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 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 can be affected by a number of factors:
|The concentration gradient||The greater the difference in concentration, the quicker the rate of diffusion.|
|The temperature||The 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 regions||The greater the surface area, the faster the rate of diffusion.|
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.
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.
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.