Every cell needs a supply of substances such as oxygen and glucose, and must get rid of wastes such as carbon dioxide and urea. In small organisms these can move to and from every cell by diffusion alone. In large organisms diffusion is far too slow, so a mass transport system is needed.Three factors decide whether diffusion is enough:1. Surface area to volume ratio (SA:V)
As an organism gets bigger, its volume increases faster than its surface area, so its SA:V falls. A small organism has a large surface for exchange compared with the volume of cells it must supply. A large organism does not have enough surface to meet its needs.Worked example: a cube of side 1 cm has surface area 6 cm2 and volume 1 cm3, so SA:V = 6:1. A cube of side 3 cm has surface area 6 × 9 = 54 cm2 and volume 27 cm3, so SA:V = 2:1.2. Diffusion distance
The time for diffusion increases with the square of the distance. Diffusion is fast over a few micrometres but takes hours or days over centimetres, so cells deep inside a large organism would not be supplied in time.3. Metabolic rate
Active organisms, especially endotherms such as mammals and birds that maintain a constant body temperature, use oxygen and glucose rapidly and produce wastes quickly, so they need a fast supply.Single-celled organisms, and flat or thin animals such as flatworms, can rely on diffusion. Larger, more active multicellular organisms need specialised exchange surfaces plus a transport system to carry substances between those surfaces and the cells.
As an organism gets bigger, its volume increases faster than its surface area, so its SA:V falls. A small organism has a large surface for exchange compared with the volume of cells it must supply. A large organism does not have enough surface to meet its needs.Worked example: a cube of side 1 cm has surface area 6 cm2 and volume 1 cm3, so SA:V = 6:1. A cube of side 3 cm has surface area 6 × 9 = 54 cm2 and volume 27 cm3, so SA:V = 2:1.2. Diffusion distance
The time for diffusion increases with the square of the distance. Diffusion is fast over a few micrometres but takes hours or days over centimetres, so cells deep inside a large organism would not be supplied in time.3. Metabolic rate
Active organisms, especially endotherms such as mammals and birds that maintain a constant body temperature, use oxygen and glucose rapidly and produce wastes quickly, so they need a fast supply.Single-celled organisms, and flat or thin animals such as flatworms, can rely on diffusion. Larger, more active multicellular organisms need specialised exchange surfaces plus a transport system to carry substances between those surfaces and the cells.