Organisms exchange substances and heat with their environment all the time, and this possibility is crucial to survival. The specific way in which this is achieved is very tightly related to the shape and structure of the specific organism, as well as its environment. For example, unicellular organisms are so small that molecules such as oxygen and water can readily diffuse in and out via the membrane, due to the short diffusion pathway. Could this be achieved by a human, or even a bee? No – they are simply too big.
Two properties are important to consider here: the volume of an organism, and the surface area of an organism. The volume is what determines the amount of substances which need exchanging, while the surface area determines the amount which can be exchanged.
Surface area describes the number of cells in direct contact with the environment. Volume describes the space occupied by all metabolically active cells.
Key principle: as the size of an organism increases, the surface area to volume ratio…