Video Transcript
Which of the following diagrams
correctly represents the structure of a saturated triglyceride? (A), (B), (C), or (D).
This question asks us about
triglycerides, which are a type of lipid. To answer this question correctly,
it would be useful to look at what lipids and, in particular, what triglycerides
are.
A lipid is a biological
macromolecule that is made up of carbon, hydrogen, and oxygen atoms, with far more
carbon and hydrogen than oxygen. Lipids have many roles within the
body, including energy storage, formation of biological membranes, and insulation,
both thermal and electrical.
Now let’s look at one group of
lipids, the triglycerides. If we look at the name
triglyceride, it gives us a clue to their structure. The tri- part of their name refers
to their three fatty acid chains. The glyceride part refers to the
glycerol molecule, their other component. Glycerol is made up of three carbon
atoms, each bound to one hydroxyl, or OH, group and then one or two hydrogens. The -ol at the end of the name
tells us that glycerol is an alcohol.
Fatty acids are molecules with a
hydrocarbon tail, in other words, a chain of carbon and hydrogen atoms which is
anywhere between two and 27 carbons long. A carboxyl group is then found at
one end.
The hydrocarbon chains can be
saturated or unsaturated, but what does this mean? A saturated hydrocarbon chain means
that none of the carbon atoms in the chain have a carbon–carbon double or triple
bond. All four carbon bonds are either
joined to another carbon or to a hydrogen atom. The bonds are saturated. In other words, the carbons have no
bonds left to bind to any other hydrogen atoms. As soon as there is a carbon–carbon
double or triple bond in the chain, it is regarded as unsaturated, as it is able to
bind to another hydrogen atom.
Now let’s look at the formation of
triglycerides so we can answer our question correctly. A triglyceride is made by
condensation reactions forming ester bonds between the hydroxyl groups on the
glycerol backbone and the carboxyl group of each fatty acid. They are condensation reactions
because each one is binding two molecules together to form one molecule, with the
loss of a small molecule. Here it is water that is lost.
We now have enough information to
come up with the correct answer to our question. We are looking for a triglyceride,
and these only contain carbon, hydrogen, and oxygen atoms. We can therefore eliminate molecule
(B) from our options as it contains nitrogen atoms. Triglycerides have three fatty
acids joined to the glycerol molecule. The hydrocarbon tails of the fatty
acids are just made up of carbon atoms joined to hydrogen atoms. We can therefore also exclude
answer choice (C), as the third molecule joined to the glycerol has hydroxyl groups
attached to the carbon atoms in the tail.
We are now left with options (A)
and (D). These molecules both contain
glycerol joined to three fatty acids. Therefore, they are both
triglycerides. Now we just have to identify the
saturated triglyceride. This means we have to find the one
which has no carbon–carbon double or triple bonds in the hydrocarbon chains. As you can see, molecule (A)
contains double bonds between some of its fatty acid carbons, so we can rule this
option out. Therefore, we are left with the
correct answer. The diagram that correctly
represents the structure of a saturated triglyceride is option (D).
