⚖ Independent
The concentration of the salt or sugar solution. This is the thing you change.
Transport in Cells » Required Practical: Osmosis in Plant Tissue
What you'll learn this session
Study time: 30 minutes
AQA spec: 8.2.3 (required practical 3)
- How to investigate the effect of salt or sugar solutions on the mass of plant tissue
- The independent, dependent and control variables, and how to work safely
- How to blot, weigh, calculate a mean and find the percentage change in mass
- How to plot the graph, read where it crosses zero and suggest improvements
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Unlock This CourseThe aim of the practical
In the last lesson you met osmosis. In this practical you see it happen. You place pieces of plant tissue in solutions of different concentrations and find out whether they gain or lose mass.
The tissue is often potato, cut into cylinders. The solutions can be salt or sugar, made up at several different concentrations. Pure water is used as the lowest concentration.
A good hypothesis uses the theory of osmosis. For example: The higher the concentration of the sugar solution, the more mass the potato cylinders will lose, because more water will move out of the cells by osmosis.
Key terms:
- Cork borer: a metal tube with a sharp edge, used to cut cylinders of equal width from a potato.
- Blotting: gently drying the surface of the tissue with a paper towel before weighing it.
The method
- Label five test tubes with the concentrations you are testing, for example 0.0 (pure water), 0.1, 0.2, 0.3 and 0.4 mol/dm3.
- Put the same volume of each solution into its tube, for example 20 cm3.
- Use a cork borer to cut cylinders from one potato. Trim them to the same length with a scalpel on a tile, and remove any skin.
- Blot each cylinder gently with a paper towel. Weigh it on a balance and record its starting mass.
- Place one cylinder in each tube (or a few cylinders per tube, kept together), so it is covered by the solution.
- Leave the tubes for the same time, for example 30 minutes, at the same temperature.
- Take each cylinder out, blot it in the same way and weigh it again. Record the final mass.
Blotting matters. A wet surface would add the mass of the solution, not the mass of the tissue, and this would make the result wrong.
Variables
📏 Dependent
The change in mass of the potato cylinder. This is the thing you measure.
🔒 Control
Things you keep the same, so the test is fair.
The control variables include:
- the type of plant tissue, and the same potato for every cylinder
- the length and width of the cylinders, so they have the same surface area
- the volume of solution in each tube
- the time left in the solution
- the temperature
- the way you blot each cylinder
Working safely
A cork borer and a scalpel are sharp. Cut on a tile, with the blade pointing away from you and your fingers. Wipe up any spills straight away so nobody slips, and wash your hands at the end. Never taste anything in the lab.
Processing the results
The cylinders do not all start at exactly the same mass, so a fair comparison uses the percentage change in mass. You calculated this in the last lesson, so here it is applied to a full set of results.
Percentage change (as in the last lesson) = (final mass − starting mass) ÷ starting mass × 100
Worked example
Three cylinders were left in 0.3 mol/dm3 sugar solution. Their final masses were 2.44 g, 2.46 g and 2.45 g.
Mean final mass = (2.44 + 2.46 + 2.45) ÷ 3 = 7.35 ÷ 3 = 2.45 g
The mean starting mass was 2.50 g, so the change = 2.45 − 2.50 = −0.05 g
Percentage change = −0.05 ÷ 2.50 × 100 = −2%
Repeating the test and using the mean makes the results more reliable. A mean also evens out small errors in weighing.
A full set of results might look like this:
| Concentration (mol/dm3) | Starting mass (g) | Final mass (g) | Percentage change (%) |
|---|---|---|---|
| 0.0 | 2.50 | 2.75 | +10 |
| 0.1 | 2.50 | 2.65 | +6 |
| 0.2 | 2.50 | 2.55 | +2 |
| 0.3 | 2.50 | 2.45 | −2 |
| 0.4 | 2.50 | 2.35 | −6 |
Plotting the graph
- Put concentration on the x-axis and percentage change in mass on the y-axis.
- The y-axis needs to go below zero, so include the negative values.
- Label both axes with units, use most of the paper, and plot each point with a small cross.
- Draw a smooth line of best fit through the points.
With this data the line slopes downwards from +10% to −6%. It crosses the x-axis between 0.2 and 0.3 mol/dm3, at about 0.25 mol/dm3.
At that point the percentage change is zero, so there is no net movement of water. The solution has the same concentration as the inside of the potato cells. Read the value across from the point where the line crosses zero.
You can also find a rate of water uptake. In pure water the cylinder gained 0.25 g in 30 minutes, which is 0.25 ÷ 30 = about 0.008 g per minute. Water moves into the cells by osmosis because pure water is more dilute than the cell contents, so the cylinder gains mass.
Errors and improvements
⚠ Possible errors
Cylinders blotted too hard or not enough. Skin left on some cylinders. Cylinders cut from different potatoes. Different lengths, so different surface areas. Using a balance that only reads to the nearest 0.1 g, so small changes are missed.
✅ Improvements
Repeat each concentration and calculate a mean. Use the same potato and cut the cylinders carefully. Blot every cylinder the same way. Use a balance that reads to 0.01 g. Test more concentrations between 0.2 and 0.3 to find the zero point more exactly.
Common mistakes
Forgetting to blot the cylinders, or blotting them differently each time. Dividing by the final mass instead of the starting mass. Plotting concentration on the y-axis. Saying that 'no water moves' at the zero point: water still moves both ways, but there is no net movement.
Exam-style question
A student cut potato cylinders and placed them in sugar solutions of different concentrations for 30 minutes.
(a) State the independent variable and one control variable. [2 marks]
(b) A cylinder had a starting mass of 3.0 g and a final mass of 3.3 g in pure water. Calculate the percentage change in mass. [2 marks]
(c) Explain why the student blotted each cylinder before weighing it. [1 mark]
(d) Suggest one way to make the results more reliable. [1 mark]
Model answer
(a) Independent variable: the concentration of the sugar solution (1). Control variable, any one from: volume of solution, time, temperature, size of cylinder, type of potato (1).
(b) Change in mass = 3.3 − 3.0 = +0.3 g (1). Percentage change = 0.3 ÷ 3.0 × 100 = +10% (1).
(c) To remove the solution on the surface (1), so that only the mass of the potato is measured.
(d) Repeat the test for each concentration and calculate a mean (1).
Exam tip
In a calculation, show the change in mass first, then divide by the starting mass. Even if the final number is wrong, you can still earn a mark for the method.