Surrounding every single cell of our body, we have a special membrane, a phospholipid bilayer membrane known as a semipermeable membrane. So what that basically means is it allows the movement of certain molecules, but at the same time, it prevents the movement of other molecules across that cell membrane. For example, polar molecules that have charged and large molecules cannot pass across the cell membrane without using a special type of protein transport molecule. Now, in the same analogous way, we can also build a semipermeable membrane, and we can use the semipermeable membrane in a process known as dialysis. So dialysis is a process by which we can purify proteins by using a semipermeable membrane. So let's see exactly what we mean by taking a look at the following three diagrams.