Lipids, Carbohydrates, and Nucleic Acids Practice Problem | MIT 7.01SC Fundamentals of Biology
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Lipids, Carbohydrates, and Nucleic Acids Practice Problem | MIT 7.01SC Fundamentals of Biology

August 31, 2019


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.

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