Diffusion, Osmosis and Active Transport (IGCSE Biology 0610)

Diffusion is the net movement of particles from a higher to a lower concentration, down a concentration gradient, by random movement. Osmosis is the diffusion of water through a partially permeable membrane. Active transport moves particles against a concentration gradient using energy from respiration. Together they make up topic 3 of Cambridge IGCSE Biology 0610.

Diffusion, osmosis and active transport compared

Diffusion

The 0610 syllabus for 2026 to 2028 asks you to describe diffusion as "the net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement". The energy comes from the kinetic energy of that random movement, which is why diffusion needs no energy from respiration.

Learn that definition word for word. The mark schemes we sampled award one mark for "net movement of particles, molecules or ions", one for "higher to lower concentration" or "down a concentration gradient", and sometimes one for "random". Leaving out "net" or saying "from high to low" without "concentration" is where students drop a mark.

You also need the four factors the syllabus lists: surface area, temperature, concentration gradient and distance. A bigger surface area, a higher temperature, a steeper gradient and a shorter distance all make diffusion faster. Questions often test surface area through agar cubes. As the cube gets bigger, its surface area to volume ratio falls and diffusion to the centre takes longer, which is why large organisms need transport systems.

Osmosis

Core candidates can define osmosis as the diffusion of water through a partially permeable membrane. Extended candidates need the full version from the syllabus: "the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane".

Water potential trips people up because the direction feels backwards. A dilute solution has a high water potential. A concentrated sugar or salt solution has a low one. Water moves from the dilute side to the concentrated side.

Plant cells in different solutions

• In pure water or a dilute solution, water enters by osmosis. The vacuole swells and the cell pushes on its cell wall. The cell is turgid, and the outward push is turgor pressure. This pressure is what supports a plant.
• In a concentrated solution, water leaves by osmosis. The cell becomes flaccid, and if enough water is lost the cytoplasm and membrane pull away from the wall. That is plasmolysis.
• Animal cells have no wall. In pure water they can swell and burst; in a concentrated solution they shrink.

The practical side comes up too. The syllabus asks you to investigate osmosis with dialysis tubing and to investigate how plant tissue, such as potato cylinders, changes in solutions of different concentration. For potato data, the concentration where mass does not change is where the tissue's water potential equals the solution's. Our Paper 6 guide covers how those results are marked.

Active transport

Active transport is "the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e. against a concentration gradient), using energy from respiration". Extended candidates also need to state that protein carriers move the molecules or ions across the membrane, and to explain ion uptake by root hairs. Soil water holds mineral ions at a lower concentration than root hair cells do, so root hairs take them in against the gradient.

Any condition that stops respiration in roots stops active transport too. Waterlogged soil has little oxygen, so root cells respire less and take up fewer ions. Mark schemes for flooding questions reward this link.

How the topic is examined

The written questions repeat themselves. Of the 23, seven ask you to define diffusion, three ask you to define osmosis and three ask how osmosis differs from diffusion. Two describe a crop field flooded by sea water and ask what happens to the plants. That means 15 of 23 written questions come down to four answers you can learn in an evening:

1. Define diffusion
   Net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, by random movement.
2. Define osmosis
   Net movement of water molecules from higher to lower water potential, through a partially permeable membrane.
3. How osmosis differs from diffusion
   Two points: osmosis moves only water, and it needs a partially permeable membrane. Diffusion moves any particles and needs no membrane.
4. Sea water floods a field
   The soil water now has a lower water potential than the root cells. Water leaves the roots by osmosis, the plants lose turgor and wilt. Waterlogged soil also has little oxygen, so root respiration and active transport fall.

The multiple-choice questions lean on applying the definitions: which way water moves between two solutions, which process needs energy, or what happens to a cell in a given solution. Practising them by subtopic is the quickest way to find which idea you keep getting wrong. You can try the diffusion, osmosis and active transport questions in the PapaMarks library, or start from the subtopics on diffusion, osmosis in animals and plants and active transport.

To see the membrane and the cell wall these processes act on, the free 3D Cell Explorer lets you take a plant and animal cell apart. For where this topic sits in the course, read the 0610 syllabus map and the most-tested 0610 topics.

FAQ

Sources

• Cambridge International, Cambridge IGCSE Biology 0610 syllabus for 2026, 2027 and 2028, version 3, topic 3: Movement into and out of cells.
• Cambridge IGCSE Biology 0610 past papers and mark schemes, as held in the PapaMarks question bank (counts read 10 November 2026).