 So hopefully David introduced you to the concept of lipids and the properties of lipids. This is actually quite old. We've known that lipid was an important component of cellular membranes for almost 100 years. The actual structure was published in the 1930s when Schmidt-Bear and Ponder in 1938. I think it was used to polarize light on red blood cells to realize that you have this ordering and an interior lipid phase that's hydrophobic and then things facing the water that are hydrophilic. This was confirmed in the 1950s with electron microscopy and eventually David Robertson did a huge amount of amazing work in the 1980s. I'll see if I have a couple of pictures about that. Here we have the red blood cell. Do you see the thin layer here? This is actually not one but two layers of lipids. And I think I have some micrographs. Yes, here again you see an electron micrograph where you really magnify things so that you can see the lipid detail. But for natural reasons most of the early studies on membranes were topped out. We started to have a membrane and see how far we could penetrate this either with light microscopy or electronic microscopy. Which is fine in many ways but just looking at this we see that there is a membrane but we do not see all the atomic detail. Exactly what are the properties of the molecules here. For a long time we've drawn this in textbooks as these plain simple lipids with two straight tails. Today we can do this in slightly modified ways by using computers and though computer simulations are severely limited in other ways that I'll be happy to tell you about at some point they are really useful as a computational microscope so we can look at the real behavior of these molecules. So this is a movie of a lipid from a simulation. It's called POPC this particular one not super important. You have two large fatty acid chains here. They're derived from fatty acids originally and they're very hydrophobic. They're connected to a group here that we call a glycerol. It was originally derived from a glycerol when we created the lipid and then eventually we have a head group here that is very polar. Hopefully David told you about the fact that it's even so polar that we occasionally call it switter ionic. You have a full minus chart here and a full plus chart here. Do you see the lipid tails here though that they are much more zigzaggy than the traditional pictures you might see in textbooks. Let's have a look at that. I have an old movie for you. It's a bit corny but I think it's a great way to introduce lipids in particular in the context of how they interact with proteins and other stuff in the membrane. You won't hear the speaker voice here. So if we zoom in on this animation you will see the lipid bilayer. This lipid bilayer is really working like a two-dimensional liquid the so-called singer-nickelson model when things diffuse freely in a two-dimensional space. The lipids themselves though are very flexible and this flexibility is what gives the self-assembling and in particular self-repairing ability that if we have small defects the lipids will heal very quickly and there will not be any water going in although you don't see the water molecules here. The membrane is not limited to lipids though that's important. You have a lipid bilayer as one component but in addition to that we have cholesterol these small gray parts here. We have some sugars sticking out and in particular what we do not show you here is that we have a ton of proteins, actual membrane proteins embedded. Depending a little bit on how we count it can be up to 30% of the mass of a membrane that is actually the membrane proteins rather than lipids and that's what I'm going to spend most of the time today talking about. A better way of viewing the lipid bilayer itself rather than those simple, I'm not even going to draw them but rather than those simple models where we have a head group and then two straight tails of this. This is a snapshot taken from an actual simulation that's probably even me who read this a few years ago. Now it's mostly my students. Do you see the chaotic nature here in the hydrophobic part? That is because all these long hydrocarbons while it might look plain nice and simple to put them with straight chains that would actually be very unfavorable from an entropic point of view. Think of this as throwing out a string. The probability that the string will be completely straight is virtually zero although it could technically happen. This in particular means that the hydrophobic part of the bilayer will not have any ability to form hydrogen bonds or anything so it's going to be difficult for us to put anything charged in here. I'll get back to that in a second. This head group part on the other hand that's almost the opposite. Not only do they like water the large charges here means that they're almost more hydrophilic than water itself. They will love to interact with charged and the hydrophilic polar things which is going to create a very specific pattern. Hydrophobic parts go in the center of the membrane while hydrophilic polar even charged parts will not just accept but love to interact with the head groups and then eventually we might have a large water soluble domain or something outside it. It's an amazing chapter just to understand transport through the membrane. Some molecules such as oxygen or carbon dioxide will actually be able to go spontaneously diffuse through the membrane because they're not polar or anything. David will likely tell you a little bit about that in the class but again the obvious example is your red blood cells. All the oxygen I'm breathing when I'm standing here that's diffusing freely through my membranes and the carbon dioxide that is then being transported back to the lungs. The reason that goes into the same red blood cells is that it can diffuse pretty much freely through the membrane. But this is roughly where we're going to stop looking at the plain lipid membranes that consist just of lipids and start considering all the complications. Do you see the parts that we have here? First we have the blue part. The blue large part roughly there is a large membrane protein. Actually it's not just a membrane protein you have one so-called domain that sits straight through the cellular membrane here then you have one part that's sitting outside one part that's sitting inside. We might have some small I think these yellow components or so that could be cholesterol. You might have a green component out here that's a so-called monotopic membrane protein I think so it's a protein that's anchored a bit into the membrane but then it sticks out on the other side and this is a far more realistic picture than the others. These things are influenced by the membrane but they also do influence the membrane themselves. Cholesterol for instance have a tendency to rigidify the membrane which is very important in some of our cells and occasionally plants too. What I'm going to tell you about today is a bunch of concepts. These are roughly the ones we're going to cover. I think it's a great idea to go back revisit this be make sure that you know what I'm speaking about when I write this down. If you do that you master this lecture and then you should be able to pretty much breeze through the study questions at the end but I'll go through the roughly one by one.