Structures and Functions in Living Organisms · 6 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 22% of your exam marks.
One of the most tested topics; osmosis definitions and explanations appear on virtually every paper.
To investigate how the concentration of an external sucrose solution affects osmosis in plant cells.
Cylinders of fresh potato contain plant cells with cytoplasm at some particular water concentration. If a cylinder is placed into a solution of higher water concentration (dilute sucrose, or distilled water), water moves into the potato by osmosis, and the cylinder gains mass. If it is placed in a solution of lower water concentration (concentrated sucrose), water moves out, and the cylinder loses mass. Somewhere in the middle there is a sucrose concentration that exactly matches the water concentration inside the potato cells, and the cylinder shows no change in mass.
percentage change in mass = ((final mass − initial mass) ÷ initial mass) × 100
Example — A potato cylinder has an initial mass of 2.50 g. After being left in 0.2 mol/dm³ sucrose solution for an hour, its mass is 2.65 g. Calculate its percentage change in mass.
Example — A second cylinder, also starting at 2.50 g, is placed in 0.8 mol/dm³ sucrose. After an hour it weighs 2.20 g. Calculate its percentage change in mass.
Plot percentage change in mass on the y-axis against sucrose concentration on the x-axis. The result is a straight-ish line falling from positive values on the left (low sucrose, water gained) through zero (no change) and on to negative values on the right (high sucrose, water lost).
The point where the line crosses the x-axis (zero change in mass) gives the sucrose concentration that matches the water concentration inside the potato cells. This is a clever way of measuring something you cannot weigh directly.
Different cylinders start at slightly different masses no matter how carefully you cut them. Using percentage change cancels out those starting differences and lets you compare cylinders fairly.