What Is Osmosis? GCSE Biology Explained

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

This content applies to both Foundation and Higher Tier AQA GCSE Biology. It is also shared with Combined Science.

How osmosis works

A dilute solution contains a high concentration of water and a low concentration of dissolved substances, called solutes. A concentrated solution contains less water and more solute.

Water molecules move randomly in both directions across the membrane. However, the net movement — the overall movement after considering both directions — is towards the more concentrated solution.

A partially permeable membrane allows some small molecules, such as water, to pass through but prevents some larger molecules from crossing.

Osmosis in plant cells

When plant tissue is placed in a dilute solution, water moves into its cells by osmosis. The cells gain water and become firm, so the tissue usually increases in mass.

In a concentrated solution, water moves out of the cells. The cells lose water and the tissue usually decreases in mass.

In the required practical

In AQA's osmosis required practical, equal-sized pieces of plant tissue are placed in a range of salt or sugar solution concentrations. Their masses are measured before and after. Keep variables such as time, temperature and tissue size the same so concentration is the factor being investigated.

Worked example

A potato cylinder has an initial mass of $5.0g$ and a final mass of $5.8g$.

The mass increased by:

$$5.8-5.0=0.8g$$

The percentage gain in mass is:

$$\frac{0.8}{5.0}\times 100=16\%$$

This suggests that water entered the potato cells by osmosis because the surrounding solution was more dilute than the cell contents.

Common exam mistake

Do not describe osmosis as the movement of any substance. Osmosis only refers to water moving through a partially permeable membrane. Diffusion can involve other particles and does not always require a membrane.