Diffusion: the difference in concentrations (concentration gradient)

The idea

The concentration gradient is the difference in concentration between two regions. Particles move randomly, but because there are more particles in the region of higher concentration, there is a net movement towards the region of lower concentration. The steeper the gradient, the faster the net rate of diffusion. If the gradient is zero, movement continues but net movement is zero. In the lungs, oxygen diffuses from alveolar air into blood because alveolar oxygen concentration is higher than that in the blood; carbon dioxide diffuses the other way. Breathing and blood flow maintain these gradients so exchange continues.

What earns the marks

• The concentration gradient is the difference in concentration between two regions.
• Net diffusion occurs from higher concentration to lower concentration.
• A steeper gradient produces a faster net rate of diffusion.
• When concentrations are equal, net movement is zero even though particles still move randomly.
• Maintaining a steep gradient, for example by ventilation or blood flow, keeps diffusion efficient.

Common mistakes and how to put them right

• Saying particles 'want' to move down the gradient.

  Put it right: The movement is random and the net effect is down the gradient.

• Believing diffusion stops at equilibrium.

  Put it right: Particles still move, but net movement is zero.

• Reversing the direction: net diffusion is from higher to lower concentration, not the reverse.

Exam tips

• Define the gradient as a difference in concentration, not just as 'high concentration'.
• State the direction of net movement clearly: from higher to lower concentration.
• Use a named example, such as oxygen moving from alveolar air into blood, to show the gradient in action.