Dissolved or gaseous substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that allows this to happen.

Diffusion occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration. Diffusion happens when the particles are free to move. This is true in gases and for particles dissolved in solutions - but diffusion does not occur in solids.

Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example.

Diffusion can be observed by placing potassium permanganate into a beaker of water

Diffusion experiment

Potassium permanganate is placed into a beaker of water

Examples of diffusion in living organisms

Products of digestion, dissolved in water, can pass across the wall of the small intestine by diffusion. Their concentration is higher in the small intestine than their concentration in the blood, so there is a concentration gradient from the intestine to the blood.

Oxygen and carbon dioxide, dissolved in water, are exchanged by diffusion in the lungs:

  • oxygen moves down a concentration gradient from the air in the alveoli to the blood
  • carbon dioxide moves down a concentration gradient from the blood to the air in the alveoli

The dissolved substances will only continue to diffuse while there is a concentration gradient.

Dexoygenated red blood cells enter the capillary.  CO2 is absorbed into the alveoli and O2 passes from the alveoli into the blood cells.  When the blood cells leave they have been oxygenated.Gas exchange in the lungs happening in the alveoli