Diffusion

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A diagram of diffusion happening. The first diagram shows particles in a liquid. The second shows the same liquid a few seconds later after the particles have spread out

Diffusion is a physical process where molecules of a material move from an area of high concentration (where there are many molecules) to an area of low concentration (where there are fewer molecules)[1]until it has reached equilibrium (molecules evenly spread).

Diffusion usually happens in a solution in gas or in a liquid. It is possible to see diffusion happening when two liquids are mixed in a transparent container. It describes the constant movement of particles in all liquids and gases. These particles move in all directions bumping into each other. Diffusion can only work with gases and liquids.

Examples[change | change source]

  • A sugar cube is left in a beaker of water for a while.
  • The smell of ammonia spreads from the front of the classroom to the back of the room.
  • Fumes of perfume rise from the bottle when the top is removed.
  • Food coloring dropped on the beaker spreads out.
  • the smell of food spread in the whole house

Molecules tend to move from places of high concentration to places of low concentration, just by moving randomly. For example, there is more oxygen in a lung than there is oxygen in the blood so oxygen molecules will tend to move into the blood. Similarly, there is more carbon dioxide molecules in the blood than in the lung so carbon dioxide molecules will tend to move into the lung. It happens in cell biology, where small molecules simply diffuse through the cell membrane, but larger molecules only get through by using energy: see active transport.

The random movement of fluid molecules makes them spread out until a boundary stops them.

Diffusion is a passive process, therefore does not require energy as it occurs down a concentration gradient.

Rate of Diffusion[change | change source]

Diffusion is affected by:

  • the concentration gradient - diffusion will be greater where gradient is larger
  • the temperatures - diffusion will happen faster when temperatures are higher as there is more kinetic energy
  • the surface area - diffusion will be greater where it's greater
  • the diffusion distance - diffusion will be greater where there is a short diffusion distance

Surface Area and Volume[change | change source]

In small unicellular organisms, simple diffusion is sufficient to enable the exchange of molecules for a suitable rate for survival. It is desirable to have a high surface area to volume ratio.

However, for multicellular organisms, they can no longer rely on simple diffusion - larger quantities need to be moved and distances become greater. They have evolved to have internal structures and systems for rapid distribution movement. eg. in humans the Lung allows for diffusion and the same happens in plants with the leaf.

Related pages[change | change source]

References[change | change source]

  1. "BBC - GCSE Bitesize: Diffusion".