Introduction to Bacterial Cell Walls - Online Tutor, Practice Problems & Exam Prep

Bacterial cell walls are crucial for maintaining structural integrity against high internal water pressure. They primarily consist of peptidoglycan, a rigid polysaccharide-protein complex unique to bacteria. Gram-positive bacteria have a thick peptidoglycan layer, while gram-negative bacteria possess a thin layer and an outer membrane. The structure includes glycan chains formed by N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked by beta-1,4 glycosidic bonds, and tetrapeptides that enhance rigidity. Understanding these differences is essential for grasping bacterial classification and antibiotic resistance mechanisms.

Introduction to Bacterial Cell Walls

In this video, we're going to begin our introduction to bacterial cell walls. It's important to note that the water pressure inside a bacterial cell is typically higher than its surroundings. There is higher water pressure inside the cell and lower water pressure outside the cell. This requires the bacterial cells to have a protective layer that surrounds the cell. This is why most bacterial cells have a cell wall. A cell wall can be defined as a semi-rigid structural layer located on the outside of the membrane, on the outside of the cell membrane, and it is part of the cell envelope. Recall from our previous lesson videos that the cell envelope is just a term that refers to all of the layers that surround the outside of the cell. The cell envelope includes a collection of the cell wall, the cell membrane, and an outer membrane as well, if that outer membrane is present.

Some bacteria, as we'll learn moving forward in our course, will have two membranes. They'll have an inner membrane, and then they'll also have an outer membrane. We'll talk more about those later in our course. In our example below, we're showing you how the cell wall protects the cell from rupturing due to high water pressure inside. Here, we show you that water is moving towards the inside of the bacterial cell. This high water pressure inside the bacterial cell creates the possibility of rupturing if there was not a cell wall. Thankfully, most bacteria have a cell wall. Notice that the cell wall is this light blue layer that you see in between here, and the cell wall is made of a molecule called peptidoglycan. We're labeling the cell wall as peptidoglycan because that is the predominant molecule that makes up the cell wall. We'll talk more about peptidoglycan as we move forward in our course.

You'll also see that part of the cell envelope, the peptidoglycan cell wall, is included in the cell envelope. The cell envelope also includes an outer membrane if it's present or a capsule. Going all the way around here, this is the outer membrane or the capsule. Then you have the peptidoglycan, which is the cell wall itself, in light blue that goes all the way around. On the inside here, what you have is the inner membrane, also known as the cell membrane. We'll just label it here as the cell membrane. The cell membrane would be this layer. I'll do it in red so that it's easier to see. It's this layer that you see all the way around here. This is showing you the outer layers that surround a bacterial cell. We'll get to talk more about all of this here as we move forward in our course. But for now, this here concludes our brief introduction to bacterial cell walls, and I'll see you all in our next video.

Which is (are) true concerning the cell wall of prokaryotes?

• A. It determines the shape of the bacteria.
• B. It is part of the cell envelope.
• C. It prevents the bacteria from bursting.
• D. All of the choices are true.

Peptidoglycan

In this video, we're going to introduce peptidoglycan. Peptidoglycan is a rigid, mesh-like polysaccharide and protein mixture that is the main component of specifically bacterial cell walls. Peptidoglycan is a molecule specific to bacteria and is not really found in archaea or in eukarya. It's important to keep that in mind. Peptidoglycan provides structural support and helps to maintain rigidity for the cell, as it is the main component of the bacterial cell walls.

If we take a look at our image down below, notice over here we're showing you a bacterial cell. We are focusing mainly on the bacterial cell wall, which is the light blue layer that goes all the way around. If you zoom into that light blue layer, what you'll notice is that the structure is mainly composed of peptidoglycan. What you see down below right here is the structure of peptidoglycan. It consists of polysaccharides—these are the polysaccharide components—but then it also has protein linkages. Peptidoglycan is really a polysaccharide and protein mixture.

We'll talk a lot more about the structure of peptidoglycan in our next video. But for now, this here concludes our brief introduction to peptidoglycan. We'll be able to get some practice in our next video.

The peptidoglycan molecule is responsible for the:

• A. Entry and exit of molecules into and from the cell.
• B. Flexibility of the cytoplasmic membrane.
• C. Motility of the bacterial cell.
• D. Genetic characteristics of the bacterial cell.
• E. Semi-rigid cell wall structure of prokaryotes.

Peptidoglycan Structure

In this video, we're going to talk more about the peptidoglycan structure. Peptidoglycan consists of repeating units of two different monosaccharides. These monosaccharides are linked to each other covalently by a special type of bond known as a beta-1,4-glycosidic linkage. The two monosaccharides are N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These NAG and NAM subunits are repetitively linked together to create a long chain known as a glycan chain. So, the glycan chain is really just a sugar chain, made up of repeating units of NAG and NAM repetitively linked together.

If we take a look at the image on the left-hand side below, it shows an image representing peptidoglycan structure. It shows multiple glycan chains; each referred to as a glycan chain. We're displaying three glycan chains here, and each of these chains consists of repeating NAG and NAM subunits. Notice that the NAG subunits are in blue and the NAM subunits are in green. In addition to these glycan chains, there's also a four amino acid chain known as a tetrapeptide. Since 'tetra' is the prefix that means four, and these are amino acids, it forms a small peptide. This tetrapeptide is attached to each of the NAM molecules and is important for the cell wall structure to provide extra rigidity.

In the image below, in addition to these glycan chains, what links these glycan chains are tetrapeptide units, which are very important for linking adjacent glycan chains together. On the right-hand side image, it zooms into this specific region. Here you can see an N-acetylglucosamine molecule, which is the NAG subunit, and an N-acetylmuramic acid molecule, which is the NAM subunit. Notice that the NAG and NAM subunits are linked to each other via a bond known as a beta-1,4-glycosidic linkage, which links monosaccharides together. Branching off of the NAM subunit is a tetrapeptide. This tetrapeptide consists of four amino acids, and each of these circles represents an amino acid. This concludes our brief introduction to peptidoglycan structure, 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.

Peptidoglycan is made up of:

• A. N-acetylglucosamine
• B. N-acetylmuramic acid & N-acetylglucosamine
• C. N-acetylmuramic acid, N-acetylglucosamine, & amino acids
• D. N-acetylmuramic acid, N-acetylglucosamine, & phospholipids
• E. N-acetylmuramic acid, N-acetylglucosamine, & ribosomes

The NAG and NAM molecules of peptidoglycan are connected by a:

• A. A tripeptide chain
• B. β-(1,4) glycosidic linkage
• C. A disulfide bridge
• D. β-(1,6) glycosidic linkage
• E. A phosphodiester linkage

Peptide Interbridge

In this video, we're going to introduce the peptide interbridge. The peptide interbridge is the cross-link between tetrapeptides of adjacent glycan chains. If we take a look at our image down below on the left-hand side, you should notice that this is the structure of peptidoglycan. It consists of these glycan chains, which are repeating NAG and NAM subunits going across here. Here, you can see three glycan chains. Branching off of these glycan chains are these tetrapeptides, which you can see at these positions. These are the tetrapeptides that we talked about in our previous lesson video. What you'll also notice is that in between these tetrapeptides are these interbridges, which you can see here and here and here and here. The gray little circles that you see there are amino acids that serve as the peptide interbridge.

Now, positive cells. Positive cells. They are not going to be found in gram-negative cells. In gram-negative cell walls, the tetrapeptides are going to be linked directly to each other. So, you won’t see peptide interbridges in gram-negative cell walls. You'll only see the peptide interbridges in gram-positive cell walls. This peptide interbridge is going to be important for these gram-positive cells because it helps to form an interconnected network of these peptidoglycan fibers. What you can see here is, once again, these little bridges that I'm highlighting here in blue are the peptide interbridges.

This image over here on the right-hand side is really just a zoom in of this specific region that you see here. When you zoom in, you can see the NAM and NAG repeating subunits. And branching off of the NAM, you have the tetrapeptides. Cross-linking these tetrapeptides is this peptide interbridge. Once again, the peptide interbridge is going to be this region that you see right here. This is going to help to give peptidoglycan a stronger structure and help to link all of these glycan chains together.

This here concludes our brief introduction to the peptide interbridge, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

The glycan chains of adjacent peptidoglycan molecules are connected by:

• A. A sugar backbone
• B. Phosphodiester linkage
• C. A lipid bilayer
• D. Polypeptide chain crosslinking
• E. Disulfide linkages

Gram-Positive & Gram-Negative Bacteria

In this video, we're going to continue our introduction to bacterial cell walls by talking about gram-positive and gram-negative bacteria. And so, bacterial cells can be generally categorized based on whether or not they take up what's known as the gram stain. Now, the gram stain is a specific staining procedure that can differentiate bacteria based on differences in their cell walls. Later in our course in a different video, we'll talk a lot more about the differences between gram-positive and gram-negative cell walls. But for now, what you guys should know is that gram-positive bacteria will absorb the Gram stain, whereas gram-negative bacteria do not absorb the gram stain. Let's take a look at our image down below to get a better understanding of this.

What you'll notice is, on the far left over here, what we have are some bacterial cells that are unstained. They are not stained. And after the Gram staining procedure, the purple stain that's used will be absorbed by the gram-positive cells. And so, the gram-positive cells will absorb this purple stain, and so they will become purple themselves. Whereas notice that the gram-negative cells do not absorb the stain, and so they do not turn purple. Once again, the reason why gram-positive absorb the purple stain and gram-negative do not absorb the purple stain has to do with differences in their cell walls. Later in a different video, we'll talk more about the differences in the cell walls of gram-positive and gram-negative bacteria. But for now, this here concludes our brief introduction to gram-positive and gram-negative bacteria. We'll be able to get some practice as we move forward. So, I'll see you all in our next video.

The Gram stain works because of differences in the ________ of bacteria.

• A. Genetic characteristics
• B. Cell membranes
• C. Cell walls
• D. Capsules

Types of Bacterial Cell Walls

In this video, we're going to continue to talk more about the types of bacterial cell walls. The two types of bacterial cells, gram-positive and gram-negative bacteria, are grouped by the structure of their cell walls. Now, gram-positive bacteria, which absorb the gram stain, have a thick peptidoglycan layer. Whereas gram-negative bacteria, which do not absorb the gram stain, only have a thin peptidoglycan layer. However, gram-negative bacteria also have a complex outer membrane that the gram-positive bacteria do not have. And so let's take a look at our image down below to get a better understanding of this. Notice on the left-hand side what we're showing you are gram-positive cells. And on the right-hand side, we're showing you gram-negative cells. What you'll notice is that the gram-positive cells over here on the right have a thick peptidoglycan layer. And so notice that the peptidoglycan layer is this layer here that's in blue. It's very, very thick. Whereas, if you take a look at the gram-negative cells over here, notice that their peptidoglycan layer is very thin in comparison. So here we can say that there is a thin peptidoglycan layer for gram-negative cells. Now, what you'll notice is that the gram-positive cells have a plasma membrane, an inner membrane, if you will, which is right here. This is the plasma membrane. But they do not have an outer membrane. So the outer membrane is absent in gram-positive cells. However, when you take a look at the gram-negative cells, they have a thin peptidoglycan layer, but notice that they also have an outer membrane that's beyond their plasma membrane. Here's the plasma membrane. The outer membrane is this one that's out here, and so the outer membrane is going to be present for gram-negative bacteria. These are some of the main differences between gram-positive and gram-negative. But there are also other differences that we'll get to talk more about as we move forward in our course. For now, this here is just a brief introduction to these two different types of bacterial cell walls. We'll be able to get some practice as we move forward and learn more as we move forward as well. So I'll see you all in our next video.

Here’s what students ask on this topic:

What is the primary function of bacterial cell walls?

The primary function of bacterial cell walls is to maintain structural integrity and protect the cell from rupturing due to high internal water pressure. The cell wall acts as a semi-rigid layer that surrounds the cell membrane, providing support and shape to the bacterial cell. It is primarily composed of peptidoglycan, a unique polysaccharide-protein complex that offers rigidity and strength. This structural support is crucial for bacteria to survive in various environments, especially those with fluctuating osmotic conditions.

What is peptidoglycan and why is it important in bacterial cell walls?

Peptidoglycan is a rigid, mesh-like polysaccharide and protein mixture that forms the main component of bacterial cell walls. It is unique to bacteria and provides structural support and rigidity. Peptidoglycan consists of repeating units of two monosaccharides, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked by β-1,4 glycosidic bonds. These glycan chains are cross-linked by tetrapeptides, enhancing the cell wall's strength. Peptidoglycan is crucial for maintaining the cell's shape and protecting it from osmotic pressure, making it a key target for antibiotics like penicillin.

What are the differences between gram-positive and gram-negative bacterial cell walls?

Gram-positive and gram-negative bacteria differ primarily in their cell wall structure. Gram-positive bacteria have a thick peptidoglycan layer, which retains the Gram stain, making them appear purple under a microscope. They lack an outer membrane. In contrast, gram-negative bacteria have a thin peptidoglycan layer and an additional outer membrane, which does not retain the Gram stain, making them appear pink. This outer membrane contains lipopolysaccharides, contributing to the bacteria's resistance to certain antibiotics and detergents.

How do NAG and NAM contribute to the structure of peptidoglycan?

N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) are the two monosaccharides that form the repeating units of peptidoglycan. These units are linked covalently by β-1,4 glycosidic bonds to create long glycan chains. Each NAM molecule is attached to a tetrapeptide, a short chain of four amino acids. These tetrapeptides cross-link adjacent glycan chains, providing additional rigidity and strength to the cell wall. This intricate structure is essential for maintaining the bacterial cell's shape and integrity.

What is the role of the peptide interbridge in gram-positive bacteria?

The peptide interbridge in gram-positive bacteria serves as a cross-link between the tetrapeptides of adjacent glycan chains in the peptidoglycan layer. This interbridge, composed of amino acids, enhances the structural integrity and rigidity of the cell wall. It is specific to gram-positive bacteria and is absent in gram-negative bacteria, where tetrapeptides are directly linked. The peptide interbridge helps form a robust, interconnected network of peptidoglycan fibers, crucial for the cell wall's strength and resistance to environmental stress.