|Figure 1: A single phospholipid|
The membrane of a cell is made up of a phospholipid bilayer. When you break those terms down, you can see it has something to do with phosphorous, lipids, and two layers. Phospholipids are two-part molecules- on one end, a phosphate group- called the head; on the other, two dangly chains of fatty acids- the tail (Fig. 1).
The phosphate head is polar, while the fat chains are non-polar. Polar simply means that the charges within the molecules are more clumped than evenly distributed. Polar molecules are attracted to other polar molecules, like, oh say, water. As a result, the phosphate heads love to snuggle up next to water, and always try to acclimate so that they are in contact with it. As for those fat chains, well, they’re lipids. And we all know that lipids (oils, fats, wax) repel water. Slap some fancy terminology in there, and we have a molecule with one hydrophilic (water-loving) end and one hydrophobic (water-fearing) end.
|Figure 2: Phospholipid bilayer that forms a cell membrane|
The end result of all this is that phospholipids tend to form sheets, where the hydrophobic fatty acids are sandwiched in between the protective phosphate heads, which are exposed to the aqueous environments inside and outside of the cell. These sheets form the shell of a sphere, and are the cell membrane (Fig. 2).