Our model for a thought experiment generalizes needs of supply and demand to satisfy metabolism as size, activity and demand increase. When bodies are small, diffusion suffices.

Conceive of a spherical bee smaller than a gnat. For energy she requires oxygen and nutrients, and at the same time she must eliminate carbon dioxide and water as waste products. Because she is spherical, we assume all of her interior points of metabolism where she consumes oxygen and fuel and produces carbon dioxide and water to be equidistant from her body's surface. If she is small enough, her body equi-permeable to the molecules so that distances for diffusion remain optimally short, and if supplies of energy in the environment around her suffice, diffusion of both oxygen and food molecules from the environment through her body wall to her tissues as well as diffusion of wastes from her body to her environment will supply her metabolic needs. Diffusion suffices as long as diffusion time and path length remain short enough.

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