They create a closed boundary. Now, the fact that the cell membranes are thin basically means they only consist of two layers of molecules, two layers of lipids, as we'll see in a future lecture. Now, on top of that, inside the cell, inside eukaryotic cells, which are cells found inside our body, we also have individual organelles that themselves contain membrane. So things like the nucleus, the Golgi complex, the endoplasm reticulum, lysosomes, paroxysomes, all these organelles also contain their own membrane that basically plays the same type of role as the cell membrane found around the actual cell. Now, property number two, cell membranes are made up of not only lipids, but also proteins and carbohydrates. And the additional proteins and carbohydrates basically diversify the functionality of that cell membrane.