Diffusion: In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells for the organism’s needs. The effectiveness of an exchange surface is increased by:

The idea

Multicellular organisms are large, so their outer surface is far too small relative to volume to supply every cell by diffusion alone. Instead, materials are exchanged at specialised surfaces and moved around by organ systems. For example, the lungs present millions of alveoli for gas exchange, the small intestine has villi for absorbing digested food, and the gills of a fish expose many filaments to water. The blood system then transports oxygen, glucose and carbon dioxide between these surfaces and the cells. Each exchange surface is adapted so that sufficient molecules enter or leave to meet the organism's needs, and the next statements list the features that raise effectiveness.

What earns the marks

• Multicellular organisms have a small surface area to volume ratio, so diffusion across the body surface alone cannot supply all cells.
• Specialised exchange surfaces provide a large area in contact with the environment, such as alveoli in lungs or villi in the small intestine.
• Organ systems transport substances between exchange surfaces and cells; in mammals the circulatory system carries oxygen, glucose and carbon dioxide in the blood.
• Exchange must be sufficient for the organism's needs, so larger or more active organisms need more effective surfaces and transport.
• Different organisms have different exchange surfaces matched to their environment, for example gills in fish and leaves in plants.

Common mistakes and how to put them right

• Thinking that diffusion alone is enough in large organisms.

  Put it right by explaining that the surface area to volume ratio is too small, so transport systems are also needed.

• Confusing the exchange surface with the transport system.

  Put it right: State that the surface is where substances enter or leave, while the organ system moves them around the body.

• Assuming all exchange surfaces look the same.

  Put it right by linking each surface to its function, such as thin alveoli for gases and folded villi for absorption.

Exam tips

• Name a specific organism and exchange surface, then state the substance exchanged, for example oxygen into the blood at alveoli.
• Use the phrase surface area to volume ratio when explaining why large organisms need specialised surfaces.
• Link each adaptation to the organism's needs rather than listing features without a function.