Phospholipids & Cholesterol – TRANSCRIPT

A bit of a precursor to this lesson is the lesson on triglycerides because that’s going to go into the structure of a triglyceride, and a phospholipid is basically a modified one of those. We’re not going to delve into huge amounts of detail on cholesterol because we don’t need to know too much about it. Okay, again, let’s look at the structure first.

I’m going to draw a simplified version of glycerol, here. And again, you do technically need to know the full detail that I’ve covered in the triglycerides lesson, although it’s not very commonly asked.

Okay, so we have the same basic structure, and I’m going to draw glycerol like this. Okay, so far, so much the same. Well, we have two fatty acids. So here, I’m gonna draw my ester bond, which is my oxygen onto my carbon with my double bond oxygen, and then that’s going to attach on to my fatty acids, which I’m just gonna draw as a line and we have two of those. So, let’s just draw another one in.

Not all fatty acids are the same length. These are hydrocarbons. They are just carbon and hydrogen, there’s no OH, there’s no charges in there. So these are nonpolar and they are hydrophobic. So I’m going to label them as hydrophobic fatty acid tails. That’s true in a triglyceride. I’m just adding that extra detail right now.

We call them tails, here. And they’re hydrophobic because they’re nonpolar, and there’s nothing for the sort of little charges on water – the delta negatives and the delta positives too sort of latch onto. The difference is that it’s not a triglyceride: We’ve only got two of these fatty acid tails, we lose one, and on the other side, we have, and I’m just going to draw here as P. So this is a hydrophilic phosphate group.

So hydrophilic means that it is charged. It will bind to water very easily. It likes water. So just note this is a phosphate group, which is technically, if I’ll put it down for you, it’s PO43- is what a phosphate group is. Not just an atom of phosphorus. If you just say phosphorus, you won’t get the mark. And they’re obviously joined by the same bond that we’ve recognized from the triglycerides lesson. This is an ester bond.

Okay. Hydrophobic fatty acid tails are non-polar and they sort of repel water. They’re hydrophobic, and hydrophilic phosphate head.

Well, what are the functions of phospholipids?

Okay, so phospholipids are used as the main molecule in cell membranes. What’s a good idea: If you’re referring to the surface cell membrane, the membrane that goes around the outside of the cell, call it the surface cell membrane because there are lots of internal membranes as well. Very occasionally exam boards will get really picky and just want you to refer to the surface cell membrane if that’s what you’re talking about.

I think technically by mass proteins make up about 50% of the cell membrane, but certainly by the number of molecules, phospholipids are by far the most dominant ones.

Okay, so lipid-soluble or nonpolar molecules can diffuse straight through the cell membrane by simple diffusion. But polar or charged molecules cannot do that. In fact, let’s give a couple of examples: steroid or lipid hormones or we could have, let’s say, oxygen or CO2.

Yes, steroid hormones. There are two types of hormones: protein hormones and lipid hormones. Steroid hormones are the lipid hormones.

So these things, they don’t have any overall charge, they can just go straight through.

Another following on point: So, polar or charged molecules cannot diffuse through. Examples of those would be any ion. So take your pick: sodium ions, chloride ions, magnesium ions, protons, or water. Water obviously has the polarity where all the hydrogen bonding comes from. All of those types of things have charge, and they basically can’t go through; they’re repelled by the non-polarity of the hydrophobic tails here.

Okay. So, when we’re forming a cell membrane, phospholipids form a bilayer, which is pretty much what it says on the tin. There’s gonna be more detail on the cell membranes lesson, but I’m just gonna sort of flip my phospholipid now, and I’m gonna draw these horizontally. So, facing outwards we have our phosphate or hydrophilic head. So I’ll draw a couple of those. And I suppose, I will add the little detail of my phosphate group. It’s not normally drawn on these things, but I’m just making sure you can map things nice and easily. And then we have our two fatty acid tails which poke out in this direction. So this is one layer.

And we have a bilayer. So what it actually means is that we have the hydrophobic tails grouped together on the inside. Basically, these are cell membranes; they’re surrounded by water, the cytoplasm, the tissue fluid. They’re in an aqueous environment, which means that the bits that are hydrophobic, so the fatty acid tails basically are out of their own element. They want to be with each other and away from water. And again, we can put our little phosphate heads in. And again, if you’re sketching these, you don’t normally need to add the phosphate heads. I’m just making sure it’s really clear what’s going on here.

So, in here, we have hydrophobic tails facing inwards. So here would be, let’s say, inside of the cell; here would be outside of the cell. I might be able to label that on here, but that’s… so they face inwards. Let’s just put outside and inside of the cell. So there’s water here and there’s water here. And so the hydrophobic bits are basically hiding away from that on the inside of the cell membrane.

The hydrophilic phosphate heads face outwards. So this basically forms a border with the cytoplasm. In fact, technically, hydrogen bonds to the cytoplasm. I’m just going to say with water…and e.g. in cytoplasm…could be the tissue fluid, whichever liquid is surrounding the cell, essentially. So that forms our cell membrane.

Obviously, I’ve drawn gaps, here. In fact, if I’m being really fancy, I could maybe just put in a few extras because it does form a continuous layer. I don’t want to give you the impression that it’s full of gaps…because they are fluid; they are able to move past one another, but they are basically… there’s not giant gaping gaps for things to move through.

Looks a little bit more like it. Then we move on to cholesterol.

So cholesterol is a molecule that’s going to fit in between the fatty acid tails of the phospholipids, and the more cholesterol you have, the less fluid the cell membrane is. It’s basically constantly moving and sliding over each other, but the cholesterol sort of binds and jams up the fatty acid tails, which means that the phospholipids jam together a bit tighter and you get less fluidity. Obviously, if you remove cholesterol, then the membrane becomes more fluid.

We can say it changes the fluidity of the membrane. More cholesterol is less fluidity.

And that’s everything you need to know about cholesterol.