May 01, 2016 at 03:28AM

Tody I Learned: 1) ...how to use Adobe Fireworks, or at least a few tricks with it. I now know how to use the magic wand tool, apply textures, make my own textures, move things around, and a few other things. Perhaps my favorite is rubber-stamping, which is essentially a brush that copies content from a location relative to the cursor (in a way you define). 2) The definition of "persistence length". Informally, persistence length is the distance over which a long, thin object remains stiff and rod-like. For example, the persistence length of rope is quite short -- only a few inches at most -- while the persistence length of spaghetti is approximately 10^18 meters. That means that at distances longer than 10^18 meters, spaghetti acts more like a string than like a rod. Who knew! Anyway, today I learned the exact definition of persistence lengh. It has to do with the average angle formed between two ends of a rod of material. As the material gets longer and longer, the end wobbles around more with respect to the beginning. Specifically, it can be shown (somehow, I don't know how) that the average cosine of the angle between the two ends falls off exponentially. The persistence length is the constant that defines the rate of that dropoff. More precisely, if A is the angle between ends and L is the length of the rod, then P is the constant such that mean(cos(A)) = e^(-L/P) This is a nice tidy definition mathematically, but it *does* make it a little hard to intuit exactly what it means. From another article (http://ift.tt/21n39YY), the persistence length is also the distance at which the average angle between ends reaches 68.4°. 3) The average mass of protein in a cell membrane can be higher than the average mass of lipid in the same membrane. Whoa. ...of course, this might be a little misleading, since membrane proteins generally have stuff sticking out of both ends of the membrane. Oh, another useful fact -- typical membrane thickness is about 4 nanometers, which means the trans-membrane domains of membrane proteins are also about 4 nm.