I just learned about this in school. They need to be small in the ratio to make them more efficent in having molecules enter and leave them quickley.
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A small cell will have a larger surface-to-volume ratio.
As a cell becomes larger the surface area to volume ratio gets smaller. The volume increases by the square of the surface area. That is the main reason that one celled organisms are small.
If the surface area of the organism is small, then there is no problem with getting all the oxygen needed. If the surface area of the organism is large, therefore a special respiratory surface is needed. An example is lungs, gills. The ratio of surface area to volume in a small organism is greater than the ratio in a large organism.
Cell have a greater surface area to volume rations than a larger cell.
Any living thing that has a low surface area per unit of volume will lose less moisture to evaporation than a living thing with a high surface area to volume ratio.