Fatty Acids – Lipids

00:00 Now one of the interesting things about unsaturation within a fatty acid, is it changes the property of the fatty acid. We can see for example if we compare the individual saturated fatty acids of myristic, palmitic and stearic acid, that as we increase the length and going from 14 to 16 to 18 carbons, that what happens is the melting point of these fatty acids increases. Now if we take that same 18 carbons and compared it to unsaturated fatty acids, as can be seen here, all these fatty acids have 18 carbons, they differ in the number of double bonds that they contain. The stearic acid with no double bonds has a melting point of 71°, whereas linolenic acid with three double bonds has a melting point of -11°.

00:48 Obviously desaturation reduces the melting points of fatty acids and consequently reduces the melting point of the fats that contain these fatty acids.

00:58 I've been describing the structure of unsaturated fatty acids, but now you can see three of the structures on the screen here. Oleic acid, linoleic acid and linolenic acid differ in the number of double bonds that they contain. You can see that oleic acid has a double bond locator to what's described as position number nine. Now when we describe the location of double bonds within fatty acids, there are two different numbering systems that are used.

01:24 The first numbering system is called the delta numbering system and it's the one that's labelled on the oleic acid above. The delta numbering system numbers carbon number one as the carboxyl group on the end of the molecule, then counting inwards nine carbons, we get to the double bond that's located in oleic acid. In linoleic acid you see both the one through twelve numbering in green below and you also see the one through six numbering from the right towards the left.

01:54 The numbering from the right towards the left is called the omega numbering system and the omega numbering systems starts carbon number one with the methyl at the end of the fatty acid. Linoleic acid is described as an omega six fatty acid and biologically omega six fatty acids have a little bit different effect on the body compared to the other omega fatty acids. Linolenic acid is the fatty acid that's described as an omega 3 fatty acid. Omega 3 fatty acids are associated, in many cases, with what people think is good health and healthy heart and so forth. So the omega system and the delta systems are different from each other and it's important to recognize that. One of the reasons that we have the delta numbering system is it allows us to describe the position of double bonds that can be made by certain organisms because the difference of position from the carboxyl group is critical in whether or not enzymes within an organism will function. So for example, oleic acid has the bond of position nine as you can see here and that bond can be synthesized by animal cells. By contrast, animal cells cannot make the bonds located at position twelve or at position fifteen. Now because of that linoleic acid and linolenic acid are what we describe as essential fatty acids. So with the nomenclature that is used here to describe these fatty acids you can see on the screen here, oleic acid is described as monounsaturated because it only has a single double bond. Linolenic and linoleic acid are both described as polyunsaturated because they have more than one double bond. Now these names that we use to describe fatty acids are also used to describe the fats that contain them.