In this video, we're going to begin our lesson on biological membranes. But first, we need to recall from some of our previous lesson videos that phospholipids are actually amphipathic molecules, and recall that the term amphipathic refers to a single molecule that has both polar groups that are hydrophilic or water-loving and nonpolar groups that are hydrophobic or water-fearing. And so, these phospholipids are also the major component of biological membranes. And it's actually the amphipathic nature of the phospholipids that allows these phospholipids to come together to form these biological membranes. Now, if you don't remember many of the details that we discussed here so far, then be sure to go back to our older lesson videos on lipids and rewatch those videos before you continue here.
Now that being said, biological membranes can be defined as a phospholipid bilayer and this is because biological membranes are going to be composed of 2 layers of phospholipids, and so the root bi- in bilayer is a root that means 2. And so, a bilayer means that there will be 2 layers of these phospholipids. However, it's important to note that biological membranes are more than just a phospholipid bilayer. And this is because biological membranes are a phospholipid bilayer with other embedded molecules as well. And these other embedded molecules can be extremely critical to the function of these biological membranes. And these other embedded molecules include proteins that are found embedded in the phospholipid bilayer, as well as cholesterol which can be found in the biological membranes of animal cells.
Now, it's also important to note that biological membranes are also commonly referred to as cell membranes or plasma membranes. And so, you'll notice that your professor or your textbook will interchangeably use these terms biological membrane, cell membrane, and plasma membrane to refer to the same exact thing. And so, these biological membranes are actually described by the fluid mosaic model. And so the fluid mosaic model describes the nature of these biological membranes. And so as its name implies, it describes the biological membranes as semi-fluid because it has a fluid-like nature to it and this is because the molecules within the biological membrane are not stuck in one single position. In fact, the molecules that are part of the biological membrane can actually move and they can shift around to different regions within the biological membrane, and so they are very fluid-like and it is described as semi-fluid.
Now, it's also described as a mosaic, and so a mosaic is really just going to be a single piece that's composed of smaller individual pieces that are different. And so that is exactly what the biological membrane is. It consists of many different, smaller pieces that come together to form it. And so, the mosaic is going to include the phospholipids within the phospholipid bilayer but again it will also include the proteins that are embedded in cholesterol and things of that nature. So the mosaic here can be thought of as the membrane-embedded proteins along with the phospholipids. And so these proteins again they can be extremely critical to the function of the biological membrane. And in fact, biological membranes can be comprised of 20 to 80% protein by mass. And so what this means is that there's quite a large percentage of proteins found in some biological membranes. And again, these proteins, they are not stuck in one position and this is why the biological membranes have a fluid-like nature. These proteins can actually move laterally within the cell membrane and again this is why they are not stuck in one single position.
And so if we take a look at our image down below, we can get a better understanding of these biological membranes. And so, notice over here on the left-hand side we're showing you a cell and as soon as you draw the perimeter of the cell, you've already drawn the biological membrane. And so if we zoom into this region on the perimeter of the cell, we can zoom into the biological membrane. That's what this image is showing you here. And what you'll notice is that the main component of the biological membrane are these phospholipids, and so notice the phospholipids are being shown by this representation that you see here, and again you can see that is the main component of the biological membrane. However, the biological membrane consists of more than just the phospholipids. Notice that there are several different types of proteins that are embedded here. These proteins are different, they can have different shapes and different functions, and notice that these proteins can also contain carbohydrate groups attached to them, and the lipids can also have carbohydrate groups attached to them. So these are the carbohydrate groups that you can see here like this. And then you'll also notice that embedded within the phospholipid bilayer there can be these cholesterol molecules, and so notice the cholesterol is being shown, like that but the cholesterol is found embedded in all of these different regions as you see throughout this biological membrane. And so again, all of these different structures that you see here that are indicated by this label, these are going to be the proteins. And so, again, these biological membranes they can serve many different functions but they pretty much serve as the skin of the cell. So just like our skin separates the outside of our bodies from the inside of our bodies, well the biological membranes they do the same for the cell and so they are going to control the passage of molecules into and out of the cell but they can also have many other functions as well.
And so this here concludes our brief introduction to biological membranes, and we'll be able to get some practice applying these concepts and learn more about biological membranes as we move forward in our course. So I'll see you all in our next video.
