Video Transcript
Which of the following best
describes the main structural difference between triglycerides and
phospholipids? (A) Triglycerides contain three
glycerol molecules; but in phospholipids, one of these glycerols is replaced by a
phosphate molecule. (B) Triglycerides contain fatty
acid molecules; but in phospholipids, these fatty acids are replaced by phosphate
molecules. (C) Triglycerides contain hydrogen,
carbon, and oxygen only; but in phospholipids, the hydrogens are replaced by
phosphates. Or (D) triglycerides contain three
fatty acid molecules; but in phospholipids, one of these fatty acids is replaced by
a phosphate molecule.
This question is asking about two
different kinds of lipids found in the human body: triglycerides and
phospholipids. So what is the difference between
these two molecules? And what makes them both
lipids?
Let’s start by recapping the
function of both. Triglycerides act mainly as a fat
storage molecule in our bodies. Phospholipids are the main
component of cell membranes. In biology, lipids are small
hydrophobic molecules. In this case, the fatty acid
components of triglycerides and phospholipids allow them to fall within this
category. Let’s start by talking about the
structure of triglycerides.
Triglycerides are lipids composed
of a glycerol backbone and three fatty acid chains, as shown here. Using just this small piece of
information, we can already narrow down our answers. We know that answer (A) is
incorrect because triglycerides contain three fatty acid chains, not three glycerol
molecules.
Phospholipids are similar in that
they contain multiple fatty acid chains and a glycerol backbone but have two key
differences. The first difference is that they
only have two fatty acid chains, while triglycerides have three. The second difference is that
phospholipids contain a phosphate, shown as the pink oval in the diagram.
Now let’s continue eliminating
incorrect answers. We can eliminate answer (B) as well
because we know that phospholipids have two fatty acid chains and only one phosphate
group, not three as this answer implies. Finally, we can eliminate answer
(C) because we know that not every hydrogen molecule is replaced by a phosphate. We can see in our diagram that each
phospholipid only has one phosphate group.
So the correct answer that
describes the main structural difference between triglycerides and phospholipids
must be (D). Triglycerides contain three fatty
acid molecules; but in phospholipids, one of these fatty acids is replaced by a
phosphate molecule.
