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

Prokaryotic cells, classified into Archaea and Bacteria, exhibit remarkable diversity in shape, size, and metabolic pathways. They lack a nucleus, containing DNA in a nucleoid as a circular chromosome, and replicate through binary fission. Most prokaryotes possess a plasma membrane and often a cell wall, with some, like cyanobacteria, having complex internal membranes. Their rapid division contributes to genetic diversity, enabling them to thrive in various environments, including extreme conditions. Understanding these features is crucial for grasping the fundamental differences between prokaryotic and eukaryotic cells.

Prokaryotic Diversity

Hi. In this video, we are going to be talking about prokaryotic cell architecture. So first, a lot of text behind me, but it's a really simple concept that I'm going to be talking about over the next few minutes, and that is that prokaryotic cells are diverse, and that's really it. They're just extremely diverse. But let me talk about some ways that they are diverse. So first is that they're classified in these two overarching domains. You've heard these before, archaea and bacteria. Sometimes archaea can be termed archaebacteria. Bacteria can also be called cyanobacteria, but essentially, it all means the same thing, and they're all prokaryotes. Now they come in all different shapes and sizes, but all of them are single-celled organisms, and you can kind of think, or at least I do. I think, oh, single-celled organism, super simple. But that's not true. Actually, they do all of these different diverse processes, which I'm going to talk about a little bit now. But they also can actually form these social groups like films, for instance, or chains. So for instance, the film on your teeth when you don't brush it, that's actually some bacteria, some prokaryotes working together in these social groups that form those films, and they can come in all different types of shapes, spherical, rod-shaped, spirals. So they're very diverse in cell type and size. Now traditionally, we actually didn't think they were that diverse, so, some traditional techniques, which include growing them in laboratories, only identified around 6,000 species, which is relatively small, but we now know that this represents less than one-tenth of 1% of all the prokaryotic species. It's so tiny, such a tiny amount. So how did we find them if all this diversity if we couldn't grow them traditionally in a laboratory? So we sequence the metagenome. So what is a metagenome? Well, that is actually the collective genome of a species in a habitat. Scroll up a little. And, so, what that means is if I were to collect the metagenome, what I, of a certain environment, I would go to that environment, say, hydrothermal vents, and I would collect the water, and then I would just DNA sequence it. And because all of that water in there contains so many different organisms, I would get all the genomic sequences of the organisms that live in the habitat and not just a particular one. And so when I get the sequences back, I have the collective genome of the species in a certain environment or habitat. And then I can say, okay, this is really diverse. This represents so many different thousands of organisms. And that allowed us to identify the prokaryotic diversity. So because there is this huge diversity, thousands upon thousands of prokaryotic species, they have diverse, metabolic pathways. We've talked about some of these that they could be photosynthetic, be aerobic, which is require oxygen, anaerobic don't require oxygen. And the reason that they're so diverse, they're more diverse than eukaryotes is because they rapidly divide. And so if you're rapidly dividing, rapidly one Now, one way that you may have heard members of their archaea domain called is this word called extremophiles, and pretty much the reason they are called that is because they tend to live in harsh environments. They tend to live near volcanoes or deep in the ocean environment. They can literally live anywhere in the world, prokaryotes. And so that also contributes to the diversity because they're not restricted to one environment. They can literally live anywhere in the world, prokaryotes can. So I just wanted to show you this example. This is not nearly, all the shapes and sizes and groups that prokaryotes can form, but you can see here just by looking at it, there are a lot of diversity just in these few organisms I'm presenting. You can see that there are the circle organisms. There are these long stringy ones. They can form social groups, and essentially, they're just all very diverse. So now let's move on to the next concept.

Prokaryotic Features

So we know prokaryotic cells are diverse, but what are some of the structural common features between them? Prokaryotic cells have specific structural features that define them. One of the biggest ones is the lack of a nucleus. "Pro" means before, and "karyotes" means kernel or nucleus. Prokaryote literally means "before nucleus," so they do not have a nucleus. However, what they do have is a plasma membrane, and most of them also contain a cell wall. They don't contain a nucleus, but they do contain plasma membranes, and for the most part, they don't contain internal membranes. These are things like organelles, mitochondria, chloroplasts that you might remember from your introductory biology class.

Now there is one exception to this, and this is called cyanobacteria. Cyanobacteria actually contain really complex internal membranes where they perform photosynthesis. That's the one exception. The rest of the prokaryotic cells do not have internal membranes, but they have to be able to support themselves. So they do have these primitive cytoskeleton elements. Remember back to introductory biology: the cytoskeleton really performs the structure of the cell. And they can move, which we kind of know just through looking at pond water or blood or any of the labs that we've done with cells, is that they actually have the ability to move. And one of the main ways that they do this is through a structure called a flagellum.

Let's look at a prokaryotic cell. You see here that there is no main nucleus, no primary nucleus. There's this thing called a nucleoid, which we'll talk about, but that's pretty much just where they store their DNA, but this is not a nucleus. They have a flagellum, they contain a cell wall, a cell membrane, but generally, they are fairly simple. If you remember back to looking at different cells in your early biology classes, prokaryotes are fairly simple. Now, let's move on to the next concept.

Prokaryotic Genetics

In this video, we're going to be talking about prokaryotic genetic features. So this has to do with DNA storage, structure, and replication. So, how a prokaryotic cell stores its DNA and how it replicates it are distinguishing features of prokaryotic cells. So remember back I said, prokaryotic cells do not have a nucleus, but they do have a structure called a nucleoid, which is just where they keep their DNA. And it's actually a circular chromosome and that's where their DNA is packaged. And so DNA is not contained inside any kind of intracellular compartment, so no nucleus. And they don't have nuclear envelopes, they also don't have nuclei. "Nuclei" is the plural for that. And then, because they don't have these specialized compartments to hold their DNA, they actually don't have that much DNA. So they have about 8,000,000 base pairs for an average prokaryotic cell, amounting to around 5,000 proteins. Now if we think of this compared to a human cell, that's so small. Human cells contain upwards of 25,000 proteins, and that's only 2% of the genome. So a huge amount of genetic material in eukaryotes, but a small amount in prokaryotes. Now, that's how DNA is stored, and how it's structured. But how is it replicated? So prokaryotic cells undergo binary fission, and a complete copy of the DNA is passed on to the daughter cell. So genetic information can also be transferred through organisms through conjugation, which is a different process. Let me go down here and talk about this. So binary fission is where DNA replication occurs and you see you get 2 copies of the entire DNA and that split and then that goes into the daughter cells where each cell contains a single copy. So that's binary fission. Now that's, DNA replication. There's this thing called conjugation and that's just genetic sharing between organisms that's not dividing, that's actually just giving some genetic material between organisms. DNA and then eventually into a protein, so expressing that gene. DNA transcription and gene expression in prokaryotic cells is very simple. We're running into a theme here, where DNA is stored in this simple nucleoid. DNA in prokaryotic cells divide through binary fission in a simple process. DNA transcription is also simple. All happens in a single compartment. It really requires very little processing, and it does use ribosomes for translation, which if you remember back to your bio 101, sort of throwing these terms out here just to review. It uses ribosomes for translation, but they are smaller and less complex than the ones you're familiar with from your intro bio class for eukaryotic cells. So, for prokaryotic cells and their genetic storage, just think simple. It's more simple. So with that, let's move on to the next concept.

Which of the follow is true about prokaryotic cells?

• A. They have a plasma membrane, nucleoid, and nucleus
• B. They have a plasma membrane, nucleoid, and no nucleus
• C. They have a plasma membrane, and reproduce slowly
• D. They have a plasma membrane, reproduce slowly, and divide through binary fission

In prokaryotic cells, where is the DNA stored?

• A. Nucleoid
• B. Nucleus
• C. Nucleolus
• D. Nucleaid

Prokaryotic cells always live alone, and never form social groups.

• A. True
• B. False

Here’s what students ask on this topic:

What are the main differences between prokaryotic and eukaryotic cells?

Prokaryotic cells lack a nucleus and membrane-bound organelles, while eukaryotic cells have a defined nucleus and various organelles like mitochondria and chloroplasts. Prokaryotic DNA is circular and located in a nucleoid, whereas eukaryotic DNA is linear and housed within the nucleus. Prokaryotes reproduce through binary fission, a simpler process compared to the mitosis and meiosis seen in eukaryotes. Additionally, prokaryotic cells are generally smaller and less complex than eukaryotic cells, which can form multicellular organisms with specialized tissues.

How do prokaryotic cells replicate their DNA?

Prokaryotic cells replicate their DNA through a process called binary fission. During binary fission, the circular DNA molecule is duplicated, and each copy attaches to different parts of the cell membrane. The cell then elongates, and the membrane pinches inward, dividing the cell into two daughter cells, each containing a complete copy of the original DNA. This process is simpler and faster than eukaryotic cell division, contributing to the rapid reproduction and genetic diversity of prokaryotes.

What are extremophiles and where can they be found?

Extremophiles are prokaryotic organisms, particularly from the Archaea domain, that thrive in extreme environmental conditions. These conditions can include high temperatures (thermophiles), high salinity (halophiles), acidic or alkaline environments (acidophiles and alkaliphiles), and high pressure (barophiles). Extremophiles can be found in diverse habitats such as hydrothermal vents, salt flats, acidic hot springs, and deep-sea trenches. Their ability to survive in such harsh conditions highlights the remarkable adaptability and diversity of prokaryotic life.

What is the role of the plasma membrane in prokaryotic cells?

The plasma membrane in prokaryotic cells serves as a selective barrier that regulates the entry and exit of substances. It is composed of a phospholipid bilayer with embedded proteins that facilitate various functions, including nutrient uptake, waste removal, and communication with the environment. The plasma membrane also plays a crucial role in energy production through processes like cellular respiration and photosynthesis in some prokaryotes. Additionally, it helps maintain the cell's structural integrity and supports the attachment of the cell wall in many prokaryotic species.

How do prokaryotic cells achieve genetic diversity?

Prokaryotic cells achieve genetic diversity through several mechanisms. One primary method is rapid reproduction via binary fission, which allows for frequent mutations. Additionally, prokaryotes can exchange genetic material through horizontal gene transfer methods such as conjugation, transformation, and transduction. Conjugation involves the direct transfer of DNA between two cells via a pilus. Transformation is the uptake of free DNA from the environment, and transduction is the transfer of DNA by bacteriophages (viruses that infect bacteria). These processes contribute to the genetic variability and adaptability of prokaryotic populations.