PROFESSOR: Hi, my name is Nicole and welcome to
recitation.
Today we are going to go over the practice problem for the
nucleic acids, carbohydrates, and lipids section of the
biochemistry module.
First, if you haven't had a chance to look at this
problem, please go back and look at it and then come back
to recitation and we'll go over it together.
Welcome back.
To start this problem, we are going to go through parts A
through D and then we'll take a break and come back and do
parts E and F.
So, this problem asks us about the molecule shown here.
First, we are asked to name this molecule.
By looking at this molecule, it's really easy to figure out
what it is.
All you have to do is notice the fatty acid tail and the
phosphate head.
If you remember from lecture, this should tell you that it
is a phospholipid.
Next, we are asked where we find these molecules in living
cells and what the function of these molecules is.
So if you remember from lecture, we are told that we
find these molecules in cell membranes.
We learned that they are an integral part of cell
membranes and their function is to control the permeability
of the cell, which is a very important function.
Their structure allows them to keep out polar molecules and
let in nonpolar molecules so they can allow the cell to
have a high solute concentration on the inside as
compared to the outside.
So their critical function is to form a permeability barrier
on the cell.
So now that we understand what the function of these
molecules is, this question asks us what would happen if
we dropped these molecules into a glass of water.
So sometimes these molecules are drawn like this, with a
circle as the phosphate head and two squiggly
lines as the tail.
So if we dropped these in water, we can get three
possibilities.
Either they can form a micelle, which is drawn with
all of the heads towards the outside, facing the water
molecules and the tails all hidden on the inside.
Or, it can form a liposome, which is two layers with a
very tight layer of phospholipids on the inside
and a larger layer of phospholipids on the outside
with the tails facing inwards and the heads facing outwards
towards the water.
And there is also water in the middle as well
facing these nonpolar--
I mean, these polar heads.
The third structure they can form is a bilayer.
This is a structure that is found that makes cell
membranes, is this phospholipid bilayer.
So this is actually a much larger structure--
I've only drawn part of it here--
and this is where you have water on both sides of it.
So you have water molecules out here and on the inside of
the bilayer.
So those are the three molecules that--
three structures that this molecule will form when you
drop it into water.
Now the most important part of this problem is asking why do
these molecules form these structures when
dropped into water.
And we can find the answer to this very easily in just
looking at the properties of phospholipids and the
properties of water.
If you look at a phospholipid, you'll see that the head, the
phosphate head, is negatively charged, so that means that
it's polar, whereas the tail is a fatty acid, and we have
learned that that means that it is nonpolar,
because it has no charge.
And if you look at water, water is a
polar molecule as well.
It has two polar bonds to hydrogen.
And the reason that they form these structures is because
the polar heads of the phospholipids want to interact
with polar water and the nonpolar tails of the
phospholipids do not want to interact with the polar water.
So they have to find the confirmations in which the
tails are hidden from water and the heads
are exposed to water.
So now that we understand why phospholipids form these
structures, we're going to take a break and come back to
the second part of the problem.
Welcome back.
Now we are going to solve parts E and F of this problem.
Part E asks us to draw another molecule that contains
phosphate that is essential to biological systems.
Right off, you should be thinking about nucleic acids.
I have written an example of a nucleic acid up here.
This, as you can see by the A in the base position, is ATP.
And you can see where phosphate is essential as the
triphosphate portion of ATP.
The next part of the question asks us what is the function
of these type of molecules?
So the molecule that I chose to draw, ATP, is very
interesting because it has multiple functions.
It serves as a building block of RNA through oligomerization
through this bond by adding multiple
nucleic acids together.
It also serves as an energy source for the cell, the
energy source for the cell, through breaking of this bond,
which is an exergonic reaction and releases energy and allows
a lot of the processes in the cell that wouldn't normally be
able to occur to occur.
So I'm going to put as a function of this molecule as
both energy and RNA.
So now that we understand the properties of phospholipids
and we've been able to identify another very
important molecule that contains phosphate, we can
move on to the rest of the biochemistry section.
In this problem, we have learned about the properties
of phospholipids as well as the importance of phosphate in
nucleic acids.
I hope you've enjoyed watching this video and I hope to see
you next time.