The fluid mosaic model of the plasma membrane is a model that helps us understand how the cell’s outer barrier acts and what its is made up of. Let’s tackle this concept term by term and with some gifs to help out.
The “Mosaic” in Fluid Mosaic Model
Take a look at the gif above. You’ve heard of mosaics, right? A mosaic describes a patchwork of small parts that come together to comprise a larger whole. It originally described the old-school art technique where artists broke small chips of glass or colored clay tiles and arranged the pieces in such a way so that the pieces made an image.
Well the plasma membrane is a lot like a mosaic. It’s made of many smaller parts, including:
In the gif above, the large globular molecules are the proteins, the sandwich looking layer is the phospholipid bilayer, and if you look closely you can see the line pointing to dark colored cholesterol molecules that are anchoring some of the proteins to the bilayer. (The quality of the gif isn’t awesome. Don’t think that the large molecules are the cholesterol!! They are the proteins).
What truly makes this a mosaic is that there are hundreds of different types of proteins and many different types of phospholipids and cholesterols. When you start thinking of the vast variations of molecules all apart of one larger whole, the “mosaic” of the fluid mosaic model of the plasma membrane suddenly makes a lot more sense.
The “Fluid” in Fluid Mosaic Model of the Plasma Membrane
The membrane is not just a mosaic, it’s a fluid mosaic. The “fluid” in fluid mosaic model of the plasma membrane refers to the fact that the plasma membrane is about as fluid as olive oil. All of the proteins, cholesterols, and phospholipids are all floating around one another, but the vast majority of them aren’t fixed in their location. They move around each other like people at a busy fair. They meander and travel freely around one another. Why is everything moving? Largely because of thermal activity. In the same way that molecules in a balloon are bouncing around, the contents of a cell are bouncing around because of thermal energy. The effects of this on the plasma membrane which, once again, has the fluidity of olive oil.
While the first gif captures the mosaic idea pretty well, this second one shows the fluidity of the plasma membrane. What you’re looking at here is the outside of the plasma membrane where a molecule is passing a signal on to a receptor that will then pass the signal through the plasma membrane and into the cell.
Why does the plasma membrane need to be fluid?
As you’ll learn later on in biology, a lot is going on in the plasma membrane. Sometimes one protein that is embedded and floating around in the plasma membrane needs to interact with another one, the only way for this to happen is if the molecules are able to freely move around so that they are able to come into contact with each other. The fluidity of the plasma membrane also allows it to be flexible enough change its shape. This can be important in doing things like allowing the cell to move or allowing the cell to take in large molecules.
Video Explanation of the Plasma Membrane
Khan Academy is always a go to for some of the beginning topics in biology. If you want a video presentation/explanation, check out Khan Academy’s phospholipid bilayer video, where Sal Khan goes into some of the chemistry of the plasma membrane. He’ll discuss the solubility of the bilayer, why the amphipathic nature of the phospholipids and the proteins matter, etc.
By the way, if you liked these gifs, you should check out The Inner Life of the Cell, which is the source of these gifs. It’s beautiful animation, don’t expect much by the way of explanation, though. It’s not much more than music, animation, and a few labels of what you’re seeing in the video.
Soon I’ll have a page up that will be about the plasma membrane in more detail, but for now, check out our learn biology online page for more resources on learning biology.