Eukaryotic Cell Architecture - Online Tutor, Practice Problems & Exam Prep

Eukaryotic cells are characterized by a nucleus enclosed by a nuclear envelope, containing organelles like mitochondria and chloroplasts, which support energy production and photosynthesis. The cytoskeleton, comprising microtubules, microfilaments, and intermediate filaments, provides structural support and facilitates intracellular transport. Eukaryotic DNA is organized into linear chromosomes, allowing for controlled gene expression through spatial separation of transcription in the nucleus and translation in the cytoplasm. Additionally, multicellular structures arise from cell connections via the extracellular matrix and plasmodesmata, enabling cell differentiation and organismal function.

Overview of Eukaryotic Features

Hi. In this video, we're going to be talking about eukaryotic cell architecture. So, this video is dealing with eukaryotic features. Eukaryotic cells are classified by a few distinct features. Now you can see there's a lot of bolded words behind me, but you know all of these terms. These are all terms that you've gone over in your intro bio class, and I'm just going to review them here, so we can all refresh and be on the same page of what defines a eukaryotic cell. So the first thing is that a eukaryotic cell stores its DNA in a nucleus, that is enclosed by a double membrane, termed the nuclear envelope. Now, this nuclear envelope contains nuclear pores, which allows for the nucleus to interact with things outside of the nucleus; things get transported between the pores. And it also contains, or at least inside the nucleus contains a specific area called nucleoli, and this is where ribosome synthesis occurs. Now remember, ribosomes are important for gene expression and protein creation.

Eukaryotic cells have organelles, so they have these internal structures where different types of processes happen and all of them are membrane-bound. They also contain cytoskeletal structural components. These are different types of proteins that really provide mechanical support to the cell just so it doesn't collapse in on itself. And they all, like all other cells, have a plasma membrane to separate the inner part of the cell and the extracellular environment. So like all membranes, this is formed by a lipid bilayer, which if you remember is made up of lipids that can interact with water in one portion and not interact with water in another portion. So this is called amphiphilic. And then, each of the terms, so if the part that can really interact with water is called hydrophilic and the part that can't interact with water is called hydrophobic.

And so like in the nuclear envelope, which contains nuclear pores to allow the cell to, or allow the nucleus to interact and transport things between the nucleus and the external environment. The plasma membrane also has membrane proteins that allow the cell to interact with the intra and extracellular environment. So here's just a very, very simple drawing of a eukaryotic cell. You can see that it has a nucleus. This is pointing specifically to the nucleolus here with this entire structure in blue is the nucleus. And you can see this orange one here is actually the nuclear envelope separating the nucleus from the inside of the cell. Now, all throughout the cell, you have these compartments. Here's one. Here's one. Here's another. Here's one. And all of these are organelles that are surrounded by membranes. And then all cells, including eukaryotic cells, have a cell membrane. This is a plasma membrane, remember it's a lipid bilayer that allows the cell to interact with the extracellular environment. So now, let's move on to the next concept.

Eukaryotic Organelles

So one of the common features of eukaryotic cells is that they all contain organelles. Eukaryotic cells contain numerous membrane-bound organelles, each with a different function. Now, you've already gone over these organelles many times; you've beaten them to death in Biology 101. So, I'm just going to refresh them here. We're going to have an image of them at the end, but if you really feel like you just don't have it down, you need to know these; these are important for cell biology. So, I suggest that if this review isn't enough, go back to your notes, go back to your textbook, and go back to some of the videos on this topic in the introductory biology class to really review the purpose and function of these different organelles, because it's crucial to understanding cell biology. Let's just review them here; maybe you'll feel refreshed at the end and think, "Oh yeah, I got this." The first one is the endoplasmic reticulum, and this is where proteins are synthesized and then exported to other compartments. There are two main types, rough and smooth, and they are called rough and smooth because that's how they look under the microscope. Rough is the place of protein synthesis, which, for protein synthesis to occur, needs a lot of ribosomes, and ribosomes make it look rough, give it an uneven appearance under the microscope. Whereas the smooth endoplasmic reticulum (ER), short for endoplasmic reticulum, is actually a place of lipid synthesis, and so it doesn't need those ribosomes, so it appears smooth.

Now the second organelle is the Golgi apparatus. This is where, once the protein has been synthesized in the ER and then goes to the Golgi if it needs to be modified, sorted, transported, or secreted. If a protein needs to get somewhere in the cell or needs to be secreted, it's got to go through the Golgi.

Now, the next two are mitochondria and chloroplasts, which are kind of unique organelles because they're really responsible for producing energy. Mitochondria, as I'm sure you've heard, are the powerhouse of the cell, and this is because they produce ATP as an energy storage molecule. Mitochondria actually contain their own DNA and their own ribosomes, which is a very unique feature of organelles. Other than chloroplasts, it is the only organelle that can do that. Chloroplasts are actually the location of photosynthesis, meaning chloroplasts aren't in human cells because we don't photosynthesize, but anything that does photosynthesize, including plants and some types of algae, have chloroplasts. Here, chloroplasts produce organic molecule sugars that are responsible for the cell's survival. It also, like mitochondria, contains DNA and ribosomes.

Now we start getting into these, I'm sure you're familiar with those, but now we get to some of the lesser-known organelles. These are lysosomes, which are responsible for intracellular digestion. So, if something needs to be broken up, it's sent to the lysosome. Then we have peroxisomes, which are actually just kind of this really interesting organelle. So, if anything really harmful, like a harmful chemical reaction needs to occur, if it occurs just in the cytosol within the cell, it could really damage some organelles or, you know, proteins or something that the cell needs. So instead, the cell says, okay, we're just gonna put all these harmful chemical reactions into a peroxisome. One example of this is the formation and breakdown of hydrogen peroxide, which cells occasionally need to do. And when they do, they just put them in peroxisomes and say, okay, you're harmful; I'm gonna store you here.

Another organelle is called a vacuole, and this is a place for temporary storage. You're probably most familiar with this in plant cells because plant cells contain a single large vacuole that stores water, and this is why it gives plant leaves their really strong structure, and that's called turgor pressure, the water pressure that allows the plant to remain, sort of vivacious and upright. When the water is out of the vacuole, it sort of wilts. Now, vesicles are an important organelle, and they transport materials to other locations. So if you remember back, the Golgi apparatus sort of processes proteins and gets them ready and sorted to their different locations, but vesicles are actually what carry them there. So, if something needs to get to another part of the cell or secreted, it's got to be carried there through a vesicle. This has major roles in endocytosis, which is entry of products into the cell, or exocytosis, which is exit of materials outside of the cell.

Now cellular organelles and, before I actually fill in this, I just want to say this is important, because people really confuse cytosol and cytoplasm. They say, "Oh, those are the same things," but they're not. So how they're different is that cellular organelles are suspended in cytosol. Any type of this aqueous gel material in a cell that's not in an organelle is called cytosol. But, there's a lot of this gel inside of the organelles as well. So the cytoplasm is the total of the liquid that's inside and outside of organelles. These are different. The cytosol is not an organelle. Cytoplasm is inside and outside of organelles. And a ton of things happen in the cytosol. That's where things are moved. It's where protein and lipid synthesis occur and so many different chemical reactions occur in the cytosol and also in the cytoplasm. Here are really two images that I want to show you. So the first is from here over, and this is this huge cell, this is a eukaryotic cell, obviously, and has all these different organelles. Now there are a lot of things we haven't talked about yet, mainly here. But it's really important that you see the nucleus, not necessarily these. But you'll see as you go through, you have the plasma membrane, the Golgi apparatus, the rough and smooth ER. Then you don't necessarily need to know actin filaments now. But you have the peroxisome, lysosome, ribosomes, mitochondria, and these are all compartments of the cell. And so they are, really crucial. They're what make up eukaryotic cells and are necessary for eukaryotic cell function. Now the second image on this side is talking about the difference between the cytoplasm and the cytosol. So cytoplasm is made up of the cytosol and the gel inside organelles. But the cytosol is just gel outside of organelles. They're not the same. Really important to keep that in mind when we use these terms in the future. So now let's move on to the next concept.

Eukaryotic Origins

So in this video, we're going to be talking about the eukaryotic origins. Multiple theories exist about the evolution of eukaryotic cells. First, and I think the basis of really all of the eukaryotic features is this idea that before there were eukaryotic cells, there were all these prokaryotic cells of different sizes existing. But eventually, these larger prokaryotic cells began taking up the smaller ones. So really, these eukaryotic cells eventually evolved from predatory prokaryotic cells. They sort of consumed these smaller ones. This first gave them a larger size because if these larger prokaryotic cells are eating or consuming all these little small ones, they have to get bigger because they have to be able to take up all these small ones. So first, it gave them the cell size, but then it also, this predation gives a foundation for what we also term the endosymbiont theory. And that explains the presence of organelles like mitochondria and chloroplasts. Mitochondria and chloroplasts are unique organelles. They actually contain their own DNA, which suggests that at some point before they became organelles, they were actually these individual organisms. And so, through this predation process, eventually, one of these smaller cells that was taken up, decided not to be destroyed, sort of stayed there, was able to survive and eventually evolve over time to become the organelle. What you can see here in this diagram, you have this bacteria, which you remember is a small prokaryotic cell. And this right here is a much larger prokaryotic cell. Now, this large one is consuming the small one, and eventually, over time, it is able to remain inside the larger prokaryotic cell and evolve to become what we now know as the mitochondria. This is how it's thought that chloroplasts also evolved. This is called the endosymbiont theory, the idea of this evolution of mitochondria and chloroplasts in eukaryotic cells. One of the things that is currently unknown is whether this large prokaryotic cell was originally bacteria or archaea. So are eukaryotic cells derived from bacteria, archaea? We actually don't know. Some people have very strong opinions on it, but right now, it's just unknown. So now that we've talked about the origins, let's move on to the next concept.

Eukaryotic Cell Structure

So this day we're going to talk about structural features that allow for the support of and transport of various materials in the cell. Eukaryotic cells are supported through a complex structural system. The three main components of this, you've already heard about really in your intro class. I just want to review them. First are microtubules, and these are hollow cylindrical proteins that are responsible for motility, cell organization, and shape. The second are microfilaments, which sometimes are called actin filaments, and these are thin. They're polarized, which if you don't know what polarized means, it just means that the two ends are different. So one end doesn't look like the start of the actin filament is different from the end of this actin filament. The proteins, these actin filaments are really responsible for muscle contraction. And then, the third type is intermediate filaments, which provide a stable scaffold for cell structure. So you can kind of imagine these as really the construction scaffolds for a big construction project is that they really provide support for the cell. Now these three structural features do a lot for the cell. One thing that they do is they provide a framework for internal transport, so things in the cell are constantly moving. They're dynamic. They need to get to one place to another, and these structural features allow for that transport to happen. For instance, motor proteins transporting vesicles containing important chemicals or macromolecules across the cell. They use these structural features. And they also provide the mechanical support for cell division. So there's a lot of movement of the cell when the cell divides. You have all these different organelles, these chemicals, and the DNA going all of these different places into the two daughter cells. And eventually, those daughter cells have to actually split from one another. The really crucial features of a eukaryotic cell that's responsible for this are these structural features, so the microtubules, the actin filaments, and the intermediate filaments. And then they also, which I don't have written down here, but they also just help the cell survive and not collapse in on itself, provide support and the basis for life. So I really love this image, and I hope you do too. I like it when I can actually see real live cells doing something cool. So this is real life cells, and these are the cytoskeletal structures. So you can see actin filaments are in red. So here actin, you see really along the edges here. And then you have microtubules are in green, which you can see them throughout the cells here. Microtubules. And I think this is really neat, because you can really see that these compartments are throughout the entire cell. Now there's another feature here which I didn't mention, it's blue, and this is just the nucleus. And usually in these images, you'll see the nucleus represented or colored in blue. So the structural features are throughout the cell, and they have a lot of important functions in eukaryotic cell biology. So now let's move on to the next concept.

Eukaryotic Genetics

In this video, we're going to be talking about eukaryotic genetic features. So eukaryotic DNA structure and storage allow for tight control of gene expression and cell division. Let's talk about how the DNA is stored. So eukaryotic DNA is formed into linear chromosomes, which I'm sure you remember this is what they look like here. These chromosomes are packaged by histone proteins. Now, you probably have never heard of histone proteins, which is fine. You don't need to know them right this second. We're going to be talking about them a lot later, just sort of know histone proteins exist, and they have something to do with DNA. So chromatin, which is a term you might be familiar with and should know, is the combination of DNA and histone proteins.

So why does the DNA need to actually interact with the proteins? Why does it need to be packaged? Because it needs to be because the genome is ginormous. I mean, it is huge. It wouldn't fit into the cell if it wasn't packaged. This is because the eukaryotic genome contains these large stretches of DNA that really just have this unknown function. It doesn't make any sense. It doesn't code for proteins. We don't know what it does. And so because of this, it makes the genome really huge. And so it has to be packaged in order for it to be able to fit into the cell.

DNA storage is stored in these chromosomes and packaged that way, but gene expression is controlled by physically separating the location of transcription and translation. So transcription occurs in the nucleus and translation occurs in the cytoplasm. Now, the fact that we have these two separate compartments where transcription and translation occur is important because that's what allows us to really control gene expression. So if something gets transcribed and it's not supposed to be, well, it's not going to get exported to the cytoplasm, then it can't be translated. It can't exert whatever function it wasn't supposed to exert. Super important gene expression control.

Just as a reminder, about eukaryotic cell division, it has these different processes where it can result in genetically identical cells, and that's termed mitosis, or genetically similar but not identical cells, and that's called meiosis. So if we look back here, at this image, we can see that, you see first the DNA double helix, which we're familiar with, but then you also see chromatin, which if you remember is the DNA, which here is in blue, but also these green proteins. These are the histones, some proteins. Now there are other things that we're not necessarily going to focus on right now, but then eventually this is packaged into a chromosome, which is a structure we're familiar with. So, this is how DNA is packaged. Let's now move on to the next concept.

Multicellular Structures

This video, we're going to be talking about multicellular structures. So eukaryotic cells are unique in the fact that they can form these multicellular structures in these organisms. And so, because of this, they have to be able to connect cells together in some way, so that they interact as one unit instead of all these different separate cells. So one of the ways they do this is through the extracellular matrix, which attaches cells together and provides extra cellular structure, which makes sense. Extracellular matrix, extra cellular structure. And so, generally, this is made up of proteins. If you want to know the specific names, they're called collagen and proteoglycans, but you don't need to know the exact term. Just know that it's made up of proteins, and it attaches cells together. But a really important feature of it is that it's not rigid. Instead, it's flexible, allowing for movement of cells or organisms. So, the fact that we can move, and I can move my hand right now is due to the fact that the, that the cells that are connected in my body, making up my hand and my elbow, these cells are flexible. They're not rigid, and they have the ability for movement. The extracellular matrix is really important for providing that flexibility.

Now, plant cells are a little bit more unique because they have cell walls, and their cell walls are actually really rigid, which is why plants don't move as much. But, they still have to be able to connect cells together and interact as a whole organism instead of individual cells. So one of the ways they do this is through a structure called plasmodesmata, which forms the cytoplasmic bridges between cell walls. So let's actually look at these. So this here is an example of plasmodesmata down here, and you can see there are these, small structures. There are these tunnels almost between the cell walls of plant cells, and they connect the cells together, and they connect through these cytoplasmic bridges. Now groups of cells together can act as one, and this actually allows for the evolution of multicellular structures, different types of tissues and organisms, which is also referred to as cell differentiation. So the ability of a cell, one cell to turn into another cell. And the presence of both of these different types of cells is due to the fact that they can connect, they can interact, and they can have different functions. So I've already shown you the plasmodesmata, but what I want to show you here is this is really the extracellular matrix, which you can write as ECM. And it's made up of all these different proteins down here that help to support these cells and connect them together as one unit instead of all of these individual cells. So now let's move on.

Which of the following is true about eukaryotic cells?

• A. They all contain a plasma membrane, a cell wall, and divide through mitosis
• B. They all contain a plasma membrane, store their DNA in a nucleus, and have membrane bound organelles
• C. They all contain a plasma membrane, have cytoskeletal elements, and do not contain a nucleus
• D. They all contain a plasma membrane, divide through mitosis, and do not contain a nucleus

Which of the following organelles is the place of protein synthesis?

• A. Golgi
• B. Vesicle
• C. Nucleus
• D. Endoplasmic Reticulum

Which of the following is not a major structural component of the eukaryotic cell?

• A. Microtubulues
• B. Actin Filaments
• C. Plasmodesmata
• D. Intermediate Filaments

The extracellular matrix is important because it does what?

• A. Allows water to flow in and out of the cell
• B. Attaches cells together and provides support for multicellular structures
• C. Connects plant cells through a cytoplasmic bridge
• D. Provides support to the plasma membrane from inside the cell

Here’s what students ask on this topic:

What are the main structural components of the eukaryotic cytoskeleton?

The eukaryotic cytoskeleton is composed of three main structural components: microtubules, microfilaments (also known as actin filaments), and intermediate filaments. Microtubules are hollow cylindrical proteins that provide cell shape, organization, and motility. Microfilaments are thin, polarized structures responsible for muscle contraction and other cellular movements. Intermediate filaments provide a stable scaffold for cell structure, offering mechanical support and maintaining cell integrity. Together, these components facilitate intracellular transport, support cell division, and prevent the cell from collapsing.

How do mitochondria and chloroplasts support the endosymbiont theory?

The endosymbiont theory suggests that mitochondria and chloroplasts originated from free-living prokaryotic cells that were engulfed by a larger host cell. Evidence supporting this theory includes the fact that both mitochondria and chloroplasts contain their own DNA and ribosomes, similar to prokaryotes. Additionally, they replicate independently of the host cell's nuclear DNA. These organelles' double membranes also suggest an engulfment process. Mitochondria are involved in ATP production, while chloroplasts are responsible for photosynthesis, indicating their specialized roles evolved from their prokaryotic ancestors.

What is the role of the Golgi apparatus in eukaryotic cells?

The Golgi apparatus plays a crucial role in modifying, sorting, and packaging proteins and lipids for transport to various destinations within or outside the cell. After proteins are synthesized in the rough endoplasmic reticulum (ER), they are transported to the Golgi apparatus. Here, they undergo further modifications, such as glycosylation, and are sorted into vesicles. These vesicles then deliver the proteins to their specific locations, including the cell membrane for secretion or other organelles for further use. The Golgi apparatus ensures that cellular products are correctly processed and directed to their proper destinations.

How is eukaryotic DNA organized and stored within the cell?

Eukaryotic DNA is organized into linear chromosomes and stored within the nucleus. The DNA is tightly packaged with histone proteins to form chromatin, which helps fit the large genome into the nucleus. This packaging also allows for the regulation of gene expression. Transcription occurs in the nucleus, where DNA is transcribed into mRNA. The mRNA is then transported to the cytoplasm for translation into proteins. This spatial separation of transcription and translation allows for tight control over gene expression, ensuring that only the necessary genes are expressed at the right time.

What is the function of the extracellular matrix in multicellular eukaryotic organisms?

The extracellular matrix (ECM) provides structural support and facilitates cell-to-cell communication in multicellular eukaryotic organisms. It is composed mainly of proteins such as collagen and proteoglycans. The ECM helps attach cells together, forming tissues and allowing them to function as a cohesive unit. It is flexible, enabling movement and dynamic interactions between cells. In plants, the ECM is more rigid due to the presence of cell walls, but it still plays a crucial role in maintaining cell structure and facilitating communication through structures like plasmodesmata.