Diffusion is the net movement of a substance from an area of high concentration to an area of low concentration. This difference in concentration is called the concentration gradient.

Increasing the rate of diffusion[edit | edit source]

  • Distance - a shorter distance will increase the rate of diffusion. This is why the walls of structures that are adapted for diffusion, such as alveoli and capillaries, are so thin.
  • Concentration gradient - A greater concentration gradient will mean the rate of diffusion increases. This concentration gradient is kept large by removing the substance as it moves across the place where it diffuses.
  • Size of molecules - smaller molecules diffuse quicker.
  • Surface area - a greater surface area is better for fast diffusion. There are many alveoli in the lung and therefore have a very large surface area.
  • Temperature - since molecules have a greater amount of kinetic energy at a higher temperature, they will therefore diffuse quicker.

Diffusion and living organisms[edit | edit source]

Diffusion is very important to all living organisms.

Diffusion of oxygen from alveoli to red blood cells[edit | edit source]

There are thousands of tiny air sacs (alveoli) in humans' lungs, around which are wrapped tiny capillaries. Inside these capillaries are many red blood cells. Oxygen molecules move randomly around inside the alveoli. Some move through the thin walls of the alveolus and then through the capillary wall. Since there are a lot of oxygen molecules inside the alveoli and not many inside the red blood cells; therefore the net movement of oxygen is from the alveoli to the blood. The oxygen has moved down a concentration gradient by diffusion.

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