Osmosis required practical

In the osmosis required practical, you put equal pieces of potato into sugar or salt solutions of different concentrations, then measure how their mass changes. In dilute solutions the potato gains water by osmosis and gets heavier; in concentrated solutions it loses water and gets lighter. Comparing the percentage change in mass shows the effect of concentration, and where the change is zero tells you roughly the concentration inside the potato cells. It is required practical 3 in AQA's GCSE Biology and practical 2 in Combined Science Trilogy.

Aim

AQA's specification states the aim as: investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue. Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane, which is why the potato's mass changes. Our guide to osmosis explains the theory.

Equipment

AQA's handbook lists for each student or group:

• a potato, a cork borer, and a sharp knife or scalpel with a white tile;
• a ruler (a mm scale is needed);
• a 10 cm³ measuring cylinder, five boiling tubes and a test tube rack;
• labels, paper towels;
• a range of sugar or sodium chloride (salt) solutions, from 0.25 to 1.0 mol/dm³, and distilled water;
• a top-pan balance accurate to at least 0.01 g.

Method

These steps follow the student sheet in AQA's handbook.

1. Use the cork borer to cut five potato cylinders of the same diameter.
2. Trim off any skin, then cut the cylinders to the same length.
3. Measure and record the mass of each cylinder.
4. Measure and record the length of each cylinder.
5. Put 10 cm³ of each sugar or salt solution into a labelled boiling tube, and 10 cm³ of distilled water into a fifth.
6. Put one potato cylinder into each tube.
7. Leave them for a chosen length of time.
8. Remove the cylinders and carefully blot them dry with paper towels.
9. Measure the new mass and length of each cylinder and record them.

AQA's handbook notes that a balance reading to 0.01 g is needed to see a measurable change in a short time, and that leaving the tubes for 30 minutes in a water bath at 30 °C gives measurable changes while letting you control the temperature. It also says the skin must be removed, because it affects water movement.

Variables

Results table

You can add the same columns for length, as AQA's sheet does.

How to analyse the results

Pieces of potato never start with exactly the same mass, so compare the percentage change in mass, not the change in grams:

AQA's specification asks students to calculate the percentage gain and loss of mass of plant tissue, and to plot and interpret graphs. If you repeat each concentration, the Practical Data Lab averages the repeats and draws error bars for you.

Worked example: using AQA's sample results

AQA's handbook gives a set of results obtained by its technician advisor with sugar solutions. Two of them:

• In distilled water, a cylinder went from 5.98 g to 6.48 g: change = +0.50 g, percentage change = 0.50 ÷ 5.98 × 100 = +8.4%. It gained water.
• In 0.25 mol/dm³ sugar solution, a cylinder went from 5.92 g to 4.45 g: change = −1.47 g, percentage change = −1.47 ÷ 5.92 × 100 = −24.8%. It lost water.

The graph and the point of no change

Plot percentage change in mass (y-axis) against concentration (x-axis) and draw a curve of best fit. Where the curve crosses zero, there was no overall movement of water, so the solution's concentration matches the concentration of the potato cell contents.

With AQA's sample data, the change goes from +8.4% at 0 mol/dm³ to −24.8% at 0.25 mol/dm³. Joining those two points with a straight line, the zero is 8.4 ÷ (8.4 + 24.8) of the way across, which is about 0.063 mol/dm³. More concentrations between 0 and 0.25 mol/dm³ would pin it down better. Notice too that the last three values level off, and 1.0 mol/dm³ gives a slightly smaller loss than 0.75 mol/dm³: real data has scatter, which repeats would help to reduce.

Here is a quick check.

Risks

Errors and improvements

• Uneven blotting. Leaving surface water on some cylinders and not others changes their mass. Blot each one the same way, for the same time.
• Cylinders not identical. Use the same cork borer and potato, and trim carefully. Percentage change helps compare pieces that differ slightly.
• Too few concentrations. Extra concentrations, especially near where the change is zero, make the estimate of the cell concentration more accurate.
• Only one cylinder per concentration. Repeat with three or more and calculate a mean, so you can spot anomalies.
• Temperature changes. A water bath keeps the temperature the same for every tube.

Common mistakes

• Comparing changes in grams. Use percentage change, because the starting masses differ.
• Getting the sign wrong. A loss in mass is a negative percentage change.
• Saying sugar moves into the potato. In this practical it is water that moves, by osmosis.
• Describing osmosis as movement from concentrated to dilute. Water moves from a dilute solution to a more concentrated one.
• Missing the units of concentration. Concentrations are in mol/dm³ (or g/dm³ if that is how they were made up).

Practise biology in a short timed paper

GCSE Biology Higher and GCSE Biology Foundation each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so osmosis and required practicals turn up regularly but not in every paper. The Cell Simulator lets you run osmosis across a membrane and watch the water move.

Read next

Biology · CellsWhat is osmosis? Definition, examples and diagramsWhat osmosis is, in exam ready wording: why water moves, whether it is active or passive, what it does to plant and animal cells, and the potato practical.