Prokaryote Cell Structures - Online Tutor, Practice Problems & Exam Prep

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.

Prokaryotic Cell Structures 1

Hi. In this video, we'll be taking a look at prokaryotes, which can be broken up into bacteria and archaea. Now prokaryotic cells are significantly smaller than eukaryotic cells. And what differentiates them mainly from eukaryotic cells is that they lack a nucleus and they lack membrane-bound organelles. Now, as you can see in this diagram of a prokaryotic cell here, the central region of the cell contains the nucleoid, which is basically just a condensed ball of most or all of the DNA that the organism contains. Now prokaryotes also contain what are called plasmids, which are small molecules of DNA that are extrachromosomal, as in separate from the chromosome of the cell. So you can see, right here, this is our nucleoid, and over here we have these little plasmids. Now you might recall that prokaryotes have circular double-stranded DNA, unlike eukaryotes, like us, who have linear DNA. Now additionally, prokaryotes have a cell wall that's made of peptidoglycan. And that's what gives the cells their shape. This rigid outer wall that you see here in red, that's what's going to give prokaryotic cells their shape. And again, this is in part because unlike eukaryotic cells which have a cytoskeleton to maintain rigidity, this cell wall is there to keep the cell from, you know, collapsing in on itself and to maintain the proper structure. Now peptidoglycan is something we discussed way back when we talked about biological molecules and you might recall that it is comprised of proteins, that's the peptide part, and also carbohydrates, that's the glycan component. And basically, what you have are these sugar chains, which I'm marking here in blue, these are sugar chains, right? That's our carbohydrate. And then we also have these little, little peptides. You can see they're not very big, they're only a few amino acids long. Right? So these are our peptides. And as the figure points out, technically, these are oligopeptides and, that determination comes from the number of amino acids in the chain, but that's really getting into the realm of biochemistry. You guys don't need to worry about that. All you really need to know about peptidoglycan is that it's these sugar chains that are crosslinked crosslinked by small peptides. And that crosslinking is what makes, what makes these cell walls so strong, right? This peptidoglycan, this is a strong resilient material. Now not all bacterial cells have, cell or have, exterior structures that are similar. In fact, there's a divide in bacteria and it's based on this staining technique called the Gram stain. So before we get into what the difference between gram-positive and gram-negative bacteria is, I just want to point out that, this is a distinction defined by a test from a long time ago. Right? So, this microbiologist, his name was Graham, where the name comes from, came up with this staining technique, which uses actually a variety of different stains. We're not going to get into the specifics of how it works, but, so don't, you know, I point that out because don't think that the Gram stain is actually just one stain. It's a type of technique that involves many stains and washing the cells and then applying new stains. It's actually, you know, kind of a longer procedure than this name implies. And essentially, there's a pigment used, right, a dye used that will be absorbed by peptidoglycan. So essentially, this staining technique allows people to observe peptidoglycan in the cell walls of prokaryotes. And, you know, this is, they're looking through microscopes, of course, to see the cells. So basically, some bacteria which have been dubbed gram-positive bacteria because they have a positive test in or they have a positive result in the gram stain, and that is because they have this thick peptidoglycan layer. Right? You can see this thick outer layer of peptidoglycan. So when the Gram stain technique is done to these cells, lots of this particular stain called crystal violet, is going to be absorbed into this thick layer of peptidoglycan. So these cells are going to have a strong purple appearance, due to that crystal violet stain. Now gram-negative bacteria actually have this outer membrane of lipopolysaccharides. And let's pause there. What do you think lipopolysaccharides are? Well, lipo. Right? That's going to be lipid. Right? And then polysaccharides. So it's, you know, again, sugar chains with lipid attachments, so, again, you know, just always be thinking about your prefixes and suffixes when you hear these biochemical names because they'll often reveal what it is we're talking about. So anyways, gram-negative bacteria have this outer membrane of lipid polysaccharides, and then inside that they actually have this thin layer of peptidoglycan. Right? So here's our peptidoglycan. Right? It's just this thin little layer. Our outer membrane, let me actually jump out of the image here, our outer membrane, you can see marked here in green, that dark green color that is made up of lipopolysaccharides. And then of course we have the plasma membrane, this light green interior structure. Right? And this blue, light blue space that you see between the peptidoglycan layer and the plasma membrane as well as the outer membrane, that's actually called the periplasmic space. And this is, literally like a gap between these coatings of the cells, so to speak. And it's actually super important for the realm of microbiology. We're not going to get into it in our discussion, just pointing out that there is a little space there and you can see that in gram-positive bacteria they have just one of those little spaces because they don't have that outer membrane layer. Anyways, so, this is a distinction often used to characterize bacteria. Are they gram negative? Are they gram-positive? And really it's just referring to, sort of how the cell organizes its outer structures. Right? Do they have this thick outer peptidoglycan layer or do they have a little thinner internal peptidoglycan layer with this outer lipopolysaccharide membrane. And again, this is not a distinction, you know, born out of out of some, you know, evolutionary trend, this this is a distinction that is based upon a laboratory test called the Gram stain. So with that, let's flip the page.

Prokaryotic Cell Structures 2

Some prokaryotic cells will actually have an additional outer coating called a capsule, or sometimes it's referred to as the slime layer. I kind of love that name, to be honest. And this is a large polysaccharide coating that will surround the cells, and it's basically a protective layer for the cell. Right? Just another protective layer for that cell. Now, some cells can form what are called endospores, and these are super resilient but dormant forms of bacteria, and they basically form in response to either a lack of nutrients in the environment or some form of really harsh conditions. For example, some bacteria will form these endospores in response to high heat, something like that. So the idea is when the dormant endospore lays low, basically like hibernate, almost for super long periods of time. And seriously, we're talking like 100, even 1,000, debatably even longer periods of time before they will reactivate. And you can see just a bunch of examples of endospores here. The idea being that they come in all shapes and sizes. Don't really need to know anything specific about any of these endospores. So just another defense mechanism that some bacteria have evolved in order to avoid dying in harsh conditions. Now the last thing I want to talk about is how prokaryotes move around and also what appendages these cells use.

So, some have what are called fimbriae. And these, I kind of like to think of them as little arms because they're basically appendages that allow bacterial cells to adhere to surfaces. And you can see these little pink strands on this cell. Those are fimbriae, and they are allowing the cell to adhere to the surface it is attached to. Now, bacteria will also have what are called flagella for plural or flagellum singular. So you can see that this cell has a number of flagella. Right? I'm pointing them out with these red arrows. And these are kind of like whips in the way they move. You can see in this image here that basically they will spin around and are used for locomotion and sensation. So, they can be used for the bacteria to actually move around, and, you know, if you think about it, it's kind of crazy but these cells are so small that the flagella are almost like boring through the medium, like water, for example. Essentially like boring through the water. So I like to think of them as like a drill or something almost. And again, that just has to do with the comparative size of the bacteria to the attractive forces of those molecules. Interestingly, flagella can also be used for sensation. So, there are some that have evolved and been modified as sensory appendages.

Now the last type of appendage I want to talk about is the pilus. Sometimes it's called the sex pilus. And this is an appendage on the surface of many bacterial cells and it's involved in this process called conjugation that we're going to talk about in more depth when we discuss bacterial reproduction. But the basic idea is that this is sort of like a tube through which bacteria can pass DNA. And again, we'll talk more about this concept later but as you can see right here, this little bacterial cell in the diagram has this little appendage, the pilus, and through this pilus, it's going to move some of the DNA in this plasmid. Alright. With that, let's actually turn the page.

Prokaryotic Cell Structures 3

Prokaryotes, bacteria, and archaea make up 60% of Earth's biomass. And by that, I mean, if you took all the living organisms of Earth and you put them on a scale, right, 60% of the weight that you'd be reading on that scale would be due to prokaryotic cells, right? These little microscopic organisms. They were the first life forms, and they also happen to be the most prolific. They're everywhere. I mean, they're inside us, they're on our skin, they're inside other creatures. If you take a scoop of ocean water, you're going to get a ton of them in there. I mean, these guys are everywhere. And they're also in crazy places too, like deep sea hydrothermal vents. They're just amazing organisms. And the reason I'm saying all of this is because it is, in a sense, tragic how little we really get to talk about them in introductory biology. Okay? So we're doing a quick review of them here, but by no means, should you take that as an indication that these are unimportant or uninteresting organisms. They're actually some of the most important, most interesting life forms on the planet. Introductory biology just tends to be a little more focused on the type of organisms that, you know, humans interact or visibly interact with on a day-to-day basis. Okay? So take microbiology, it's truly fascinating.

And now, we are going to briefly, you know, in an almost criminally briefly discuss archaea, which are prokaryotes similar to bacteria, but they have certain features that differentiate them from bacteria and of course, eukaryotes. So they're of a similar size and shape to bacteria. But unlike bacteria, they do not use peptidoglycan in their cell walls. The chemical composition of their membranes is distinct from both bacteria and eukaryotes. Now, just like bacteria, they have a circular loop of DNA. Right? That's their chromosome. You'll find it in a nucleoid in the cell. They also lack membrane-bound organelles and a nucleus. Right? So they are, you know, prokaryotes, they have all those defining prokaryotic features. However, there are some interesting differences between them and bacteria. For one, their genetic machinery that they use for transcription and translation, right, gene expression, happens to be more similar to eukaryotes than to bacteria. Which is pretty fascinating and it's also one of the reasons that eukaryotes are thought to have evolved from archaea not bacteria.

Additionally, archaea reproduce asexually similar to bacteria, but like bacteria, they're capable of forms of gene transfer and, we're going to get into how that works in a different video. So the takeaway is archaea are similar to bacteria, but there are some biochemical differences that separate them as a class of organism. Now the thing most people tend to know about archaea is that they're extremophiles, but this is actually anecdotal. I mean, certainly, there are many species of extremophiles within archaea, but there are many, many types of archaea that don't live in extreme environments. Right?

Really the whole 'archaea or extremophiles' is not a good generalization. Now looking at this image here, these are some really famous archaea which you will find in Yellowstone. This is what's known as the Grand Prismatic Pool. It is a sulfur-rich hot spring, and the archaea that live in there are known as thermophiles. Right? They like heat. And they will live in these hot springs, which are, you know, close to boiling point roughly. They can also be found in these deep sea hydrothermal vents that exude incredibly hot temperatures. I mean, way beyond boiling point. Right? So, they don't all just like the most extreme temperatures. They like a range of temperatures. And that's really the point I'm trying to impress in general is that archaea can be found in all different types of environments.

Some cool ones that I happen to like are the halophiles, and these like to live in salty environments, including environments as salty as the Dead Sea. Right? Which has such a high salinity level that, if you were to swim in it, you'd feel the difference in density of the water. You know, it just feels very thick. Now methanogens are a really cool type of archaea. They produce methane as a byproduct of their metabolism, and these are found all over the place. They live in swamps. Right? That's that kind of funky gross smell that you smell in swamps is, from methane in part, and that's due to methanogens. And also they live in the guts of animals. And, cows for example, or ruminants, have, these methanogens that live inside them, and that's, where, the methane that they fart comes from.

Last last thing I want to point out is, you know, main theme of biology, structure fits function. Right? So here we have the normal sort of phospholipid bilayer. Right? This is composed of, you know, what you can kind of think of as like generic or vanilla phospholipids. Right? And this you'll find in, you know, regular old prokaryotes, eukaryotes, whatever. Here, on the other hand, we have our phospholipids from the hot spring bacteria, and you'll notice that the actual chemical structure of these is pretty different. Right? And it's made of these isoprene units. That's what these are called. Don't worry about the name. The main point is, these phospholipids will stick together much more tightly, which is how the membranes of these hot spring prokaryotes will resist breaking down at those high temperatures. So again, like structure fits function and you're going to see these biochemical differences between the cells of archaea and the cells of bacteria. Alright. That's all I have for this video. See you guys later.

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.