The rate of diffusion

The rate of diffusion can be affected by several factors:

FactorHow the factor affects the rate of diffusion
Concentration gradientThe greater the difference in concentration, the quicker the rate of diffusion
TemperatureThe higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly
Surface area of the cell membrane separating different regionsThe greater the surface area, the faster the rate of diffusion

Diffusion, surface area and volume

For a bacterium, substances diffuse into and out of the bacterial cell across its surface. Once inside, because of the bacterium's size, substances will need to diffuse 1 μm or less to where they are needed.

For simple multicellular organisms, such as small plants such as mosses, substances diffuse into the leaves and simple roots over their surface. Again, once inside the plant, they don’t need to move far. Substances move into and around the moss plants by diffusion and osmosis.

Simple organisms therefore take in substances over their body surface. Their needs are determined by their volume. As organisms increase in size, their surface area does not increase at the same rate as their volume. For example, the surface area to volume ratio of a puppy is several times greater than that of an adult dog.


Suggest why puppies are more at risk of losing body heat than adult dogs.

Dogs lose heat over their body surface. Puppies have a larger surface area to volume ratio than adult dogs, so will lose heat more readily.