Diffusion, osmosis and active transport

Where this is in your specification

Spec points: AQA 4.1.3.1 to 4.1.3.3 (required practical 3; Trilogy required practical 2), Edexcel Topic 1, OCR B2.1

Diffusion

Diffusion is the spreading out of the particles of a gas or a substance in solution, resulting in a net movement from an area of higher concentration to an area of lower concentration. Oxygen diffuses into cells and carbon dioxide out of them; urea diffuses from cells into the blood.

• A bigger concentration gradient (difference) makes it faster.
• A higher temperature makes it faster.
• A larger surface area of membrane makes it faster.

Surface area to volume ratio

Small organisms have a large surface area compared with their volume, so diffusion across their surface is enough. Large organisms need exchange surfaces and transport systems. Good exchange surfaces have a large surface area, a thin membrane (a short distance), a good blood supply and, in animals, are ventilated: think of the alveoli in the lungs and the villi in the small intestine.

Osmosis

Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane. In a more concentrated solution an animal cell loses water and shrinks; in pure water it can swell and burst. A plant cell's wall stops it bursting: it becomes firm (turgid).

The osmosis practical

• Cut equal-sized cylinders of potato and blot and weigh each one.
• Put each in a different concentration of sugar or salt solution for the same time.
• Blot dry and reweigh, then work out the percentage change in mass.
• Plot percentage change in mass against concentration. Where the line crosses zero, the solution has the same concentration as the potato cells.

Active transport

Active transport moves substances from a more dilute solution to a more concentrated one, against the concentration gradient. It needs energy from respiration. Root hair cells use it to take in mineral ions from dilute soil water, and the gut uses it to absorb sugar into the blood even when the blood has more sugar.

Worked examples

Common mistakes

• Describing osmosis without saying 'water' and 'partially permeable membrane'.
• Saying water moves from a high concentration solution. It moves from the dilute solution (high water concentration) to the concentrated one.
• Forgetting that active transport needs energy from respiration.
• Explaining percentage change as 'more accurate'. It is used because the cylinders may start at different masses, so it lets you compare them fairly.