absorb mineral ions from very dilute solutions in the soil, and lets sugar molecules be absorbed from the gut into the blood. Cells that carry it out have many mitochondria.
Exam questions
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The whole of cell biology on one page, so you can see where this subtopic fits.
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Quick check questions
Name three factors that affect the rate of diffusion.
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The concentration gradient, the temperature and the surface area of the membrane.
Why do multicellular organisms need exchange surfaces and a transport system?
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They have a small surface area to volume ratio, so diffusion across the outside alone cannot supply all of their cells.
Define osmosis.
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The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
How is active transport different from diffusion?
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It moves substances against a concentration gradient and needs energy from respiration.
Give an example of active transport in plants.
Show answer
Absorption of mineral ions into root hairs from very dilute solutions in the soil.
Frequently asked questions
How does osmosis differ from diffusion?
Osmosis is the diffusion of water only, from a dilute solution to a concentrated solution through a partially permeable membrane. Diffusion is the net movement of particles of any gas or dissolved substance from a higher to a lower concentration. Neither process needs energy from respiration, whereas active transport does.
How does osmosis affect plant cells?
In a dilute solution, water moves into plant cells by osmosis and the tissue gains mass. In a concentrated solution, water moves out of the cells and the tissue loses mass. Where the solution has the same concentration as the inside of the cells, there is no net movement of water, so the mass does not change.
Why is the surface area to volume ratio important for cells?
A single-celled organism has a large surface area to volume ratio, so diffusion across its surface can supply its needs. As organisms get bigger, volume increases faster than surface area and the ratio falls. Multicellular organisms cannot rely on diffusion across their outer surface alone, so they need exchange surfaces and a transport system.
Why is active transport needed in plant roots?
Active transport is needed because mineral ions in the soil are in a very dilute solution, so root hair cells must absorb them against a concentration gradient. This requires energy from respiration, which is why root hair cells have many mitochondria. Plants need these mineral ions for healthy growth.
Why does osmosis not require energy?
Osmosis does not require energy from respiration because it is a type of diffusion. Water simply moves from a dilute solution to a more concentrated solution through a partially permeable membrane. Only active transport, which moves substances against a concentration gradient, needs energy from respiration, supplied by mitochondria.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026