 Welcome back everybody. In the previous gargantuan video, we ended at this slide, but I'm going to start here again because it's a lot of information and it's worth repeating. There are 20 universal amino acids. That means there are 20 amino acid molecules that are found in all living organisms. Amino acid molecules are connected to each other by a special type of dehydration synthesis reaction. The bond that gets made when you connect amino acids to each other is called a peptide bond. If you attach enough amino acids to each other, the molecule you make is called a polypeptide. This is also known as a protein molecule. So your cells use these 20 different amino acid molecules to make protein molecules for you. So now I'm going to show you the 20 universal amino acids, or at least their names I think. Here they are. They are listed alphabetically starting with alanine, arginine, da da da da da. All the way down. Number 20 is valine, if you at least do it alphabetically. In the previous video I said sometimes if you have a bunch of amino acid molecules connected to each other, you don't want to draw every single atom. So sometimes you can just write the names of the amino acids like this and you can say oh this is the part, this is the end that has the end terminus, this is the part that has the C terminus. This was on the previous slide. But if you have 100 or 1000 amino acids connected to each other like this, even this is tedious, right? That's not the easiest thing to keep doing 100 or 1000 times. So chemists and biologists have come up with abbreviations for the names of the amino acids. There's a three letter abbreviation for every single one of the 20 amino acids and there they are. I don't expect you to memorize these, but I will give them to you on the last exam. You'll have them at your disposal, you'll have a table or you'll be able to look it up in your book. I'll expect you to understand what those abbreviations mean. So instead of saying end terminus, glycine connected to serine connected to tryptophan, C terminus, we could use these three letter abbreviations. And instead of writing that, we could write NGLY because glycine, three letter code is GLY, serine, S-E-R, and then tryptophan is TRP. And that's a little bit easier, right? If you saw that, it means the same thing as this up here, except it took fewer letters to write everything. Even this can get tedious after a while, so guess what? People have come up with one letter abbreviation for all of the amino acids. So if I wanted to write the same molecule, well, glycine, one letter abbreviation is G, serine, one letter abbreviation is S, and tryptophan, one letter abbreviation is W because T got used somewhere else. And in this case, you actually don't write the N and the C on either end. The reason is because N and C are being used as one letter abbreviations. So the convention is if you're going to use the one letter abbreviations, you always write the N terminus end on the left, you write the C terminus end on the right. And everybody assumes that you're doing that. And this is even easier, right? If you use the one letter abbreviation and you had to tell someone, oh, I have polypeptide or I have a protein molecule that is made of 1,000 different amino acid molecules connected to each other, you would probably want to use the one letter abbreviation because it's just going to be easier to write. Again, I don't expect you to memorize these, but I expect you to be able to use them and to be able to interpret them. If you see me writing, if you see something like this and I say this is a protein molecule, what amino acids is it made of, you should be able to use a table like this to say, oh, why is tyrosine? R is arginine, et cetera, et cetera. So that's what I expect you to be able to do with these different types of abbreviations for the 20 universal amino acids. As I mentioned, there are 20 universal amino acid molecules. They are all different from each other, but you can put them into groups because we are humans and we like to put things into groups. There are many different ways that you can put the amino acids into groups, but I'm just going to put them into one of two groups, and this is all I want you to know. There are certain amino acids that have hydrophobic side chains. Remember, side chains are the collection of atoms that make the amino acid molecules different from each other. Some of those amino acid molecules have side chains that do not like to mix with water, and those are called hydrophobic. Informally, sometimes people call those side chains in those molecules greasy, but hydrophobic is more proper. Every amino acid molecule here is considered to have a hydrophobic side chain, so that's why it says hydrophobic here. If you look, right, here's glycine. This is an amino acid called alanine, valine, et cetera, et cetera. The parts that all of these molecules have that's highlighted in purple, this part here, this part here, those are the parts where that all of the amino acid molecules have in common with each other. In other words, they all have this, you can see it in all of them. They all have this part. The part that is highlighted in yellow is the side chain. These are all different from each other. All I am telling you is that these amino acids, the ones on this page, the side chains are considered to be hydrophobic. In other words, those little parts in yellow, they don't like to mix with water very much. They will if they have to, but they're not too happy to do it. I don't expect you to memorize which amino acids have hydrophobic side chains. You can if you want, but you certainly don't have to, but you should be able to consult your book during a quiz or a test and figure out which ones are hydrophobic and hydrophilic, because this table is in your book, it's in chapter 16. There are other amino acids. All of the other amino acids are generally considered to be hydrophilic. Hydrophilic means that the side chains like to mix with water. You can break them into subcategories, but I am not going to. Every amino acid on this slide has a hydrophilic side chain. So this little bit here, these are the side chains again. All of those side chains in general like to mix with water. And again, the parts in purple here, those are the parts that all of the amino acids have in common with each other. So there are basically two categories of amino acids. One's with hydrophobic side chains. They don't like to mix with water. And one's with hydrophilic side chains. These are not all of the hydrophilic amino acid molecules. There's another bunch on the next slide. All of these amino acids are also hydrophilic. In other words, this likes to mix with water. So does this, this, this, all the way down the line. And again, the purple parts here are identical between all of the different amino acid molecules. Again, please don't memorize which amino acids are hydrophobic and which amino acids are hydrophilic. But you should be able to look at these tables in your book and just look up which ones are hydrophilic, it says right there, and which ones are hydrophobic. Who cares? It's probably what everybody is staring at on this video. Probably nobody, but there is some relevance to human disease and other things, which I will get to in a later video. So we're going to come back to who cares. This is going to be the end of the video. Again, another summary slide. There's a lot of information, so there will be a fair number of summary slides. Remember this, I've said it about 6,000 times. There are 20 universal amino acids. Each amino acid of the 20 has its own special side chain or R group that makes it different from the other ones. And each different amino acid has a slightly different behavior because of the side chain. But you can put them into categories. There are amino acids that have hydrophilic side chains, in other words, the side chains like the mix of water. And I don't care about the distinction here. Just think hydrophilic or there are amino acids that have side chains that are hydrophobic. That means the side chains do not like to mix with water. You can put the amino acids into either one of those two categories depending on which amino acid it is. And again, the big point, protein molecules or polypeptides are just chains of amino acid molecules attached to each other by what's called a peptide bond. So that's the end of this video. See you in the next one.