8.4B Describe the factors affecting the rate of diffusion, including…

The idea

The rate of diffusion is how quickly particles spread from a region of higher concentration to a region of lower concentration. Three factors named in this specification change that rate. A larger surface area allows more particles to cross at once, so the rate increases. A steeper concentration gradient, meaning a bigger difference in concentration between two regions, drives faster net movement. A shorter diffusion distance means particles travel less far, so diffusion is quicker. These factors combine in exchange surfaces: alveoli have a huge total surface area, thin one-cell-thick walls give a short distance, and breathing plus blood flow keep the gradient steep.

For example: Doubling the surface area tends to increase the rate, while thickening a barrier increases distance and slows diffusion.

What earns the marks

• A larger surface area increases the rate of diffusion because more particles can cross the surface at once.
• A steeper concentration gradient increases the rate of diffusion.
• A shorter diffusion distance increases the rate of diffusion.
• Increasing diffusion distance or reducing surface area slows the rate of diffusion.

Common mistakes and how to put them right

• Thinking a steeper gradient slows diffusion.

  Put it right: A greater concentration difference increases the rate.

• Confusing surface area with diffusion distance, for example saying a thicker wall increases the rate.

• Assuming only one factor matters, when surface area, gradient and distance all affect the rate together.

Exam tips

• Identify which single factor the question changes, then decide whether that change raises or lowers the rate.
• Use the relationship directly: more surface area and steeper gradient speed diffusion, while greater distance slows it.
• Reject options that describe a factor but give the wrong direction of effect.