Video Transcript
Which of the following best
explains the difference between a saturated and an unsaturated fatty acid? (A) Saturated fatty acid chains
have only single bonds between the carbon atoms, whereas unsaturated fatty acid
chains have one or more double bonds between the carbon atoms. (B) Unsaturated fatty acid chains
have only single bonds between the carbon atoms, whereas saturated fatty acid chains
have one or more double bonds between the carbon atoms. (C) A saturated fatty acid chain
contains double the number of carbon atoms that an unsaturated fatty acid chain
contains. Or (D) saturated fatty acids
contain a higher proportion of oxygen than unsaturated fatty acids.
This question asks about the
difference between saturated and unsaturated fatty acids. So, first let’s remove the answer
choices so we have some space to remind ourselves about the basic structure of fatty
acids. As you can see here, fatty acids
are composed of a carboxyl group attached to a hydrocarbon chain and are a major
component of lipids. The carboxyl group is hydrophilic;
it’s charged and will interact with charged or polar molecules, like water. On the other hand, the hydrocarbon
chain doesn’t contain any charge and is therefore hydrophobic. This diagram is specifically
showing the structure of a saturated fatty acid, but what does this mean? Let’s take a closer look at the
atoms in the hydrocarbon chain to understand this better.
As the name suggests, the
hydrocarbon chain of a fatty acid only contains carbon and hydrogen atoms. Saturated fatty acids only contain
single bonds between adjacent carbon atoms in the hydrocarbon chain. This means that the hydrocarbon
chain is fully saturated with hydrogen atoms. In other words, they contain the
maximum number of atoms of hydrogen, and hydrogen atoms are bonded to a carbon atom
in every available location on the chain.
Because of their chemical
structure, fats containing saturated fatty acids can lie flat against each other and
form a solid at room temperature. Unsaturated fatty acids have at
least one double bond between the carbon atoms in the hydrocarbon chain. Each double bond between carbon
atoms takes the place of two hydrogen atoms, resulting in fewer hydrogen atoms in
the hydrocarbon chain overall and causing the structure to kink. This results in lipids that are
liquid at room temperature because the kink in the fatty acid hydrocarbon chain
means they do not fit together as easily as saturated fatty acids.
With this information, let’s take a
look at the answer choices again. First, we can eliminate answer
(C). The classification of these
molecules into saturated or unsaturated fatty acids has nothing to do with the
number of carbon atoms in the hydrocarbon chain. In fact, both saturated and
unsaturated fatty acids come in a wide range of lengths. Next, we can eliminate answer (D),
as both fatty acid types have the same proportion of oxygen atoms in their carboxyl
group.
This means that the correct answer
is (A), as (A) and (B) are the exact opposite of each other and (A) correctly
describes the difference between the carbon-carbon bonds in saturated and
unsaturated fatty acids. Saturated fatty acid chains have
only single bonds between the carbon atoms, whereas unsaturated fatty acid chains
have one or more double bonds between the carbon atoms.
