This allows sufficient transport of molecules into and out of the cell to…

The idea

Because a single-celled organism has a large surface area to volume ratio, its cell surface membrane provides enough area for diffusion to supply the whole cell. Substances such as oxygen and glucose diffuse in, and carbon dioxide and other waste products diffuse out, fast enough to meet the organism's metabolic needs. The diffusion distance is short because the cell is small, so molecules do not have far to travel. This is sufficient for a single-celled organism, but a large multicellular organism has a smaller surface area to volume ratio and a longer diffusion distance, so diffusion alone cannot meet its needs; it requires specialised exchange surfaces and a transport system.

What earns the marks

• State that the large surface area to volume ratio provides sufficient membrane area for exchange.
• Explain that oxygen and glucose diffuse into the cell while carbon dioxide and waste products diffuse out.
• Link the short diffusion distance in a small cell to the speed of supply to the cytoplasm.
• State that diffusion alone meets the needs of a single-celled organism without a transport system.
• Contrast this with large multicellular organisms, where a small surface area to volume ratio and long diffusion distance make diffusion alone insufficient.
• Recognise that the rate of diffusion must match the organism's metabolic demand for the transport to be sufficient.

Common mistakes and how to put them right

• Claiming that single-celled organisms do not need to exchange gases.

  Put it right by stating that they still respire and must take in oxygen and remove carbon dioxide.

• Saying diffusion is 'fast' without linking it to the large ratio and short distance.

  Put it right by explaining both reasons.

• Assuming a large organism can rely on diffusion alone.

  Put it right by explaining that its small surface area to volume ratio and long diffusion distance require specialised surfaces and transport systems.

Exam tips

• Use the phrase 'sufficient to meet the needs of the organism' and then justify it with the ratio and diffusion distance.
• Name at least one substance entering and one leaving the cell to show the transport is bidirectional.
• When comparing with a multicellular organism, refer to both surface area to volume ratio and diffusion distance.