ABSTRACT
Diffusion time In small and slow creatures, this works fine. The one shown below has sensory cells (S) that control motor cells (M) by releasing a chemical transmitter or hormone into the common fluid space. But as an organism gets bigger,
things get difficult, in two ways. First of all, time: how long it takes for the message to get from the sender cell to its recipient depends on diffusion. Depends rather
dramatically, because diffusion time is proportional to the square of the distance travelled – Einstein’s law – so it takes a disproportionately longer time for a chemical signal to travel over increasing distances: 1 mm takes 0.5 ms, but 10 mm takes 15 hours. If speed of response is
not particularly important, this kind of communication may still be satisfactory even in very large organisms: our own hormonal control systems are of course precisely of this kind. Circulation helps, of course, but even so your blood takes about a minute to go once round the body. If a tiger suddenly burst into your room, although you would instantly release a burst of adrenaline into your bloodstream, it would take about a minute for it to reach its target tissues. By that time you might well have been devoured.
