Factors affecting the rate of diffusion

If a crystal of a coloured chemical, eg potassium manganate(VII), is placed in water, the particles spread out and mix with the water particles.

  • The potassium manganate(VII) is the solute.
  • The water is the solvent.
  • The potassium manganate(VII) has dissolved.
  • The mixture that results is the solution.
The particles have moved from a region of high concentration in the crystal to a low concentration in the water. This difference in concentration is called a concentration gradient.

The particles have moved from a region of high concentration in the crystal to a low concentration in the water. This difference in concentration is called a concentration gradient. Particles will move down a concentration gradient, from a high concentration to a low concentration.

As well as diffusion occurring between different regions, it also occurs across membranes, between the outside and inside of cells.

The rate of diffusion

The rate of diffusion can be affected by a number of factors:

FactorReason
The concentration gradientThe greater the difference in concentration, the quicker the rate of diffusion.
The temperatureThe higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly.
The surface area of the cell membrane separating the 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 example, for respiration.

For simple multicellularorganisms, such as small plants like mosses, substances diffuse into the leaves and 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 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.

Question

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.