 Back in the day, R.H. Whitaker's five-kingdom system became one of the most well-made classification systems to ever exist. It classified all known organisms as accurately as possible and it also addressed several issues that the previous systems had. But then, in the 1970s, Carl Vos found out that not all Monerans were the same. Turns out some special bacteria had genes and enzymes closely related to that of eukaryotes. Something that other prokaryotes didn't have. These special bacteria later on came to be known as Archaea. Archaea. And because of how different Archaea were from the other bacteria, it didn't make any sense to keep the two of them together in the same kingdom. So, Vos came up with a revolutionary change. He assigned something called domains, which was a rank higher than kingdoms. And he went on to divide all Monerans into two major domains. Archaea and bacteria. In this video, we're going to talk all about Archaea and how unique they are from the rest of the living organisms on our planet. Archaea are prokaryotes, just like bacteria. But they have certain characteristics that set them apart from bacteria. For example, they're cell walls. Let's bring this a bit more into focus. Now, bacterial cell walls are made up of peptidoglycan. Archaea have cell walls too, but they are not made up of peptidoglycan. In fact, there is absolutely no peptidoglycan present anywhere in their bodies. Then, what are Archaea cell walls made up of? Well, different Archaea cell walls have different chemical compounds or chemical components in them. One such chemical or one such component is something called Pseudo-Peptidoglycan. But this has only been found in some methanogens, which is a type of an Archaea. We'll talk more about methanogens in some time. Now, the unique features of Archaea don't just end at cell walls. Their cell membranes are equally different. So, let's take a look at that. Now, let's bring the cell membranes into focus. Now, the top one right over here is the structure that you're going to find in an Archaea cell membrane. And the one below is something that you're going to find in a bacterial or eukaryotic cell membrane. So, this is something we're going to see in eukaryotes as well. Now, all cell membranes are made up of lipids. These lipids, in turn, are made up of hydrocarbon chains like fatty acids. So, this long wavy chain that you can see, this is a hydrocarbon side chain, which, in our case, are fatty acids. So, we're going to write that here as well. Now, this is a fatty acid chain and this is linked to a glycerol molecule. So, this purple-coloured blob that you can see in both the cases, this is a glycerol molecule right over here. Now, this red-coloured blob, if you're wondering what that is, this is nothing but phosphate and we're not really going to talk about this. So, we'll just keep it there as it is. Now, back to our side chains and our glycerol molecules. In bacteria and even in us eukaryotes, what you're going to find is that the hydrocarbon side chain over here, it is linked to the glycerol molecule via something called an ester linkage. So, this blue-coloured circle or this blob here, this is an ester linkage. In an ester linkage, let's take a closer look at this. In an ester linkage, you will see that carbon forms a double bond with an oxygen right over here, a single bond with a carbon right here, which is the carbon of the side chain here, and then it also forms a single bond with an oxygen, which further bonds with the carbon of the glycerol. This is essentially what an ester linkage looks like. This is a carbon which is bound to two oxygens and one carbon. Now, this is, however, not the linkage that you will see in an archaeal cell membrane. The linkage between the hydrocarbon side chain, which is this right over here, this is the hydrocarbon side chain, and the linkage between this and the glycerol molecule is not an ester linkage. It is called rather an ether linkage. Now, if you compare between the two, if you compare the ester linkage and the ether linkage, then you will see that this double bonded oxygen is completely missing from the ether linkage. So, if we put this over here, then it would be an ester linkage, but since we are, it's an ether one, there's nothing. So, it's just one oxygen right over here, which is attached to two carbons on either side. So, this is essentially what the ether linkage looks like. Now, because of this change in the chemical composition, the cell membrane of archaea are more chemically stable than that of bacteria or eukaryotes. Archea also have quite complex RNA polymerases, the prime enzymes of transcription, aka the making of an RNA copy from a DNA segment. Bacteria have a very simple RNA polymerase, which is made up of like four polypeptides only. Archea, on the other hand, have different types of RNA polymerases and each of them are made up of multiple polypeptides. I'm talking like more than eight polypeptides and that's very similar to us eukaryotes. We also have multiple types of RNA polymerases and they are also made up of 10 to 12 polypeptides and that is why we say that archaeal RNA polymerases are more closely related to that of eukaryotes compared to that of bacteria. Now that we know how different archaea are from other organisms, let's talk about their habitats for a bit, some of which are way too extreme than the rest. Many archaea are found in places like hot springs or very, very saline environments or even acidic environments for that matter. Yes, trust me, I know how crazy that sounds. Archea that thrive in hot areas like hot springs are called thermophiles. So these are the heat-loving archaea, you can say. One example is methanopyrus canleri, which is known to be the hottest thermophile out there. Salt-loving archaea are called halophiles. These salty fellows are found in any place with very high salt concentrations. Most organisms would die there but our beloved halophiles, they will thrive. A very common halophile is this archaea called halobacterium salinarum. Then we have the acidophiles. Let's move this a little bit. Acidophiles or acid-loving archaea. Think of those sulfur springs which are teeming with sulfuric acid. We'd probably give ourselves a burn if we accidentally dip our hands in it but these acidophiles, well they love that. Not the burning of course, the acidic environment. One such acid-loving archaea is the species sulfolobus. And finally we have the methanogens. Archea that produce methane. Now we've already talked about this one before. Remember how some archaea have pseudopeptidoglycan in their cell walls? Yep, those are methanogens. Naturally you would find methanogens in wetlands like swamps and marshes and also in the gut of herbivorous animals. Herbivores feast only on plants right? And these methanogens which are present in their gut, they help the animal to digest the cellulose of the plants. But that's not all. Methanogens also help us. But how exactly? Well methanogens are strict anaerobes, meaning that they grow only in the absence of oxygen. As they digest or break down organic matter in the absence of oxygen, they produce two things. One is methane as you already know and the other thing is carbon dioxide. And together these two things make up something called biogas. And biogas can be used as a fuel or a biofuel for cooking and electricity. That's pretty cool, huh? An example of a methanogen is methanobacterium which is also an archaea which is used or which is added to organic matter so that we can commercially produce biogas as well. Now honestly we have barely scratched the surface of the topic that is archaea. But doesn't this give you a faint idea of how interesting and unique this whole domain can be?