To interpret experiment data, read the trend, quote numbers from the table, and explain with osmosis. The isotonic point is where the change in mass is zero.
This lesson is part of movement across membranes. It uses the ideas from explaining concentration gradients.
What is the routine?
- Trend: does the percentage change fall, rise or stay constant as concentration rises?
- Numbers: quote at least two values from the table.
- Reason: link the change to water movement by osmosis.
- Point: find where the change is zero, if asked.
Worked example: potato strips in sucrose
An original dataset: potato strips were kept in sucrose solutions for 1 hour.
| Sucrose (M) | Change in mass (%) |
|---|---|
| 0.0 | +10.0 |
| 0.2 | +5.0 |
| 0.4 | −3.0 |
| 0.6 | −9.0 |
| 0.8 | −13.0 |
The trend: as sucrose concentration rises, the percentage change in mass falls from a gain to a loss. At 0.0 M the strips gain 10.0%, and at 0.8 M they lose 13.0%.
The reason: in dilute solutions water enters the cells by osmosis and the strips gain mass. In concentrated solutions water leaves and the strips lose mass.
How do I find the isotonic point?
The change crosses zero between 0.2 M (+5.0) and 0.4 M (−3.0). The gain is 5.0 and the loss is 3.0, so the zero point is 5.0 ÷ (5.0 + 3.0) = 0.625 of the way from 0.2 to 0.4.
That gives 0.2 + 0.625 × 0.2 = 0.325 M, or about 0.33 M. At this concentration there is no net water movement, so the solution is isotonic to the potato cells.
The mistake that loses the conclusion mark
A student picks 0.4 M as the isotonic point because −3.0 is the change closest to zero. But it is not zero, and the true zero lies between 0.2 M and 0.4 M.
A second mistake is a conclusion with no numbers: “Higher concentration causes mass to decrease.” Add “from +10.0% at 0.0 M to −13.0% at 0.8 M”.
How do I describe a fair test?
Change one variable: the concentration. Keep the potato type, strip size, volume, temperature and time the same, and dry the strips the same way before weighing.
Repeat the test and take an average, because one strip may differ from another.
A self-check question
In a second test, the changes were: 0.1 M, +8.0%; 0.3 M, +2.0%; 0.5 M, −6.0%. Estimate the isotonic concentration.
Answer
The change crosses zero between 0.3 M (+2.0) and 0.5 M (−6.0). The fraction is 2.0 ÷ (2.0 + 6.0) = 0.25, so the point is 0.3 + 0.25 × 0.2 = 0.35 M. At that concentration there is no net movement of water.
Use the graph evidence comparison lab to practise reading trends. For a longer set, try the practice questions.
If data questions still feel hard, a teacher in SPM Biology one-to-one tuition can build the routine with you on new datasets. You can start with the one-hour trial class (from RM50).