1.3.5 Required Practical: Osmosis · 1. Cell Biology | Aqa GCSE Combined Science Trilogy 8464: Biology

Section 1 of 9Aim and principle

The aim of this AQA required practical is to investigate the effect of a range of concentrations of salt or sugar solution on the mass of plant tissue, usually potato or other root vegetable.

Aim and principle

If the solution outside is more dilute than the cell contents, water enters the tissue by osmosis and its mass increases.

If the solution is more concentrated, water leaves and the mass decreases.

If the solution is the same concentration as the cell sap, there is no net change.

Section 2 of 9Method

Prepare a range of sugar solutions (for example 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³) by diluting a stock solution with distilled water.

Use a cork borer to cut several cylinders from the same potato. Trim them to the same length (e.g. 3 cm), removing the skin, which would limit water movement.

Initial mass

Blot each cylinder dry with a paper towel and measure its initial mass on a balance.

Place each cylinder into its own boiling tube containing the same volume of one solution, and label it.

Leave for a set time (for example 30 minutes or 24 hours) at the same temperature.

Final mass

Remove each cylinder, blot dry and measure its final mass.

Repeat for each concentration, using three cylinders per concentration, and calculate means.

Section 3 of 9Calculations

Because cylinders may start at slightly different masses, use percentage change in mass rather than the raw change:

percentage change = (final mass - initial mass) / initial mass x 100

For example, a cylinder going from 2.50 g to 2.75 g changes by +0.25 g, so percentage change = 0.25 / 2.50 x 100 = +10%. A decrease from 2.50 g to 2.25 g gives -10%.

Plot a graph of percentage change (y-axis) against concentration (x-axis) and draw a smooth curve or line of best fit. The point where the line crosses the x-axis (0% change) estimates the concentration of the solution that is isotonic with the potato cells, which means the concentration of the cell sap.

Concentration / mol/dm³	Initial mass / g	Final mass / g	% change
0.0	2.50	2.85	+14.0
0.4	2.50	2.55	+2.0
0.8	2.50	2.20	-12.0

Section 4 of 9Variables and control

Keep sample volume, reagent volume, temperature and heating time constant.

Independent variable

Independent variable concentration of the solution.

Dependent variable

Dependent variable percentage change in mass of the tissue.

Control variables

Control variables type and age of potato (ideally the same potato), length and surface area of cylinders, volume of solution, temperature, time left in solution, and how thoroughly they are blotted.

The 0 mol/dm³ solution (distilled water) acts as a comparison showing the maximum gain. Repeating with several cylinders and calculating a mean makes the results more reliable, and anomalous results should be identified and left out of the mean.

Section 5 of 9Evaluation and safety

Sources of error include differences in blotting (surface water adds mass), cylinders not being cut to identical size, and different potato tissue.

1

Using a balance that measures to 0.01 g improves resolution.

2

Safety: take care with the sharp cork borer and scalpel, cutting on a tile and cutting away from the body, and wipe up any spillages.

3

Using a water bath improves temperature control.

4

If the percentage changes are small, a longer time or a bigger range of concentrations will make the trend clearer.

Section 6 of 9Presenting and interpreting results

Once percentage changes are calculated, present means in a table with clear headings and units.

Presenting and interpreting results

On the graph, plot the independent variable on the x-axis and use a sensible scale that fills at least half of the grid.

Because some points represent gain and others loss, the y-axis must run through both positive and negative values.

Draw a smooth curve of best fit rather than joining points with straight lines.

Describe the pattern in words

'As the concentration of the sugar solution increased, the percentage change in mass decreased from a gain to a loss.' Explain the pattern using osmosis: at low concentrations, the solution is more dilute than the potato cells so water enters;
- at high concentrations, the solution is more concentrated so water leaves.
If the curve crosses zero at 0.3 mol/dm³, that is the concentration at which there is no net water movement, so the cell sap has this same concentration.

Conclusions should refer to the evidence only. You can say that the results support the idea that the potato cells are about 0.3 mol/dm³, but you cannot say this is exact because of the limited range of concentrations tested.

Section 7 of 9Exam-style questions

A potato cylinder has an initial mass of 3.20 g and a final mass of 2.88 g. Calculate the percentage change in mass. [2]2 marks
Explain why the potato cylinders are blotted dry before they are weighed. [2]2 marks
Describe how the results could be used to find the concentration of the cell sap of the potato. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Percentage change	Change in value divided by initial value, multiplied by 100.
Cork borer	Tool used to cut equal-sized cylinders from a potato.
Isotonic point	Concentration where there is no net change in mass.
Independent variable	The variable that is deliberately changed.
Dependent variable	The variable that is measured.
Control variable	A variable kept the same.
Anomalous result	A result that does not fit the pattern.

Section 9 of 9Exam technique

1

Using the raw change in mass instead of percentage change when starting masses differ.

2

Forgetting that the negative sign means a loss in mass.

3

Not controlling surface area, so differently sized cylinders give unfair comparisons.

4

Leaving out the distilled-water control.