Cambridge IGCSE Biology 0610 · Chapter 3
Movement Into and Out of Cells
Every living cell has to exchange substances with its surroundings — oxygen, carbon dioxide, water, dissolved substances and mineral ions all need to enter or leave, but they don’t all move the same way. This chapter is really three separate answers to one question, and almost every mark in it depends on choosing the right one and describing it precisely.
Cambridge splits this chapter into three named processes: diffusion, the net movement of particles down a concentration gradient with no energy cost to the cell; osmosis, the movement of water specifically, through a partially permeable membrane; and active transport, the movement of particles against a concentration gradient, which only happens because the cell spends energy from respiration to make it happen. They are related — osmosis is really diffusion applied to water, and active transport exists precisely because diffusion cannot do everything a cell needs — but they are not interchangeable, and examiners test the boundaries between them just as often as they test any one process on its own.
Students tend to lose marks here less because they don’t understand what is happening biologically, and more because they reach for the wrong process, or the wrong direction, when a question is phrased slightly differently from how they first learned it. “Water moves from high concentration to low concentration” sounds like the same idea as diffusion, but at Supplement level it is exactly the kind of sentence that costs a mark, because it doesn’t say whether “concentration” means the water or the solute dissolved in it. The pages below are written to close that specific kind of gap — not more facts, but the precise phrasing and reasoning a Cambridge mark scheme is actually built around.
What’s in this chapter
| Page | Level | What it covers |
|---|---|---|
| Diffusion | Core | The exact definition, why no energy from respiration is needed, and how a concentration gradient decides which way a substance actually moves. |
| Factors affecting diffusion | Core | Surface area, temperature, concentration gradient and distance — and how to design a valid investigation into any one of them. |
| Osmosis | Core & Supplement | Water as a solvent, partially permeable membranes, and the precise water-potential language Cambridge expects at Supplement. |
| Osmosis experiments | Core practical | Dialysis tubing and plant tissue investigations, plus how to compare results fairly with percentage change in mass. |
| Osmosis in plant cells | Core & Supplement | Turgid, flaccid and plasmolysed cells, and why water pressure is what actually holds a non-woody plant upright. |
| Active transport | Core & Supplement | Moving particles against a concentration gradient using energy from respiration — and why root hair cells need to do exactly that. |
Core or Supplement?
Diffusion and factors affecting diffusion are entirely Core, so every candidate needs them regardless of tier. Osmosis, osmosis in plant cells and active transport each carry a Core layer that every candidate needs — the basic definitions, the practical investigations, and the idea of turgor and plant support — with a Supplement layer built on top: water potential, plasmolysis, why active transport matters, root hair cells, and protein carriers. If you’re not sure which tier you’re sitting, it’s worth checking before spending time on the Supplement sections, since none of the Core content on any of these pages depends on them.
Diffusion, osmosis and active transport together
These three processes all move substances into or out of cells, but they answer different questions, and the safest way to identify which one an unfamiliar example is describing is to ask a short sequence of questions rather than trying to recall which substance “usually” uses which process.
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| What moves? | Particles, including gases and solutes | Water molecules only | Molecules or ions |
| Direction | Higher concentration → lower concentration | Higher water potential → lower water potential | Lower concentration → higher concentration |
| Gradient | Down a concentration gradient | Down a water-potential gradient | Against a concentration gradient |
| Membrane required by the definition? | No | Yes — a partially permeable membrane | Yes — a cell membrane |
| Energy from respiration required? | No | No | Yes |
| Protein carriers in the 0610 requirement | Not specified | Not specified | Yes — Supplement |
To identify an unfamiliar example, ask, in this order:
- Is it water moving through a partially permeable membrane? Then it’s osmosis.
- Is a substance moving from higher concentration to lower concentration? Then it’s diffusion.
- Is a substance moving from lower concentration to higher concentration, using energy from respiration? Then it’s active transport.
That relationship between what is moving, the direction of the gradient, and whether energy is needed is the single idea the whole chapter is built around. Start with diffusion, since osmosis and active transport both make more sense once that base case is solid.