Cell Envelope & Biological Membranes: Study with Video Lessons, Practice Problems & Examples

The cell envelope encompasses all surface layers surrounding a cell, including the cell membrane and cell wall, which vary among cell types. All cells, whether prokaryotic or eukaryotic, possess a cell membrane, a phospholipid bilayer integral to biological membranes. These membranes, described by the fluid mosaic model, consist of phospholipids, proteins, and cholesterol, allowing lateral movement of proteins. This structure is crucial for cellular functions, emphasizing the importance of understanding membrane composition and dynamics in biological systems.

Biological Membranes

This video, we're going to begin our lesson on the cell envelope and biological membranes. The term "cell envelope" refers to all of the surface layers surrounding the cell, which includes structures such as the cell membranes and cell walls, and some other possible structures as well that we'll get to talk more about later in our course. The specific structures that compose the cell envelope may actually vary in different types of cells. Depending on the cell type, the cell envelope may also change. However, cell membranes are always included in the cell envelope, because all cells have a cell membrane, regardless if they are prokaryotic or eukaryotic cells. All cells have a cell membrane. That's part of the cell envelope. If we take a look at our image down below, we can get a better understanding of this term, cell envelope. Notice on the far left-hand side, we're showing you a representation of a bacterial cell. We're zooming into the perimeter or the surface of this cell here, to get this image. Notice that on the perimeter of the cell, there is the cell membrane, which we're showing you right here. But then there can also be what's known as a cell wall. Here we can label the cell wall. Collectively, the cell membrane and the cell wall can be referred to as the cell envelope. The envelope again is just referring to all of the surface layers surrounding the cell on the perimeter of the cell. This here concludes our brief introduction to the cell envelope, and we'll be able to talk more about the cell envelope and biological membranes as we move forward. So I'll see you all in our next video.

Components of the cell envelope include:

• A. Cell membrane.
• B. Cell wall.
• C. Cytoplasm.
• D. a, b, & c.
• E. a & b.

Intro to Biological Membranes

In this video, we're going to begin our introduction to biological membranes. First, recall from our previous lesson videos that phospholipids are amphipathic molecules. Recall that the term "amphipathic" just means that the molecule has both hydrophilic or water-loving and hydrophobic or water-fearing groups. These amphipathic phospholipids are the major component of biological membranes, which can be defined as phospholipid bilayers, consisting of two layers of phospholipids. Embedded within the phospholipid bilayer are other molecules such as proteins and cholesterol. These biological membranes are also called cell membranes or just plasma membranes. It's good to know that these terms refer to the same thing.

If we take a look at the image below, notice, on the left-hand side, we are showing you a SEM (scanning electron micrograph) of a membrane, represented by the border in the image. We are zooming into a particular section of the membrane, as shown in the image. The major component of this biological membrane is the phospholipids, structured in a bilayer. Embedded within this phospholipid bilayer are also many other types of molecules, such as proteins and cholesterol molecules, present in these positions. This biological membrane consists of an assortment of many different types of molecules, with phospholipids being the major component, but also includes proteins and cholesterol.

Scientists use what is known as the fluid mosaic model to describe the nature of biological membranes. Biological membranes are somewhat fluid, being semi-fluid because they are not rigid. They can have some flexibility, and molecules are able to move past one another almost like in a phospholipid ocean where a protein might not necessarily be fixed in one position but can move throughout the membrane in either direction. This gives the membrane a fluid type of nature. The membrane is also a mosaic, where mosaics are art pieces made up of a collection of smaller individual pieces. The membrane is a single structure made up of a bunch of smaller individual pieces brought together, such as proteins, phospholipids, and cholesterol.

It is also interesting to note that proteins can make up a significant portion of these biological membranes, with some membranes comprised of anywhere between 20% to 80% protein by mass. This shows how proteins can make up a large percentage of the membrane. These proteins are not fixed in position; they have the ability to move laterally within the cell membrane. Any of the individual proteins shown are able to diffuse laterally or horizontally throughout the membrane in either direction.

This concludes our brief introduction to biological membranes, and we will be able to discuss them more as we move forward in our course. But for now, I'll see you all in our next video.

Cell Envelope & 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 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 cell envelope and what structures does it include?

The cell envelope refers to all the surface layers surrounding a cell, including the cell membrane and cell wall. The specific structures that make up the cell envelope can vary depending on the type of cell. For instance, bacterial cells have both a cell membrane and a cell wall, while eukaryotic cells may have additional structures like an outer membrane. Regardless of the cell type, all cells possess a cell membrane, which is a phospholipid bilayer essential for various cellular functions. The cell envelope plays a crucial role in protecting the cell and maintaining its structural integrity.

What is the fluid mosaic model of biological membranes?

The fluid mosaic model describes the structure of biological membranes. According to this model, membranes are composed of a phospholipid bilayer with embedded proteins and cholesterol molecules. The term 'fluid' refers to the semi-fluid nature of the membrane, allowing lateral movement of proteins and lipids within the bilayer. The 'mosaic' aspect highlights that the membrane is made up of various individual components, such as proteins, phospholipids, and cholesterol, creating a dynamic and flexible structure. This model helps explain the membrane's functionality, including its role in cell signaling, transport, and maintaining homeostasis.

What are amphipathic molecules and why are they important in biological membranes?

Amphipathic molecules have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. Phospholipids, the major component of biological membranes, are amphipathic. Their hydrophilic heads face the aqueous environment, while their hydrophobic tails face inward, away from water, forming a bilayer. This arrangement is crucial for the membrane's structural integrity and function. The amphipathic nature allows the membrane to form a barrier that separates the cell's interior from its external environment, while also enabling the incorporation of proteins and other molecules that facilitate various cellular processes.

How do proteins contribute to the structure and function of biological membranes?

Proteins are integral to the structure and function of biological membranes. They can make up 20-80% of the membrane by mass. These proteins are not static; they can move laterally within the membrane, contributing to its fluid nature. Membrane proteins serve various functions, including acting as receptors for signal transduction, transporters for molecules across the membrane, and enzymes catalyzing specific reactions. Their ability to move within the membrane allows for dynamic interactions and responses to environmental changes, making them essential for maintaining cellular homeostasis and communication.

What role does cholesterol play in biological membranes?

Cholesterol is an important component of biological membranes, particularly in eukaryotic cells. It is embedded within the phospholipid bilayer and plays a crucial role in modulating membrane fluidity and stability. Cholesterol helps to maintain the membrane's integrity by preventing it from becoming too rigid or too fluid. It also assists in the formation of lipid rafts, which are specialized membrane microdomains involved in cell signaling and protein sorting. By influencing the physical properties of the membrane, cholesterol is essential for proper cellular function and response to environmental changes.