Cell Structure and Processes PDF

In this lesson, we're going to talk about cell structure and cellular processes and kick that off. We've got to talk about the two major types of cells, those are prokaryotes and those of eukaryotes. Now, very quickly, we're talking about prokaryotes. We're talking about bacteria. We're talking abou...

In this lesson, we're going to talk about cell structure and cellular processes and kick that off. We've got to talk about the two major types of cells, those are prokaryotes and those of eukaryotes. Now, very quickly, we're talking about prokaryotes. We're talking about bacteria. We're talking about eukaryotes. We're talking about fungi, plant cells, animal cells, things of that sort. Now, if we look, one of the principal differences between these two is the presence or absence of a nucleus. So prokaryotes, they don't have a nucleus. And eukaryotes, there is indeed a membrane-bound nucleus, and that's kind of where the name comes from, that principal difference. So prokaryote, a karyon, is a Latin word, I believe, Latin, or nucleus. And so prokaryote means before the nucleus, whereas eukaryote, the EU there, that U, means true, and so eukaryote means true nucleus, and indeed eukaryotes have a membrane-bound nucleus. Now, that's a principal difference, but it's not the only difference. So in addition to having no membrane-bound nucleus, no nucleus whatsoever, there's also no membrane-bound organelles in prokaryotes whatsoever. So that means no mitochondria, no chloroplasts, no endoplasmic reticulum, no Golgi apparatus. So whereas in eukaryotes, you've got all of those, or at least the chance of having all those plants are the only ones that actually have the chloroplasts. So what all the rest are going to have mitochondria, endoplasmic reticulum, and Golgi apparatus. So here, we've got a single circular chromosome in prokaryotes, and it's just in the cytoplasm, again, because there's no nucleus, where here in eukaryotes, we're going to have multiple linear chromosomes. They're going to be in that membrane-bound nucleus. And because they're linear, they have ends, right? So they're going to end in telomeres, typically, again, with prokaryotes, no ends, no telomeres. Also there's multiple, not just one, so typically the result, like in humans, we have 23 pairs of 46 total chromosomes. In prokaryotes, you also potentially have some extra chromosomal DNA called plasmids, usually much smaller than the actual chromosome, and don't have these plasmids in eukaryotes. Typically, it's these plasmids that carry bacterial resistance, and those plasmids can be shared via conjugation and stuff like that, so a big problem for us in fighting bacterial infections. Also, reproduction, so a difference between fission and mitosis. Now, all the things you think about with mitosis, and going through, like, you know, prophase, metaphase, and all the different phases and stuff, so think about the spindle apparatus in the microtubule organizing centers, the centrozomes and the centrozomes, and the centrozomes, like that. None of that exists in fission, so there's no centrozomes, no centrozomes, no microtubule organizing centers present in prokaryotes whatsoever. So, and finally, a difference in cell walls, and I don't want to say like one has cell walls and one doesn't, so you do find typically cell walls present in prokaryotes, and you may or may not find them present in eukaryotes. So, typically, plants and fungal cells have cell walls, but animal cells do not, but we will see a difference. We'll take a little closer a little bit later. We will see a difference in the composition in the different cell walls as well. So, I'm going to take a little closer look at an actual bacterial cell here, and you can see that again, there's no nucleus. So, your big circular chromosome here, which is all supercoiled up and stuff, it's just right in the cytoplasm. So, no membrane-bound nucleus, and also, again, no membrane-bound organelles whatsoever. Notice the only real organelles you're finding here are ribosomes, ribosomes are not membrane-bound, but there's no mitochondria, no chloroplasts, no antoplasmic reticulum, and no Golgi apparatus whatsoever. Also, you do have a plasma membrane around there, cell membrane. Outside of that, you have a cell wall, and then outside of that, you often have a capsule. Now, that cell wall is typically made of peptidoglycan, which is kind of a combination of polysaccharides with some short peptides, and outside of that, you have a capsule, which is often composed of polysaccharides as well, so a kind of hydrated polysaccharide layer outside the cell wall. Also, notice a couple things on here, pillis and flagellum, pillis is the singular, pilli is the plural, and these pilli might serve a little bit of a function in locomotion movement, stuff like that, but with bacteria, we want to more commonly associate these pilli with cell adhesion. Oftentimes, if you've got some sort of pathogenic bacteria, it's often going to use these pilli to bind to the host cells and things of this sort. Then finally, the flagellum here, you're going to find for the flagellum both in prokaryotes and eukaryotes. So, use for locomotion in both cases, but we're going to find out that you're definitely made of different components. In bacteria, they're made of flagellum, approaching called flagellum, so whereas eukaryotes will find out that they're going to be made up of microtubules. So, then we're going to focus on the eukaryotic cells, and really specifically, we're going to focus on animals versus plants. So, we're going to leave the fungi for another lesson in the future, and get into a biological diversity here. But, one common here, they're both eukaryotes, so one common when I first just point out a couple of key differences. So, plants being capable of photosynthesis, well, then have the organelle that's capable of photosynthesis, next to chloroplast, animal cells do not have that chloroplast. That's one of those membrane-bound organelles. Also, plants don't have centrosomes, so it turns out these centrosomes that are composed of a couple of centrioles only present in animal cells, not present in plant cells whatsoever. Now, notice both of these reproduce via mitosis, so they both need microtubule organizing centers. Well, again, the pair of centrioles in the centrosome are going to serve as those microtubule organizing centers in animal cells, but it turns out, well, we still are going to have these microtubule organizing centers in plants. There's no distinct organelle they're associated with, so there's no centrosomes in plant cells. Cool, so there's a couple of big key differences from here on out, we're really going to spend a little more time looking at the animal cell, but notice, mostly the organelle's we're going to look at, we're going to present both in animal cells as well as plant cells. So, the first organelle we'll take a look at is the nucleus inside eukaryotes here. Your nucleus, that's where you're going to store the genetic information. All your DNA is stored there, and because that's where your DNA is, that's also where transcription is going to take place. When you're making messenger RNA from the DNA, messenger RNA is going to get sent out of the nucleus eventually, out to the ribosomes, where transcription, sorry, where translation is going to take place. Just keep that in mind. It is surrounded by a nuclear envelope, and that nuclear envelope is not just a single lipid bilayer, it's actually two layers, but also nuclear pores, so where you can traffic big molecules and the messenger RNA will leave through these pores and things of this sort. Also the nucleolus, so typically when you're standing a cell, you're going to see the nucleus, and you're going to see a dark spot on that nucleus, that's the nucleolus, easy to identify when stained properly. And basically, it's just where you have intense ribosomal RNA synthesis taking place, and that's why I ended up standing so dark. Next, we're going to talk about ribosomes, and ribosomes responsible for protein synthesis, so they actually catalyze the reaction that joins the amino acids together, making those peptide bonds, so ribosomes specifically are going to catalyze the production of those peptide bonds. These are present in both prokaryotes and eukaryotes, one of the major things they both have in common, notice that both prokaryotes and eukaryotes need proteins to function, so it makes sense that they both have ribosomes. The ribosomes are going to fall in one of two classes, those are going to be associated with the rough endoplasmic reticulum, so in specifically, those are going to be used to make either membrane proteins or proteins that are going to be secreted outside the cell, but then you're also going to have some free ribosomes that are not associated with the endoplasmic reticulum, and those are usually going to be used to make cytoplasmic proteins instead. So, when I'm talking about ribosomes, we mentioned the rough ER, so let's talk about that endoplasmic reticulum, and endoplasmic reticulum is one of those membrane-bound organelles, and I say membrane-bound organelles, I mean it is really just a big canal system of membrane enclosures, so, and we've got both the rough ER and the smooth ER, the difference, and the reason we call the rough ER is it's dotted with ribosomes, right, where the smooth ER is not dotted with ribosomes. And the rough ER, so this is where glycoprotein synthesis is going to take place, and again these glycoproteins are proteins that are going to get modified with polysaccharides, and when we say perform modified with sugars, we'll find out that that actually happens in conjunction with the Golgi here in just a second. So, again, from the rough ER, these proteins are either destined to get bound in the membrane or secreted out of the cell entirely. Now, the smooth ER, on the other hand, involved in the synthesis of phospholipids as well as steroid hormones, also involved in the breakdown of toxins in liver cells specifically, so those liver cells we often associate with problems for drug addicts or alcoholics and stuff like that. That's because that's where toxins end up, so liver that's supposed to kind of take and filter them out of your blood, and then your liver's got to break those down, and that actually happens in the smooth ER. And then finally, your Golgi apparatus is really working in conjunction with your rough endoplasmic reticulum. So, when you're typically going to have either a membrane bound or a protein that's destined for secretion first synthesized by the ribosomes on the rough ER, they're threaded right into the rough ER, and from there, a vessel is going to snap off, if you will, pinch off the rough ER, and then it's shipped off to the Golgi apparatus, and then from the Golgi, which we like to think of as the shipper and packager of the cell, you might see some further modification, further glycosylation, and then basically it's going to be transported wherever it needs to go. If it's going to the cell membrane, it'll be transferred to the cell membrane. If it's destined for secretion, it'll be destined to fuse with the cell membrane in that case as well, releasing the contents outside the cell, so they can also be shipped to other cellular vessels like lysosomes as well. So typically, they're going to have some sort of marker that indicates where these proteins are destined to go. Next we're going to take a look at our mitochondria. Mitochondria, mitochondria is the singular mitochondria is the plural, and again, only present in eukaryotes, not present in prokaryotes, and a big role here is to make ATP, these are involved in ATP synthesis. In prokaryotes, all you've really got is glycolysis for the breakdown of glucose. So, in eukaryotes, you can take that a step further, right? So at the end of glycolysis, you end up with pyruvate, and if you're a prokaryote, you're just going to have to undergo fermentation to kind of regenerate some NAD from that, stuff like that. But with eukaryotes, you can take that pyruvate, turn it into acetyl-CoA at the pyruvate dehydrogenase complex, ship it off to the citric acid cycle, AKA creb cycle, AKA tricarboxylic acid cycle. So, and then take the results of that and take advantage of the electron transport chain to make a boatload more ATP. So it turns out your pyruvate dehydrogenase complex, all the enzymes associated with the citric acid cycle, and then all the major complexes in the electron transport chain, they are all located inside of mitochondria. So it's also where beta-oxidation. So fatty acid metabolism is going to take place. And so lots of opportunities for ATP production that are not available to prokaryotes. We like to say that prokaryotes are only doing anaerobic respiration, so it's just glycolysis in conjunction with fermentation. So whereas eukaryotes are capable of aerobic respiration in addition, where we can take pyruvate that's made in glycolysis and get a whole lot more ATP out of it. So just two net ATP per glucose molecule in anaerobic respiration, so but over 30 possible in aerobic respiration. So big advances here between prokaryotes and eukaryotes for ATP production. You should also know that mitochondria have their own DNA and it's circular DNA. So and they also have their own ribosomes inside the mitochondria as well for making their own proteins and stuff like this. So this is one of the bases for the endosymbiotic theory, which kind of says that maybe both mitochondria and chloroplasts, which also, chloroplasts and plants also have their own DNA, maybe those are actually derived from bacteria that got swallowed up by some larger cell or something like this. So and they have their membrane bound for one, and bacteria membrane bound, they have circular DNA just like bacteria have circular DNA. And so it's one of the bases for me to take a look at the endosymbiotic theory, maybe the where did eukaryotes actually come from? Where did they get their mitochondria? Next, we're going to very briefly talk about lysosomes and these are involved in degradation. So old organelles, phagocytose material that's been brought into the cell, other things that need recycling going to take place in the lysosomes. You should know that inside the lysome, the pH is roughly five, so much lower, much more acidic than the rest of the cell is definitely membrane-encloser result. And it is produced by a pinching off of a vesicle from the Golgi apparatus. The digestive enzymes that it contains that are going to carry out this degradation, they operate maximally right around a pH of five, which makes sense if it's a pH five in there. So not so effective at pH seven. So if they somehow magically got released in the rest of the cell, it wouldn't be the worst thing in the world because those enzymes are not so active at pH seven, but definitely optimally active at pH five. Now we'll talk about peroxisomes, which have some similarities to the lysomes, but some different functions as well. So they get their name from hydrogen peroxide because it's one of the things they break down. So they break down not only hydrogen peroxide, but a variety of reactive oxygen species that can cause different forms of cellular damage and stuff like that. So peroxone serving a vital purpose there, but also involved in the breakdown of fatty acids, amino acids, so as well as a variety of toxins that can happen in the peroxisomes as well. And then a special function in the seeds of plants, in the seeds of plants. This is also where the glyoxylate cycle takes place in germinating seeds. And the glyoxylate cycle has some similarities to the citric acid cycle. So but instead of being involved in catabolism, here's actually going to serve an anabolic function and these germinating seeds are going to use this glyoxylate cycle to actually produce some larger carbohydrates and things of the sort. Next, we'll talk about the centrosomes and centrosomes composed of a pair of centrioles. Again, these serve as your microtubule organizing centers often abbreviated as MTOCs that create the spindle apparatus for microtubules used for cell division during mitosis. And again, one thing to note is again, while you do have microtubule organizing centers in plant cells, you do not have centrosomes present in plant cells. All right, let's talk about vacuoles for a second. Note that the plant cell picture up there. And when people hear the word vacuole, they actually usually associate it specifically with plants. And there's a reason for that. But you should know that actually both plant cells and animal cells can have vacuoles. And they're membrane bound vegetables used for the storage of nutrients, again, in both plants and animals. So then why do people often associate vacuoles only with plants? Well, it turns out that plants often have a large central vacuole. So and this one is usually just storing a bunch of water, not so much so big on the storage of nutrients, but water. So they're to maintain trigger pressure and stuff like that, but also storing water and maybe some nutrients on top of that. So in this case, there's my large central vacuole. And the truth is, though, they can get much larger than this. In fact, they can, you know, kind of dwarf the rest of the cell, often taking up more than half of the volume of the cell in that central vacuole. And then finally, again, picture the plant on there because we're going to briefly here talk about chloroplasts. And again, this is where photosynthesis is going to take place. Again, a membrane bound organelle has its own DNA, also often used as good evidence for the endosymbiotic theory and things of this sort. And we'll definitely talk more about this when we get to the lesson on photosynthesis specifically. All right. So you want to take a little closer look at the cell walls. And everything we're about to say, we actually mentioned earlier in this chapter in talking about like polysaccharides and stuff. So it's worth mentioning again in this context. It's present in plant cells, present in fungal cells, present in prokaryotes, bacterial cells as well. And you kind of want to know what the difference is. And it's usually about what is its principal component, what are they made of? And so you should know that in plant cells, they are made of cellulose. Let's make that look a little better. So made of cellulose. And that's glucose polymer. In fungi, they're made of chitin. And again, you're supposed to know that chitin is made of a polymer of an acetyl-glucosamine, a glucose derivative. So but in bacteria, they're made of peptidoglycans, so which is a polysaccharide with some peptide, short peptides attached as well. So, archaeobacteria just briefly mentioned, they're made of some other polysaccharides, but not peptidoglycans, worth noting, and again, not present in animal cells. One thing to note, in plant cells, these cell walls are going to provide tensile strength as well as mediating some mechanical osmotic stress. So you might know that if you take, say, red blood cells and put them in pure water. So due to osmosis, you're going to have to just water rush in the cell and eventually leading to the cell down to go lysis and stuff like that. But in plant cells, the cell wall gives enough rigidity to prevent that from actually happening. So it prevents sort of cell lysis due to osmotic stress.