making heads or tails of phospholipids

Cells are, in some ways, measurable units of life. Life started out as single, functioning, living, cells. In school, they teach you to associate cells with fancy words like “endoplasmic reticulum,” “organelle,” and “Golgi apparatus.” Look past those biology tests from high school, and you can appreciate what a cell is on a deeper level. It is a division from the environment. Outside of the cell is non-living space, and inside of the cell is life. The structures that are ticking within the cell must be housed, separate from the watery dead-space around them. That housing must also provide a means of transport for waste and nutrients to leave and enter the cell, as we know a living system is not a closed system. How does the membrane of a cell work?

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).

Now, let’s apply this setup to reality. The phospholipids are not cemented in place, and in fact shuffle amongst each other like tightly-packed floating ducks in a bath tub. This non-static membrane system is described by the fluid mosaic model.  That basically says that cell membranes are fluid (non-static) and mosaic in nature (made up of sub-units). Ah lah, a membrane that is made up of subunits that float and shift to form an ever-changing cell surface. 

I like this cell membrane business because it shows how a bunch of non-living molecules like phosphates and fatty acids can work together to form a system that is living. Because of the fluid mosaic model and it’s important duty of dividing non-life from life, the spectrum of “living” spans from an itty bitty amoeba to a T-rex. As complex as we are as vertebrate organisms, it’s important to remember that we are complexities made up of even more complexities. Complexities like phospholipids, that are flashes of our humble beginnings as single cells- a collection of working parts, housed together.