Investigate how the concentration of a sugar or salt solution affects the mass of plant tissue.
This is a relatively simple practical that has much wider implications. You are going to find out the concentration of the solutions in plant cells. You need to take accurate measurements and plot a graph in order to discover the concentration of the unknown within the plant material. This teaches us other applications within scientific investigations.
Water moves by osmosis through a partially permeable membrane from a dilute solution to a more concentrated one. Plant cells gain or lose water depending on surrounding concentration.
Measure mass before and after; blot every sample consistently; calculate percentage change; identify zero change from a graph.
Potato
cork borer
ruler
scalpel or cutting tile
balance
five or more labelled tubes
sugar solutions of known concentrations
distilled water
paper towels
timer
Collect 5 boiling tubes and place them into the boiling tube rack.
Clearly label them with the appropriate solution (usually, 1.0, 0.75, 0.5 and 0.25 mol/dm3 sugar solution and a final one with distilled water.
Collect a potato (other vegetables may be used later) and carefully bore out your samples. Take care, the borers can be sharp.
Carefully measure the length and mass of each sample of potato and place it into the boiling tube. It is essential that you do not mix them up and record the lengths and masses accurately.
Place the boiling tubes into a water bath at 30°C for 30 minutes. While waiting, complete your table and prepare for your graph.
After the time has elapsed, remove the potato cylinders, remeasure the length and after gently patting dry, find the new mass. (For consistency, it is essential that you treat each core the same way).
Calculate the percentage change in mass and length.
When you have plotted the graph, you will be able to work out the concentration within the cells of the potato.
| Type of variable | What is measured or controlled? |
|---|---|
| Independent variable | Concentration of sugar or salt solution |
| Dependent variable | Percentage change in mass of potato cylinders |
| Control variables | Initial cylinder size; tissue source; solution volume; immersion time; temperature; blotting method |
Follow your teacher's instructions and your school's risk assessment. This page is not a substitute for a formal risk assessment.
| Hazard | Risk | Precaution |
|---|---|---|
Cork borer or scalpel |
Cuts when preparing potato cores |
Use a cutting tile and the demonstrated safe cutting technique. |
Glassware and solutions |
Spills, breakages or slips |
Keep tubes stable and report breakages or spills. |
Table: concentration (mol/dm³), initial mass (g), final mass (g), mass change (g), percentage change (%). Plot concentration on x-axis and percentage change on y-axis.
Percentage change = ((final mass − initial mass) ÷ initial mass) × 100.
Dilute external solution generally causes mass gain; concentrated external solution causes mass loss.
Repeat each concentration, calculate means, standardise blotting and cylinder dimensions, and use more concentrations around the zero crossing.
The point where percentage change in mass is approximately 0% estimates the external concentration at which there is no net movement of water.
Mass change and percentage mass change are different. Use the initial mass in the denominator.
Answer each question in full before opening the model answer. Check your work against the marking points and look for scientific key words.
Variables
Identify the independent variable, dependent variable and one control variable.
Sugar/salt concentration (1); percentage change in mass (1); equal length/time/temperature or sample source (1).
Calculation
A potato cylinder changes from 4.0 g to 3.4 g. Calculate its percentage change in mass.
Change = −0.6 g (1); −0.6 ÷ 4.0 × 100 = −15% (2).
Analysis
What does the concentration where percentage mass change is zero indicate?
It is the concentration where there is no net movement of water (1), giving an estimate of the plant tissue cell sap concentration (1).
Evaluation
Suggest one improvement to make the investigation more reliable and explain why it helps.
Repeat measurements and calculate a mean (1), reducing the influence of random variation/anomalies (1). Alternative valid improvement specific to the experiment with explanation can gain both marks.
This page was updated on: 9th June 2026
