How to answer osmosis questions
The definition that gets full marks, how to work out which way water moves, and how to handle percentage change in the required practical.
What the examiner is testing
Two things, and they are marked separately. First, the definition — worth full marks only if it names water, the direction, and the membrane. Second, whether you can predict which way water moves and describe the effect on the cell.
The definition that earns the marks
Osmosis is the movement of water molecules from a dilute solution (high water potential) to a concentrated solution (low water potential) through a partially permeable membrane.
Say "particles" instead of "water" and you lose a mark. Osmosis is about water specifically — solute movement is diffusion or active transport.
Working out the direction
Water always moves towards the more concentrated solution — that is, towards more solute and less water.
| Cell placed in | Water moves | Plant cell | Animal cell |
|---|---|---|---|
| Pure water / dilute solution | Into the cell | Turgid | Bursts (lysis) |
| Same concentration | No net movement | Flaccid | No change |
| Concentrated solution | Out of the cell | Plasmolysed | Shrivels (crenation) |
The difference in the last column is the cell wall. Plant cells have one, so they resist bursting; animal cells do not.
The required practical
Potato cylinders are left in sugar solutions of different concentrations, then reweighed.
Percentage change in mass:
$$ \% \text{ change} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100 $$
Worked example. A cylinder starts at 4.2 g and finishes at 4.9 g.
$$ \frac{4.9 - 4.2}{4.2} \times 100 = \frac{0.7}{4.2} \times 100 = +16.7\% $$
A gain in mass means water moved in, so the solution outside was more dilute than the cell contents.
Reading the graph. Plot percentage change against concentration. Where the line crosses zero, there was no net movement — so at that point the sugar solution has the same concentration as the cell contents. That crossing point is what the question is usually driving at, and it is worth stating explicitly.
The three mistakes that lose marks
1. Saying "particles move" instead of "water moves". Osmosis moves water. Be specific.
2. Getting the direction backwards. Water moves to the concentrated solution. A useful check: salt draws water out of things, which is why salted food dries.
3. Using change in mass instead of percentage change. The cylinders start at different masses, so raw change cannot be compared between them. Divide by the initial mass, not the final one.
30-second recap
Water, dilute to concentrated, partially permeable membrane — all three for the definition. Water moves towards the concentrated side. Percentage change is calculated from the initial mass, and the concentration where it reads zero equals the concentration inside the cell.