Diffusion: Students should be able to explain how different factors affect the rate of diffusion.

The idea

The rate of diffusion depends on several factors. A steeper concentration gradient increases the rate because more particles move in a given time. A shorter diffusion distance increases the rate because particles have less far to travel. A larger surface area increases the rate because more particles can cross at once. Higher temperature increases the rate because particles have more kinetic energy and move faster. In living organisms, exchange surfaces often have a large surface area, a thin membrane and a good blood supply to maintain steep gradients.

For example: Alveoli are numerous and thin-walled, giving a large surface area and short diffusion distance for oxygen and carbon dioxide.

What earns the marks

• A steeper concentration gradient increases the rate of diffusion because more particles move down the gradient in a given time.
• A shorter diffusion distance increases the rate of diffusion because particles travel less far to cross the exchange surface.
• A larger surface area increases the rate of diffusion because more particles can cross the surface at the same time.
• A higher temperature increases the rate of diffusion because particles gain kinetic energy and move faster.
• Exchange surfaces in organisms are adapted by having a large surface area, a thin membrane and a good blood supply to maintain steep concentration gradients.
• The rate of diffusion can be compared by measuring how quickly a substance moves or how much crosses a surface in a set time.

Common mistakes and how to put them right

• Saying that a larger surface area decreases diffusion because there is more to cover.

  Put it right by explaining that a larger surface area allows more particles to cross at once, increasing the rate.

• Confusing concentration gradient with diffusion distance.

  Put it right by stating that gradient is the difference in concentration, while distance is how far particles must travel.

• Thinking that temperature affects only the particles on one side.

  Put it right by explaining that all particles gain kinetic energy, so both sides move faster and the rate increases.

Exam tips

• For each factor, state the direction of change and then explain why the rate changes.
• Use comparative language such as 'steeper', 'shorter' or 'larger' rather than simply naming the factor.
• Apply the factors to a named exchange surface, such as alveoli or villi, to show understanding in context.