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PROFESSOR: Hi, my name is
Nicole and welcome to

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recitation.

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Today we are going to go over
the practice problem for the

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nucleic acids, carbohydrates,
and lipids section of the

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biochemistry module.

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First, if you haven't had
a chance to look at this

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problem, please go back and look
at it and then come back

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to recitation and we'll
go over it together.

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Welcome back.

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To start this problem, we are
going to go through parts A

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through D and then we'll take
a break and come back and do

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parts E and F.

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So, this problem asks us about
the molecule shown here.

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First, we are asked to
name this molecule.

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By looking at this molecule,
it's really easy to figure out

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what it is.

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All you have to do is notice
the fatty acid tail and the

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phosphate head.

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If you remember from lecture,
this should tell you that it

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is a phospholipid.

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Next, we are asked where we find
these molecules in living

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cells and what the function
of these molecules is.

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So if you remember from lecture,
we are told that we

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find these molecules
in cell membranes.

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We learned that they are an
integral part of cell

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membranes and their function is
to control the permeability

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of the cell, which is a very
important function.

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Their structure allows them to
keep out polar molecules and

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let in nonpolar molecules so
they can allow the cell to

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have a high solute concentration
on the inside as

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compared to the outside.

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So their critical function is to
form a permeability barrier

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on the cell.

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So now that we understand what
the function of these

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molecules is, this question asks
us what would happen if

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we dropped these molecules
into a glass of water.

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So sometimes these molecules
are drawn like this, with a

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circle as the phosphate
head and two squiggly

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lines as the tail.

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So if we dropped these in
water, we can get three

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possibilities.

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Either they can form a micelle,
which is drawn with

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all of the heads towards the
outside, facing the water

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molecules and the tails all
hidden on the inside.

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Or, it can form a liposome,
which is two layers with a

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very tight layer of
phospholipids on the inside

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and a larger layer of
phospholipids on the outside

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with the tails facing inwards
and the heads facing outwards

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towards the water.

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And there is also water
in the middle as well

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facing these nonpolar--

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I mean, these polar heads.

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The third structure they
can form is a bilayer.

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This is a structure that is
found that makes cell

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membranes, is this phospholipid
bilayer.

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So this is actually a much
larger structure--

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I've only drawn part
of it here--

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and this is where you have water
on both sides of it.

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So you have water molecules out
here and on the inside of

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the bilayer.

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So those are the three
molecules that--

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three structures that this
molecule will form when you

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drop it into water.

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Now the most important part of
this problem is asking why do

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these molecules form these
structures when

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dropped into water.

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And we can find the answer to
this very easily in just

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looking at the properties
of phospholipids and the

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properties of water.

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If you look at a phospholipid,
you'll see that the head, the

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phosphate head, is negatively
charged, so that means that

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it's polar, whereas the tail is
a fatty acid, and we have

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learned that that means
that it is nonpolar,

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because it has no charge.

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And if you look at
water, water is a

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polar molecule as well.

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It has two polar bonds
to hydrogen.

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And the reason that they form
these structures is because

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the polar heads of the
phospholipids want to interact

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with polar water and the
nonpolar tails of the

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phospholipids do not want to
interact with the polar water.

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So they have to find the
confirmations in which the

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tails are hidden from
water and the heads

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are exposed to water.

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So now that we understand why
phospholipids form these

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structures, we're going to take
a break and come back to

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the second part of
the problem.

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Welcome back.

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Now we are going to solve parts
E and F of this problem.

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Part E asks us to draw another
molecule that contains

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phosphate that is essential
to biological systems.

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Right off, you should be
thinking about nucleic acids.

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I have written an example of
a nucleic acid up here.

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This, as you can see by the A in
the base position, is ATP.

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And you can see where phosphate
is essential as the

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triphosphate portion of ATP.

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The next part of the question
asks us what is the function

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of these type of molecules?

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So the molecule that I chose
to draw, ATP, is very

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interesting because it has
multiple functions.

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It serves as a building block of
RNA through oligomerization

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through this bond by
adding multiple

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nucleic acids together.

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It also serves as an energy
source for the cell, the

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energy source for the cell,
through breaking of this bond,

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which is an exergonic reaction
and releases energy and allows

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a lot of the processes in the
cell that wouldn't normally be

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able to occur to occur.

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So I'm going to put as a
function of this molecule as

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both energy and RNA.

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So now that we understand the
properties of phospholipids

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and we've been able to
identify another very

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important molecule that contains
phosphate, we can

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move on to the rest of the
biochemistry section.

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In this problem, we have learned
about the properties

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of phospholipids as well as the
importance of phosphate in

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nucleic acids.

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I hope you've enjoyed watching
this video and I hope to see

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you next time.