Diffusion: the surface area of the membrane.

The idea

The surface area of the membrane is a factor that affects the rate of diffusion. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. A larger surface area of membrane provides more space through which particles can pass in a given time, so the rate of diffusion increases.

For example: The villi in the small intestine have a large surface area to absorb digested food molecules quickly, and alveoli in the lungs provide a large surface area for gas exchange.

What earns the marks

• States that increasing the surface area of the membrane increases the rate of diffusion.
• Explains that a larger surface area provides more space for particles to cross in a given time.
• Links larger surface area to more particles diffusing per unit time.
• Uses a suitable example, such as villi in the small intestine or alveoli in the lungs.
• Describes diffusion as net movement from higher to lower concentration.
• Recognises that surface area is one of several factors affecting diffusion rate.

Common mistakes and how to put them right

• Thinking that surface area changes the concentration gradient.

  Put it right: Surface area affects the rate, while the concentration gradient is a separate factor.

• Confusing surface area with volume.

  Put it right: Surface area is the total area of the membrane available for diffusion, not the space inside the cell.

• Assuming that a larger surface area always means faster diffusion regardless of other factors.

  Put it right: Temperature, concentration gradient and diffusion distance also affect the rate.

• Believing that surface area affects only the direction of diffusion.

  Put it right: It affects the rate at which particles cross the membrane.

Exam tips

• Link surface area to the number of particles crossing the membrane per unit time.
• Use a named biological example, such as villi or alveoli, to show how surface area aids diffusion.
• When comparing structures, state which has the larger surface area and explain the effect on diffusion rate.
• Remember to mention that diffusion is passive and does not require energy.