Required practical 2 · Biology · Biology Paper 1, both tiers

Osmosis in plant tissue

Investigate the effect of a range of sugar or salt solution concentrations on the mass of plant tissue.

The exam trap. Compare percentage change, not grams. Where the line crosses zero, the solution matches the cells' concentration.

Variables

Independent (you change): Concentration of the sucrose solution (mol/dm³)
Dependent (you measure): Percentage change in mass of the potato cylinder
Control (you keep the same): Length and diameter of the cylinders (same cork borer)
Volume of solution in each tube
Time left in solution
Temperature
Type and source of potato

Cut potato cylinders with a cork borer and trim them to the same length with a scalpel.
Why Equal size means equal surface area, so differences come from concentration only.
Blot each cylinder and record its starting mass on a balance.
Why Surface water would add mass that is not part of the tissue.
Put each cylinder in a boiling tube with the same volume of a different concentration of sucrose solution.
Why The concentration is the independent variable; the volume is controlled.
Leave all the tubes for the same time, for example 24 hours, at room temperature.
Why Osmosis continues over time, so time must be the same.
Remove each cylinder, blot it dry in the same way and reweigh it.
Why Blotting the same way each time removes surface solution consistently.
Calculate the percentage change in mass for each and plot it against concentration.
Why Percentage change allows a fair comparison because starting masses differ.

Hazard	How to reduce the risk
The scalpel and cork borer are sharp.	Cut downwards onto a tile, away from fingers.

Concentration in mol/dm³	Start mass in g	End mass in g	Change in mass in %
0.0	2.50	2.95	+18
0.2	2.48	2.63	+6
0.4	2.52	2.42	−4
0.6	2.50	2.20	−12
0.8	2.45	2.06	−16

Percentage change = (end mass − start mass) ÷ start mass × 100. Keep the sign.
Where the line of best fit crosses zero change, the solution has the same concentration as the potato cells (about 0.32 mol/dm³ here).
Below that concentration water enters the cells by osmosis; above it water leaves.

Comparing raw changes in grams when the cylinders started at different masses.
Instead Always compare percentage change.
Naming both variables when asked for the independent variable.
Instead The independent variable is only the one you change: the concentration.
Saying sugar moves into the cells.
Instead It is water that moves, from the more dilute to the more concentrated solution.