Fick's Law

Fick's Law describes the relationship between the rate of diffusion and the three factors that affect diffusion. It states that 'the rate of diffusion is proportional to both the surface area and concentration difference and is inversely proportional to the thickness of the membrane'.

Fick's law can be written as:

\[\text{Rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{thickness of membrane}}\]

\(\propto\) means 'is proportional to'.

The rate of diffusion will double if:

  1. surface area or concentration difference is doubled or
  2. thickness of the exchange membrane is halved.

All of the exchange surfaces consist of cell membranes which are very thin. This explains why diffusion is very fast across membranes.


The cell membrane has a thickness of 10 nm.

Describe what happens to the rate of diffusion of oxygen into the cell if the thickness changes to 20 nm.

The rate of diffusion will halve.

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