Biological Membranes - Online Tutor, Practice Problems & Exam Prep

Biological membranes, primarily composed of phospholipids, are amphipathic molecules featuring hydrophilic and hydrophobic regions. These membranes form a fluid mosaic model, allowing proteins and cholesterol to move laterally, contributing to their dynamic nature. Proteins can constitute 20-80% of the membrane's mass, playing various roles. The structure includes embedded carbohydrates, enhancing functionality. Understanding these components is crucial for grasping cellular processes and membrane dynamics, which are essential for maintaining homeostasis and facilitating communication within and between cells.

Biological Membranes

In this video we're going to begin our introduction to biological membranes. And so first we need to recall from our older lesson videos on lipids that phospholipids are amphipathic molecules, and recall this word amphipathic just means that the phospholipids contain both water loving or hydrophilic groups, as well as water fearing or hydrophobic groups. Also recall from our previous lesson videos that phospholipids are the major component of biological membranes. If this information here does not ring a bell for you, then make sure to go back and check out those older lesson videos on lipids before you continue here with this video.

Now that being said, a biological membrane can be defined as a phospholipid bilayer, because the phospholipids will come together to form 2 layers. But biological membranes are more than just a phospholipid bilayer, because biological membranes also have other embedded molecules such as proteins and cholesterol, for instance. Biological membranes are also commonly referred to or called cell membranes or plasma membranes as well. And so, really, cell membranes, plasma membranes, and biological membranes are synonyms of each other.

Biological membranes are commonly described by the fluid mosaic model. The reason for that is because the fluid mosaic model basically says that biological membranes are fluid. They're very, very fluid, which means that the molecules that are embedded inside of the membrane are capable of shifting past each other and shifting around within the membrane. Thus, the molecules are not stuck in place. They are very fluid. Additionally, biological membranes are a mosaic that are made up of lots of tiny pieces that come together to form the larger structure. They are a mosaic of these membrane-embedded proteins. In fact, biological membranes can comprise anywhere between 20 to 80% protein by mass. This goes to show that there are quite a lot of proteins that are found inside of biological membranes. Once again, these proteins are still able to move laterally within the cell membrane or the biological membrane which is once again why biological membranes are described as being fluid.

Let's take a look at our image down below to clear up some of these ideas that we've talked about. Over here on the left-hand side, notice that we're showing you a scanning electron micrograph (SEM) of a biological membrane. Notice here that this circle represents the biological membrane that's visualized using the scanning electron micrograph. In this image on the right, we're zooming in to this specific region of the biological membrane, and if we were to do that, it would look somewhat like what we see right here. Notice that the major component of the biological membrane are these phospholipids that we see in the membrane. Just one of these circles here, one of these circles with two tails here and here represents a single phospholipid. All of these phospholipids make up the major component of the biological membrane as we indicated up above. However, what you'll also notice just by looking at this image is that biological membranes are more than just the phospholipid bilayer. They also have other embedded molecules as well, such as proteins and cholesterol.

When you look at this image down below, notice that all of these purple structures that you see throughout the image represent proteins. There are quite a lot of proteins that are embedded in biological membranes. Proteins by mass can make up somewhere between 20 to 80% of the mass of the biological membrane. There are a lot of proteins embedded in the membrane. The functions of these proteins can vary, and we'll get to talk more about the proteins that are embedded in the membranes as we move forward in our course. These membrane proteins that you see here are not stuck where you see them. They can actually shift around within the membrane laterally left and right. And so that makes the membrane very fluid-like. Also, because all of these components are coming together to create the single structure, it makes the membrane a mosaic because it's made up of a bunch of smaller pieces that come together to create the single structure.

Notice that we're also labeling cholesterol molecules that are embedded within the membrane as well. You'll also see that we have these greenish structures that are sticking up here, and these structures are supposed to represent carbohydrates. Essentially, biological membranes have lots of other embedded molecules as well. So, this here concludes our introduction to biological membranes, and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Biological Membranes Example 1

Alright. So here we have an example problem that's asking, membranes are a fluid mosaic of what major components? And we've got these 4 potential answer options down below. And so of course we know from our last lesson video that phospholipids make up the major component of biological membranes, and so we would expect that one of the major components listed for the answer should be phospholipids. But when we take a look at answer option a, notice that it has a lot of molecules listed but no phospholipids, which is the major component, and so option a is not going to be the best answer option here.

And notice that all of the other answer options do mention phospholipids, so very nice. Now we have the major component considered in those three answer options. Now what we need to realize next is that nucleic acids are not going to be a major component of biological membranes. In fact, they're not really a component of biological membranes at all. And so for that reason, we can go ahead and eliminate answer option c.

Now when we take a look at option b and option d, what you'll notice is the real difference between them is cholesterol or glucose. Now glucose, we did not mention is going to be a major component of biological membranes, and in fact, it turns out that glucose is not a major component of biological membranes. But we did mention in our last lesson video that cholesterol is a major component of animal cell membranes or biological membranes. And so b here is going to be the correct answer. Phospholipids, proteins, and cholesterol are major components of cell membranes.

And so we can go ahead and indicate that b here is the correct answer for this example. And if we go back up to our image from our previous lesson video, you can see that proteins are certainly in, a major component of the membrane. Cholesterol is also a major component of the membrane. And once again, these little structures that we see here with the two tails, these are all phospholipids, and so they're the major component of biological membranes. So once again, b here is the correct answer to this example and that concludes this example so I'll see you all in our next video.

The fluid mosaic model of the membrane proposed that membranes:

• A. Consist of a single layer of phospholipids and proteins.
• B. Consist of a phospholipid bilayer composed of a variety of fatty acids.
• C. Consist of protein molecules embedded in a dynamic bilayer of phospholipids.
• D. Consist of a phospholipid bilayer between two layers of hydrophilic proteins.

Here’s what students ask on this topic:

What is the fluid mosaic model of biological membranes?

The fluid mosaic model describes the structure of biological membranes. It suggests that membranes are fluid, meaning the molecules within them can move laterally. This fluidity is due to the amphipathic nature of phospholipids, which form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward. The 'mosaic' part of the model refers to the diverse proteins, cholesterol, and carbohydrates embedded within the phospholipid bilayer. These components can move and shift, contributing to the dynamic nature of the membrane. This model is crucial for understanding membrane functions like transport, signaling, and maintaining cellular integrity.

What are the main components of biological membranes?

Biological membranes are primarily composed of phospholipids, which form a bilayer. This bilayer is amphipathic, featuring hydrophilic heads and hydrophobic tails. In addition to phospholipids, membranes contain proteins, which can make up 20-80% of the membrane's mass. These proteins serve various functions, including transport, signaling, and structural support. Cholesterol is another key component, helping to maintain membrane fluidity and stability. Carbohydrates are also present, often attached to proteins or lipids, playing roles in cell recognition and communication.

How do proteins contribute to the function of biological membranes?

Proteins embedded in biological membranes play several crucial roles. They can act as transporters, facilitating the movement of molecules across the membrane. Some proteins function as receptors, allowing cells to receive and respond to external signals. Enzymatic proteins catalyze reactions at the membrane surface. Structural proteins help maintain the cell's shape and stability. Additionally, proteins can be involved in cell recognition and adhesion, which are essential for tissue formation and immune responses. The ability of these proteins to move laterally within the membrane contributes to its fluidity and dynamic nature.

Why are phospholipids considered amphipathic molecules?

Phospholipids are considered amphipathic molecules because they contain both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. The hydrophilic part is the phosphate head, which is attracted to water. The hydrophobic part consists of the fatty acid tails, which repel water. This dual nature allows phospholipids to form bilayers in aqueous environments, with the hydrophilic heads facing outward towards the water and the hydrophobic tails facing inward, away from the water. This arrangement is fundamental to the structure and function of biological membranes.

What role does cholesterol play in biological membranes?

Cholesterol plays a crucial role in maintaining the fluidity and stability of biological membranes. It is interspersed among the phospholipids in the bilayer, where it helps to modulate the membrane's fluidity. At high temperatures, cholesterol stabilizes the membrane by preventing it from becoming too fluid. At low temperatures, it prevents the membrane from becoming too rigid by disrupting the regular packing of phospholipids. This balance is essential for proper membrane function, including the movement of proteins and lipids within the membrane and the overall integrity of the cell.