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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. 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:

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 instance 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.