 So there are going to be a ton of different ways to classify amino acids. Basically there are 20 of them and they're all different. But to simplify things and avoid having to look at each individual amino acid, we can try to group them. And there are a couple of different groupings that I like to use. This one is an obvious one. There are some non-polar ones and they're corresponding like small hydrophobic ones. Remember the hydrophobic effect that's going to be important. There are amino acids that are not charged but they are polar. Think of that, things like they're going to be easy to solvate in water. And then we have charged amino acids. Some of them with negative charge and some of them with positive charge. Another way to refer to those is basic versus acidic residues that has to do with this leaving or taking a proton from water. That's not the only way to divide amino acids. So you could imagine for instance saying that some of these are aliphatic, that is linear hydrocarbons, while some of them are aromatic. Phenylalanine and tyrosine in particular, they have these benzene-like rings. There is also a significant size difference here. Glycine is a very small amino acid while tryptophan is a gigantic one. I'll share that with you in a second. So depending on what you're interested in these properties means that in many cases you can take one amino acid that nothing bad will happen if you replace that for another similar amino acid in the same place in a protein. While if I take a small hydrophobic amino acid and replace it with a gigantic water soluble even charged amino acid there will actually be a significant difference. So I don't necessarily expect you to know the formula for each amino acid by heart but you need to have a gut feeling for instance if alanine is a hydrophobic or hydrophilic or in this case kind of intermediate amino acid and a tryptophan is a big bulky one. But why does that matter? These are just small chemical compounds. Why are the side genes here so important and why are they more important than other things? That has to do with the protein part that I've only hinted this for but it's a pivotal result in molecular biophysics. So when we move on from amino acid to proteins there are a ton of proteins too. Sorry about that and there's a huge difference in size. So starting from something small here the cytochrome domain or a ribonuclease cool protein we'll get to that in a second. They're tiny they're in the ballpark of 100 amino acids. Maybe not so tiny. But they're certainly tiny compared to the titin which is a component in my muscles. You see there it's like 30,000 amino acids and that's one single long stretched out chain. That protein factory we looked at the ribosome that reads messenger RNA uses TRNA and crates of protein that causes a 58 different protein chains in many cases and RNA too for that matter. So they can be very very complicated and each of these has a unique function. But that function might be created by some sort of special life process that breeds the life air into the ribonuclease or titin. And you might laugh at that but that's not at all such an obvious result. In the 1950s we didn't know that until Christian Anfenzen proposed something.