Recap Discussion Sept 2

To recap my impressions - one day later - from this week's discussion, we established some important points.

1. The Reynolds number gives the relative importance of inertial vs. viscous forces for an object moving through a fluid.
2. The small Reynolds numbers experienced by bacteria (and even more so, biomolecules) means that their mechanics are very different from objects on our scale (a car, for example).

One of the consequences of observation 2 is that is is incorrect to think of the energy from a chemical event (ATP bond breaking, for example) at a given site in a protein is the "same" energy that is used to accomplish something at a distant part of the protein. This would be an inertial transfer, which cannot happen in water on the scale of molecules. Rather, molecular motors work through a "Brownian Ratchet" action, where the energy is used LOCALLY to make a change that alters the energy surface on which the system is diffusing. The energy that is harvested at a distant location is not the "same" energy as was originally liberated, it is harvested instead from the thermal motions, which now occur on a BIASED surface.

En route to explaining this, we reviewed Feynman's "thought-ratchet" and his result that you cannot harvest fluctuations with a ratchet if the entire ratchet - both the handle and the ratchet itself - are in the same bath. You can harvest energy, however, if the handle and the ratchet are in different baths. This helps explain why membranes are so important to biology - they separate baths with different states, and allow energy harvest.

-Seth