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Multicellular organisms

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shvarz:
Imagine two water-filled sacs covered in velcro tape (both hooks and pads mixed on the same surface).  The surface of contact is very flexible, but quite sturdy, although each individual connection is not very strong (a single hook of velcro).  That would be a very good imitiation of real life.  To make the picture complete you can imagine different kinds of velcro in small patches - some are very strong, others are weak, some stick only to their own kind others are pretty general.

Numsgil:
Which probably only really works in real life if the area of surface contanct is greater than a single point.  Another limitation of abstraction ;)

shvarz:
Yes, of course.  A lot of things in real biology depend on "summing up" a large number of small signals.  The velcro is an example of cooperative binding - attachment of one hook helps attachment of the next and so on.  The final force becomes very large.  

These systems are usually characterized by sigmoidal shape.  If you plot number of "hooks" connected versus the total affinity (also called avidity) then it is very small at low numbers and increases slowly but after some threshold it goes up exponentially and then plateaus.   This approach allows a lot more control over the avidity.  Imagine different velcros with different density of hooks.  The more hooks/area a velcro has, the faster the exponential phase appears.  Very easy for cells to control the strength of binding to another cell - just change the number of adhesion molecules on the surface.

Similar mechanisms are used by immune system to detect patigens and by neurons to detect signals and by rodes and cones in the eyes to detect light.

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