Students should be able to explain how different factors affect the rate…

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, as a result of random motion. To explain how factors affect its rate, link each factor to particle behaviour or to the geometry of the exchange surface. A steeper concentration gradient, a higher temperature, or a larger surface area of membrane each increases the rate. For example, oxygen diffuses from alveolar air into blood because alveolar air has a higher oxygen concentration than deoxygenated blood; breathing keeps this gradient steep. Explaining means giving a causal mechanism, not merely naming the factor, and applying it to a named context such as a root hair cell or a villus.

What earns the marks

• State that diffusion is the net movement of particles down a concentration gradient, from higher to lower concentration, caused by random particle motion.
• Explain that a steeper concentration gradient increases the rate because more particles move in one direction than the other, giving a greater net movement per unit time.
• Explain that a higher temperature increases the rate because particles have more kinetic energy and move faster, so collisions and net movement are more frequent.
• Explain that a larger surface area of membrane increases the rate because more particles can cross the membrane at the same time.
• Apply the factors to a named biological context, such as oxygen uptake across alveoli or absorption of mineral ions by root hair cells.
• Use the relationship rate proportional to surface area multiplied by concentration difference divided by distance, treating it as a qualitative guide rather than a calculation requirement.

Common mistakes and how to put them right

• Saying particles 'want' to move or move 'to be equal', which is teleological.

  Put it right by stating that random motion produces net movement down a concentration gradient.

• Confusing diffusion with active transport.

  Put it right by noting that diffusion is passive and requires no energy from respiration.

• Claiming that a higher temperature slows diffusion because particles 'stick together'.

  Put it right by explaining that increased kinetic energy speeds up particle movement and raises the rate.

Exam tips

• Name the factor, state whether the rate increases or decreases, then give the particle-level reason in one linked sentence.
• When a question supplies data, quote a relevant value or trend from the table or graph before explaining it.
• Use a named example such as gas exchange in alveoli to show application rather than describing diffusion in the abstract.