April 30, 2016 at 12:40AM

Today I Learned: I've been reading Ron Milo and Rob Phillips' "Cell Biology by the Numbers", which has given me more than three TIL-worthy facts in the last twenty minutes... so today y'all get four facts! The book can be read for free here, I highly recommend picking some random chapters and reading through them if you're into cell biology stuff: http://ift.tt/1AvhcfE 1) The plasma membrane (the lipid bilayer that defines the outside of a cell) actually only contains less than 10% of the total membrane in a typical eukaryotic (i.e., human) cell. The rest is mostly bound up in mitochondria and the endoplasmid reticulum, though it varies quite a bit by cell type. 2) Mitochondria are usually pictured in textbooks as little pill-shaped organelles, roughly the size and shape of a bacteria. Which makes sense, given that mitochondria are descended from bacterial endosymbionts. However, if you look at mitochondria under a microscope, you'll see that they actually look more like a continuous net, like the outside of a morel mushroom (some example 3D reconstructions here: http://ift.tt/1Un6cPY). The reason mitochondria look like little pill-shaped things in all the EM pictures in textbooks is that those EM pictures are all 2D slices through that net. Well, today I learned that mitochondria sometimes *do* take on little separate pill-like shapes. Specifically, when yeast are grown in ethanol, their mitochondria split up and form little balls. I'm a little suspicious about this, though. Ethanol is a pretty stressful condition for yeast, and the blebbing of mitochondria reminds me (at least superficially) of what happens to cells when they apoptose. Perhaps that mitochondrial morphology is just a precursor to apoptosis? 3) Chloroplasts in a plant cell will often move away from light to avoid photodamage! How ironic, given how much effort plants put into moving towards light. 4) Speaking of chloroplasts (and mitochondria, for that matter), one of the most fascinating events in the history of life is the acquisition of cyanobacteria by some ancient Eukaryote, making the first chloroplast and the first photosynthetic Eukaryote (similar to what happened with the ancestor of all Eukaryotes when it first acquired a mitochondrion). One thing that's been seen repeatedly during endosymbiotic events like that is that the symbiont's genome slowly migrates to the nucleus, until the symbiont is left with a tiny little core genome of things that can't move for one reason or another. Here's a question -- how long should we expect that process to take? Some enterprising scientists performed an experiment to try to get at least some of the way towards an answer to that question. They engineered chloroplasts to hold a reporter gene (don't ask me how the heck they did that), then introduced those chloroplasts into hundreds of thousands of pollen grains (again, how?!?!), mated those pollens with plant eggs, and checked the resulting embryos for nuclear versions of the reporter. It turns out that the reporter was moved to the nucleus in about 1:10,000 mating events. That seems pretty darned fast! Of course, other events would also have to happen to make those genes *functional*, but it gives some hint about how long it should take to move a chloroplast's genome to the nucleus.