Diffusion

In the syllabus and the exam

Diffusion is one of the three ways particles move across the cell membrane that the syllabus compares directly against each other, and exam questions frequently present all three (diffusion, osmosis and active transport) in a single table or structured question asking candidates to state which process is occurring and justify the choice using the given information. Candidates must be able to identify diffusion from a scenario describing any particle, such as oxygen, carbon dioxide or a dissolved mineral ion, moving down its own concentration gradient without the use of energy, and to explain how factors such as temperature, surface area and the steepness of the concentration gradient affect its rate. The most common source of lost marks is failing to distinguish diffusion from osmosis: diffusion applies to any type of particle, while osmosis is reserved specifically for the movement of water molecules across a partially permeable membrane, so an answer describing water movement should use the term osmosis, not diffusion, once a membrane is involved. Another frequent error is describing diffusion as requiring energy from the cell; because it moves particles down a concentration gradient, diffusion is a passive process, and stating that it needs ATP is a factual error that examiners specifically look for. Everyday examples such as the smell of food spreading across a room, or perfume becoming noticeable moments after it is sprayed, are commonly used in Paper 1 multiple-choice items to test whether candidates can recognise diffusion outside a strictly biological context.

Example

When a drop of purple potassium permanganate is placed in still water, the colour spreads outward over time by diffusion, moving from where it is concentrated to where it is more dilute.

Related terms

OsmosisActive transport

Source:SRC-DSKP-EN