Well, let's start with some definitions, again - mega important in A level biology. I will highlight the ones where you will have to give a definition in an exam. For pretty much all the key terms, we need to be able to recognise, know what they mean, and be able to use them, but most of the time we don't have to give the definitions.

So condensation: Joins two molecules together, with the formation of a chemical bond, and it involves the elimination (or the removal) of a molecule of water.

Okay, so I'm going to draw some examples here using carbohydrates, but this could well be lipids…any of the biological molecules: could be triglycerides, proteins, it could be nucleic acids…pretty much anything. There are only two types of reactions. So these are both types of reaction and if you get asked what type of reaction is this, you've got a choice of two: is it condensation joining things together or hydrolysis splitting things apart?

I'm gonna start by drawing the molecules (and see how neatly I can do them). I'm going to draw the bits that are involved in the bond in red, but actually let’s complete the molecule first. I'm just leaving a little bit of a gap there for a reason…these are just, it's an OH group that's just like any other. And right next to it. I'm gonna draw my other molecule of glucose, so that we're going to attach onto it here in this condensation reaction.

Okay, so here I've got two molecules. It happens to be alpha glucose, and you can see I've highlighted some of…I could have chosen both the OH here and just the hydrogen here, doesn't matter which way around but what you can't do is take one of the single hydrogens and bond it on to the OH group over here. It's always between the two OH groups. And in this case, I'm just exemplifying it this way around.

So these are the ones that are involved in forming that molecule of water. O, oxygen, two hydrogens: H2O gives us our molecule. So this is glucose and technically it's alpha glucose. This is Alpha (Greek symbol) alpha glucose and alpha glucose, and they react together to form maltose, which I'll draw next. So again, I'm starting from this end of the molecule, drawing it from left to right, so here I'm drawing the left…and then the bond is…this is a carbon atom onto an oxygen atom onto the carbon atom on the next molecule.

And this molecule is maltose. If I was asked to draw this in the exam I would not have full marks at this point because, what have I forgotten? I have eliminated two hydrogens and oxygen and I have not drawn them in and so they need to be included. If you get asked the question to draw this, which is not a common question, but if you do, then if it's a condensation reaction, you'll have to draw the molecule of water and if it's a hydrolysis reaction, you'll have to include the molecule of water there as well. So these, and this is water.

Okay, this name of this bond we’ll dive into more in another video. But this is a glycosidic bond.

If ever you've got a bond joining any of the carbohydrates, then it's a glycosidic bond. And you don't really need to know its full structure. And if I do go into that, it'll be in another video.

Okay. So what's the opposite of a condensation reaction? Well, it's a hydrolysis reaction.

So this breaks a chemical bond between two molecules using a molecule of water. Okay, so the difference here: we eliminate one, we produce one in other words: It's one of the products, and here we're going to use one. It's going to be one of the reactants, if you want anything in it from a chemistry perspective, again, I'm going to draw my molecule of maltose. It's going to be exactly the same as the one I've drawn up here.

Okay, this is my maltose. What do we need to add in to go back to where we were, well, we're going to need a molecule of water. And now we're gonna draw two molecules of alpha glucose. So this is good practice, this, it's not a very common question, but you are required to know the basic structure of alpha glucose. So it's good practice to see if you can draw these ones or just have a quick glance, see what you've got going on up here cover it up, try and do it from your memory and write it down on your own, or at least maybe sketch them on a side pad and then get your neat copy of the notes correct.

So we've got… And let's label them up. In order to make this reaction happen in reality, (we're talking about the subject of biology and living organisms), there's going to be an enzyme involved, called maltase. So we've got maltose, which is our molecule, here. And we're gonna yeah, the difference, let me write it out because it'll be more obvious. And in fact, I'm gonna write this in block capitals, a tiny little bit of exam technique for you, here.

We would also use the enzyme maltase and obviously if I'd written this as a lowercase ‘a’ and a lowercase ‘o’, there's very little difference between them. The exam boards are super lenient in accepting misspellings, as long as it's obvious what you're trying to write, but if they can't read your handwriting and A's and O's can easily be confused. You can see they will not give you the benefit of the doubt, there. So it's sometimes a good idea if you're writing an answer involving maltose and maltase, (which is reasonably common), to write maltose or maltase in capital letters, or at least the A in the O in such a way that they can definitely tell what you are talking about.

Okay, so quick summary: condensation joins two things together. We eliminate (or get rid of) a molecule of water. You have to draw it in, should you get asked to do that. This would be the case for any of the biological molecules and it's one of the only two types of reaction you need to know. Hydro lysis, we can break this word down: lysis means to split or to break (so to split) and then it's using the thing that's in front of the word lysis. So if you have hydro lysis using water to split apart another molecule. Later on in the course, in photosynthesis will come across photo lysis or photolysis and that is splitting using light energy and we might have osmotic lysis which is when cells burst due to osmotic pressure. So lysis means to split and the thing that comes in front of the lysis is what's causing the splitting to happen. Be careful with your ‘Ose’s and your ‘Ase’s that comes up really commonly and, yeah, that in a nutshell is condensation and hydrolysis reactions.

Okay, well the words are going to give us a lot of clues, here, as always in biology: So ‘mono’ is one, ‘di’ is two so a monosaccharide plus a monosaccharide equals a disaccharide. And we're going to look at the ones that you need to know for the course, so, conveniently they all contain glucose.

So, we have:
glucose plus glucose is going to give us a molecule of (and, again, see if you can get these before I write them down) the disaccharide which is maltose and, again, be really careful with your O’s and your A's if we're involving enzymes at all.
We also have glucose, reacted with fructose, and this gives us the disaccharide called sucrose.
And finally, you have glucose plus galactose which gives us the disaccharide of lactose. Obviously, the clue is very much in the name, with galactose forming lactose.

So the three disaccharides that you need to remember are maltose, sucrose and lactose. You need to know which monomers make them up: So, glucose and glucose. (Maybe let's put some pluses in here). Nice and easy so far.

Okay. Let's look at glucose in a little bit more detail.

So you probably know from GCSE: It has six carbon atoms, so we therefore call it a hexose (which is a generic name for a carbohydrate). ‘Ose’ tells us it's a carbohydrate: if it's an ‘ose’ it's a sugar, it's therefore a carbohydrate and hex just means six carbon atoms.

It contains the elements carbon, hydrogen and oxygen, often abbreviated to CHO. In the exam, you are allowed to use abbreviations for elements. If it's one of the known…well, if you know it, then it's likely to be accepted, but you can't make them up. They have to be the official ones.

When they join together, or when any carbohydrates join together, we form a glycosidic bond. I think I'm actually going to put that information over here. I'm gonna say that it has two forms, which are alpha glucose and beta glucose. That's my version of the Greek letter Alpha.

I'm going to draw the generic structure: the bits that are exactly the same between the two, in the centre, here, in full atomic detail. I get asked for this - It's not something that you get asked to draw in the exam. If you check your copy of the specification, there will be the molecule of glucose and it will show you how the exam board represent it in the spec and that's the level of detail they're looking for, but I'm just doing this middle version just so that there's no confusion about what I mean when I draw the slightly more simplified version.

So, probably the easiest place to start with your molecular glucose is the oxygen atom which is in the ring. It's a hexagonal ring. So, that gives us our central structure. We're going to have carbons at all the other points of the hexagon. But, obviously, we've got one, two, three, four, five carbon atoms because one of the hexagonal sides is taken up by the oxygen. So there is an additional carbon atom off the top and this is just a carbon with hydrogen, hydrogen, oxygen, and it's nearly always… I literally couldn't find the example online of it drawn out in full… So it's nearly always written as CH2OH on the top, there. And again, this is the mega mega detailed version that you don't really need to know. And then, these carbons I'm going to leave empty for reasons that we're going to see in just a moment. And then we have a hydrogen here, with an OH, here. And again, this is the detail that we don't really require in the exam, but I'm drawing it in because I get asked for it.

And then this carbon is going to make two bonds. Every carbon atom always makes four covalent bonds. So you can see this one has got one, two, three, four. This one has got one, two, three, four, and that's always the case in organic chemistry. And that's that.

Okay, so then we've got what goes on here and here, well, this is going to be basically, this is the difference between alpha and beta glucose. I'm just gonna put them in as empty boxes right now.

The other thing I'm going to delve into the detail here is that we're going to number these carbon atoms, which occasionally we touch upon when we're looking at how the bonds form. So, we simply are going to give them a number... full A level chemistry will tell you exactly how you pick which one is carbon atom number one, but I'm gonna tell you that this is carbon atom number one carbon atom number two, three, four. This one is number five and then this one is number six.

So, if I said which one is carbon atom number four, it is that one right there. Okay, so now I'm going to draw alpha glucose and beta glucose. And I can draw the simplified structure which is the one that if you get asked to draw it, again, very unlikely that ever comes up, that this is the one that you would draw.

So now I'm going to simplify this diagram here and just have it as my oxygen.

Okay, so now, I guess I should put on my alpha glucose (this is obviously what's going on inside these boxes here). We have hydrogen on the top and OHs on the bottom. And we can see here, if I were to just accentuate where the carbon atoms are (and this is why I've drawn the full structure out), we'd have a carbon atom here, carbon atom here, here, here and on the end up here as well. That's where our six carbon atoms would be. But again, it's normally just drawn as straight lines - standard practice in organic chemistry.

The way I like to remember this is: we have our Ho Ho Ho's, our OHs, our hydroxide groups. I'm going to say… I use my alpha…So, these are A's: Santa, that says, Ho Ho, and he goes down the chimney. So, that tells us that our OH groups are on the bottom on both sides in alpha glucose. This is by far the most common one that you're going to come across. If people just refer to glucose, they're generally referring to alpha glucose, and this is involved in, therefore, maltose, sucrose, lactose, starch, and glycogen. So, they’re our disaccharides, and then our polymers that contain alpha glucose are starch, which again is a mixture of amylose and amylopectin. I'll go into that in more detail in the Polymers of Carbohydrates. Starch and Glycogen.

Now, beta glucose, on the other hand, is only found in cellulose.

And again, can you try and sketch out the structure before I draw it for you? And start with our oxygen…Okay, so so far it's identical. But, what we're going to flip here is the up-down direction of the hydroxide, the OH, and the hydrogen on the right-hand side, so next to carbon atom 1. If we remember carbon atom 1, that's the one that's flipped upside down. And so, if we remember that Santa goes down the chimney, and therefore we've got both OH groups, OH, Ho Ho, on the bottom, then beta glucose has one of them flipped, and one of the OHs is on the top.

And I'm just going to add in a tiny little detail about the glycosidic bonding. So, this is a bond formed by a condensation reaction joining molecules in a carbohydrate.

So, if it's a bond joining carbohydrates together, you can bet your bottom dollar it's a glycosidic bond. That's a rule. Typically, we're going to sort of draw it as an abbreviation, and it's going to look like this: carbon atom, oxygen atom, carbon atom. If we were to draw that in full detail, we would end up with something joined onto the carbon atom here, and then we're going to end up with our oxygen and then our carbon. And then that's joining on somewhere else, and we might have a hydrogen up here and a hydrogen down here. To be honest, whether the hydrogens go on the top or the bottom isn't particularly important. You're never going to get asked to draw a glycosidic bond in this detail. It's nearly always going to be recognizing one in the exam or just simply knowing that the bonds in a carbohydrate are glycosidic bonds.

Alright, so there's going to be a pretty big table. It's going to condense so much information from the textbooks. This is everything you need to know about the polymers of carbohydrates: in the textbook, probably pages and pages. So buckle up, sit in, and get ready.

I'm going to draw a table. We'll get it sped up, and then we can start filling it in.

Okay, let's start off with the titles. I've left a bit of a funky gap here, but there is a reason for that and again, I'm going left to right in a specific order. So first of all, we've got cellulose. Then these two are starch, but starch is actually a mixture of two different things. So I'm going to put starch in at the top. And the two compounds that starch is actually made of are amylose and amylopectin. Again, we've got ‘ose’ here: Any O's or any saccharide is a carbohydrate. So these are all polysaccharides. And then finally, we have glycogen. Basically, amylopectin and glycogen are more or less identical apart from amylopectin is found in plants and glycogen is found in animals, and there's a bit more branching going on in glycogen, and we'll touch upon that in a moment. Okay, well, let's start with their function.

Cellulose is a bit of the black sheep here. It doesn't really fit the rest of the pattern and we'll see why, so this provides rigidity and strength to plant cell walls. The rest of these, all three of them, store glucose for respiration. So amylose and amylopectin can do that in plants and glycogen does that in animals.

Okay, so, basic function done. Next row. So what are they made of? Well, again cellulose is a bit of the black sheep, here. This is beta glucose and it's the only time you're really going to come across beta glucose is for cellulose. And beta is sometimes abbreviated as the symbol (β). The rest of them are going to be Alpha, and if we're just talking about glucose without saying Alpha or Beta, it's nearly always alpha glucose. And the Greek letter here looks a bit like that. It's a really good revision activity. Obviously, if you're learning this, you're not going to know it. If you are coming back to this as revision, can you just print the template note for this page and just fill it in? There should be content knowledge that should be in our brains by the time we do the exam in an ideal world.

Alright, okay, so because of this type of glucose this is going to control its structure and a very common question is linking structure to function. That's typically, cellulose and starch is the classic, but they can ask it any way around.

We also need to look at the type of bonding that's going on in here. So I touched upon this a little bit in some of the introduction to carbohydrates videos. The carbon atoms in glucose, we can number them. We know that it's got six carbon atoms and we can number them starting from the oxygen going around clockwise. One, two, three, four, five, and then six being the little one on the side chain. So when I say this has 1-4 glycosidic bonds, that means it connects carbon atom 1 on one molecule to carbon 4 on the other molecule. In other words, it goes basically in a straight line and as I draw the diagram, that will be much more obvious. In amylose, we also have 1-4 glycosidic bonds. And in these two, we have two types of bonding which basically means that we're forming a branch or a fork and therefore we have a branched molecule. So we have 1-4, all these have 1-4, and these ones also have 1-6 glycosidic bonds. Same for glycogen, as I said, these two are going to be basically the same. You can write this as 1,4 if you prefer or if you see that and you're not sure what it means, it's the same.

Okay, well, this is the majority of the information and we've got two other sections, really, we've got a diagram and we've got the properties linking the structure to its function. So I'm going to divide this up into two boxes. I think I might do the diagram next just so that you've got a bit more visualisation to add context.

Let's start with cellulose and I'm going to draw some of that with beta glucoses. So, I'm generally going to summarise our single glucose molecule like this. We have our oxygen and the extra little side chain. So that's one glucose monomer. Now, the difference in cellulose is that every other monomer is inverted so we can see we've got the oxygen on the top, oxygen on the bottom and then it's going to just keep repeating like that: oxygen on the top, oxygen on the bottom. And basically, each one of these bond angles is not perfectly straight. There's a bit of a kink, which means it kinks one way and then it kinks the other way. It kinks one way and it kinks the other way. So, because each of these is going in sort of opposite directions each time, we end up with this straight line. And so we end up with this straight molecule.

And we call this chain, which I'm now going to just simplify as a straight line. We call that the cellulose molecule, and we actually form layers of these cellulose molecules. And between the layers, we get hydrogen bonding. You don't need to know specifically what's doing it, but it's the OH groups: They've got sort of polarity, little delta charges, the same as water, and that allows us to form hydrogen bonds. So basically, this is the molecular structure. And if I sort of do this as a zoomed-in version, then if we were to zoom in on that little cellulose molecule, that's what we'd see at the molecular level.

So let's label some of this up. Obviously, it's beta glucose. These would be the cellulose molecules, which we could either label as these ones up here, and that's signifying the same as this, here. We also have the name for the sort of bundle of a bunch of cellulose molecules together, and that's a microfibril. And then we obviously have the hydrogen bonds, which hold cellulose molecules together to form a microfibril. Technically, the microfibrils bundle up into macrofibrils, but we don't need to know that. Microfibril is the term that we need for the bundle of cellulose molecules.

Okay, great. So that is cellulose. Let's move on to amylose, as we move across. Well, I'm going to start with another simplified structure of glucose. But this time, the oxygen atoms, they're all the same orientation, the same way up, which means we have this little kink here and we have a little angle going on here. And then the next one joins in the same plane, and we have another little angle, and all these little angles kind of add up.

And you can see that we're now ending up sort of going around in circles. So if I simplify this, we are simply going to form a giant helix or a coil. But because we've only got 1-4, so when I say carbon atoms 1, we start the oxygen. This is carbon atom 1, 2, 3, 4, 5, and 6. Or we can have 1, 2, 3, 4, 5, and 6. So we can see here that it's carbon atom number 1 of this molecule and carbon atom 4 of this molecule. So we've got a 1-4 bond, but only 1-4 bonds, which means we're going basically in a single chain. Now we're going to do exactly the same over here for our amylopectin, and we're gonna try and replicate what I've drawn without being too different over here because all I'm drawing right now is exactly the same.

But if we now count, let's say this atom here we’ve got 1, 2, 3, 4. So that's our 1-4. 5, 6 is this extra little branch on the top here. So, if we can form a 1-6 bond as well, then we can put a branch in, up here. And then this one is obviously going to form a little bit of an angle. So we're going to kink off over here. Something like that. And so we're gonna end up with again this sort of macro spiral, and we're gonna have maybe a spiral coming off it, and so that is our macrostructure of amylopectin.

And the only difference between amylopectin and glycogen is that the glycogen is more branched. So if I'm just gonna draw the simplified version here, I think you get the picture here with the general. So this spiral is basically exactly the same as this spiral, except we're going to add some extra branching onto it, and we're going to have more branching in glycogen. And this is obviously a diagrammatic simplification. I'm sure if you look at it in a crazy atomic structure, it's a little bit different to this, but you get the idea. We've got more branching going on here than we do for amylopectin.

Right, the final few bits of details, we're gonna say, well, we've got 1-4 branching only, so that's one connecting to one other glucose in one place. So this is unbranched. It's not possible to have a branch because they've only got 1-4 bonding.

It's also a straight chain because we've got that as a bond angle in one direction, bond angle in the other direction, and because the oxygen atoms or because the monomers are inverted, so it is a straight chain or straight chained. I suppose we could say, cellulose is a long polymer, long polysaccharide.

Insoluble, I mean some mark schemes accept this one and some don't. It's stupid for me for them to not accept it because if cellulose was soluble then plants would dissolve in the rain. They have a high tensile strength, and it's better to say that than just to say strong.

They are flexible, so they provide rigidity, but they are flexible. So it's a bit of a weird thing there, but trust me, this is all based on the mark schemes. So these are the words to use to describe it. And it's relatively inert or unreactive.

Okay, let's move on with, well, we've got 1-4 bonding. Is it branched or unbranched? It is unbranched. But it is coiled because of the structure and the way that it bonds. This coiling means that it's compact, which means you can store lots of glucose in a small space. So, sometimes you can say coiled and compact. I'm gonna add compact as its own point, and I'm going to add a detail there to say storage of glucose or ‘stores lots of glucose in a small space’ is what I'd like to write, but I'm going to be quite tight on space.

It is also a large molecule. You could probably get away with saying cellulose…cellulose because it forms a straight line ‘long’ is more appropriate and because amylose forms a more complex structure, large is a better term and the reason why large is important: it can't cross the cell membrane by simple diffusion. And it is also insoluble. And this is really important because glucose (single molecules of glucose) is soluble. And so if it's soluble that affects the water potential it would make a cell that was storing lots of starch or lots of glycogen have a very low water potential if it was all stored as single glucose molecules. That would mean loads of water would rush in by osmosis and that would cause the cell problems especially if it's an animal cell such as with glycogen: if we stored glucose in the form of glucose in animals, then it would basically cause the cells to burst which is obviously not ideal. So this is important because it doesn't affect the water potential. So it has no effect on osmosis.

Okay, so amylopectin we've got the 1-4 and the 1-6, so we're forming this branch here. So we have a branched molecule. Now the importance of this…if we think about an enzyme coming in and we want to convert our stored glucose into a single molecule of glucose for respiration…then we need to basically break one off from the ends. And that's more or less how it happens, the enzyme comes in and chops off the end one. So the more ends that we have, the faster we can break down this molecule. If this was a thousand molecules long and we can really only access the two on the end, well that's only so useful to us, whereas if we've got hundreds of ends, because we've got lots of branches, then we can access the glucose more quickly. So, more ends for rapid hydrolysis.

It is also compact. It is also large. And it is also insoluble. So, each of these reasons would still apply: so it's effective/ It's good for storage because it's compact; it's large: It can't cross the cell membrane by simple diffusion; and it's insoluble which means it doesn't affect the water potential of the cell.

And the only difference I'm going to put here between amylopectin and glycogen is that…it I'm going to say that it is very branched. Therefore…there is more/ all of these have 1-4 bonds which makes up the the basic polysaccharide and the 1-6 is the branching. So we’ve got more 1-6 bonding going on in glycogen than we do in amylopectin. Which means it can be hydrolyzed even faster, which means we can access the glucose more quickly and that's because animals have a faster rate of metabolism: They need to access their glucose because they move around, they need to run away from things. Whereas plants can respond in…generally, they don't have such a rush when they need their glucose. They can take their own sweet time about it.

It is still compact, it is still a large molecule and it's still an insoluble molecule. This would be a particular problem for animals because if it was soluble, it would have a massive effect on osmosis: you’d get very very negative water potential - loads of water would rush into the cell and, because animal cells don't have a plant cell wall, those cells would burst and that term would be osmotic lysis, (but, you don't really need to go into that in this detail here). So let's just do a quick summary.

Cellulose: the black sheep of the group. It provides rigidity and strength as plant cell walls. It's beta glucose: The only one you're going to come across for beta glucose. It's unbranched, straight chained, it's long, insoluble, has a high tensile strength, and it's flexible and unreactive. Its structure is the beta glucose molecules are inverted or you could say flipped: alternate monosaccharides are in opposite orientations. You can choose whichever language you like, there, and the examples will accept all of them. Basically the oxygen atom on the top, oxygen atom on the bottom. We form the cellulose molecules which are these straight chains that are joined by hydrogen bonds and a few of them joined together forms a microfibril.

Starch is amylose and amylopectin. All these are all alpha glucose and all for storing glucose for respiration: Glycogen in animals, amylopectin and amylose in plants. We have branching depending upon the bonding. They're compact, they’re large and insoluble and this is their rough structure and a really common question to make sure that you practise is linking this or connecting the structure to the function, which is basically these answers here…which is a great summary of the polymers of carbohydrates. You are most welcome!

Okay, we're gonna start off with some of the summary, basic structure, look at the function and the differences between saturated and unsaturated fatty acids, which are one of the components. Okay, let's start with the basic structure.

They contain three elements: carbon, hydrogen, and oxygen. And just those three elements, no nitrogen, no sulphur, nothing like that.

So, triglycerides are made up of three fatty acids and a molecule of glycerol. I'm going to probably colour-coordinate these, actually and do glycerol (for reasons you're gonna see) do glyceryl in green.

Three fatty acids and the general arrangement is going to look like, (and this is just a tiny little sketch here): We have glycerol in green. It's actually joined by an ester bond and I might actually just be a little bit fancy and put…oh no, it's gonna be too tight if I put my ester bonds in…you're gonna get it in full detail in just a moment. And then, these are my fatty acids. I'm gonna do this one with a slight kink in it. And again, all will be revealed.

The bonds that join them together here are called ester bonds. So they're joined by three fatty acids, one for each. This is joined by three ester bonds.

And obviously, they're made by condensation reaction. It's the type of reaction, if you ever get asked: What type of reaction is it? It's either condensation or hydrolysis. Are they doing joining together? In this case, we're joining the glycerol to three fatty acids. So it's a condensation reaction. It produces three molecules of water.

And the fatty acids can come in various different forms and they are either called saturated or unsaturated. In fact, I'm going to put all the fatty acids as a key term. I've drawn it in blue up here just because I wanted to make it clear on the diagram.

Okay. So again, we're going to explain what all of these things mean.

Well, let's draw out this tiny little diagram here in a little bit more detail. So, we have this molecule of glycerol, and we do need to know the atomic structure of these things, which is why I am drawing them out just here. So I'm gonna start off with…it's got three carbon atoms. (I'm just thinking about my spacing a little bit).

I'm gonna do my carbons a little bit more spaced down. (So I've already made a mistake there but I think I can correct it). These are actually…I'm going to make this into an oxygen. These are OHs, but I'm going to do the H, which takes part in this condensation reaction, so I'm going to label them in red. And then the three fatty acids, one for each of these carbon atoms, essentially.

So again, we have…and my fatty acids in blue, so I’m matching my colours up. This is our carboxylic acid group. So it's a carbon onto a double bond oxygen.

And then I'm going to just represent this by an R Group, which means that this is the bit, the carboxylic acid bit, is the bit that is common to all fatty acids. The R is going to be a hydrocarbon tail. So carbons and hydrogens, and we're going to look in more detail at saturated and unsaturated in a moment. So, we have glycerol plus three fatty acids produces triglyceride plus three molecules of water because there are three condensation reactions taking place here between these groups.

And so what do we get? Well, the green part of the glycerol isn't going to change. So, all the hydrocarbons are just carbon and hydrogen…so the only thing we need to worry about with glycerol is just these OH groups, but it's not going to be an issue, here. I'm just gonna think which ones do I want to represent inside of my bonds. I think I'm gonna do all of my…okay…So the newly formed bond I'm going to highlight. We've got the oxygen here, which bonds onto a carbon which is double-bonded to an oxygen and that is our ester bond.

And then we have our R group, which is going to be a hydrocarbon. So this is going to be a carbon atom in reality, but we're going to call it R because it's going to change depending on which type of fatty acid it is. Okay. So this is our ester bond. If you’re asked to circle it, it's important that we include both the carbon that's double-bonded to the oxygen and the oxygen that's attached to that. So it's the carbon and both oxygens.

And I'm just going to label that. I guess I need to draw my H2Os. We've got obviously one, two, three and they're forming three molecules of water up here. This is my ester bond.

Okay, so far so good. Glycerol, three fatty acids. I suppose we can put some little arrows in here…forms a triglyceride and three molecules of water by condensation reaction.

Okay, so what are the functions of these things? Well, why do we bother with triglycerides? Well, they are an energy source for aerobic respiration.

You could say that they are a respiratory substrate, but that's getting more into the respiration side of things later on the A-level. But yeah, we can break these things down and we can respire them to release energy and make ATP.

They act as thermal insulation. A little bit of fat in winter keeps us a little bit warm. So I'm gonna put, e.g., polar bears have obviously a decent layer of fat to keep them protected from the cold.

We also have electrical insulation. They do not conduct electricity. An example, again we’ll come across this later in the course, is the myelin sheath.

We also have some buoyancy. If I can spell it…U O Y A N C Y…correct me in the comments if my spelling is not correct. Yeah, they’re pretty lenient with spelling unless it can be confused with another key term or it's not clear what you mean mitosis, meiosis, maltose, maltase, that kind of thing. So if it is similar to another word, be careful that your writing is nice and neat.

So…seals…obviously spend some time floating on the surface and fat helps them to do that. And there's also some physical protection around organs. So, if you get punched, then you don't just damage your organs because they're unprotected. They have a layer of fat around them which helps cushion any blows.

Okay. Well now let's dive in and look at these little fatty acids a little more that we've just signified so far as an R Group. So, they are gonna be a hydrocarbon chain with this carboxylic acid group on the end and a carboxylic acid group is a C double bond O with an OH on the end.

So, let's look at saturated fatty acids. Saturated means that they are full up and they are full up of hydrogen. They cannot contain another atom of hydrogen no matter what you do to them. So, to be a fatty acid, they need to have the carboxylic acid group. So that's gonna be our C double bond O and our OH. So this is our carboxylic acid group and now we've got just a hydrocarbon chain.

So I'm just going to draw a couple just for demonstration. Let's draw three carbon atoms. Let's draw actually a fourth one here. And this one has hydrogens all around it. So this is like our ending carbon.

And you might get question, it’s very rare, that's like: what's the generic formula for the structure of this fatty acid: a saturated fatty acid or they give you a random fatty acid. Is it saturated or unsaturated? Well, we can just look at…the end here is slightly different. We've got a carbon with three hydrogens on it. And we've got the carbon with the C double OH on it and all the other carbons in between are saturated, which means they have single carbon, carbon bonds and all the other bonds are hydrogen.

So, if we were looking at the generic formula, we can sort of treat this end piece as CH3. We can look at the carboxylic acid group on the end and then we can look at everything in the middle of it...it depends on how many carbon atoms are in the middle. But there's two hydrogen atoms for every carbon atom that's not included on either end that might seem a little bit complicated, but it really isn't too bad.

Okay, I think I'm gonna draw my unsaturated so we can compare it for differences.

Obviously, this chain of carbon atoms could be a lot longer…could be a hundred carbon atoms in that hydrocarbon. I'm only drawing a few just for convenience sake. So, again, to be a fatty acid, It must have this carboxylic acid group on the end. So that is a given. And now we're going to draw a couple of carbon atoms. These ones are going to have hydrogens on either side. Each carbon atom can make four, these are covalent bonds. So you can see this one has got one two, three, four. Now, this one is going to double bond onto the next carbon and this actually happens at a bit of an angle. So, I haven't drawn that particularly well, but if you imagine that just sloping down a little bit down here.

Now this carbon has got one, two, three. It's already got three so it can't do a hydrogen on either side. So, it can only do one more hydrogen here and same with this one. These bonds are shared. So this is one, two, it's gonna make a carbon down here…this one is straight. This is where the kink is supposed to be, and again, we can only add one more because I've got one, two, three, four carbon atoms there and let's say that this is our last one so it's just got hydrogens on all sides. Okay.

Let's label a few things here. So this is our carboxylic acid group. You can call it a carboxyl group/ carboxylic acid. It's an acid because it can dissociate some of its hydrogen, but that's more chemistry than biology, so let's not get too involved in that.

And every fatty acid is going to have one of those. The length of the carbon atom chain is gonna vary. Let's put a little bit of notes down around our saturated fatty acid. So it does not have carbon-carbon double bonds. Do NOT have…you could say double bonds between carbon atoms or you can call it a carbon-carbon double bond. So if we were drawing that it would look something like this: carbon double bonded onto carbon. It doesn't have any of those. Every one of these bonds is a single which means you can't fit any more hydrogen in.

If we were to break this double bond, we could add a hydrogen here. We can add a hydrogen here and hence this one being unsaturated with hydrogen.

They basically cause an increase in cholesterol and low-density lipoproteins, so they're not ideal for our health.

What are lipoproteins? Well, sometimes these are called LDLs…Basically lipids are insoluble. Triglycerides are lipids and they're not soluble in water or the blood. So in order to transport them, fatty acids and triglycerides are transported by these lipoproteins. We bind them onto a protein and then we can sort of move them around a little bit more straightforwardly.

Alright, then unsaturated. What have we got down here. Well, they DO have carbon-carbon double bonds. Which obviously we are representing, here. They contain less hydrogen than saturated fatty acids.

If it were to have more than one carbon-carbon double bond, let's say there was a carbon-carbon double bond here and here, we would call it a poly unsaturated. Obviously, this would be a mono unsaturated and it could be a polyunsaturated. It's basically never examined, but it's good to just be familiar with some of that language.

Also, we might get…if when we do get the double bonds…the double bonds have a kink because the bonding angle isn't straight. I'm going to say they are kinked at each double bond. It's a bit lazy just to call it a double bond because obviously we're specifically referring to the double bond between carbon atoms. Not to, for example, this double one between the carbon and the oxygen over here in the carboxylic acid group, because every fatty acid has those as well. So really you should refer to it as carbon-carbon double bond or a double bond between carbon atoms and this gives us our little kink which is why these carbon atoms are off at an angle, and these ones are in a straight line.

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.