Professor Dave here, let's learn about lipids.
As we continue our survey of biomolecules, we arrive at lipids.
Lipids include things like fats, oils, and steroids, and the key characteristic that
defines them is that they are largely nonpolar, composed predominantly of long
hydrocarbon chains or ring systems, and unlike the two types of polymers we've
discussed, lipids are usually not big enough to be classified as
macromolecules. Let's take a look at some different types of lipids.
First, triacylglycerols, or TAGs.
These are molecules with three ester groups, and examples include peanut oil,
corn oil, butter, and lard. TAGs that are liquid at room temperature are called
oils, and if they're solid, they're called fats. These molecules can differ in the
type of acyl groups they possess, which is this portion of the molecule, which
will always be a hydrocarbon chain of some length. If these ester groups are
hydrolyzed, meaning a hydroxide attacks each carbonyl, kicking off this portion
of the molecule, we get glycerol and three fatty acids. They're called fatty acids
because they are carboxylic acids but they also have long nonpolar hydrocarbon
tails, which can vary depending on the TAG the fatty acid is derived from. Most of
the time this tail is unbranched and contains an even number of carbons. If it
contains all carbon-carbon single bonds, this would be a saturated fatty acid
because it contains the maximum number of hydrogen atoms possible. If there's a
double bond
it's an unsaturated fatty acid. Sometimes there are two or three double bonds,
which would make it a polyunsaturated fatty acid. In naturally occurring fats
the double bonds are almost always cis. When we hear about trans fats, these are
the ones that seem to be harmful to the human body, as their linear shape gives
them a higher melting point, so they are more likely to remain solid and more
difficult to metabolize.
These are associated with cardiovascular disease. In general, saturated fatty acids
have higher melting points than unsaturated ones, because the double
bonds restrict rotation and therefore cause a kink in the chain, which
interferes with crystal packing. Saturated fatty acids can pack neatly
and maximize van der Waals attractions.
That's why butter, which is fully saturated, is a solid at room temperature
whereas olive oil, which has a degree of unsaturation in one of its hydrocarbon
chains, is less able to form an orderly solid lattice and is therefore a liquid
at room temperature. An omega-3 fatty acid is one where the third carbon from
the end of the chain is participating in a double bond. When we talk about
hydrogenated vegetable oils, these are derived by hydrogenating some of the
double bonds in polyunsaturated fats. One problem with partial hydrogenation is
that it can sometimes isomerize double bonds, turning cis into trans, resulting
in trans fats, which as we said, should be avoided. In general fats like TAGs are
used as energy storage. When these are metabolized in our bodies, they release
twice as much energy as carbohydrates, since there are so many carbon-hydrogen bonds.
Furthermore, polysaccharides like glycogen only provide energy for about a
day, whereas the fats we store that make up around twenty percent of our body
mass provide long-term energy storage, which would keep us alive for up to a
month without any food intake if necessary.
Still, there is much discussion about how much saturated fat and unsaturated fat
should be in our diet, but we will have to examine that another day.
Another interesting aspect of fatty acids is that when the carboxyl group is
deprotonated to form the sodium salt, you get soap molecules, which have a polar
head and a nonpolar tail. These work by forming micelles in solution, where many
soap molecules orient themselves with the negatively charged carboxylate
groups all facing outwards to maximize ion-dipole interactions with
water molecules, while the hydrophobic hydrocarbon chains point inwards by
exclusion, making van der Waals interactions with one another.
Any nonpolar dirt or grease which is normally not water-soluble can get
trapped inside and willl then wash away with the micelle, which is water-soluble.
That's why washing your hands with just water alone won't do anything, but add
some soap and the dirt gets transported away in little spheres of soap molecules.
Let's discuss another type of lipid: the terpenes and terpenoids. These are
polymers made up of repeating isoprene units, which is this 5 carbon diene.
Here we can see myrcene, a terpene consisting of two isoprene units.
Farnesene, found in apples, has three. The units can be linked to form rings such
as in limonene, which is found in lemons. These can get much larger such as with
beta-carotene, comprised of eight isoprene units, which is found in various
plants and fruits. Terpenoids are similar to terpenes but they contain oxygen
atoms, such as with menthol, found in peppermint. Those are just a few types of
lipids found in our bodies.
Let's continue with lipids as we take a look at steroids.
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