Fatty Acids: Study with Video Lessons, Practice Problems & Examples

Fatty acids are hydrocarbon chains with a terminal carboxylic acid group, categorized into saturated and unsaturated types. Saturated fatty acids contain only single carbon-carbon bonds, while unsaturated fatty acids have one or more double bonds, creating kinks in their structure. The melting point of fatty acids is influenced by chain length and saturation; longer chains and fewer double bonds result in higher melting points. Saturated fats are typically solid at room temperature, while unsaturated fats are liquid, impacting their biological roles and applications in nutrition.

Fatty Acids

Alright. In this video, we're going to explore our first category of lipids, the fatty acids. And so first notice that down below over here, we have a different version of our original lipid map. And you can see that the lipids are going to be broken up into the fatty acid based lipids, which is this entire left branch over here and then we also have the isoprene based lipids and notice that the Isoprene based lipids are being abbreviated with this Isoprene molecule and we're going to continue to do that until after we've explored all of the fatty acid lipids in their entirety. And then again, we'll go and explore the isoprenes last. And so what in the world are these fatty acids? Well, fatty acids, which are also sometimes generally called just fats occasionally, these are just hydrocarbon chains of varying length that have a terminal carboxylic acid group. And so if we take a look at our image down below right here, notice we have a fatty acid molecule. We have this long hydrocarbon chain, and at the very end of the hydrocarbon chain, we have this carboxylic acid, just as we described up above. Now, these fatty acids are gonna be used as building blocks for more complex lipids such as glycerolipids, sphingolipids, and all of these other ones down below, waxes and eicosanoids as well. And so, what's important to note is that the carbon atoms of fatty acids, they're usually numbered, starting at the carboxyl carbon atom. And so notice over here we have the carboxyl carbon atom, and so this would be carbon number 1. And what's also important to note is that the alpha carbon designated by just the Greek symbol alpha is going to be the carbon atom that is adjacent or right next to the carboxyl carbon atom. And so this carbon atom right here in blue, because it's adjacent or right next to the carboxyl carbon atom, this is going to be the alpha carbon atom, and you can think the a in alpha is just for the a in adjacent right next to. And then, of course, the omega carbon atom symbolized by the Greek letter omega is going to be the carbon atom that is furthest from the carboxyl carbon atom. And so, of course, this carbon atom over here is the one that's furthest away from the carboxylic carbon. And so this is going to be the Omega Carbon. And so what's important to note is that before we actually move on to talking about Glycerolipids, we're we're actually gonna talk about fatty acids for a little while. We're gonna talk about different types of fatty acids such as unsaturated and saturated in our next lesson video. And then we'll talk about nomenclature and some other fatty acids as well. So we're gonna stick on this fatty acids for a little bit before we actually get to glycerolipids. And so this here concludes our introduction to fatty acids and again, in our next lesson video we'll be able to talk about unsaturated and saturated fatty acids. So I'll see you guys there.

Which fatty acid chain would you expect to be least soluble in water?

a) 16-Carbon fatty acid. 

b) 4-Carbon fatty acid.

c) 22-Carbon fatty acid.

d) 12-Carbon fatty acid.

• A. 16-Carbon fatty acid.
• B. 4-Carbon fatty acid.
• C. 22-Carbon fatty acid.
• D. 12-Carbon fatty acid.

Fatty Acids

In this video, we're going to differentiate between saturated and unsaturated fatty acids. These fatty acids that we've been discussing can be categorized into 2 different groups based on the nature of their hydrocarbon chains. These two groups are the saturated fatty acids and the unsaturated fatty acids. Now, as the name implies, the saturated fatty acids are going to be fully saturated with hydrogen atoms. That means their hydrocarbon chains will only contain Carbon-Carbon single bonds, and there will not be any Carbon-Carbon double bonds. If we take a look at our image down below, over here on the left-hand side, notice we're showing you an example of a saturated fatty acid. You can see that it's got a carboxylic acid group and this long hydrocarbon chain, which only has Carbon-Carbon single bonds, meaning the hydrocarbon chain is fully saturated with hydrogen. Over here, we have a different 3D representation of the same exact fatty acid, and this is actually the fatty acid stearate, which is one of the most abundant saturated fatty acids in animals.

Now, the second group of fatty acids are the unsaturated fatty acids. As their name implies, they are not going to be fully saturated with hydrogen atoms. This is due to the presence of greater than or equal to 1 Carbon-Carbon double bond in the hydrocarbon chain. The prefixes mono- and poly- respectively indicate the presence of just 1 or more than 1 double bond. The double bonds present in unsaturated fatty acids are almost always in the cis conformation, which creates kinks in the chains of unsaturated fatty acids. If we take a look at our image down below, over here, notice we're showing another fatty acid, but this one is an unsaturated fatty acid because there's the presence of a double bond. Notice that the double bond is almost always in the cis conformation, which creates a kink in the chain of the unsaturated fatty acid. Because there's only one double bond present here in this specific unsaturated fatty acid, oleate, this makes it a monounsaturated fatty acid, indicating the presence of just one double bond. Over here on the right, we have another unsaturated fatty acid, linoleate, which has the presence of 3 double bonds. Because it has more than one double bond, this would be a polyunsaturated fatty acid. The double bonds are almost always in the cis conformation. This creates kinks in the chains of unsaturated fatty acids, and that will be important in our next lesson video when we talk about the melting points of these fatty acids.

This here concludes our introduction to saturated and unsaturated fatty acids, and we'll be able to get some practice applying these concepts in our next video. So I'll see you guys there.

What is the molecular formula of Linolenic acid, an 18-carbon polyunsaturated fatty acid with 3 double bonds?

a) C₁₈H₃₂O₂

b) C₁₈H₃₀O₂

c) C₂₈H₃₀O₂

d)C₁₈H₃₄O₂

• A. a) C₁₈H₃₂O₂
• B. b) C₁₈H₃₀O₂
• C. c) C₂₈H₃₀O₂
• D. d)C₁₈H₃₄O₂

Fatty Acids

In this video, we're going to talk about the melting points of fatty acids, and really there are two primary factors that affect the strength of the hydrophobic interactions between the hydrocarbon chains of fatty acids, and the strength of the hydrophobic interactions is actually what affects the melting point of the fatty acids. And so these two factors, by affecting the strength of the hydrophobic interactions, they're also affecting the melting point of the fatty acids. And so the first of these two primary factors is going to be the length of the hydrocarbon chains or the amount of carbon atoms that are present in the hydrocarbon chains. And so the longer these hydrocarbon chains are, the higher the melting point will actually be. And so if we take a look down below at our image at table number 1, which corresponds with number 1 up above, you'll notice that we have the fatty acid name in the first column and the second column practically has the number of carbon atoms or the length of the hydrocarbon chain and the 3rd column has the corresponding melting point in degrees Celsius. And you'll notice that as we increase the number of carbon atoms in the hydrocarbon chain or as we increase the length of the hydrocarbon chain, the melting point is also increasing and so, it's true that the longer the chain is, the higher the melting point will be.

Now, the second primary factor that affects the strength of hydrophobic interactions, and thus the melting point of the fatty acids, is going to be the degree of saturation of the hydrocarbon chains or the amount of double bonds that are present in the hydrocarbon chain. And so this time, it turns out that the more double bonds there are in the hydrocarbon chain, the lower the melting point is going to be. And so if we take a look at our second table, table number 2, which corresponds with number 2 up above. Notice again, we have the fatty acid chain name in the first column and then we have the degree of saturation or the number of double bonds in the second column and again, the melting point in degrees Celsius in the 3rd column. And notice that this time, as we increase the number of double bonds, that the melting point is actually decreasing. And so we start off with positive 16 degrees, and we go down to negative 50 degrees. So, the melting point temperature is actually decreasing as we increase the number of double bonds. Just like what we said up above. The more double bonds, the lower the melting point. And so, therefore, because this here is true. This means that unsaturated fatty acids, which are gonna end up having kinks in their chain due to the double bonds, are gonna also end up having lower melting points than saturated fatty acids.

And so if we take a look at our image over here, what you'll note is on the left hand side, we have saturated fatty acids and on the right hand side, what we have are unsaturated fatty acids. And so taking a look at the, saturated fatty acids over here on the left hand side, notice that they are pretty highly packed. And that's because they don't have any double bonds and they don't have any kinks in their chain. And so their fatty acid chains are able to be pretty linear and, they're able to be pretty highly packed. And they're so highly packed, the more packing there is, the more hydrophobic interactions that can form between all of these hydrophobic chains. Higher the higher the melting point will be. Now, on the other hand, taking a look at the unsaturated fatty acids, which again are gonna have these double bonds, the double bonds are ultimately going to create kinks in the chains of these unsaturated fatty acids. And so notice that these chains are not linear, they're actually, forming these kinks and that makes them a lot harder to pack tightly so they are going to be less packed and if they're less packed, that means that they're going to be less hydrophobic interactions forming between them and less hydrophobic interactions corresponds with having a lower melting point.

Now, a higher melting point means that at room temperature, these are are going to be solids at room temperature such as butter, which are again solids at room temperature. And on the other hand, having a lower melting point means that you're gonna melt a lot easier at room temperature and so they're going to be liquids at room temperature such as oils. And so down below, we've defined fats as a general term referring to lipids or fatty acids that are solids at room temperature and then, of course, oils are going to be liquids at room temperature. And what you'll notice is that the, fats such as butter here, they're typically extracted from animals and that's because although animals have both saturated and unsaturated fatty acids, they tend to have a higher percentage of saturated fatty acids. And so fats are typically extracted from animals. And on the other hand, plants, and oils, I'm sorry, oils which are liquids at room temperature, they tend to be extracted from plants and that's because plants, again, although they have both saturated and unsaturated fatty acids, they tend to have a higher percentage of unsaturated fatty acids, and that's what allows, oils to be extracted from plants, typically. And so this here concludes our lesson on the melting points of fatty acids and we'll be able to get some practice applying all of these concepts in our next couple of practice problems. So I'll see you guys there.

What aspect of each of the 18-carbon fatty acids in the table below is correlated with their melting point?

a) The charge of the carboxylic acid group.

b) The length of the hydrocarbon chain

c) The number of double bonds

d)The polar hydrocarbon chains

• A. The charge of the carboxylic acid group.
• B. The length of the hydrocarbon chain
• C. The number of double bonds
• D. The polar hydrocarbon chains

What happens to the melting point in fatty acids as the hydrocarbon length increases?

a) It increases.  

b) It decreases.

c) It stays the same.

d) There is no direct correlation.

• A. It increases.
• B. It decreases.
• C. It stays the same.
• D. There is no direct correlation.

Unsaturated fatty acids: 

a) Usually contain a double bond with cis stereochemistry.

b) Are found in both plants and animals. 

c) Sometimes contain multiple double bonds. 

d) Have lower melting points than the analogous saturated fatty acids.

e) All of the above are correct.

• A. Usually contain a double bond with cis stereochemistry.
• B. Are found in both plants and animals.
• C. Sometimes contain multiple double bonds.
• D. Have lower melting points than the analogous saturated fatty acids.
• E. All of the above are correct.

Which of the following are correct with regard to saturated fatty acids?

a) They are generally solid at room temperature. 

b) The carbon backbone contains at least one double bond, creating a kink in the chain. 

c) Come primarily from vegetable products. 

d) They are only hydrocarbon chains, making them liquids at room temperature.

e) Cannot be present in other lipids, such as phospholipids.

• A. They are generally solid at room temperature.
• B. The carbon backbone contains at least one double bond, creating a kink in the chain.
• C. Come primarily from vegetable products.
• D. They are only hydrocarbon chains, making them liquids at room temperature.
• E. Cannot be present in other lipids, such as phospholipids.

Match each of the fatty acids with the appropriate melting point:

a) CH₃(CH₂)₁₈COOH ______

b) CH₃(CH₂)₁₄COOH ______

c) CH₃(CH₂)₁₀COOH ______

d) CH₃(CH₂)₇CH=CH(CH₂)₇COOH ______

e) CH₃(CH₂)₄CH=CH-CH₂-CH=CH(CH₂)₇COOH ______

Alright, so this practice problem wants us to match each of the fatty acids that we see down below right here with the appropriate melting point that we see over here. And notice that we're given the room temperature as 25 degrees Celsius. Now, the first thing that we need to notice here is that for all of these fatty acids, notice that they all have a carboxylic acid group that I'm highlighting in green. Really, the rest of this that we see are all just hydrocarbon chains, and so these are indeed fatty acids. The next thing that you guys should note is that these first three fatty acids do not have any double bonds being indicated, which means that these first three are going to be saturated fatty acids. And so what we can do is go ahead and label these first three as being saturated. And then what you should notice is that these down here actually do have the presence of double bonds and so because they have double bonds these 2 are going to be of course unsaturated. And then, of course, what we need to recall is that saturated fatty acids tend to be solids at room temperature and unsaturated fatty acids tend to be liquids at room temperature. And so, if you are going to be a solid at room temperature, that means that your temperature must be above the room temperature and so, notice that there are 3 temperatures that are above the room temperature of 25 degrees Celsius and so these three temperatures must correspond with these three fatty acids and then we have 2 temperatures that are below the room temperature and so these would correspond with being liquids at room temperature. And so these 2 must correspond with these two down below. These two temperatures must correspond with these, down below. And so, the next thing that we need to do is, for these saturated fatty acids is consider the total number of carbon atoms or the length of the chain. Now notice that for fatty acid a here, there's, 1 plus 18 plus 1, which is 20 carbon atoms in the first one. So we'll put 20 c here. For fatty acid b, we have 1+14+1 which is 16 carbon atoms and then for fatty acid c down below we have, 1+10+1 which is 12 carbon atoms. And so recall from our previous lesson videos that the greater the length of the chain, the higher the melting temperature is going to be. And so, of these 3 here, that we've designated for these positions, the highest one is 76 degrees and that's gonna correspond with, the one that has the longest chain. So we can go ahead and cross that off. The next highest is 63 degrees and that's gonna correspond with the next highest chain. So, we can cross that off and then, the next highest is going to be, 45 degrees. So, that's gonna correspond with this chain right here, and so we can cross that off. And then looking at these down below here, what we need to notice is, count the number of carbon atoms as well. So, for fatty acid D here, we have 1, plus 7, which is 8. Here's 9, 10, plus another 7 is 17, and then plus 1 is 18. So this has 18 carbons. And then counting the carbon atoms here in fatty acid e, we have 1 plus 4, which is 5, plus 1, which is 6, here's 7, 8, 9, 10, plus another 7 is 17, plus another one is, 18. And so notice that they both have 18 carbons, which means that their length, the length of the chain is exactly the same, and that's not what's going to differentiate these 2 melting points. And so instead, what we need to take a look at is the number of double bonds. Notice that fatty acid d only has one double bond, whereas fatty acid e actually has 2 double bonds. And recall that the greater the degree of saturation or the more double bonds there are, the lower the temperature is. And so because fatty acid e has more double bonds, it's gonna have the lower melting temperature of negative 5 degrees Celsius. And then of course, that leaves the 13 degrees Celsius for this last one and that concludes our practice problem here, and so I'll see you guys in our next video.

Here’s what students ask on this topic:

What are fatty acids and how are they classified?

Fatty acids are hydrocarbon chains with a terminal carboxylic acid group. They are classified into two main types: saturated and unsaturated fatty acids. Saturated fatty acids have only single carbon-carbon bonds, making them fully saturated with hydrogen atoms. Unsaturated fatty acids contain one or more double bonds, which introduce kinks in their structure. These kinks prevent tight packing, affecting their physical properties and biological roles. Saturated fats are typically solid at room temperature, while unsaturated fats are liquid. This classification impacts their nutritional and functional roles in biological systems.

What is the difference between saturated and unsaturated fatty acids?

Saturated fatty acids contain only single carbon-carbon bonds, making them fully saturated with hydrogen atoms. This results in straight chains that can pack tightly, leading to higher melting points and solid state at room temperature. Unsaturated fatty acids have one or more double bonds, usually in the cis configuration, creating kinks in the chain. These kinks prevent tight packing, resulting in lower melting points and a liquid state at room temperature. The presence of double bonds in unsaturated fatty acids also influences their biological functions and nutritional properties.

How do the length and degree of saturation of fatty acids affect their melting points?

The melting point of fatty acids is influenced by two main factors: the length of the hydrocarbon chain and the degree of saturation. Longer hydrocarbon chains have higher melting points due to increased hydrophobic interactions. Conversely, the degree of saturation affects melting points inversely; more double bonds (unsaturation) result in lower melting points. This is because double bonds introduce kinks in the chain, preventing tight packing and reducing hydrophobic interactions. Therefore, saturated fatty acids have higher melting points and are solid at room temperature, while unsaturated fatty acids have lower melting points and are liquid.

Why are saturated fats solid at room temperature while unsaturated fats are liquid?

Saturated fats are solid at room temperature because their hydrocarbon chains contain only single carbon-carbon bonds, allowing them to pack tightly together. This tight packing increases the strength of hydrophobic interactions, resulting in higher melting points. In contrast, unsaturated fats have one or more double bonds in their hydrocarbon chains, creating kinks that prevent tight packing. This loose packing reduces the strength of hydrophobic interactions, leading to lower melting points and a liquid state at room temperature. This difference in physical state impacts their biological roles and nutritional properties.

What are the biological roles of fatty acids?

Fatty acids play several crucial biological roles. They are key components of cell membranes, providing structural integrity and fluidity. Fatty acids are also involved in energy storage; triglycerides, composed of three fatty acids and glycerol, are stored in adipose tissue and can be metabolized for energy. Additionally, fatty acids serve as precursors for bioactive molecules like eicosanoids, which are involved in inflammation and signaling pathways. Their saturation level influences membrane fluidity and function, with unsaturated fatty acids contributing to more fluid membranes compared to saturated fatty acids.