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The role of randomness: Biological implications (2013)

Page history last edited by Joe Redish 4 years, 5 months ago

Class content I > Coherent vs Random Motion





A critical component of an understanding of fundamental biological processes is the idea that all matter is made up of tiny particles -- atoms and molecules -- in continuous motion as a result of thermal energy. Although eighteenth-century physicists imagined that, in principle, if one knew everything about the positions and velocities of the molecules of matter, one could use Newton's laws to predict all their future motions (Laplace's demon), in practice it would be impossible. (And twentieth-century physicists showed through the development of quantum mechanics and chaos theory that even in principle it's impossible.)


A better model is to treat the motions of the molecules of matter as if they are moving in random directions. Whatever direction a molecule might be moving in right now, in a very short time it will interact with another molecule and head off in a different direction. As a result of this continual activity, some molecules move from one place to another, some are moving faster (have more kinetic energy) than others, and there is no rhyme or reason to it -- from the point of view of any individual molecule.


The surprising thing is that as one steps back and looks at lots of molecules, regularities can arise. The motions are constrained by universal principles such as the conservation of energy -- the 1st law of thermodynamics, conservation of charge, .... Even more interesting, statistical laws of physics emerge -- Fick's law, the second law of thermodynamics, .... These lead to some of the fundamental mechanisms by which biological systems function. Although some of the molecular-scale processes of biology appear to be controlled at the molecular level, at their core they all function with the driving force of randomness underlying them. Processes that seem strongly directed are often "ratchet-like" -- incrementing in response to some random changes and not going backwards when the random change is opposite. This is one way that random inputs can lead to directed results. Another apparently directed result coming from randomness is diffusion -- the transfer of material from a high concentration to a low concentration without any directed forces. 


While the random thermal motion of molecules can provide small biological systems with the energy and chemicals they need, the time and distance scales of these processes limit both the size and responsiveness of an organism. Major steps in the evolutionary history of life on earth are associated with developing structures that can seek out higher concentrations of needed energy, materials, and organization. We have already seen in our analysis of the worm (How big is a worm?) that diffusion rates of oxygen can limit the growth of an organism, producing evolutionary barriers that need to be eliminated by the creation of new structures. This shows that the quantitative description of what one can do with randomness is essential in understanding how organisms evolved the way they did and how they function today.


In this section of the class we will be developing and analyzing the equations that show what randomness can do for an organism and how much.




Joe Redish 12/1/11

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