Particles - molecules and ions - in a liquid and a gas move continuously. Because of this movement, particles will spread themselves evenly throughout a liquid or a gas.

If there is a situation where particles of a substance are in a higher concentration, they will therefore move from this region to where they are in a lower concentration. This is called diffusion.

It is important to remember that the particles:

  • will move in both directions, but there will be a net movement from high to low concentration
  • will end up evenly spread throughout the liquid or gas, but will continue to move

Importance to life

Diffusion is crucial in both single-celled and multi-celled organisms. In both single-celled organisms like prokaryotic bacteria and eukaryotic algae, oxygen and carbon dioxide molecules move by diffusion through cell membranes. Larger multi-cellular organisms such as vertebrates have specially adapted exchange surfaces, like lungs or gills. In these organisms, diffusion is still vital because it is this process that allows carbon dioxide and oxygen to move between the exchange surfaces.

Examples of diffusion in biological systems

Some substances move into and out of living cells by diffusion.

Diffusion in a leaf

Diagram showing the process of diffusion in a leaf

Diffusion in the lungs

Diagram showing how gases pass through the alveoli and oxygen is transported around the bodyCarbon dioxide is carried by blood plasma. Oxygen is carried by haemoglobin in red blood cells. The haemoglobin inside the red blood cells becomes brighter red in colour when oxygen binds to it.