Diffusion: Factors which affect the rate of diffusion are:

The idea

Diffusion is the net, random movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. This statement introduces the factors that change how quickly that net movement happens. The main factors are the concentration gradient, temperature, and the surface area of the exchange surface, together with the diffusion distance and the size or mass of the particles involved. A steeper gradient, a higher temperature, a larger surface area and a shorter distance all tend to increase the rate. Smaller, lighter particles also diffuse faster than larger, heavier ones. In cells and organisms, these factors explain why exchange surfaces such as alveoli and villi are thin, moist and richly supplied with blood.

What earns the marks

• Diffusion is the net movement of particles from higher to lower concentration, driven by random motion.
• A steeper concentration gradient increases the rate of diffusion.
• A higher temperature increases particle kinetic energy and therefore the rate of diffusion.
• A larger surface area over which diffusion can occur increases the rate.
• A shorter diffusion distance increases the rate because particles travel less far.
• Smaller or lighter particles diffuse faster than larger or heavier particles.

Common mistakes and how to put them right

• Thinking diffusion stops when concentrations become equal.

  Put it right: It does not stop, but net movement becomes zero because movement is balanced in both directions.

• Confusing diffusion with active transport.

  Put it right: Diffusion is passive and needs no energy from respiration, whereas active transport uses energy and can move substances against a gradient.

• Assuming only concentration matters.

  Put it right: Temperature, surface area, distance and particle size also affect the rate and should be considered.

Exam tips

• Name the factor and then state the direction of its effect, for example 'a steeper gradient increases the rate'.
• Use comparative language such as 'faster', 'slower', 'steeper' or 'larger' rather than vague words like 'better'.
• Link each factor to a biological example, such as oxygen diffusing into blood in the alveoli, to show understanding.