Diffusion: having a large surface area

The idea

A large surface area increases the number of places where particles can cross a membrane at the same time, so the rate of diffusion rises. This is why exchange surfaces are folded, flattened or branched. Alveoli give the lungs a huge area for oxygen and carbon dioxide, villi and microvilli increase the area for absorbing digested food in the small intestine, and root hair cells extend the area for taking in water and mineral ions. When you compare surfaces, remember that rate depends on area as well as concentration gradient and thickness, so a large area alone is not enough; it must be combined with a short diffusion path and a maintained gradient.

What earns the marks

• A larger surface area provides more space for particles to diffuse across at the same time, increasing the rate of exchange.
• Folding, branching or flattening increases the area without greatly increasing volume, as in alveoli, villi and root hair cells.
• Surface area is one factor affecting diffusion rate; concentration gradient and thickness of the exchange surface also matter.
• Calculating or comparing surface area to volume ratios shows why small, thin structures exchange materials effectively.

Common mistakes and how to put them right

• Saying a large surface area increases the distance particles travel.

  Put it right by stating it increases the number of particles crossing per unit time.

• Treating surface area as the only factor.

  Put it right: Explain that a steep concentration gradient and a thin surface are also needed for a fast rate.

• Confusing total surface area with surface area to volume ratio.

  Put it right: State that the ratio compares area with volume and predicts how easily an organism exchanges materials.

Exam tips

• Quote a named example, such as alveoli or villi, and state how its shape increases surface area.
• Use comparative language, for example a larger surface area allows a faster rate of diffusion.
• When asked to explain, link the large area to more collisions or more particles crossing per second.