In this required practical you cut potato cylinders, weigh them, leave them in sugar solutions of different concentrations, then reweigh them. Water moves in or out by osmosis, so the cylinders gain or lose mass. You calculate the percentage change in mass and plot it against concentration.
What is the practical actually investigating?
The independent variable is the concentration of the sugar solution; the dependent variable is the percentage change in mass of the potato tissue. The point is to find out how the concentration of the surrounding solution affects the direction and amount of water movement into plant cells.
Osmosis is the movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane. The potato cell membrane is partially permeable: water can cross it, but sugar molecules largely cannot.
- In a dilute solution (low sugar), water moves into the cells. The cylinder gains mass and becomes firm.
- In a concentrated solution (high sugar), water moves out of the cells. The cylinder loses mass and becomes floppy.
- At one particular concentration, water moves in and out at the same rate and there is no net change in mass.
What is the method?
- Prepare a range of sugar solutions of known concentration — commonly 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³ sucrose. The 0.0 solution is pure water and acts as your control.
- Cut potato cylinders using a cork borer, then trim them all to the same length with a scalpel on a cutting tile. Use one potato so the tissue is the same throughout.
- Blot each cylinder dry with a paper towel and record its starting mass using a balance, to the same number of decimal places every time.
- Place one cylinder into each solution, using the same volume of solution in each boiling tube, and label the tubes.
- Leave for a set time — often around 24 hours, or a shorter fixed period if lesson time is limited. Every tube must be left for exactly the same length of time.
- Remove each cylinder, blot it dry in the same way, and record its final mass.
- Calculate the percentage change in mass for each concentration and plot a graph.
Why do you calculate percentage change in mass?
Because the cylinders never start at exactly the same mass. A cylinder that begins at 4.0 g and one that begins at 4.6 g cannot be fairly compared using the raw gain or loss in grams. Converting to a percentage makes every result comparable:
$$\text{percentage change} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100$$
Worked example: a cylinder starts at 4.20 g and ends at 4.62 g.
- Change in mass = 4.62 − 4.20 = +0.42 g
- Percentage change = (0.42 ÷ 4.20) × 100 = +10%
A second cylinder starts at 4.50 g and ends at 3.96 g.
- Change in mass = 3.96 − 4.50 = −0.54 g
- Percentage change = (−0.54 ÷ 4.50) × 100 = −12%
Keep the sign: a positive value means water moved in, a negative value means water moved out.
How do you read the graph?
Plot concentration of sugar solution on the x-axis and percentage change in mass on the y-axis, then draw a line of best fit. The line slopes downwards: as the surrounding solution becomes more concentrated, the change in mass moves from positive, through zero, to negative.
The point where the line crosses the x-axis — zero percentage change — is the concentration at which there was no net movement of water. At that point the solution has the same concentration as the cell contents of the potato, so it gives you an estimate of the potato's own internal concentration. This is the highest-value conclusion in the practical, and it is why the graph must include enough points on both sides of zero to see the crossing.
What are the control variables?
To make it a fair test, keep these the same in every tube:
| Variable | Why it must be controlled |
|---|---|
| Volume of solution | A larger volume changes concentration less as water moves, affecting the result |
| Time left in the solution | More time means more net water movement |
| Temperature | Affects the rate of diffusion of water molecules |
| Size and shape of cylinders | Determines the surface area available for osmosis |
| Source of potato | Different potatoes (or varieties) have different internal concentrations |
| Blotting method | Surface liquid adds mass that has nothing to do with osmosis |
What are the common mistakes?
- Not blotting, or blotting inconsistently. A film of solution left on the surface adds mass and produces a falsely positive result.
- Using cylinders from different potatoes. Their internal concentrations differ, adding a second variable.
- Recording the change in grams only. The mark scheme wants percentage change, for the reason above.
- Squeezing the cylinders while blotting, which forces water out of the tissue.
- Reading the balance to different precisions for the initial and final masses.
- Describing osmosis as the movement of sugar. It is the movement of water, and it is the sugar that cannot cross the membrane.
Frequently asked questions
Why does the potato cylinder become firm in water and floppy in strong sugar solution?
In pure water, water enters the cells by osmosis until the cell contents press outwards against the cell wall. The cells become turgid, and the whole cylinder feels rigid — this is what keeps a plant's leaves and stems held up. In a concentrated sugar solution, water leaves the cells, so they become flaccid and, if enough water leaves, the membrane pulls away from the cell wall, which is called plasmolysis. The cylinder feels bendy and looks slightly shrunken.
Can you use salt solution instead of sugar solution?
Yes, and many schools do. Sodium chloride solutions produce the same pattern of results, because what matters is the concentration of dissolved solute rather than which solute it is. Sucrose is often preferred because it does not readily cross the cell membrane and does not affect the tissue chemically, so the mass change is due to water movement alone. Whichever is used, the concentrations must be stated and kept consistent throughout the investigation.
Why is a 0.0 mol/dm³ (pure water) tube included?
It acts as the control. Pure water is the most dilute solution possible, so it should give the largest gain in mass, and if it does not, something has gone wrong with the method — the timing, the blotting or the balance. It also anchors one end of the graph, making the line of best fit and the crossing point far easier to identify than if every tube contained sugar.
How could the practical be made more accurate?
Repeat each concentration at least three times and use the mean percentage change, which reduces the effect of any one unusual cylinder. Use a balance reading to two decimal places, control temperature with a water bath, use a cork borer so every cylinder has the same diameter, and use narrower intervals of concentration around the point where the line crosses zero, so that the potato's internal concentration can be estimated more precisely.
For Socratic GCSE biology with Professor Darwin — who bridges the scales from cylinder to cell to water molecule — visit aitutors.me.