Diffusion: Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.

The idea

Diffusion is the passive spreading out of particles of a gas or of any substance in solution. Particles move randomly, but because there are more particles in a region of higher concentration, the overall or net movement is from higher to lower concentration. This continues until the particles are evenly spread or the concentration gradient is removed. For example, a drop of ink in water spreads until the colour is uniform, and oxygen diffuses from the air in an alveolus into the blood because the oxygen concentration is higher in the alveolus. Diffusion does not require energy from respiration. The rate of diffusion is affected by factors such as concentration gradient, temperature, surface area and diffusion distance, which are important in exchange surfaces.

What earns the marks

• Defines diffusion as the spreading out of particles of a gas or of any substance in solution.
• States that the net movement is from an area of higher concentration to an area of lower concentration.
• Explains that random particle motion produces an overall net movement even though individual particles move in all directions.
• Describes diffusion as a passive process that does not require energy from respiration.
• Applies diffusion to a named example, such as oxygen moving from an alveolus into the blood or ink spreading through water.
• Identifies factors that affect the rate of diffusion, including concentration gradient, temperature, surface area and diffusion distance.

Common mistakes and how to put them right

• Saying particles stop moving once concentrations are equal.

  Put it right: Particles continue to move randomly, but there is no net movement because the concentrations are equal.

• Defining diffusion only for gases.

  Put it right: Diffusion also occurs for particles of any substance in solution.

• Stating that diffusion moves particles from low to high concentration.

  Put it right: Net diffusion is from higher to lower concentration down a concentration gradient.

• Claiming diffusion requires energy.

  Put it right: Diffusion is passive and relies on the kinetic energy of particles, not ATP from respiration.

Exam tips

• Include the phrase net movement from higher to lower concentration in definitions to show that you understand the overall direction despite random motion.
• When explaining a rate of diffusion, link each factor to particle movement, for example a steeper concentration gradient means more particles move in one direction per unit time.
• Use a familiar example such as a smell spreading across a room or oxygen entering the blood to make your explanation concrete.
• Avoid saying particles move because they are pushed by other particles; describe random motion and the statistical result of more particles in one region.