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Diffusion limits the size of cells. A single protein diffuses across a bacterium in just 10 ms, but diffuses across a human cell in 10 seconds, because the human cell has a diameter 20x larger. TIL that there are very large bacteria, such as Thiomargarita magnifica, that are 1cm across and visible to the naked eye. They get around this diffusion limit by filling their cytoplasm with a "vacuole that occupies most of the cell (65–80% by volume) and pushes the cytoplasm to the periphery of the cell," where diffusion can occur. Biology finds ways around human-perceived limits.