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The role of randomness: Biological implications (2012)

Page history last edited by Joe Redish 11 years, 5 months ago

Class content > The micro to macro connectionThermodynamics and Statistical Physics

 

Prerequisites:

 

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, and molecular complexes -- in continuous motion as a result of thermal energy. 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).

 

But after multiple collisions, in many cases we can't predict the future position and velocity of atoms and molecules or other small objects - the calculations are too sensitive to the exact starting points and we can never know those exactly. So what can we say about their motion? It turns out that even though we can say nothing about the future position and velocity of an individual atom or molecule, we can say a lot about the behavior of a large group of them.  This means that we will need to measure the motion of lots of atoms or molecules and describe the motion of large groups with graphs, histograms or equations.  

 

How can we make predictions about the behavior of large groups?  A good 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 when one looks at lots of molecules, regularities arise.  Even for groups of molecules universal principles such as the conservation of energy -- the 1st law of thermodynamics, conservation of charge,....apply and constrain the average results. Even more interesting, statistical laws of physics emerge -- Fick's law, the second law of thermodynamics, and more. 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 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 understand 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.

 

Follow-ons:

 

Joe Redish 12/1/11

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