This topic explains how substances move into and out of cells: by diffusion, by osmosis and by active transport. It matters because almost every later topic, from plant nutrition to gas exchange and kidney function, uses these ideas. Check the current Cambridge syllabus for your exam year to confirm the exact content.
The topic sits inside Cambridge IGCSE Biology, after the structure of cells and before the chemistry of life.
What should you know first?
You should be comfortable with what a cell membrane is and with the idea that cells exchange substances with their surroundings, from cells and microscopy. You also need simple percentages and ratios, because the data questions use them.
Being able to read a table and describe a trend in one sentence will help more than memorising extra facts.
One orienting example
The data are invented. A cell contains 0.15 units of an ion and sits in a solution with 0.02 units. Suppose the cell’s uptake of this ion slows when oxygen is reduced.
Ask two questions.
First, which way does the ion move relative to the gradient? It enters the cell where the concentration is higher, so it moves against the gradient.
Second, does it need energy? The slowing when oxygen is reduced suggests respiration is involved.
Together these point to active transport. If the ion had moved into a region of lower concentration with no energy needed, it would have been diffusion. The habit of asking these questions every time is the main skill of this topic.
In what order should you study it?
- Explain diffusion using a concentration difference: start here, because every other lesson builds on net movement down a gradient.
- Predict water movement in a stated osmosis model: osmosis is diffusion of water, so it comes straight after.
- Distinguish active transport from diffusion: now you can compare the two, using direction and energy.
- Interpret a mass-change dataset: this puts osmosis into numbers, with percentage change and estimates.
- Separate a cell response from a whole-organism claim: this teaches you to size conclusions to the evidence.
- Movement across membranes practice set: mixed questions that do not tell you which idea to use.
What are the common traps?
- Flipping water direction. Water moves from the dilute solution to the concentrated one. Follow the water, not the solute.
- Leaving out “net” and “gradient” in definitions.
- Saying that speed decides the process. Direction and energy decide active transport, not how fast it goes.
- Comparing raw masses instead of percentage change when starting masses differ.
- Writing claims that go beyond the data, such as saying a cell result proves something about a whole plant.
How should you use the practice set?
Attempt the practice set after at least the first three lessons, then again after the whole topic. Write full answers on paper and read the worked solutions to compare your reasoning. The mistake log and retest queue helps you record slips by type and retest them a few days later.
If you mix up the direction of water or what counts as active transport, a teacher in online one-to-one Biology tuition can go through your answers with you. It starts with a paid one-hour trial.