If two plant tissue samples gain the same mass, the smaller starting sample has the larger percentage gain. Divide each sample’s change by its own initial mass. Then decide what the comparison actually shows about osmosis.
The 2027 SEC G3 Biology syllabus (K325) includes investigating the effects of osmosis on tissues. The example below separates a measured change from an explanation about water movement and from a judgement about the experimental design.
- Subtract starting mass from final mass.
- Divide each change by the starting mass of that sample.
- Check whether the setup supports a comparison between solutions.
Worked example | Both samples gain 0.4 g
A fictional table records Sample A changing from 4.0 g to 4.4 g and Sample B from 8.0 g to 8.4 g after the same immersion time. Both gain 0.4 g. Relative to their starting masses, A gains 0.4 ÷ 4.0 × 100% = 10% and B gains 0.4 ÷ 8.0 × 100% = 5%.
| Sample | Initial mass | Final mass | Percentage gain |
|---|---|---|---|
| A | 4.0 g | 4.4 g | 10% |
| B | 8.0 g | 8.4 g | 5% |
A correct description is that the two samples gained the same mass in grams, but A gained twice as much as a percentage of its starting mass. Use “percentage mass gain” when making that comparison.
What can you say about osmosis?
If net water entry caused the gain, water moved into cells through partially permeable membranes, from higher water potential outside the cells to lower water potential inside. Treat that as an explanation consistent with the mass data.
Percentage change makes the figures relative to the starting masses. It does not equalise sample shape, exposed surface area relative to volume, or how much surface liquid remained when each sample was weighed. Those differences matter if you want to attribute the result to solution concentration.
For a stronger comparison in a school investigation, use comparable pieces from the same tissue, control time and temperature, and blot each piece in the same way before the final weighing. Compare repeat samples to see how much the results vary.
Check a mass loss the same way
Sample C falls from 5.0 g to 4.5 g and Sample D from 10.0 g to 9.5 g. Both lose 0.5 g. C changes by −10% and D by −5%. C has the larger percentage loss, while −5% is the larger signed number.
Ask a study partner to give the result first in grams and then in percentages. Next, ask which controlled condition would strengthen a comparison between solutions.
Frequently asked questions
Which mass belongs in the denominator?
Use the original mass for each sample.
Does zero percentage change mean no water crossed the membranes?
It indicates no measured net mass change. Water can still move in both directions.
Official information
SEAB’s 2027 SEC G3 Biology syllabus (K325), Movement of Substances and Practical Assessment. Checked 9 November 2026.