Episode 10: The Cell

[MUSIC] Hello, my name is James Foto and you're listening to The Science of Everything Podcast. Here I discuss a wide variety of topics in the natural and social sciences in an attempt to better understand the world around us. This is episode number 10 and the topic for today is The Cell. So in this episode we'll take a look at the cell that is, you know, animal cells mostly and we'll look at the history of early discoveries of the cell and the different types of cells. And then I want to just go through and look at the different structures and functions of the various organelles and other parts of the cell. So I've divided these into three main categories which I think are helpful in sort of getting a grip of the main things that the cell does. So three groups I have are structural items or structural structures within the cell that keep it together and move things about within the cell. Then there's protein production which is one of the biggest things that the cell does and we'll talk more about that later. And thirdly energy production, making energy to power everything else that the cell needs to do. So we'll look at each of those in turn but first of all we'll start with an overview and brief history. So basics, what is a cell? A cell is just the smallest unit that can carry out the processes of life. It is the basic unit of all living things and all organisms are made up of at least one cell, the one or more cells. Organisms like bacteria or amoeba are made up of a single cell where it's human beings are made up of trillions of cells. All cells, regardless of whether they're in bacteria or amoeba or plants or animals, all carry out very similar processes and they're all actually quite similar in their structures and general layout. And functions of cells include transporting materials, obtaining energy, disposing of waste, replicating and also responding to their environment. So that's pretty basic stuff. Let's have a look at the history of cells and how they were discovered. The word cell was first used by Robert Hook, who was a British biologist and early person who worked on microscopes. He looked at thin slices of cork, which is taken from a trunk of a particular tree under a microscope and observed structures that to him looked like the cells that monks lived in, sort of little boxed rooms and so he called themselves. A little bit later in the 1670s, a Dutch merchant called Antony Van Lewenherk used microscopes to observe many small microbes and body cells. It was the first to observe bacteria, blood cells, sperm, all sorts of interesting things. He was really good at making exceptionally high quality microscope lenses, so he saw a lot of things that other people weren't able to at the time. And so he made many of these early discoveries of cells. That was only a few years after Robert Hook's work in first coming up with the word cell. So a few discoveries continued, but some interesting stuff really happened going in the 19th century. In 1839, Theodor Schwann and Matthias Jakob Schleden elucidated the principle that plants and animals are all made of cells and also concluded that cells are the common structure of all types of life. So effectively cell theory, as we know it today, was founded around the mid 19th century. So only slightly earlier than the theory of evolution, which is kind of interesting. In 1858, Rudolf Vichau proposed that cells come only from other cells, and this was part of a sort of a growth and understanding around that time that the old theory of spontaneous generation that life just sort of springs up out of non-life is false. And in fact cells, which comprise all living things, only come from other cells. So by the end of the 19th century, light microscopes, that is microscopes using visible light, had come to reach kind of the end of their useful resolution limits in terms of viewing cells. And so it was only since the early 20th century, as we got electron microscopes that we've been able to observe, the fine details of cells like organelles and protein structures and other details of cells. And so many of the discoveries that I'm going to talk about today about the specific structures in cells and what they do are fairly new, 50 plus years old. So that's a brief history of cells. Now, one interesting question, why are cells so small? Well, basically the reason is because that as a cell grows, as it gets bigger, its volume increases more quickly than its surface area, the surface area being the area around the outside surface of the cell. And that's just because of the formula form volume, which increases with the cube of radius, whereas surface area increases with the square of the radius. So volume increases more rapidly than surface area. Now, why is that a problem? Well, because volume, the volume of cell determines the amount of nutrients it needs, you know, the amount of stuff that it needs to bring in in order to keep running. But the surface area determines how much food or nutrients it can get in, because obviously all those nutrients have to pass from the outside to the inside of the cells that they need to pass through the cell, the surface of the cell. So basically the bigger the cell gets, the harder it gets for it to obtain all the nutrients it needs. And so that's why this kind of a limit to how big cells can get. However, cells have kind of pushed that limit upwards by having lots of folds in their surface membranes so that you can increase the surface area relative to volume. Interestingly, eggs are an exception to this, because eggs are actually just one cell. Or more accurately, there's kind of a normal sized nucleus and other bits and pieces in there, and then just a whole bunch of proteins and other carbohydrates and other nutrients there for the animal to feed on as it develops. And in a sense, an egg is just a single cell, and that means the largest cell in the world is actually the ostrich egg, which is like two kilograms or something, pretty big. But ostrich eggs aside, most cells are very small. Cells can come in many different shapes. Nerve cells, as you may have seen pictured before, are very long and thin. Red blood cells are by concave, so they kind of look like doughnuts, except they don't quite have a hole in the middle, they just have a depression. There are rod shaped bacteria cells, which are just like rectangles. Some cells are oval shaped, pollen grains, if you see them, are very spiky. And the reason they have all these spikes is because it helps them to stick onto other plants and animals and things to spread around. So anyway, cells can come in all sorts of different shapes and sizes as well. Now, there are two main different types of cells, pro-cariotic and eukaryotic cells. These words sound a little bit complicated, but basically the name is just referring to the nucleus. So karyot actually comes from the Greek word nut, and that's referring to the nucleus. And so a pro-cariotic cell is a cell that is before the nucleus, sort of being before, eukaryotic is like a real nucleus. So pro-cariotic cells are simpler, eukaryotic, more complex. Pro-cariotic cells have no nucleus, and they also have very few organelles. So very few structures in the cell. Pro-cariotic cells are things like bacteria and archaea, so they're very simple organisms. Their genetic material is held as a single loop of DNA, so they don't have a nucleus that don't have chromosomes. They also have a cell wall, which most eukaryotic cells do not have. The other main difference between pro and eukaryotic cells is that pro-cariotic cells are very small, about one-tenth or less of the size of a eukaryotic cell. Eukaryotic cells are the cells that are in plants, animals, pretty much any complex organism. They have many more organelles, and other bits and pieces inside the cells are specialized functions. They're much bigger, as I said, more complicated, and they also evolved much more recently. It's thought that they evolved about two billion years ago, whereas pro-cariotic cells probably evolved about four billion years ago. So it took a very long time for eukaryotic cells to evolve, which just goes to how complicated they actually are. Because if you think about it, it took far longer for eukaryotic cells to evolve from pro-cariotic cells than it took, say, for human beings to evolve from the first multicellular organisms. Okay, so that's it for the overview and history of the cell. Now we're going to go and look at the structures within the cell. What is the cell? What's in it? What does it do? So as I said, I've divided these into three areas, structure, structural things, protein production, and energy production. So we're going to start with structure. The first concept that you really need to understand is that of the cytoplasm and the plasma membrane. We'll start with the plasma membrane. The plasma membrane is a double layer of specialized lipid molecules, called phospholipids, that surrounds the outside of the cell. So it's kind of like a bubble, really, inside of which resides everything in the cell. Now what is a lipid? A lipid molecule. A lipid molecule is just a particular kind of macro molecule. If you refer back to the matter and molecules podcast, I mentioned the concept of macro molecules, just really big molecules with lots of atoms in them, particularly carbon and oxygen and hydrogen. Lipids are special molecules that have, or particularly phospholipids, they have basically one end which is soluble in water and a long tail, which is not. And so the head, which is soluble in water, will, it'll face water, it'll tend to stick around with water, whereas the tail will kind of point away from water. So what these lipid molecules do is that they kind of line up. So if you imagine that the, suppose you had a whole lot of phospholipid molecules lying on the surface of a little bit of water, they would kind of all stick upwards there. Their heads would be facing down and their tails would point upwards away from the water because the tails, the fatty acid tails of the molecules, are not soluble in water, so they tend to be pushed away from the water. And so what happens is you have kind of one row or one wall of these molecules with their tails facing one way and then one second one with their tails facing the other way. So the two tails kind of face each other and the two heads have pointed in opposite directions. So it's kind of like you form this wall with water or a solution of water on one side inside the cell and water on the other side in the extra cellular fluid. And the phospholipid membrane plasma membrane in between. And the lipid, the two lipid tails, which point towards each other inside the middle of the membrane, are not soluble in water, so water molecules and most other things can't pass through the membrane. And that's why it forms a membrane. It's actually not, that's not quite true because some things can pass through it, so that's why we say a membrane is semi permeable. And if you find it hard to picture what I'm saying, just look up like phospholipid membrane or something on Google and you'll just see some good diagrams. Okay, so we've got the plasma membrane. The purpose of the plasma membrane is just to separate the inside from the outside of the cell. So the special reactions and the other stuff can happen inside this. So we're just kind of like your skin keeps you together and separates you from the outside world. Inside the plasma membrane is what it's called siteoplasm. The siteoplasm is just everything inside the plasma membrane membrane. And we can define that into sort of two categories of stuff. There's organelles, which are structures that do stuff basically, things like mitochondria, the nucleus, ribosome, stuff like that. So there are the organelles and there's also the cytosol. The cytosol is just the, it's mostly water, but it's also also has heaps of other stuff dissolved in it. Things like ions, salts, proteins, organic molecules, enzymes, all these bits and pieces are dissolved in the cytosol. So that's the inside liquid stuff that makes up the interior of the cell and that's where many reactions go on that are important for protein production and making energy and stuff like that. Okay, I should also mention that inside the plasma membrane, lots of structures, protein structures, so these are molecules that permit certain things to travel across the membrane. And so some of these will lie, they sit sort of like boys in water that they're, they're studded in the plasma membrane. So these protein structures allow some molecules or some types of molecules to pass through and some not to. So some proteins will let say a certain type of ion pass through the membrane, some will let maybe oxygen go through, et cetera. And the different proteins are specialized for different functions. So in this way, the, using these, these membrane proteins, the cell is able to control what goes in and what goes out. Okay, so that's the basic structure of the cell. Now moving on to another area of structure called the cytoskeleton. And this, this is literally the skeleton or the scaffolding of the cell that keeps the cell together. These are made of mostly proteins, long thin protein fibers. A protein is just a particular type of macro molecule. So it's an organic molecule, big long organic molecules. And they put these protein molecules together in particular ways, which allow them to sort of twirl and wind up together and form long strong fibers, which sort of keep the cell together. There are different kinds of these, of these fibers forming the cytoskeleton, including micro tubules, microfilaments and intermediate filaments. I won't go into all the details of those, but suffice it to say some of them are sort of thicker than others, some of them are more flexible than others, and they form different functions. Some of them keep the organelles in place, some of them give the cell rigidity. Some of them can sort of move around to help the cell move if it needs to. Others are useful for moving things around the cell. So the filaments can attach to something and sort of drag it along by contracting and bending. They're also useful for cell division where they sort of pull the different parts of the cell apart as the cell divides. So that's a cytoskeleton. Protein molecules that keep everything in place and kind of move things around. So I think that basically covers all of the structural elements. So I want to move on to what I think is the more interesting stuff, which is about protein production. Now protein production is, in a sense, the main purpose of cells. And why do I say that? Because pretty much all of the important tasks that a cell does require proteins. Now remember, a protein is just a particular type of macromolecule. But it so happens that cells use protein molecules to do pretty much everything. Like for example, well, the plasma membrane and the nucleus are kind of exceptions. But apart from that, the enzymes that make energy for the cell are made of proteins. The cytoskeleton, as I said, is made of proteins. Messenger molecules that cells send to one another to send signals are generally made of proteins. As I mentioned before, the sort of gatekeeper molecules that sit in the plasma membrane and determine what can pass through those proteins. So almost always when you're talking about a cell doing something, carrying out some kind of function, a protein is, it'll be a protein that's doing it, or at least a protein is crucial to the operation of that. Proteins are kind of like the basic building blocks of a cell. So that's why protein production is so important. Because to do anything, a cell basically has to make proteins. And so most of the structures inside a cell, and certainly most of the important ones, are concerned with making proteins, protein production. And there's kind of a chain, if you like, that you can go through. I mean, I'm simplifying a lot here, but there's sort of like a, that assembly line that you can see a start and a finish to the process of protein production. It starts in the nucleus and kind of ends up in the Golgi apparatus and goes through several different structures and organelles in between. And so I want to take you through those. And I think it's helpful to look at this in terms of protein production, because often if you'll refer to an introduction to the cell, it'll just say, it'll just point to all the different bits inside the cell and say, "This does this, and this does this, and this does this." And it gets a bit confusing. But putting it in sort of this broad framework of making proteins, and then later on making energy, the two main functions of a cell, I think is very helpful. Okay, so protein production, we're going to start with the nucleus, which is kind of where it all starts, it all begins. Nucleuses are only found in eukaryotic cells, and a nucleus is a membrane in closed structure, so it's surrounded by a phospholipid membrane just like the cell itself, so it's kind of like a mini cell within a cell, in a sense. Inside the nucleus is found most of the genetic material of the cell, so this genetic material stores information to make proteins, basically. And as you'll probably know, genetic material is basically DNA, the oxo ribonucleic acid. And once again, DNA is just a particular type of macromolecule, so big long molecules with lots of different atoms in them, mostly carbons, oxygens, phosphorus, and stuff like that. Now, the nuclear membrane has tiny holes called pores in it, which allow something to move in and out the exchange of materials, particularly it allows proteins to go into catalyzed reactions that happen inside the nucleus, and RNA to go out. RNA is a messenger molecule that sort of carries the information from the nucleus to the outside of the cell to make proteins. Now, the topic of DNA, genetics, and how proteins are made, and so on, is, that's for another podcast, but basically, the basic idea is that the DNA molecules inside the nucleus hold the information to make proteins, and this information is held in the form of a gene. So, a gene is just a short segment of a DNA molecule, which contains information that it codes for a particular protein, and the way it codes for a protein is just the order of little molecule segments within the DNA. So, it's literally the arrangement of the atoms within the DNA that determines the information that it holds to make proteins. So, the function of the nucleus is just to store all of these DNA molecules, all of this genetic material, so that they can keep that information for making proteins. And obviously, that information is very important, so that's why the nucleus is well protected. It has its own membrane around it. DNA, so DNA is just molecules that hold the genetic material. They are sort of wound up and bound up in very complicated ways to protect it and to keep it intact, and sort of these structures that they're wound up in are called chromosomes. And so, you can think of it as all the DNA material that we have is sort of wound up and coiled up into a few different chromosomes. So, we have, I think, 23 chromosomes, and in human beings, different animals have different numbers. Yeah, so the purpose of the nucleus is just to protect these chromosomes and keep them safe and to safeguard that genetic material, because if that genetic material is lost or damaged, the cell will lose the information and needs to make certain proteins. And without those proteins, it won't be able to carry out some particular function, and so the cell will die, or not be able to replicate, or something bad will happen. Okay, so that's where this all starts, and once you extract the information from the genes to make proteins, and the RNA carries that information, and the RNA is just another type of molecule. I'll go into this in much more detail on our later podcast, but the RNA carries the information out of the nucleus, and we start making proteins. Now, the organelles that are responsible for making proteins are called ribosomes. Now, ribosomes just look like small little round balls, if you see them through a microscope or a diagram, and they're scattered right throughout the cytoplasm. Some of them are, some of them are just located by themselves, all throughout the cytoplasm, but a lot of them are docked onto another special organelle, which is called the endoplasmic reticulum. But before we get into that, just a bit more on ribosomes, ribosomes are made of an RNA molecule. Now, you'll remember that the RNA molecule is that transfer molecule that takes the information from the DNA to the site of protein synthesis. Ribosomes are also made of RNA, but it's kind of a different type of RNA. But anyway, so ribosomes are made of both that RNA molecule and associated proteins. So, once again, we see proteins coming in to it, and the ribosomes take the RNA from the nucleus and use that as a template for making proteins. And the details of that are kind of complicated. We'll go over those in another podcast, but it's faster to say ribosomes make proteins using information from the nucleus as a template. Now, as I said, many ribosomes are docked onto an organelle called the endoplasmic reticulum. The endoplasmic reticulum is a network of membranes, phospholipid membranes, just like the cell membrane and the nuclear membrane. These membranes form hollow tubes, flattened sheets, rounded sacs. It's kind of like a big three-dimensional maze of all of these tubes and tunnels of membrane. And the purpose of the endoplasmic reticulum is to assist the process of protein synthesis. So, you've got a lot of these ribosomes that docked onto the sides of the endoplasmic reticulum. The ribosomes synthesize the proteins and often excrete them into the endoplasmic reticulum, into the interior of it inside all of the membranes. And then the endoplasmic reticulum acts as a kind of a highway. It's hollow inside to kind of direct the proteins to where they're supposed to go. Also, the interior of the interior environment of the endoplasmic reticulum provides a suitable environment for the proteins to fold correctly and do various other bits and pieces that they need to do. So, basically, the endoplasmic reticulum just helps this synthesis of proteins, helps it to proceed properly and then transports them to different parts of the cell. There are two types of endoplasmic reticulum. One's called the rough endoplasmic reticulum, the other the smooth. And the reason for this difference is the rough ER endoplasmic reticulum has ribosomes studded all over it. The smooth one does not. And so, all of the ribosomes give it a rough ER, a rough appearance that's hence the name. The smooth ER, by the way, is the site has lots of different functions which can be kind of complicated including lipidsynthesis, calcium ion storage, drug detoxification, all sorts of other bits and pieces. But the rough ER, as I said, transports the proteins. Once the proteins kind of reach the end of the, bits of the membrane, the membrane of the endoplasmic reticulum sort of pinch off into small little compartments which are called vesicles. And I'll talk more about that later. And the proteins sit in these vesicles and are transported to different parts of the cell. And many of the proteins will actually be transported in this manner into another organelle, which is separate from the ER, which is called the Golgi apparatus. The Golgi apparatus is a pretty big organelle and similar to the endoplasmic reticulum, it's also made of lots of membrane covered discs. The Golgi apparatus takes the proteins made by the ribosomes near the ER or in the ER and modifies them, sorts them, packages them, and then transports them to where they need to go into the cell. So when I say the Golgi apparatus modifies them, the Golgi apparatus contains many enzymes, which themselves are proteins, as I said before, which can add things like carbohydrates or phosphates or other bits and pieces onto the protein molecules and sorts them into different categories. And then kind of puts markers on them, which tell the cell essentially where they're supposed to go. So the analogy that's often used is that the Golgi apparatus is kind of a post office that it takes all of the proteins made elsewhere, sorts them, alters them a bit, and then sends them out where they need to go. I mean, that analogy is kind of useful. I don't like analogies like that generally because they imply that there's some kind of intelligence in the cell doing this. Of course, it's not. It's all just chemical reactions with atoms and molecules proceeding, you know, down their concentration gradients and into lower energy levels and according to electrochemical reactions and stuff like that. But the post office analogy is still somewhat useful. Just remember that there's no intelligence behind all this. It just happens. Okay. And once the alterations have been made, pieces of the Golgi membrane pinch off into vesicles, just like they did from the endoplasmic reticulum and the proteins go into those vesicles and are transported around the cell to where they need to go. So that's the process of protein production. Basically, the information comes from the nucleus in the chromosomes. It goes to the ribosomes, which make the proteins. The proteins or many of them are excreted into the endoplasmic reticulum where they're folded and transported via vesicles to the Golgi apparatus and the Golgi apparatus. They're altered, carbohydrates and phosphates and other bits and pieces added and markers are placed on them so that they are carried to where they need to go in the cell. That's the very rough outline, very, very simplified, but still helpful, I think, in understanding what's going on. Now, we'll move on to the last major function of cells, which is to produce energy. The reason cells need to make energy is because protein production requires energy, so there's something to get that from somewhere. As you may know, animal cells have little organelles called mitochondria, which are responsible for the production of energy, whereas plant cells rely mostly on chloroplasts, which are the side of photosynthesis. Both mitochondria and chloroplasts are actually kind of like mini cells in and of themselves. They have their own plasma membrane around them, and they even have a small amount of DNA in them, which is used to make some proteins, which they need ready access to. They don't have all of their DNA in the but chloroplasts and mitochondria do have a little bit of DNA. It's thought that both mitochondria and chloroplasts actually began as separate organisms, something like a bacteria, and then entered a symbiotic relationship with the larger cell of which they're now embedded, and now they're so interconnected that they can't live separately from each other. So that's a very interesting theory, called endosymbiosis, and I think I'll do a future podcast on that because there's a lot to say about it, but for now, just remember that chloroplasts and mitochondria are kind of like mini cells within the cell. They even have their own DNA. The mitochondria takes in energy in the form of carbohydrates and other organic molecules and bits and pieces from the outside, and through a chain of very complicated chemical reactions, produces energy in the form of ATP. ATP is a special molecule which holds energy in its chemical bonds, and I'll go over here's yet another podcast we need to do about how energy is made in cells, but that suffices it to say. They take in inputs, mess them around with lots of chemical reactions, and those chemical reactions store the energy in a convenient form of ATP molecules. Chloroplasts obviously do it a bit differently in that they convert light energy into the energy in chemical bonds. That's another complicated process involving many reactions which needs, we yet again, a whole podcast to itself. So we've got mitochondria and chloroplasts both making energy. Another aspect that I want to talk about here is vesicles and vacuals which is kind of related to energy production and energy storage. Now as I mentioned before, a vesicle is just like a small spherical compartment which is separated from the cytosol, so from the rest of the interior of the cell by one or more lipid by layers. So it's just kind of like mini compartments with membranes around them. The reason that they're separated from the cell is to provide a chemically different environment to the cytosol, and this can be useful for all sorts of things including storing proteins or storing energy. They can be used for storing waste products which will then be excreted out of the cell. The vesicle kind of moves through the cell and then merges with the external membrane and pushes its waste products outside of the cell. The reverse can also happen. The vesicles collect materials from the outside of the cell and kind of bring them in and move them about to where they need to go in the cell. Some vesicles also contain enzymes which are used to break down or digest various harmful products, waste materials or other things from inside or outside of the cell. So your vesicles are very useful kind of like trucks that carry around the stuff that you need inside the cell and also provide that separate chemical environment where different reactions can take place that might not be possible inside the cytosol itself. Because chemical reactions heavily dependent upon things like concentration of given ions, pH, temperature, all that sort of thing, and if you need specialized chemical reactions to occur you might need a different chemical environment and that's what the vesicles provide. Vascular is just like a really big version of a vesicle and they have a sort of a similar function. The main thing is that in plant cells, plant cells have very very large vacuals in the center which often hold essentially what we'd call sap and that serves as a site of storage for energy and stuff like that. Animal cells have vacuals as well but not nearly as large as ones in plant cells. A plant cell vacual can make up to 90% of the plant cells volume so they're very big. Plant cells also have cell walls as I mentioned before bacteria have cell walls which are rigid structure which keeps the shape of the cell much more so than just the membrane would. Animal cells don't have cell walls and as I also mentioned before plant cells have chloroplasts that make energy. Obviously animal cells don't have those. So that's about all I wanted to talk about in terms of the basics of the cell. As I mentioned there will be many future podcasts going into more detail about protein synthesis, genetics, energy production within cells, photosynthesis, all that sort of stuff there's certainly a lot to go into but this is just the basic outline. If you enjoyed this podcast please spread the word by posting a review on iTunes or another podcast aggregator site or sharing with the podcast with a friend. If you have any questions, comments or suggestions please email me my email address is FODZ12 that's [email protected] Thanks for listening and I'll talk to you next time. [Music]