Diffusion is the movement of particles (atoms, ions or molecules) from a region in which they are in higher concentration to regions of lower concentration. A good example of diffusion is food colouring. If you place a drop of red food colouring in a beaker of water eventually the entire beaker of water will have a red tint. The food colouring moved through the water until it was equally distributed throughout the beaker. Diffusion takes place along a concentration gradient. A concentration gradient exists until the diffused substance is evenly distributed.

Other everyday examples of diffusion are:

1. Sugar will diffuse through tea until the entire cup of tea is sweet. (We stir the tea to speed up the diffusion.)

2. The odour of food cooking diffuses throughout the kitchen. If you open the kitchen door it will spread into the next room.

The movement of these molecules is said to be passive. No energy is needed to be provided. The natural kinetic energy of the particles supplies the energy.

Examples of diffusion in science are:

1. Carbon dioxide entering the stomata of leaves.

2. Oxygen diffusing out of the stomata and lenticels of leaves.

When the molecules are even throughout a space – it is called EQUILIBRIUM.

Concentration gradient – a difference between concentrations in a space.

Molecules will always move down the concentration gradient, toward areas of lesser concentration. Think of food coloring that spreads out in a glass of water, or air freshener sprayed in a room.


Osmosis is a special example of diffusion. It is the diffusion of a substance through a semipermeable membrane from a more dilute solution to a more concentrated solution. This process is alsopassive since no external energy is needed.

A semipermeable membrane is a barrier that permits the passage of some substances but not others. Cell membranes are described as selectively permeable because not only do they allow the passage of water but also allow the passage of certain solutes (dissolved substances).

Some major examples of osmosis:

  1. Absorption of water by plant roots.
  2. Reabsorption of water by the proximal and distal convoluted tubules of the nephron.
  3. Reabsorption of tissue fluid into the venule ends of the blood capillaries.
  4. Absorption of water by the alimentary canal — stomach, small intestine and the colon.

Osmosis – the diffusion of water (across a membrane).

Water will move in the direction where there is a high concentration of solute (and hence a lower concentration of water.

A simple rule to remember is: Salt Sucks!

Salt is a solute, when it is concentrated inside or outside the cell, it will draw the water in its direction. This is also why you get thirsty after eating something salty

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