Why cells stay tiny: skin grows with the square, the cell with the cube
30seconds
Double a cell's width and its skin grows 4 times, but the cytoplasm it must feed grows 8 times. Add diffusion, where time grows with the square of the distance, and a ball of cytoplasm soon cannot get fuel to its middle fast enough.
Drag a cell from a 1 µm bacterium to a 2 cm ball and watch which part gets fed in one second, then see how red blood cells, nerve cells, eggs and a centimetre-long bacterium get round the limit.
Deep dive · 5 min
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