I don’t have a deep topic to write about today because I’m busy at work, but I thought I’d write on a subject that I’ve been studying myself, partly in order to make sure I understand it.

Phase separation is a common and easily understood way that matter segregates: when water boils the gaseous vapor and the liquid water are separated from each other due to differences in their density. A liquid-liquid phase separation is the same thing, only it’s two liquids separating and not two different states of matter. One example of this is oil and water, as everyone knows oil and water will separate from each other when placed in a glass. This is in part due to their preference to aggregate with each other, water is hydrophilic and so interacts strongly with water and not well with hydrophobic things, while oil is hydrophobic and interacts strongly with itself and not well with hydrophilic things. What’s less understood is how liquid-liquid phase separation is also important in cell biology.

If you remember what a cell looks like from a high school textbook, you can probably remember that it has things like a nucleus, a mitochondria (the powerhouse of the cell!), and some weirder things like an endoplasmic reticulum. These are all examples of membrane-bound organelles. Just like our organs all perform special duties within the body, so too do organelles perform special duties in the cell. The membranes that surround these organelles spatially separate their inside contents from the outside cell, and so allow them to more efficiently perform their functions. But there are also specialized parts of a cell that do not have a surrounding membrane. The nucleolus in particular is a specialized area of the nucleus that has its own special functions, but it is not separated from the nucleus by any membrane whatsoever. So how does the nucleolus prevent all its contents from diffusing into the nucleus and ruining whatever process it is performing? It does so through a liquid-liquid phase separation.

It’s a bit too technical to explain here, but just as oil and water have chemical properties that separate them in a glass, so too does the nucleolus have chemical properties that separate it from the wider nucleus. And it turns out that a large number of cellular functional areas segregate from the cell through liquid-liquid phase separations, all segregated not by a membrane-defined boundary, but by the physical properties of the medium in which their reside. These phase separations allow many different areas of the cell to undergo their own specialized functions without needing to constantly make and remove cell membranes around them, and thus allow for more efficient activity inside the cell.

So that’s an overview of liquid-liquid phase separation, I’m still learning it myself so I hope I got everything correct. But if I miswrote anything, cell biologists out their feel free to correct me.