Biocurious is a weblog about biology, quantified.

Stir a cup of coffee with a small enough spoon, and the coffee might just stir you!*

by PhilipJ on 8 September 2005

I am often asked what it is about biological systems that physicists are now finding so interesting—it isn’t like interesting things haven’t been coming out of biology all along! Not surprisingly, a lot of the biophysical systems being studied right now are directly related to things in more traditional physics that are poorly understood, in particular non-equilibrium thermodynamics and statistical mechanics.

An example of this is the title of this post. With macroscopic spoons and a cup of coffee, I can stir my coffee vigorously and increase its temperature slightly. I can not, however, heat the coffee via some other means, and have it stir my spoon on its own. The random thermal fluctuations can not be harnessed to do useful work. However, in the tiny, non-equilibrium systems that biophysicists can now study in the laboratory (such as molecular motors like kinesin, or synthetic nanomotors like these), using random thermal fluctuations to do useful work is actually observed, and in some cases critical for proper function.

For an excellent (and free!) non-technical review, this article in Physics Today a couple of months ago goes over the kinds of non-equilibrium systems being studied, and some of the non-equilibrium fluctuation theories being tested. For a more technical look at things, this week’s Nature has an article (by the same group, requires a subscription) looking at the refolding of RNA molecules, showing the ability to detect changes in the free energy when even a single base pair is changed: Verification of the Crooks fluctuation theorem and recovery of RNA folding free energies Nature 437, 231-234 (2005).

* While this has probably been said many times, the most recent is in this News and Views article in this week’s Nature.

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