Gram-Positive Cell Walls - Online Tutor, Practice Problems & Exam Prep

Gram-positive bacteria possess a thick peptidoglycan layer in their cell walls, which retains the gram stain. This layer allows small molecules like sugars and amino acids to pass through. Unique to gram-positive bacteria are teichoic acids, which are complex polymers that act as cation reservoirs, binding positively charged ions like magnesium and calcium. There are two types: wall teichoic acids linked to peptidoglycan and lipoteichoic acids that extend to the plasma membrane. The periplasmic space lies between the peptidoglycan layer and the plasma membrane, contributing to the overall structure and function of the cell wall.

Gram-Positive Cell Walls

This video, we're going to talk more details about gram-positive cell walls. Recall from our previous lesson videos that gram-positive bacteria will actually absorb and retain the gram stain, unlike the gram-negative cells. Also recall that gram-positive cell walls are made up of a thick layer of peptidoglycan, unlike gram-negative cell walls which have a thin layer of peptidoglycan. Even though gram-positive cells have this thick layer of peptidoglycan, really small molecules such as sugars and amino acids can pass freely across this thick layer of peptidoglycan.

Now, in between the cell wall and the plasma membrane is a gel-like substance called the periplasm. The periplasm is also sometimes referred to as the periplasmic space. It's the region that has this gel-like substance.

There are also these complex polymers of sugars called teichoic acids. These teichoic acids are these complex polymers of sugars that are only found in the cell walls of gram-positive bacteria. The teichoic acids are not generally found in the cell walls of gram-negative bacteria.

If we take a look at our image down below, we can get a better understanding of this overview of the gram-positive cell wall structure. Over here, notice that we're showing you a gram-positive cell that has a gram-positive cell wall. We know that it's going to have a thick layer of peptidoglycan, which is the layer that is in blue. If we zoom into this particular region, which is what you see here, you can see that the peptidoglycan layer is really thick. Embedded within this peptidoglycan layer, you'll find these teichoic acids, which are running vertically here through this part of the image.

Once again, these teichoic acids are these complex polymers of sugars that are only going to be found in gram-positive cell walls, not in gram-negative cell walls. So that is one difference that you want to make sure that you are aware of. Notice that in between the peptidoglycan layer that's above and the plasma membrane that's below, right in between the two is the periplasm. The periplasm is also sometimes referred to as the periplasmic space.

This here concludes our brief introduction to the gram-positive cell walls, and we'll be able to get some practice applying these concepts as we move forward. Then we'll talk about the Gram-Negative Cell Walls. So I'll see you all in our next video.

The cell wall of Gram-positive bacteria:

• A. Contains a thin layer of peptidoglycan.
• B. Contains a thick layer of peptidoglycan.
• C. Is an excellent barrier to most small molecules.
• D. Contains an outer membrane.

___________ are sugar polymers found in the cell wall of gram-positive bacteria.

• A. Lipoproteins.
• B. Lipopolysaccharide.
• C. Teichoic Acids.
• D. Slime layers.

Teichoic Acids

In this video, we're going to talk more about the teichoic acids that are found in gram-positive cell walls. Again, these teichoic acids are complex polymers of molecules called glycerol or ribitol that are connected by negatively charged phosphate groups. Now these negatively charged phosphate groups are going to help the teichoic acids act as a cation reservoir, and recall cations are ions that are positively charged. This is because these negatively charged phosphate groups can bind to positively charged molecules, such as magnesium and calcium, for example. This idea here is going to be really important later in our course when we talk about the gram staining procedure in detail. That's why we talk about it here. Now, there are two types of teichoic acids. There are what are known as wall teichoic acids, which are only going to be linked to the peptidoglycan layer found in the cell wall. Then there are also lipoteichoic acids, which are going to be linked to the cell membrane of the cell and span the entire peptidoglycan layer. If we take a look at our image below, we can get a better understanding of this. Notice that we're focusing on a gram-positive cell wall. At the top, we have the peptidoglycan cell wall layer here, which we know is going to be a thick peptidoglycan layer. Embedded within this thick peptidoglycan layer, we have these teichoic acids running vertically. There are different types of teichoic acids. The lipoteichoic acids linked to the cell membrane go all the way down and link to the plasma membrane at the very bottom. We have those here and here as well. Then we also have the wall teichoic acids, which are only linked to the peptidoglycan wall and do not run all the way down to the cell membrane. Again, you can see that the periplasm is over here, in between the peptidoglycan layer and the plasma membrane. If we zoom into one of these teichoic acids here, right into this region that you see here, that's what you see. You'll see that there are complex polymers, of either glycerol or ribitol, abbreviated here and linked by these phosphate groups that are in between. Because the phosphate groups are negatively charged, once again, this allows the teichoic acids to act as a cation reservoir because they can bind to positively charged molecules. Once again, these teichoic acids are specific to gram-positive cells. Gram-negative cells do not have teichoic acids in their cell walls. In gram-negative cells, as we'll see moving forward, the cell wall is going to be anchored to the membrane using a specific lipoprotein. We'll be able to talk more about this later in our course. But for now, this here concludes our brief lesson on teichoic acids of gram-positive cell walls, 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.

Teichoic acids are typically found:

• A. In the cell wall of gram-negative cells.
• B. The outer membrane of gram-negative cells.
• C. In the cell wall of gram-positive cells.
• D. The outer membrane of gram-positive cells.

Here’s what students ask on this topic:

What are the main components of gram-positive cell walls?

Gram-positive cell walls are primarily composed of a thick layer of peptidoglycan, which is a polymer consisting of sugars and amino acids. This thick peptidoglycan layer is responsible for retaining the gram stain. Additionally, gram-positive cell walls contain teichoic acids, which are complex polymers of glycerol or ribitol linked by negatively charged phosphate groups. These teichoic acids can be categorized into wall teichoic acids, which are linked to the peptidoglycan layer, and lipoteichoic acids, which extend to the plasma membrane. The periplasmic space, a gel-like substance, lies between the peptidoglycan layer and the plasma membrane.

How do teichoic acids function in gram-positive bacteria?

Teichoic acids in gram-positive bacteria serve several important functions. They are complex polymers of glycerol or ribitol connected by negatively charged phosphate groups. These negatively charged groups allow teichoic acids to act as cation reservoirs by binding positively charged ions such as magnesium and calcium. This binding helps stabilize the cell wall and regulate ion transport. There are two types of teichoic acids: wall teichoic acids, which are linked to the peptidoglycan layer, and lipoteichoic acids, which extend to the plasma membrane. These acids are unique to gram-positive bacteria and are not found in gram-negative bacteria.

What is the role of the periplasmic space in gram-positive bacteria?

The periplasmic space in gram-positive bacteria is a gel-like substance located between the thick peptidoglycan layer and the plasma membrane. This space, also known as the periplasm, plays a crucial role in various cellular processes. It contains enzymes and proteins that are involved in nutrient acquisition, cell wall synthesis, and the breakdown of toxic substances. The periplasmic space also helps in the transport of small molecules, such as sugars and amino acids, across the cell wall. Although it is more prominent in gram-negative bacteria, the periplasmic space in gram-positive bacteria still contributes significantly to the overall structure and function of the cell wall.

Why do gram-positive bacteria retain the gram stain?

Gram-positive bacteria retain the gram stain due to their thick peptidoglycan layer in the cell wall. During the gram staining procedure, the crystal violet stain penetrates the cell wall and forms a complex with iodine. This complex is trapped within the thick peptidoglycan layer, making it difficult to wash out with alcohol or acetone. As a result, gram-positive bacteria appear purple under a microscope. In contrast, gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, which allows the crystal violet-iodine complex to be easily washed out, causing them to appear pink after counterstaining with safranin.

What is the difference between wall teichoic acids and lipoteichoic acids?

Wall teichoic acids and lipoteichoic acids are two types of teichoic acids found in gram-positive bacteria. Wall teichoic acids are linked exclusively to the peptidoglycan layer of the cell wall and do not extend to the plasma membrane. In contrast, lipoteichoic acids are anchored to the plasma membrane and extend through the entire peptidoglycan layer. Both types of teichoic acids are composed of glycerol or ribitol polymers connected by negatively charged phosphate groups, which allow them to bind positively charged ions like magnesium and calcium. This binding helps stabilize the cell wall and regulate ion transport.