Diffusion

Diffusion is the net movement of molecule ions down a ocncentration gradient. When a uniform concentration is reached, equilibrium is established. Molecules still move, but there is not net movement in any particular direction. (Fig. 1)

Rate of diffusion (Flick's law)

(Surface area x Difference in concentration) / Lenght of diffusion path

The rate is also affected by the nature od the membrane (eg. permeability), size and type of molecule.
This law applies to situations in which there is no barrier to the movement of substances. The diffusion is a way of passive transport, that's to say, it does not require the expenditure of energy.

Adaptations of tissues to increase the rate

• Maintaining a steep concentration gradient


• Having a high surface area to volume ratio

Being thin

Cell membranes are partially permeable: many substances can pass through them but others can't.

Facilitated diffusion

It is the passive movement of molecules down a concentration gradient, but involves special carrier proteins in the cell membrane (Fig. 2). It is important because glucose, nucleic acids, amino acids and proteins are not soluble in lipids and don't pass the bilayer of phospholipids. Therefore, this process takes place. Facilitated diffusion of relatively large molecules such as glucose is specific and involves a protein called permease. It changes shape when glucose binds to it, and can transport it in either way although it is always down a concentration gradient. Like enzymes, it can be inhibited.

Surface area to volume relationship

As the size of an organism increases, its surface area to volume ratio decreases. This relationship has important consequences on organisms. Some processes such as the rate of diffusion depende on the surface area, while others such as metabolic rate, depend on volume. Small organisms can satisfy their oxygen needs by simple diffusion. Large organisms, instead, have evolved special respiratory exchange surfaces suchs as lungs, gills and tracheae to compensate for their relatively small body surface areas.

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