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There are two types of cells found in living things. In bacteria and archaea we find prokaryotic cells. But if you're a plant or a fungus or an animal like me and Sammy, then you're made of eukaryotic cells, and that's what we're here to talk about today. So stay tuned! (To Sammy) Are you gonna stay tuned watch any of my videos?
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Hi and thanks for tuning in! Today we're going to be talking about eukaryotic cells. Now in a previous video we talked about the prokaryotic cell. The smaller cousin of the eukaryotic cell that's found in bacteria and archaea. But when we talk about eukaryotic cells, we're talking about cells that make up the rest of life. We're talking about cells that are found in plants and animals, in protists and in fungi. Now unlike the prokaryotic cells, eukaryotic cells contain
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lots of internal structures. We call those things organelles and organelles are these little internal cell structures that specialize in a particular function. And these organelles are actually the key to multicellular life. And in fact, eukaryotic cells are the only type of cell that can form multicellular life. It's these internal structures that allow cells to differentiate and specialize in a particular role, which is one of the key things about multicellular
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life is that cells can differentiate and become interdependent on each other. Prokaryotic cells can't do that because they don't have the proper structures to do so. Eukaryotic cells can, so in this video we're going to talk about all the different things that are found inside of all eukaryotic cells, as well as some things that are found in only a select group of eukaryotic cells. So let's start with the outside of the cell and talk about the plasma membrane. The plasma
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membrane in eukaryotic cells is almost identical to that which is found in bacterial cells. It's going to be a phospholipid bilayer. So remember: phospholipids have that unique amphipathic property they've got that charged polar phosphate group that is hydrophilic, while they have those hydrophobic tails. And when they get together in a biological context they form these lipid bilayers with the hydrophilic heads facing outwards either towards the extracellular space or into the
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cytoplasm, both of which are aqueous, and then we have the hydrophobic tails interacting with each other inside the middle. So it's almost like a little sandwich there with a hydrophobic sandwich surrounded by hydrophilic phosphates there to form this bilayer. Now remember: the bilayer, this lipid bilayer, provides what we call selective permeability. Most things can't freely pass across this plasma membrane. This is actually a double-edged sword. We'll talk about
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this in another video about transport, which is how things make it back and forth across the plasma membrane. We'll also talk in detail about the plasma membrane in another video as well.
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Moving inside of the cell, what are some things that we're going to find inside of all eukaryotic cells? Well for one, just like all cells we're going to find cytoplasm. Cytoplasm is this water-based, gelatinous solution in which all cellular components are actually dissolved. So if you if you broke down the cytoplasm you'd find all kinds of dissolved proteins and you'd find dissolved carbohydrates and lipids and so on and so forth contained inside the cell.
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But the cytoplasm is also the home to all of these specialized internal structures that we call membrane-bound organelles that are only found in eukaryotes. So let's talk about the thing that defines a eukaryotic cell. Let's start with the nucleus. So the nucleus is actually a double membrane structure. There's actually an outer and an inner nuclear membrane that forms what we call the nuclear envelope, and this nuclear envelope provides protection
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to the DNA that can be found inside. Now, just like we find with the plasma membrane, the nuclear membranes are selectively permeable, but there are pores inside of it. They're called nuclear pore complexes that do allow things to traffic into the into the nucleus and out of the nucleus when needed. These pores seem to be regulated mainly by the size of the thing. They permit certain things that are small enough to pass through
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while they prevent other things from making it into the nucleus or out of the nucleus. So large things like organelles can't really make it into the nucleus, but smaller things like RNA can actually make it out of the nucleus, and this is actually really important, and we'll talk about that later in the video and in other videos. So if you zoom into the nucleus, you're going to find a couple of key features here. First, you're going to find the genetic information. So you're going
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to find the eukaryotic chromosome or in most cases chromosomes. So unlike the prokaryotic chromosome that is circular, the eukaryotic chromosomes - first off there's often going to be multiple chromosomes as opposed to one, and those chromosomes are going to be linear. The other thing that's interesting about the double-stranded DNA that makes up the eukaryotic chromosomes is that it is wrapped around proteins. They are wrapped on proteins that are called histones
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and these histone proteins combined with the DNA form a DNA protein complex that is referred to as chromatin. And this chromatin offers extra protection - is what coupled with the nuclear envelope, which works together to make sure that the eukaryotic DNA is highly protected from what we would call chemical insults or physical insults. In other words, eukaryotic DNA is much better protected than the prokaryotic DNA which lacks nucleus or proteins
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to protect it. We'll also find a structure called the nucleolus, and the nucleolus is where certain RNAs are produced, particularly your ribosomal RNAs. This is found inside of the nucleus.
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Now, if we move outward from the nucleus when we start to produce that - when DNA needs to be sort of activated - when DNA needs to be turned into a functional product, we are going to have to convert that into a messenger RNA and that messenger RNA is going to have to leave the nucleus and go outside of the nucleus into the cytoplasm to find a ribosome. So we talked about the fact that ribosomes do in fact exist inside of prokaryotic
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cells, and they also exist in eukaryotic cells as well. And from a superficial standpoint, the bacterial and the eukaryotic ribosome are very similar. You've got a large ribosomal subunit and a small ribosomal subunit. They come together kind of looking like a hamburger bun in order to do the process of translation, which is the conversion of that messenger RNA into a protein. Just like we find in the bacterial ribosomes it is about 50/50 ribosomal RNA and
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protein that make up the ribosome., but at the molecular level there are significant differences. The eukaryotic ribosome is what we call an 80s ribosome. It is bigger than the smaller 70s ribosome of prokaryotes and at the molecular level there are significant differences. We talked about in our video about prokaryotic cells that the prokaryotic ribosomes make excellent drug targets, especially for drugs like tetracycline and erythromycin that are able to target the parts
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of the bacterial ribosome that are different from that in the eukaryotic ribosome. Nevertheless, the eukaryotic ribosome does function very similarly using tRNAs to decode that messenger RNA resulting in a protein product. Now, some ribosomes in eukaryotic cells are found in the cytoplasm we call those free ribosomes. But there are other ribosomes that are bound to the membrane of another organelle called the endoplasmic reticulum.
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Bound ribosomes are found in a particular portion of the endoplasmic reticulum called the rough ER. Now, all ribosomes actually begin as free ribosomes, but in some cases when that messenger RNA begins to undergo translation, the first amino acids that join a protein form something called a signal particle, and this signal particle causes the ribosome to be carried from the cytoplasm to
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the membrane of the endoplasmic reticulum. This is particularly important because these proteins that are translated by bound ribosomes are proteins that are either destined to be inserted into a biological membrane or to be exported from the cell. They're bound to be secreted. So they end up going into the rough endoplasmic reticulum where they can begin their process of being modified in order to reach their final destination or perform their final functions. So the endoplasmic
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reticulum is a sprawling organelle that is actually physically attached to the outer membrane of the nucleus, and when proteins are translated by bound ribosomes, they are either inserted directly into the membrane of the rough ER or secreted into the lumen, the internal portion of the rough ER. In both cases, those proteins are likely destined for further modification before being trafficked to their final destination. The other thing that the rough ER does is in addition
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to serving as a site of protein translation it also is sort of our membrane maker. This is where the phospholipids that largely helps you build our biological membranes are actually created.
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There is another part to the endoplasmic reticulum and in most cells this is the diminutive part, it's the smaller portion, it's barely noticeable in many cells, called the smooth ER. And the smooth ER doesn't handle protein translation at all or it appears smooth because when you look at an electron micrograph of that particular portion of the ER there are no bumps, there are no ribosomes. The smooth ER is largely involved in doing things like
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in terms of producing carbohydrates, detoxifying certain toxins that can enter the cell, and ion homeostasis. So it also plays a very important role in the cell, but in many cases, in most cells it's actually quite small. In other cells that need a lot of ions or do a lot of detoxification is actually quite large. Okay? This gets to the point that certain organelles, certain cells, can specialize depending on what their job in the body actually is. Now, those proteins that
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are produced in the rough ER are going to go to one of two places. If they are fully modified by the time they're done being translated, then they will leave the ER in the form of a vesicle, which is a membrane-bound structure that will bud off in the ER and go to their final destination. But, in most cases proteins that are translated by bound ribosomes are actually going to have to go for further processing in another organelle this organelle is called the Golgi apparatus.
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And the Golgi apparatus is a series of stacks that where protein modifications actually occur you can think of the Golgi apparatus as sort of the the cellular postal service. It's where things get sort of modified and tagged and then sent to their final destination.
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So what will happen to these proteins that need to go to the Golgi for further modification? They will bud off of the endoplasmic reticulum in the form of a vesicle. You can think of a vesicle as just a small sort of spherical membrane structure that encloses cargo. That vesicle will then go to the what we call the cis face, or the receiving face, of the Golgi apparatus. It will then, the membrane of the vesicle, will then fuse with the membrane of the Golgi apparatus.
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That protein will then get progressively modified as it progresses through the Golgi stacks and eventually it will reach the trans face, which is sort of the shipping face of the Golgi apparatus. That's the outgoing mail and that modified protein will then be shipped in the form of a vesicle to its final destination. Now that could be going to another organelle like the mitochondria or a lysosome, or it could be going to the plasma membrane, or it could be destined to be secreted.
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It could be something like a neurotransmitter or a hormone like insulin that needs to be secreted out into the extracellular space. Either way, that requires modification in both the endoplasmic reticulum and then eventually into the Golgi apparatus. So let's talk briefly about vesicles. Vesicles are these small membranous structures that are created usually by budding off of another organelle, typically the ER or the Golgi. They can also be formed at the plasma membrane.
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Vesicles are transient structures. That means they don't persist. They persist long enough to get their cargo from point A to point B, and then they sort of dissolve into whatever membrane they are being trafficked to. So they're transient, they're not a permanent structure of the cell, but they are necessary for getting things around the eukaryotic cell. Now what other structures do we have that are involved in proteins and sort of removing stuff from the cell? Well let's talk
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briefly about the lysosome. The lysosome you can think of as the eukaryotic cell stomach. The lysosome has an internal pH around four, so it's very acidic, and this is where things go to be broken down. So for example, if a cell consumes something through a process called endocytosis or phagocytosis, what will happen is that will be ingested in the form of an endocytic vesicle. That vesicle will then fuse with the lysosome dumping its cargo inside of the lysosome
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where the specialized enzymes of the lysosome will then break down those food particles and and turn it into useful products. The lysosome is also where old organelles go to die. So for example, if you have a mitochondrion that's not behaving and producing energy the way it should or certain very large protein complexes that can't be destroyed other ways, they will be brought to the lysosome to be broken down so that their constituent parts can be recycled and reused.
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In cells like the macrophage, for example, whose dedicated job is to go around eating things, particularly foreign invaders that are bad for your body, they're part of the immune system, they are literally chalked full of lysosomes because that's their job. But if you look at a cell like a muscle cell they're only going to have a few lysosomes because their main job isn't to eat things. Their job is to help the body get from point A to point B, so again you
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can see lysosomes are one of those organelles that can be present in large numbers or small numbers depending on the particular specialty of that given eukaryotic cell. So now let's move on to some very unique organelles. One of which is found in all eukaryotic cells and the other one is only found in a certain group of eukaryotic cells. The first one is the mitochondrion, or the mitochondria (plural). As you probably know, mitochondria are the powerhouse of the cell.
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They are a double membrane organelle. They have an inner and an outer membrane, and they contain many of the enzymes required for cellular respiration to occur. So they're going to include the enzymes inside of its gelatinous interior which is called the matrix, which is basically the cytoplasm of the mitochondria. Inside the matrix you have the enzymes to do things like pyruvate oxidation.
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You'll also find the enzymes for the citric acid cycle. Inside of the inner mitochondrial membrane you'll also find the protein complexes that are involved in electron transport. So basically, about two-thirds of everything that happens in cellular respiration, or production of energy, inside of eukaryotic cell happens inside the mitochondrion. Of particular interest about the mitochondria is that they are likely of what we call endosymbiotic origin. In other words,
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they used to be a free-living species of microbe. We believe at one point they were a free-living species of bacteria that were able to respire. They were ingested by another prokaryote at some point and rather than digesting them as food they began to enter into what we call a symbiotic relationship. Over time, that relationship persisted and became so interdependent that mitochondria can no longer exist as an independent or as an independent microbe. They now exist
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specifically as a dedicated organelle used to produce energy. And this particular event happened very early on in the formation of eukaryotic life. It is now found in almost all eukaryotic cells. There are a couple groups that have lost their mitochondria through reductive evolution, but that's a separate story for another day. Evidence to point to the endosymbiotic theory is this. First off, mitochondria have two membranes. Second off, they have their own
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DNA and that DNA is circular like that we find in my in bacteria. Furthermore, the structure of the genes are actually bacterial in nature as well. Thirdly, your cells can't actually produce mitochondria. A mitochondrion can only come from an existing mitochondria. So what happens if you need more mitochondria? They divide on their own through binary fission, like prokaryotes. So there's a mountain of evidence to suggest that the endosymbiotic theory is correct and supports
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the idea that now that mitochondria are dedicated and dependent organelles within our cells, at one point they were a free-living species of respiring microbe. The same thing can actually be said of chloroplasts. Now chloroplasts are also an energy producing organelle that are found exclusively in algae and plants. Chloroplasts host the photosynthetic material needed to undergo photosynthesis. That's where you're going to find your chlorophyll in plant species.
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And again like we see with mitochondria, chlorophyll have two membranes. They have their own DNA that is circular in nature. They reproduce through a process very similar to binary fission. Inside of the chloroplast you're going to find a gelatinous substance called the stroma and internal structures called the thylakoids. The thylakoids, in the thylakoid membrane is where you're going to have the light reactions of photosynthesis take place. And inside the aqueous
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stroma, that's where you're going to find the the enzymes that perform the Calvin cycle to complete the photosynthetic process. So while all eukaryotes are going to have mitochondria, including plants and algae, only algae and plants are going to have chloroplasts in order to do their photosynthesis. Here's some other structures that we typically find only in algae and plants. One of them is what we call the central vacuole. So some protist species like algae have a central
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vacuole. In terms of animals, plants, and fungi, typically only plants have these vacuoles. So vacuoles are a membranous structure. Don't confuse them with vesicles. Unlike vesicles, vacuoles are a permanent structure. Plants quite often have a central vacuole whose job is to sort of retain water. So in order to prevent water loss or to retain water in cases of times where they undergo drought, these cells hold water inside of this and they can secrete that water to reinforce
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and make sure their cytoplasm stays at the right consistency when water isn't freely available.
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Another structure we find on the outside of some eukaryotic cells, but not all is a cell wall. So animal cells specifically lack cell walls, as do most protist species. So protist species like the amoebas, for example, do not have a cell wall. However, plants and algae are going to have a cell wall that provides rigid structural support, not unlike we see in bacteria and archaea. The difference there is that the cell wall of plants and algae are made out of a carbohydrate called
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cellulose. This is an indigestible polysaccharide that consists of repeating glucose subunits. If we look at fungi, another eukaryotic species, they have a cell wall. But theirs is going to consist of of chitin. Chitin is a polysaccharide, also indigestible structural polysaccharide, that consists of repeating n-acetyl-glucosamine monomers all bound together to form the cell wall. Again, animals don't have them as well as other protist species that lack them. So in order to
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maintain structural integrity, as well as to allow for things like movement, eukaryotic cells have evolved a network of proteins that are referred to as the cytoskeleton. So the cytoskeleton exists inside the cell and it consists of three major types of protein structures: microfilaments, intermediate filaments, and microtubules. So let's talk about what each one of those do. So let's start with the microfilaments. Microfilaments consist of actin
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monomers. So these actin proteins polymerize and form these long tracks along which motor proteins like myosin do their work. This allows for movement. So microfilaments are particularly important for amoeboid movement. For example, so if you see how amoebas move, or some of your cells move through the production of pseudopods, it's called cytoplasmic streaming, that's largely the result of actin. Actin fibers are also very important for mitosis,
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the stretching of the cell. And the other thing is that actin filaments are also very important and actin filaments working with their motor proteins are very important for muscle contraction. So when you contract a muscle, one thing that's happening is the myosin heads are working actively alongside the actin filaments to contract that muscle. Let's move to the intermediate filaments. So
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intermediate filaments really don't have any role in movement at all. Intermediate filaments you can think of as like steel girders providing rigid structural support on the inside of the cell. Okay? The big thing that intermediate filaments actually do is this: they actually act as like steel girders that we can anchor cell structures to. So for example, if you don't want your nucleus to move around a lot, which you don't, inside of your cells there are lots of fibers that
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attach the nucleus and sort of strap it to the intermediate filaments to keep it locked in place from just bouncing around inside the cell. Same thing with lots of other organelles like your mitochondria or your lysosomes. They sort of act as a way to sort of tether lots of your organelles to hold them in place so they're not bouncing all over the cell. They also resist compression, so it prevents the cell from squeezing together too much. They resist compression in the cell to
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give it sort of an elasticity if you will. The last one are the microtubules. Microtubules are composed entirely out of tubulin proteins that form these little tiny hollow structures, and microtubules are very important as tracks for movement. So if you need an organelle to get from point A to point B or if a vesicle needs to go from the ER to the Golgi apparatus, they're going to walk along, they're going to be carried along these microtubules by motor proteins like dynein.
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Microtubules are also essential for the separation of chromosomes during mitosis as well. So microtubules do a lot as well. So those are the three major components of the cytoskeleton. Now those tubulin proteins are also very important for some of the locomotive structures found on some eukaryotic cells, particularly flagella and cilia. Both of these consist of what we call a nine plus two microtubule arrangement where you have a circle of nine microtubule fibers producing a
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hollow tube and then two of these fibers running up the middle. Nine plus two arrangement, you can see it here in this electron micrograph. Flagella and cilia work very similarly. Flagella, most cells typically only have one, if you're eukaryote. And this is going to act to propel the whole cell wherever the cell needs to move to, whether it's chemotaxis or whatever. On the other hand, cilia typically line the entire cell and they can either work together in a coordinated
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fashion to move that cell. For example, if you look at a paramecium you see lots of cilia. Or they can function on the outside of a cell to move things along the surface of a cell. So for example a lot of your lung cells for example, your lung epithelial cells have these cilia that help to move particles and trap particles and move them back up what we call the mucociliary escalator to remove them from your body. So they can either move the cell or move things along the surface of
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the cell. Either way, they're both constructed entirely out of tubulin proteins in eukaryotes.
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Another thing that we see on the exterior portion of a lot of eukaryotic cells is what we call the extracellular matrix. So if you go back to our conversation about prokaryotes you remember some of them have glycocalyces that are either called capsules or slime layers. In eukaryotes, this is what we call the extracellular matrix. It's just like the glycocalyx in eukaryotic terms. So the extracellular matrix is important for several reasons. One major reason is that
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the extracellular matrix is what is produced and secreted by cells to hold tissues together. So if you've got a lot of different eukaryotic cells in a multicellular organism working together to form a tissue, they have to be held together in some way. One way this is done is by secreting the extracellular matrix, which consists almost entirely of proteins and carbohydrates working together in order to be able to sort of coordinate and maintain that tissue.
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The other thing that happens in the extracellular matrix is through this particular extracellular matrix you're going to find protein receptors that extend from the plasma membrane out through the extracellular matrix, and this is going to allow communication with the outside world as well. The last thing we're going to talk about are cell junction complexes. So as I mentioned before, in multicellular eukaryotes cells often come together to form a tissue.
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Now the extracellular matrix does a good job of helping to maintain that structural integrity, but these cell junctions are essential for truly maintaining cell integrity within tissues. One example found exclusively in plants of these particular cell junctions what are called plasmodesmata. So plasmodesmata are little proteinaceous channels that allow dissolved solutes from the cytoplasm of one cell to flow into the cytoplasm of a neighboring cell.
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So in a very real sense, the cells in the deepest roots of a plant are connected directly via these plasmodesmata to the cells in the uppermost shoots of those particular plants. The equivalent structure found in animal cells are what are called gap junctions. So gap junctions are proteinaceous channels that directly connect the cytoplasm of one cell to its neighboring cell. So like we see with the plasmodesmata, dissolved solutes, particularly ions,
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are a very common thing to be shared between the cytoplasm of neighboring cells. Now gap junctions are actually extremely important for a type of cell signaling known as gap junction signaling. Gap junction signaling is important for coordinating the activity, for example, of muscle contractions in your heart muscle cells. So what happens in order for your heart to beat properly is the cells of that tissue need to contract in a very coordinated fashion. They can't all
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contract at once. They have to contract in order and this is done by the sharing of calcium ions from the cytoplasm of one cell to the next, to the next, to the next, to cause that contraction to actually occur. Without this, your heart would not beat appropriately. So this is what gap junctions do. The last two cell junctions we'll talk about do not allow the transport of material from one
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cell to the next. Instead, they help to hold the cells together. The first one are what we call tight junctions. So tight junctions form water-tight seals between cells to prevent them from essentially leaking, to prevent the tissue from leaking. They're very commonly found in tissues made of epithelial cells, so for example, like your bladder or your skin.
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The last one is what we call a desmosome. Don't be confused by the name. Desmosomes are not related to plasmodesmata. You can think of desmosomes as sort of spot welds promoting the integrity of the tissue. Essentially, what desmosomes do is they connect the cytoskeleton of one cell to its neighboring cell. They're very important for tissues that need to contract or need elasticity. So things like your heart, for example, and also again your skin and your bladder
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have lots of different desmosomes to maintain the structural integrity of those tissues. So those are what happen when we have cells that come together to form a multicellular organism, is you have to have these cell junctures together to maintain the integrity of the tissue. Thank you guys so much for tuning in today. As you can see, eukaryotic cells have a lot going on - lots of dedicated structures and organelles that make eukaryotic cell life possible.
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But remember, it's that complexity of the eukaryotic cell that allows for the complexity of eukaryotic organisms as a whole. Without those dedicated organelles specialized in their functions eukaryotic cells themselves could never have specialized. Without specialized cells we would have no multicellular life and without multicellular life there would be no you and me, and none of these videos! Thank you guys so much for tuning in. I hope
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to see you guys next time. I hope you guys learned a lot. I'll talk to you real soon. Bye!
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