Diffusion: a membrane that is thin, to provide a short diffusion path

The idea

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. The rate of diffusion depends on several factors, including the distance over which particles must travel. A thin membrane reduces this distance, so particles cross it quickly and the diffusion path is short. For example, the alveolar epithelium separating air in an alveolus from blood in a capillary is extremely thin, so oxygen diffuses rapidly into the blood and carbon dioxide diffuses rapidly out. Similarly, villi in the small intestine have thin walls, allowing digested nutrients to reach the blood quickly. A short diffusion path therefore increases the rate of diffusion and helps exchange surfaces work efficiently.

What earns the marks

• Diffusion is the net movement of particles down a concentration gradient, from higher to lower concentration.
• A thin membrane means particles have a shorter distance to travel between two regions.
• A shorter diffusion path increases the rate at which particles cross the exchange surface.
• The thin alveolar epithelium is an example where a short diffusion path speeds up gas exchange.
• Thin walls of villi in the small intestine allow nutrients to diffuse rapidly into the blood.
• A thin membrane is one adaptation of an efficient exchange surface, alongside a large surface area and a steep concentration gradient.

Common mistakes and how to put them right

• Thinking that a thin membrane increases the concentration gradient.

  Put it right: Thinness reduces the diffusion distance, while the gradient depends on concentration differences.

• Stating that diffusion stops when a membrane is thin.

  Put it right: Diffusion continues until concentrations are equal, and a thin membrane simply makes this happen faster.

• Writing that particles are pushed through the membrane by pressure.

  Put it right: Particles move randomly and the net movement is down the concentration gradient.

Exam tips

• Link the word thin directly to short diffusion path in your answer, because examiners look for the cause-and-effect relationship.
• Use a named example such as the alveolus or a villus to show that you can apply the idea, not just recall it.
• If asked to explain an adaptation, state the feature, then explain how it increases the rate of diffusion, then link it to the organism's need.