The need for diffusion


Particles - molecules or ions - in a liquid and a gas are constantly moving randomly. Because of this movement, particles will spread themselves evenly throughout.

If particles of a substance are in a higher concentration compared to another area, their random movement leads to a net movement of particles from this region to the region of 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 not stop moving

The importance of diffusion in biology

Cells carry out chemical reactions, such as respiration and synthesis of biomass. For example, in aerobic respiration cells need a supply of glucose and oxygen. The process also creates carbon dioxide, a toxic substance that needs to be removed from cells.

Diffusion is one of the processes that is used to get substances into and out of cells. Substances also need to enter or leave whole organisms and this often requires diffusion too.

Some examples of diffusion in biological systems

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

In a leaf

Carbon dioxide is needed in the leaf for photosynthesis, and so diffuses into the leaf from a region of higher concentration in the air.

Diagram showing the process of diffusion in a leaf

In the lungs

Oxygen from the air passes into the bloodstream in the lungs through structures called alveoli. It diffuses to a region of lower concentration in the bloodstream.

Diagram showing how gases pass through the alveoli and oxygen is transported around the body

In liver cells

The liver breaks down excess amino acids, caused by too much protein in the diet. This produces urea which is a toxic substance. Urea diffuses from a high concentration in liver cells to a lower concentration in the bloodstream.

Some examples of diffusion in biological systems