Prokaryotic Cell Structure: Study with Video Lessons, Practice Problems & Examples

Prokaryotes, including bacteria and archaea, are essential to Earth's biomass, comprising 60% of all living organisms. They lack a nucleus and membrane-bound organelles, featuring a nucleoid with circular DNA and often plasmids. Bacteria are classified as gram-positive or gram-negative based on their peptidoglycan cell wall structure. Some possess capsules for protection and can form resilient endospores. Movement is facilitated by fimbriae and flagella, while archaea differ biochemically from bacteria, lacking peptidoglycan and thriving in diverse environments, including extreme conditions.

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 on the inside of a bacterial cell is typically higher than its surroundings. There is higher water pressure on the inside of the cell and lower water pressure on the outside of 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 cell membrane. It is part of the cell envelope. Recall from our previous lesson videos that the cell envelope is just a term that's referring 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. As we'll learn moving forward in our course, some bacteria 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. Below in our example, we're showing you how the cell wall protects the cell from rupturing due to the high water pressure found on the inside. Here, we're showing you that water is moving towards the inside of the bacterial cell, resulting in high water pressure inside.

This high water pressure on the inside of the bacterial cell could create the possibility of rupturing if there was not a cell wall. But thankfully, most bacteria have a cell wall. Notice that the cell wall is this light blue layer that you see in between here, and it is made of a molecule called peptidoglycan. We're labeling the cell wall as peptidoglycan because it 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, that the peptidoglycan cell wall is part of the cell envelope, and 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. And, then you have the peptidoglycan, which is the cell wall itself, in the light blue that goes all the way around. And then, on the inside, here, what you have is the inner membrane, also known as the cell membrane.

We'll label it here as the cell membrane. The cell membrane is this layer that you see all the way around here. Really, this is showing you the outer layers that are surrounding 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 that is specific to bacteria and is not found in archaea or eukarya.

That's something important to keep in mind. Peptidoglycan provides structural support, and helps to maintain rigidity for the cell since it is the main component of bacterial cell walls. If we take a look at our image below, notice over here we're showing you a bacterial cell. We're focusing mainly on the bacterial cell wall, which is again, 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 is the structure of peptidoglycan. It consists of polysaccharide components, but then it also has these protein linkages. Peptidoglycan is a polysaccharide and protein mixture. We'll talk a lot more about the structure of peptidoglycan in our next video.

But for now, this 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.

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. Bacterial cells can generally be categorized based on whether or not they take up what's known as the Gram stain. 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 in more detail 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 on the far left over here is some bacterial cells that are unstained. And after the gram staining procedure, the purple stain that's used will be absorbed by the gram-positive cells.

The gram-positive cells will absorb this purple stain, and so they will become purple themselves. Notice that the gram-negative cells do not absorb the stain and do not turn purple. The reason why gram-positive absorb the purple stain and gram-negative do not has to do with differences in their cell walls. Later in our 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, and 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

This video, we're going to continue to talk more about the types of bacterial cell walls. And so the 2 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. And what you'll notice is that the gram positive cells over here on the right, they have a thick peptidoglycan layer. And so notice that the peptidoglycan layer is this layer here that's in blue.

It's very, very, very thick. Whereas, if you take a look at the gram negative cells over here, notice that their peptidoglycan layer is very, very thin in comparison. And 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, they do 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. So 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. And so 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 the brief introduction to these 2 different types of bacterial cell walls. And so 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.

The Glycocalyx: Capsules & Slime Layers

In this video, we're going to begin our lesson on the glycocalyx along with capsules and slime layers. Most bacteria are surrounded by a layer of a sticky gel-like substance called the glycocalyx. This glycocalyx is a complex polysaccharide and creates a layer surrounding the outside of the cell. This glycocalyx layer is represented in the image down below as a yellow border surrounding the cells.

They could be surrounding either gram-negative cells or gram-positive cells. Once again, this is the glycocalyx. This glycocalyx promotes the adhesion of cells to solid surfaces and to other cells as well, allowing cells to stick onto surfaces and to each other. It also plays an important protective role. It protects the cell from dehydration by helping to retain water, and it can also collect nutrients from the environment for energy purposes.

Since it's so sticky, it can stick and collect nutrients. If we take a look at this image down below, once again, the glycocalyx is the yellow border that is surrounding the perimeter of the cell. If you zoom in, what we have here is the glycocalyx, shown as a complex polysaccharide structure that is going to be surrounding the perimeter of the cell. In our images, we represent the glycocalyx with a yellow background as you see here. Now, there are actually different categories of the glycocalyx based on the composition of its structure.

We'll be able to talk more about the different categories of the glycocalyx, like capsules and slime layers, in our next video. So, I'll see you guys there.

What is the function of the glycocalyx in bacterial cells?

• A. Attachment of cells to surfaces and other cells
• B. Protein synthesis
• C. Phagocytosis of other cells
• D. DNA replication

__________ is the slime-like layer of polysaccharides on the outer surface of bacterial cells.

• A. The outer membrane
• B. The cytoplasm
• C. The glycocalyx
• D. The periplasm
• E. The S-layer

Capsules vs. Slime Layers

In this video, we're going to briefly distinguish between capsules and slime layers, which are really two categories of the glycocalyx, two forms that the glycocalyx can take on. And so capsules are highly organized and dense layers of polysaccharides that are tightly anchored to the cell. Now slime layers, on the other hand, are unorganized layers of polysaccharides, and they are going to be easily removable from the cell because they are not as tightly anchored. And so if we take a look at the image down below, notice that the top half of the image is focused on capsules, and the bottom half of the image is focused on slime layers. And notice that both of them are represented as a yellow border that is surrounding the perimeter of the cell.

However, notice that the capsule is very highly organized, whereas the slime layer is not nearly as organized, and it's going to be easily removable from the cell, whereas the capsules on top are going to be tightly anchored. And so, zooming in, you can see the organized structure of the capsule and the unorganized structure of the slime layer. Now, regardless if the glycocalyx takes on a capsule or slime layer form, the glycocalyx is going to be important for the formation of what are known as biofilms. And so, we'll get to talk a lot more about biofilms in our next lesson video.

But for now, this here concludes our brief lesson here on capsules versus slime layers, 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.

___________ are an organized layer of polysaccharides tightly anchored to the cell wall.

• A. Capsules
• B. Periplasms
• C. Slime-layers
• D. Teichoic acids
• E. Lipopolysaccharides

Which of the following is not a function of slime layers and capsules:

• A. Protection against dehydration.
• B. Collect nutrients from its surroundings.
• C. Attachment to other cells & solid surfaces.
• D. Anchors the outer membrane to peptidoglycan.

Pili

In this video, we're going to introduce pili. The surface of bacterial cells can have relatively long filamentous protein structures that are called pili. Pili is actually the plural form of the word. The singular form is pilus. These pili are protein filaments that extend from the surface of the cell.

These extensions can have varied functions. If we take a look at this image down below, notice that we're showing you a bacterial cell right here, and extending off is this long filamentous protein structure that we call a pilus. The pilus can have varied functions depending on the cell type. We'll get to talk about some of the main types of functions of pili in our next lesson video. I'll see you all there.

Functions of Pili

In this video we're going to talk about some of the functions of pili. Pili typically number in only 1 to 2 per cell. Although pili can have many different functions, generally, pili have 2 primary functions that we have numbered down below, 12. The first primary function of pili is cell motility. Motility is really just referring to the ability of an organism to move through its environment.

You can see that in this image, the bacterial cell here is using its pilus to move from the starting point to the finish line over here. Pili can be used for cell motility, for a cell to propel itself and move itself through its environment. Now the second primary function of pili is for DNA transfer. The pilus can be used as somewhat of a bridge to connect one cell to a neighboring cell. This pilus can be used as a bridge or a tunnel to transfer DNA from one cell to a neighboring cell.

You can see here that the DNA is almost like a gift, and bacterial cells are able to exchange DNA with neighboring cells by using their pili. This is something that we'll be able to talk more about as we move forward in our course. But for now, this here concludes our brief introduction to the function of pili. I'll see you all in our next video.

Which of these are true about pili?

• A. Pili are short filaments of pilin on the surface of bacterial cells that allow neighboring cells to adhere to one another.
• B. Pili allow cells to 'crawl' across a surface.
• C. Pili are short filaments on the surface of archaea cells that allow neighboring cells to adhere to one another.
• D. Pili allow cells to share genetic information through a process called DNA transfer or conjugation.
• E. A and B
• F. B and C
• G. A and C
• H. B and D

The presence of fimbriae on a bacterial cell is most likely to have a critical role in

• A. Conjugation
• B. Chemotaxis
• C. Biofilm formation
• D. DNA replication

Endospores

In this video, we're going to begin our lesson on endospores. Recall from way back in our previous lesson videos that a scientist named John Tyndall discovered a heat-resistant form of bacterial cells, which were later termed endospores. These endospores can be defined as a dormant cell that is only produced by some bacteria and are highly resistant to damaging conditions. Because these endospores are resistant to damaging conditions, this means that they're able to survive in conditions where most normal cells would not be able to survive. And because these endospores are dormant, what that means is that they have lowered activity and they are not going to be able to perform the same functions as normal cells would.

Once again, they are highly resistant to damaging conditions such as extreme amounts of heat, toxic chemicals, and nutrient depletion. These endospores are able to survive a lot of conditions where most normal cells would not be able to survive. It's also important to note that endospores are not a form of replication, and so it does not allow a cell to replicate and reproduce. Instead, endospore formation is going to start and end with a single cell, and it does not form more cells, so it's not a form of replication.

Instead, endospore formation is a form of survival. It allows cells to survive these damaging conditions where they might not otherwise be able to survive. A vegetative cell is the scientific way to refer to a normal cell that is going to be replicating and is not dormant. We'll be able to see how vegetative cells can actually form endospores when nutrients are depleted down below in this image. Notice on the left-hand side over here, what we're showing you is a vegetative cell, the normal bacterial cell under normal conditions.

There are nutrients available in the environment, represented by hexagons that indicate glucose. However, when there are no nutrients available, the vegetative cell can undergo a process called sporulation, which we'll talk more about later. This process forms an endospore. When nutrients are depleted, the cell can begin to form an endospore. The endospore is still alive. However, it is a dormant cell and is going to have lowered activity and is not going to be able to perform the same functions as the vegetative cell.

It is able to survive the conditions when there's no nutrients. Once nutrients become available again, the endospore can undergo a process called germination, which we will talk more about later in our course as well. This process by which an endospore reverts back into a vegetative cell. Endospores are usually only produced by gram-positive cells, such as Bacillus and Clostridium, two examples of bacteria that are gram-positive and produce endospores.

One way to help you remember that it's gram-positive cells that typically form endospores is that you could think of endospores as dormant like winter clothes in the summer at the back of your closet. What you can think is when the temperature is positive Celsius, so when the temperature is warm like in the summer, your winter clothes are going to be in a dormant state in the back, and you're not going to be using them very often. By thinking of the positive Celsius here, that can help remind you of gram-positive cells that form endospores. This here concludes our brief introduction to endospores, and we'll be able to apply the concepts that we've learned here as we move forward.

So I'll see you all in our next video.

Endospores are:

• A. A dormant cell-type
• B. A type of vegetative cell
• C. A form of reproduction
• D. Sensitive to damaging environmental conditions

Formation of endospores _________.

• A. Allows bacterial reproduction.
• B. Occurs when the cell is in thriving environmental conditions.
• C. Is called germination.
• D. Can be triggered by adverse environmental conditions.
• E. Occurs in all bacterial cells.

What 2 bacterial genera produce endospores?

• A. Escherichia and Bacillus
• B. Staphylococcus and Streptococcus
• C. Clostridium and Bacillus
• D. Enterobactor and Clostridium
• E. Citrobacter and Staphylococcus

Here’s what students ask on this topic:

What are the main differences between prokaryotic and eukaryotic cells?

Prokaryotic cells, such as bacteria and archaea, lack a nucleus and membrane-bound organelles. Their DNA is circular and located in a region called the nucleoid. They also have plasmids, which are small, extra-chromosomal DNA molecules. In contrast, eukaryotic cells have a defined nucleus containing linear DNA and various membrane-bound organelles like mitochondria and the endoplasmic reticulum. Prokaryotes have a cell wall made of peptidoglycan (in bacteria) or other materials (in archaea), while eukaryotic cells may have a cell wall (in plants and fungi) made of cellulose or chitin, respectively, or lack a cell wall entirely (in animals).

What is the function of the peptidoglycan layer in bacterial cell walls?

The peptidoglycan layer in bacterial cell walls provides structural support and shape to the cell. It is a strong, resilient material composed of sugar chains cross-linked by small peptides. This structure helps maintain the cell's integrity, preventing it from collapsing and protecting it from environmental stress. In gram-positive bacteria, the peptidoglycan layer is thick and absorbs the crystal violet stain, giving the cells a purple appearance. In gram-negative bacteria, the peptidoglycan layer is thinner and located between the plasma membrane and an outer membrane of lipopolysaccharides.

How do gram-positive and gram-negative bacteria differ in their cell wall structure?

Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain, giving them a purple appearance under a microscope. This thick layer is the primary component of their cell wall. In contrast, gram-negative bacteria have a thinner peptidoglycan layer located between the plasma membrane and an outer membrane composed of lipopolysaccharides. This outer membrane does not retain the crystal violet stain, so gram-negative bacteria appear pink or red after a Gram stain procedure. The presence of the outer membrane in gram-negative bacteria also contributes to their increased resistance to certain antibiotics.

What are endospores, and why are they important for some bacteria?

Endospores are highly resilient, dormant forms of bacteria that can survive extreme environmental conditions, such as high heat, desiccation, and lack of nutrients. They form in response to harsh conditions and can remain dormant for long periods, even hundreds or thousands of years. When favorable conditions return, endospores can reactivate and return to their vegetative state. This ability to form endospores allows certain bacteria to endure and persist in environments that would otherwise be lethal, ensuring their survival and continued proliferation.

What are the unique features of archaea compared to bacteria?

Archaea differ from bacteria in several key ways. They do not use peptidoglycan in their cell walls; instead, they have unique membrane compositions, often featuring ether-linked lipids. Their genetic machinery for transcription and translation is more similar to eukaryotes than bacteria, suggesting a closer evolutionary relationship with eukaryotes. Archaea are known for thriving in diverse environments, including extreme conditions like high temperatures, high salinity, and anaerobic environments. Despite these differences, both archaea and bacteria are prokaryotes, lacking a nucleus and membrane-bound organelles.