Diffusion: Students should be able to explain the need for exchange surfaces and a transport system in multicellular organisms in terms of surface area to volume ratio.

The idea

Multicellular organisms are large, so diffusion alone is too slow to supply every cell. As an organism gets bigger, its volume rises faster than its surface area, so the surface area to volume ratio falls. A cube of side 1 cm has a surface area of 6 cm² and a volume of 1 cm³, a ratio of 6:1; a cube of side 3 cm has a surface area of 54 cm² and a volume of 27 cm³, a ratio of 2:1. The larger cube therefore has less surface per unit of living tissue, and the distance from the outside to the centre is greater. Exchange surfaces such as the lungs and small intestine increase the area available for diffusion, while a transport system moves substances quickly to and from all cells.

What earns the marks

• States that as size increases, volume increases faster than surface area, so the surface area to volume ratio decreases.
• Explains that a low surface area to volume ratio means diffusion over the whole body would be too slow to meet the demands of all cells.
• Explains that a greater diffusion distance in a large organism slows the movement of substances to the centre.
• Describes how exchange surfaces increase the surface area available for diffusion of oxygen, carbon dioxide, nutrients or waste.
• Explains that a transport system, such as the circulatory system, carries substances rapidly between exchange surfaces and all body cells.
• Uses a numerical example, such as comparing cubes of different side length, to support the argument.

Common mistakes and how to put them right

• Saying that surface area decreases as an organism grows.

  Put it right: Surface area increases, but volume increases faster, so the ratio falls.

• Confusing surface area to volume ratio with surface area alone.

  Put it right: The ratio compares area with volume, not just the size of the surface.

• Claiming that large organisms have no diffusion at all.

  Put it right: Diffusion still occurs, but it is too slow to supply the whole organism on its own.

Exam tips

• Always link the ratio to a consequence: a low ratio means diffusion is too slow, which is why exchange surfaces and transport are needed.
• Use a quick calculation, such as 6:1 for a 1 cm cube and 2:1 for a 3 cm cube, to make the explanation concrete.
• Write the ratio as surface area to volume and keep the two quantities clearly separate in your answer.