Next, we want to talk about lipids. Lipids are a really important component of cell membranes, and that's the main reason we're going to talk about them. They consist of carbon, hydrogen, and oxygen, and their key feature is that they are non-polar and insoluble in water.
If you took a bunch of cells and you put them in a blender and you basically ground them all up, and then you added an organic solvent that could extract the organic molecules, what would come out of that mess of cell would be all of the organic molecules and then all of the things that were soluble in water would stay with the water. So these are the lipids are the greasy things the things that actually don't dissolve in water are the lipids and because basically this includes all of the greasy things you end up with a really diverse kind of set of molecules. So you have things like hormones, cholesterol, which is a part of eukaryotic cell membranes.
but also hormones like progesterone and testosterone and estrogen. So these are lipids. We consider these lipids because they don't dissolve in water. Now for what we want to talk about, we're going to talk a little bit about simple lipids. These are triglycerides and their role in cells is energy storage.
And then we're going to talk about the complex lipids because these are really important parts of cell membranes. Just a review of a triglyceride. Triglycerides have a glycerol group. So that's what this is. And then they have three fatty acids hooked to it.
So here's one fatty acid. There's two fatty acids. And here's the third fatty acid.
Fatty acids come in two main flavors. So they can be saturated fatty acids. This means that carbon has as many hydrogens as possible. Notice that each of these carbons, remember it only forms four bonds. Each of these carbons is bonded to another carbon.
The other two bonds are both taken up by hydrogen. So this would be saturated. In an unsaturated fatty acid, what you have is carbons that have double bonds between them instead of extra hydrogens. So you can see there's fewer hydrogens in an unsaturated fat. This gives it some different properties.
It causes a kink in the three-dimensional structure, and this actually results in lower melting points. So things like olive oil, you might have that in your cupboard and it's a liquid that you pour, whereas something like lard or butter even... At room temperature, we usually keep them in the fridge to prevent oxidation, but even at room temperature, they're a solid, maybe a soft solid, but certainly you wouldn't pour them like you could pour olive oil.
So they have much higher melting points. And then this has to do with the characteristic of being saturated versus unsaturated. Now, in contrast to energy storage, we also have a very similar structure that's used to make up cell membranes.
And the back... is the same they both contain triglycerides and fossil lipids both contain this glycerol backbone fossil lipids contain a typically a unsaturated fatty acid and then one unsaturated fatty acid you can kind of see the kink here for the unsaturated fatty acid and then where the third fatty acid would be in a triglyceride in a fossil lipid you have actually a charged phosphate group right here And this gives these molecules really interesting properties because they're greasy, hydrophobic. They want to be with lipids on one end, but they want to be with water on the other end.
And so when you put these into a solution, they tend to orient themselves spontaneously so that the head groups, the fossil lipid charged head groups are on the outside. And the greasy tails, they want to be together because they're greasy. They don't like water. They're going to orient themselves to the inside.
And what this will do is... In a cell membrane, this is the outside of the cell, this is the inside of the cell, and then you have this greasy layer on the inside. And so the head groups are happy because they're lined up with water, and the tails are happy because they're greasy, they don't want to be around water.
And now you have this barrier. Molecules like glucose can't just go through this because they're charged. They dissolve easily in water, but they're not going to be soluble in the lipid bilayer. And this allows the cell to control who goes in and out. It can put proteins in this layer that allow specific molecules to come in or to leave, but not everyone will be able to go in and out.
And this gives cell membranes the property of being semipermeable, and it has a lot to do with just the function of the cell membrane, which is to control what gets in and out of the cell. This is a picture of a cell membrane. It's a beautiful picture, and it's showing the different kinds of phospholipids, which include things like... phosphatidylcholine and sphingomyelin, phosphatidylethanolamine, phosphatidylserine.
So these are all different. fossil lipids that are present. You can see the head groups on the outside and the inside of the cell and then the greasy lipid bilayer on the inside here. And then this also illustrates some of these proteins, right?
So here's a protein. This is a transmembrane protein. You can see it goes all the way through. And this might be a transporter. This might be a molecule that allows a molecule like glucose to get into the cell from the outside because it's an allowed molecule, whereas other molecules wouldn't be able to use that same transporter.
Okay. There are also receptors. So for example, this looks like a seven transmembrane receptor.
It would receive a chemical message from the outside and it would trigger a signaling cascade inside the cell and transmit that message to the inside. So these are proteins embedded in the phospholipid based cell membrane that allow the cells to communicate with the other cells in their environment. All right, next on our list are the proteins.