We usually start this topic by saying that the cell is “the smallest functional unit of life.” This means that nothing smaller than a cell is considered to be alive. There are a few criteria for determining if something is alive, including whether or not it can replicate and carry out its own metabolism. Viruses can’t do either of those things, so they technically aren’t alive.
But that does nothing to explain what a cell really is. We all know that the body is filled with water. And that cells are filled with water. So, how do you separate the water inside a cell from the water outside of a cell? It all starts with the Plasma Membrane or PM. You may see some textbooks call this the Cell Membrane.
You already know that oil and water don’t mix. If you haven’t tried mixing them before, just look at a bottle of vinaigrette salad dressing. The oil and the water separate out every time. This is because water is polar and has a slight charge, while the oil is neutral and has no charge.
The Plasma Membrane is made up of lipids, somewhat similar to those in oils. These lipids are called phospholipids. Phospholipids have a negatively charged head and a neutral tail. This means that like a compass, the head is always going to be drawn toward water and the tail will move away from water.
What does that mean? When these lipids are added to water, they will naturally form a layer where their heads are facing out (toward the water) and their tails are facing in (toward each other).
Plasma Membranes are more than just lipids, though. Proteins are essential to the stability and function of the PM. Some textbooks will tell you that the PM is 80% lipid and 20% protein. Here, they’re counting the number of molecules in the membrane. Others will tell you that the PM is 50% lipid and 50% protein. This is measured by weight. Proteins are fewer in number, but they’re bigger and heavier than lipids.
The main job of the Plasma Membrane is to control what gets into or out of the cell. Yes, it serves as a barrier between two different environments, but it also serves to move things from one area to another. For example, some bacteria have drug efflux pumps in their Plasma Membrane, which pump antibiotics out of the cell and make them resistant to that drug. We’ll talk more about how Human cells handle transport in the next post.