Using analogy to understand how chameleons change colour

The video shows artificial models of the key structures in a type of cell called a melanophore. This is from a nice paper by Aoyama et alMelanophores and similar cells are how animals like chameleons change colour. The blobs that show up as bright here in the fluoresence microscopy images are actually dark brown under natural conditions. They contain eumelanin, the brown pigment that makes brown hair brown.

The basic idea is as follows: You have say a green cell underneath one of these melanophore cells. Then, if as at the start of the movie the blobs (brown in real life) are distributed all over the cell, then they block the light and so that part of the chameleon looks brown.

But if these blobs are brought into an aggregate as we see during the movie, then light can pass through the cell and be reflected from the green cell behind it. This part of the skin then changes from brown to green.

This is a cute biological system, which has been studied a lot. But I have been wondering whether or not it is analogous to another, very familiar system: the formation of liquid droplets from a vapour.

A liquid is defined in terms of two properties. The first is that it has to be dense, liquid water is about a 1000 times denser than the air in the atmosphere. The second is that despite being dense it must flow, it can’t be solid. This is roughly equivalent to saying that the molecules in the liquid must be moving about.

The aggregates of the red blobs that form are dense, and you can see the blobs moving around in them, so maybe the process seen in the movie where they form is analogous to water molecules condensing into droplets of liquid water. If so it might help us to understand how chameleons change colour.

Our understanding of how liquids form has benefitted a lot from an analogy between it and how magnets like iron become magnetic, so this not so far fetched an idea. But there are differences between liquid droplets and the aggregates in melanophores, both the real ones in cells and the model ones above. The molecules in water droplets move due to the thermal energy all molecules have at temperatures above absolute zero (-273 C), while the movements in the movie are due to energy-burning nanoscale machines, called molecular motors. This difference is bound to introduce some differences but I don’t yet have a good feel for what they are.

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