Particles in liquids and gases are always moving randomly. Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated, until they are spread evenly.

Cells rely on diffusion. Oxygen diffuses from the blood into cells, where it is used in respiration, and carbon dioxide diffuses out. Diffusion is faster when the temperature is higher, the concentration difference is larger and the distance is shorter.

The cell membrane is partially permeable: it lets small molecules such as water through but stops larger ones. Osmosis is the movement of water across a partially permeable membrane from a dilute solution (more water) to a concentrated solution (less water).

Plant cells that take in water become firm, or turgid, because the cell wall resists. Plant cells that lose water become soft, or flaccid. Animal cells have no cell wall, so they can burst if too much water enters.

Students cut three potato strips, each with a mass of 5.0 g. They placed one strip in each solution for 30 minutes, dried them gently and measured their mass again.

Results: pure water 5.5 g; medium salt solution 4.6 g; strong salt solution 4.2 g.

Percentage change = (final mass − starting mass) ÷ starting mass × 100