When a question gives you a cell observation and asks “explain”, the marks are for a chain: observation, then movement of water, then reason for the movement, then link back to the data. A membrane explanation is that chain written in order.
This lesson belongs to biological mechanisms in mixed tasks. The data below is invented for teaching and is labelled as such.
What does the membrane explanation need?
Water moves by osmosis, which is the net movement of water molecules through a partially permeable membrane from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a more concentrated solution).
So every good answer has three parts:
- Direction: which way did water move, into or out of the cells?
- Reason: the solution outside was more dilute or more concentrated than the cell contents.
- Evidence: a number from the data that shows the direction.
Worked example (invented data)
A student places four identical potato cylinders, each 2.00 g at the start, in sugar solutions of different concentration. After the same time, each is dried on paper and weighed.
| Sugar solution (mol/dm³) | Start mass (g) | End mass (g) | Change (g) | Percentage change |
|---|---|---|---|---|
| 0.0 | 2.00 | 2.30 | +0.30 | +15.0% |
| 0.2 | 2.00 | 2.10 | +0.10 | +5.0% |
| 0.4 | 2.00 | 1.90 | −0.10 | −5.0% |
| 0.6 | 2.00 | 1.80 | −0.20 | −10.0% |
Step 1, calculate: percentage change = change ÷ start mass × 100. For 0.0: 0.30 ÷ 2.00 × 100 = +15.0%.
Step 2, find the pattern: as the sugar concentration rises, the percentage change falls from +15.0% to −10.0%.
Step 3, find the balance point: the change falls by 10 percentage points for each 0.2 mol/dm³. It is +5.0% at 0.2 and needs to fall by 5.0 more, which is half a step, so the cells are in balance at about 0.30 mol/dm³.
Step 4, explain: in pure water the solution outside is more dilute than the cell contents, so water moves into the cells by osmosis through the partially permeable membrane and the mass rises. At 0.6 mol/dm³ the outside solution is more concentrated, so water leaves and the mass falls. At about 0.30 mol/dm³ there is no net movement.
The mistake to watch for
Mistaken answer: “The mass went up because sugar entered the cells.”
The sugar molecules are too large to cross the membrane quickly.
The data cannot show sugar moving at all. It shows a change in mass, and the simplest cause is water. The correction is to name the particle that moves (water), the membrane property (partially permeable) and the concentration difference.
A second slip is comparing raw gains when the start masses differ. Always convert to percentage change first.
Check yourself
1. A piece starts at 2.50 g and ends at 2.75 g. Find the percentage change.
Show answer
Change = 2.75 − 2.50 = 0.25 g. Percentage change = 0.25 ÷ 2.50 × 100 = +10.0%. Check: 10% of 2.50 is 0.25, so it matches.
2. A piece goes from 1.80 g to 1.62 g. Is water moving in or out, and what is the percentage change?
Show answer
Mass fell, so water moved out by osmosis. Change = −0.18 g. Percentage change = −0.18 ÷ 1.80 × 100 = −10.0%.
3. In one sentence, explain why a tissue loses mass in a very concentrated sugar solution.
Show answer
The solution outside is more concentrated than the cell contents, so water moves out of the cells through the partially permeable membrane by osmosis.
Where this leads next
Next, link plant exchange with a supplied environmental dataset to practise the same chain with leaves. When you are ready, try the mixed practice set, and use the scientific investigation critic to test your own method descriptions.
If your explanations lose marks at the “reason” step, our teachers can work through them with you in online one-to-one Combined Science tuition.