Diffusion: A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.

The idea

A single-celled organism, such as an Amoeba or a bacterium, is just one cell, so its outer membrane is the only exchange surface it has. Because the cell is very small, its surface area is large compared with its volume, giving a high surface area to volume ratio. Diffusion across the membrane is therefore fast enough to supply oxygen and nutrients to the whole cell and to remove carbon dioxide and other wastes, so no transport system is needed. As a cell gets bigger, volume grows faster than surface area, so the ratio falls and diffusion alone becomes too slow to meet demand. This is why large organisms need specialised exchange surfaces and transport systems.

What earns the marks

• State that a single-celled organism consists of one cell, so the cell surface membrane is the only surface available for exchange with the environment.
• Explain that a small cell has a large surface area compared with its volume, giving a high surface area to volume ratio.
• Link the high ratio to a short diffusion distance and sufficient rate of diffusion of molecules into and out of the cell.
• Conclude that diffusion alone meets the organism's needs, so no specialised transport system or exchange organ is required.
• Contrast with larger organisms, where the surface area to volume ratio is smaller and diffusion alone would be too slow.

Common mistakes and how to put them right

• Saying a single-celled organism has a large surface area in absolute terms.

  Put it right: A large surface area relative to its volume, so the ratio is high.

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

  Put it right: A large organism can have a big surface area but a low ratio because its volume is much greater.

• Claiming diffusion is faster because the cell is warmer or more active.

  Put it right: The key reason is the short diffusion path and high surface area to volume ratio.

Exam tips

• Always write the comparison as a ratio, for example surface area : volume, and state whether it is high or low.
• Use the phrase 'sufficient transport of molecules into and out of the cell' when explaining why diffusion alone is enough.
• If asked to compare, give the single-celled organism a high ratio and a large multicellular organism a low ratio, then link each to whether diffusion alone is sufficient.