Osmosis in plant tissue
GCSE Biology (8461) · Required practical 3 — method, variables, the marks examiners report students losing.
Investigate the effect of a range of concentrations of sugar or salt solutions on the mass of plant tissue (potato).
Apparatus
- Potato and a cork borer / knife to cut equal-sized cylinders
- A range of sugar (or salt) solutions of known concentration, plus distilled water
- Balance (to 0.01 g), boiling tubes and a measuring cylinder
- Paper towel, ruler and a marker pen
Method
- 1Cut several potato cylinders of equal length using a cork borer, blot them dry and measure the starting mass of each.
- 2Place one cylinder in each solution, from distilled water (0 mol/dm3) up to a concentrated solution, using the same volume in each tube.
- 3Leave for the same length of time (e.g. 30 minutes) at the same temperature.
- 4Remove each cylinder, gently blot off surface water, and measure the final mass.
- 5Calculate the percentage change in mass for each concentration.
Variables
Independent
Concentration of the sugar/salt solution
Dependent
Percentage change in mass of the potato
Control
- Type, size and surface area of the potato pieces
- Volume of solution and time left
- Temperature
Results & processing
- Percentage change in mass = (final mass - initial mass) / initial mass x 100.
- In dilute solutions the mass increases (water enters by osmosis); in concentrated solutions the mass decreases (water leaves).
- Where a graph of % change against concentration crosses zero, the solution concentration equals that inside the potato cells (no net movement).
Where students lose marks
Comparing actual mass change instead of percentage change.
Fix: Use percentage change so pieces with slightly different starting masses can be compared fairly.
Not blotting the cylinders before weighing.
Fix: Blot gently to remove surface solution, otherwise the final mass is too high; blot each piece the same way.
Leaving pieces in for different times.
Fix: Time every tube the same and use a timer, or the comparison is unfair.
Improve the method
- Repeat each concentration with several cylinders and take a mean percentage change.
- Blot each cylinder in the same standardised way to reduce random error.
- Use narrow concentration intervals near the crossover point to find the internal concentration more accurately.
Try it — exam-style
A potato cylinder starts at 4.0 g and ends at 4.6 g. Calculate the percentage change in mass.
A potato cylinder placed in a concentrated sugar solution loses mass. Explain why.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on osmosis in plant tissue?
Percentage-change maths and the osmosis explanation are where marks go missing — I drill both, and your first lesson is free.