Surface area to volume ratio (3.3.1)

Small and single–celled organisms have:

• High surface area to volume (SA:V) ratio – can rely on diffusion to obtain oxygen and nutrients and remove wastes.
• Low metabolic activity – requirements for oxygen and nutrients are lower and production of waste is slower.

Large organisms have:

• Low SA:V ratios – diffusion cannot provide the organism with enough oxygen and nutrients.
• Higher metabolic activity – higher requirements for oxygen and nutrients and faster production of toxic waste.

Larger organisms require specialised exchange surfaces to meet their oxygen, nutrient and waste removal needs.

The larger the SA:V ratio, the faster the rate of diffusion can occur.

Organisms can often be shapes that make it difficult to calculate their SA:V ratio. Their shapes are often modelled as simple geometric shapes such as spheres, cylinders or cubes to enable their SA:V ratio to be estimated more easily.

Organisms with higher metabolic rates require a higher rate of transport of substances. These organisms are adapted with higher surface area to volume ratios to increase the rate at which oxygen and nutrients are absorbed, and waste products are removed.

Efficient exchange surfaces have the following features to increase the rate of diffusion:

• high surface area
• thin layers
• maintaining a diffusion gradient
• selectively permeable.

The factors that affect the rate of diffusion can be combined into an equation called Fick’s law to determine: