Osmosis

Cell biology · Transport in cells · note 4 of 5

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OsmosisSpec 4.1.3.2

In short

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. In the required practical, plant tissue gains mass in dilute solutions as water moves in, and loses mass in concentrated solutions as water moves out. Where the mass does not change, the solution matches the concentration inside the cells.

Water may move across cell membranes via osmosis.

Osmosis: The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

In osmosis, water moves from the dilute solution to the concentrated solution through a partially permeable membrane.

Common mistake:

Osmosis is only about water. A partially permeable membrane lets small molecules such as water through but not larger solute molecules.

Required practical:

Investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue. (Required practical 3 in Triple and required practical 2 in Combined Science: Trilogy.)

Cut identical cylinders of plant tissue, for example potato, using a cork borer, and trim them to the same length.
Blot each cylinder dry and measure its mass using a balance.
Place each cylinder in a different concentration of sugar or salt solution, including one in pure water. Use the same volume of solution in each tube.
Leave for the same length of time at the same temperature.
Remove the cylinders, blot them dry in the same way and measure their mass again.
Calculate the percentage change in mass for each cylinder.

Independent variable: the concentration of the solution.
Dependent variable: the change in mass (as a percentage).
Control variables: type and size of plant tissue, volume of solution, temperature, time left in the solution, how the tissue is dried.

In dilute solutions, water moves into the plant cells by osmosis and the mass increases. In concentrated solutions, water moves out of the cells and the mass decreases. Where the mass does not change, there is no net movement of water, so the solution is the same concentration as the inside of the cells.

Safety: take care when using the cork borer and scalpel, cutting on a tile and away from your hand.

Calculations and graphs

percentage change = (final mass − initial mass) ÷ initial mass × 100

A positive answer is a gain in mass and a negative answer is a loss in mass. You should be able to use simple compound measures of rate of water uptake, for example the change in mass or volume per minute (g/min or cm³/min).

Percentage change in mass

A potato cylinder has a mass of 4.0 g at the start and 3.4 g after 30 minutes in a salt solution. Calculate the percentage change in mass and the rate of mass change in g per minute.

Change in mass = 3.4 − 4.0 = −0.6 g.
Percentage change = −0.6 ÷ 4.0 × 100 = −15%.
Rate = −0.6 ÷ 30 = −0.02 g per minute.

Answer: −15% (a 15% loss in mass), a rate of −0.02 g/min.

Plot the concentration of solution on the x-axis (the independent variable) and the percentage change in mass on the y-axis. Draw a smooth line or a line of best fit through the points. The point where the line crosses the x-axis shows the concentration with no change in mass.

Maths skill:

Calculate percentage change using the initial mass, not the final mass, as the denominator. Show the sign: a loss is negative.

Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026

Frequently asked questions

How does osmosis differ from diffusion?

Osmosis is the diffusion of water only, from a dilute solution to a concentrated solution through a partially permeable membrane. Diffusion is the net movement of particles of any gas or dissolved substance from a higher to a lower concentration. Neither process needs energy from respiration, whereas active transport does.

How does osmosis affect plant cells?

In a dilute solution, water moves into plant cells by osmosis and the tissue gains mass. In a concentrated solution, water moves out of the cells and the tissue loses mass. Where the solution has the same concentration as the inside of the cells, there is no net movement of water, so the mass does not change.

Why is the surface area to volume ratio important for cells?

A single-celled organism has a large surface area to volume ratio, so diffusion across its surface can supply its needs. As organisms get bigger, volume increases faster than surface area and the ratio falls. Multicellular organisms cannot rely on diffusion across their outer surface alone, so they need exchange surfaces and a transport system.

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