IGCSE Biology / Blood and circulation

Why small organisms manage without a transport system and large ones cannot revision notes

Edexcel International GCSE Biology (4BI1), chapter 5: Blood and circulation, specification point 2.51, 2.52. Free lesson.

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Small organisms can exchange substances by diffusion, but large organisms cannot meet their needs this way. Their smaller surface area to volume ratio, longer diffusion distances and higher demand mean they need a transport system and specialised exchange surfaces.

How to calculate surface area to volume ratio

A cube is useful for showing how size affects the ratio. For a cube with side length 1 cm, the surface area is 6 cm² and the volume is 1 cm³, so the ratio is 6:1, or 6. Compare the surface area with the volume by dividing the surface area by the volume. For a cube, surface area grows with the square of its side length, while volume grows with the cube of its side length.

Why the ratio falls as an organism gets bigger

When the side of a cube doubles, its surface area increases four times but its volume increases eight times. Volume therefore grows faster than surface area. A larger organism has less surface area compared with the amount of living material inside it, so its outer surface is less able to supply all its cells. A 2 cm cube has a surface area of 24 cm² and a volume of 8 cm³, giving a ratio of 3:1. A 4 cm cube has a surface area of 96 cm² and a volume of 64 cm³, giving a ratio of 1.5:1.

Why diffusion alone is not enough in a large organism

Diffusion moves substances from a higher concentration to a lower concentration. It is enough for a single cell because substances have only a short distance to travel. In a large organism, diffusion alone cannot supply the whole body quickly enough. A large organism has a small surface area to volume ratio, so there is less surface available per unit of volume. Its inner cells are a long distance from the outside. Diffusion is quick over a fraction of a millimetre but too slow over many centimetres. It has a high demand for oxygen and food because millions of cells respire.

Why a flatworm can manage without a blood system

Overall length does not determine whether diffusion is enough. Thickness matters because it affects how far the cells are from the surface. A flatworm several centimetres long can still exchange substances by diffusion because it is thin. A mouse of the same length is thicker, so its inner cells are too far from the surface. In the flatworm, each cell is within a fraction of a millimetre of the surface, and its surface area to volume ratio stays high. In the mouse, cells deep inside are centimetres from the surface, so diffusion cannot supply them in time.

What makes an effective exchange surface

Large organisms use a transport system to carry substances between exchange surfaces and their cells. Lungs, gills and villi have similar features because they need to move substances efficiently into or out of the body. A large surface provides more area for exchange. A thin surface gives substances a short distance to diffuse. A moist surface allows substances to dissolve. A good blood supply keeps the concentration gradient steep, helping diffusion continue.