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OK.  So the thing that we're going
to talk today about are

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macromolecules.  And often
these are polymers.

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Now, all macromolecules comprise of
multiple carbons joined together in

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00:00:37,000 --> 00:00:44,000
some way.  But the real definition
of a polymer is that there is some

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kind of monomer that is joined
together in a repeated way to make

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00:00:51,000 --> 00:00:59,000
some kind of multimer that forms the
polymer.  OK.

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00:00:59,000 --> 00:01:03,000
Now, integral to understanding all
of this is the notion of chemical

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bonds.  And you're going to,
in this week's Section you're going

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to be addressing,
remembering, understanding what

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chemical bonds are.
And here are some concepts that are

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essential that you understand for
chemical bonds.

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Let me list them on the board.
And then we'll go through them in

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some more detail.  And then
I'm going to move on.

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And you are going to deal with this
more in Section,

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but this is really integral to
understanding the rest of the course.

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So one of the terms you need to
know is electronegativity.

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Anyone want to give me an
electronegativity definition?

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I may be able to find another
chocolate fish for an

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electronegativity definition.
Where?  A little hand.

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Yeah?  Say it again.  OK.
Good.  Yeah.  How strongly an atom

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attracts electrons.
This is good.  You see,

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you get to practice catching and I
get to practice throwing.

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So the relative affinity of a
particular atom for electrons.

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OK?  So that's one think you need
to know.  Valance.

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Super important.
Valance.  Yeah?

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How many electron slots available
for connections.

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Try to rephrase.
Yes, but rephrase it a bit.

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We're talking about chemical bonds.
So?  OK, how many bonds.  I did not

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give that away.
OK.  How many bonds a molecule can,

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an atom can make.
OK?  OK.  And then we get into the

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bonds per se.  And I'm going to list
them here.  I'm going to list them

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in order from strong to weak.
Covalent bonds.  Ionic.  I'll go

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through some slides in a moment.

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Hydrogen and something I'll call
hydrophobic.  OK.

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And covalent bonds are the
strongest and these so-called

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hydrophobic bonds are the weakest
bonds.  Ones that we are

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particularly interested in,
actually, we're interested in all of

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00:03:47,000 --> 00:03:51,000
them, but I would say covalent,
hydrogen and hydrophobic bonds are

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particularly important.
And you will see when we talk about

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DNA, hydrogen bonds are
very important.

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So let's go through some pictures.
This is a table from your book, I'm

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not going to dwell on too much,
but you really need to be familiar

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with this, you need to understand
what is in this.

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And it will indicate for you what
the nature of the different

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bonds are.  OK.
Covalent bonds,

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really, the two atoms become very
closely joined together.

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00:04:36,000 --> 00:04:40,000
And, again, you're going to deal
with this.  Let me just move onto,

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00:04:40,000 --> 00:04:44,000
in your recitation.  Let me deal
with two things that I want to dwell

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on a little bit.
The first is the notion of polar

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and nonpolar molecules.
OK?  So polar molecules have got

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00:04:53,000 --> 00:04:57,000
differential charge distribution.
All right?  So we can add that

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00:04:57,000 --> 00:05:02,000
actually to our list.
If you want to go back and add

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00:05:02,000 --> 00:05:07,000
something to your list,
its part and parcel of the notion of

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electronegativity,
but let's also write polar versus

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00:05:11,000 --> 00:05:16,000
nonpolar molecules.
In a polar molecule,

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the charges in the molecule are
asymmetrically distributed.

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Whereas, in the nonpolar molecule
the charges are uniformly

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00:05:25,000 --> 00:05:30,000
distributed and there is no net
asymmetry of charge in that

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00:05:30,000 --> 00:05:35,000
particular molecule.
And that will become important in a

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00:05:35,000 --> 00:05:40,000
little bit.  Here is one that,
again, you will dwell on in Section

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00:05:40,000 --> 00:05:44,000
but I will introduce now.
Hydrogen bonds are between a

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hydrogen and an oxygen,
and the hydrogen or something else,

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00:05:49,000 --> 00:05:54,000
something that is more
electronegative,

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00:05:54,000 --> 00:05:59,000
the hydrogen, it's the hydrogen,
this is not a covalent bond.

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And the hydrogen interaction is
integral to the structure of DNA,

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as we will discuss in this lecture
and when we talk about molecular

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biology.  OK.  So that is a very
cursory introduction to something

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that you will be dwelling on more in
Section this week.

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OK.  So I want to talk now about
polymers.

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And I want to talk how they form
biologically.  And I'm going to tell

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you about two types of reactions
that you should understand.

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Condensation reaction which forms a
polymer and a hydrolysis reaction

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00:06:49,000 --> 00:07:06,000
which breaks a polymer.

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00:07:06,000 --> 00:07:12,000
So let's look at a diagram here that
I've taken from your book.

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Here are two monomers.  And you can
see there is a hydroxyl group and a

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hydrogen group and a hydrogen.
And those two things react with one

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another and, with the use of energy,
water is released, and the two

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monomers are joined to one another.
OK?  Again, you're going to have

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plenty of practice at this.
Don't try to draw this diagram here.

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00:07:38,000 --> 00:07:42,000
OK?  You can,
we'll deal with it in a little bit.

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00:07:42,000 --> 00:07:46,000
Just look at it and understand what
I'm saying at the time and it really

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is sufficient.
So, here, you've got the exclusion

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00:07:50,000 --> 00:07:54,000
of water, the use of energy to get
two monomers joined up,

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and then the process continues and
you get a polymer being built up of

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00:07:58,000 --> 00:08:03,000
more and more monomers.
Now, the converse of that is

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00:08:03,000 --> 00:08:09,000
hydrolysis where you add water and
also use energy to break up a

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00:08:09,000 --> 00:08:15,000
polymer and release a monomer.
So either hydrolysis reaction or a

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condensation reaction are energy
requiring reactions.

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And this energy has to come from
somewhere.  And we will spend a

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lecture talking about where the
energy comes from.

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So I want to draw the analogy,
and it may seem hokey to you, but

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it's actually quite useful,
a useful way for me to think about

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this, so I want to share it with you.
I want to draw the analogy to the

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cell as a factory.
And I want to begin by talking

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about the materials that the factory
uses to synthesize whatever it is

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it's going to synthesize.
And then as we move through the

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biochemistry section I'll talk about
the machinery within the cell that

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makes the synthesis take place.
So we're going to talk about the

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materials that are present in the
cell.  And particularly we're going

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to talk about four groups of
macromolecules,

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the big four from which everything
else is built and that really are

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pivotal for life.  These
are proteins.

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And, actually,
let me not list them in that order.

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Let me list them in the order in
which I'm going to talk about them.

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I'm going to talk about lipids,
carbohydrates,

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nucleic acids.  And I'll talk about
those three today.

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And then next time I'll talk about
proteins.  So if you look at the dry

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mass of a cell it's
very interesting.

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Lipids comprise about 5% of the dry
mass, carbohydrates about 25%,

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00:10:06,000 --> 00:10:13,000
nucleic acids 10%, and proteins
about 55%.  OK.

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So that's what we're going to be
talking about,

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these four groups of macromolecules
that make up the components

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of the cell.
Now, two things that I want to point

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out as we talk about these
macromolecules is that very often in

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biology the polymers have two
properties that are very important.

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One, they have polarity.  That
means one end is different from the

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other end.  OK?
And the other thing that they very

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00:11:00,000 --> 00:11:05,000
often have, not always but some
classes in particular,

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is a linear order.  OK?
So that's, both of those properties,

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if you give this a moment's thought,
you will see are very similar to

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sentences.  There's a beginning and
an end of a sentence,

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and within a sentence the words come
in a particular order.

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And that gives you an information
content.  That gives you a way to

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carry out or to store information.
So I'll point this out.

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This is particularly true for the
proteins and the nucleic acids.

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It's also somewhat true for
carbohydrates and less so for lipids.

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So these two properties of
biological polymers are fascinating

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and they are crucial for life to
proceed.  All right.

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So let's move onto one of the big
four.  Lipids.

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Lipids.  So lipids are a somewhat
large and almost amorphous group of

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molecules in that I cannot give you
a chemical formula,

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really, to say this is definitely a
lipid and this is not a lipid.

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They have one property that is
crucial for defining them as lipids,

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and this is that they are
hydrophobic --

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-- or nonpolar.

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And this property of being
hydrophobic, so lipids in foods.

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What are the lipids just, you know,
in your food pyramid?  It's just to

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make sure we're all talking about
the same stuff here.

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Fats and oils, right.
Yes.  And you can come and get a

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fish later.  OK.
So fats and oils.

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They're used for a couple of things.
One of the things about fats and

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oils is that they store a lot of
energy.  So they serve as a

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long-term energy source for the cell.
And the other thing they do really

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has to do with their insulation
properties.  And their insulation

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properties fall into multiple
categories.  The most important one

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we touched on last time when we
talked about the components

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of a cell.
We talked about the fact that the

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cell is a cell,
and the organelles in the cell are

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organelles because they are membrane
bound.  And I threw out at you the

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term lipid bilayer,
and I'll throw it out at you again

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in a moment.  This is something made
of lipids that insulates the cell

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from its environment or subcellular
compartments from one another.

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So part of the insulation property
are membranes.

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00:14:04,000 --> 00:14:09,000
And, again, this term,
I'll write on the board this time,

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lipid bilayer.  And the other aspect
of insulation or the more classical

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things you think about,
you don't get cold because you've

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got a layer of fat under your skin,
and that stops yourself from losing

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heat.  So heat loss.
And, for example,

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birds use lipids to waterproof their
feathers so that they can stand in

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the rain and not get wet.
OK.  Another aspect that I'll throw

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out at you also here is that lipids
also make things called hormones,

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which are substances that control
various aspects of body function.

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I'll have a little more to say
about them in a couple of minutes,

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and then will have quite a bit more
to say about them when we talk about

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reproduction.
So let me see what is next here.

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OK.  So let's talk about some
typical lipids and some examples of

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lipids.  This is a diagram from your
book, so you can go and look at it

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00:15:16,000 --> 00:15:22,000
at your leisure,
of a triglyceride.

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00:15:22,000 --> 00:15:28,000
Now, what I want you to take from
this is that, so triglycerides,

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tri meaning three.
And the glyce- part is from glycerol,

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which is this molecule up here,
onto which three long carbon chains

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are added.  And these long carbon
chains are called fatty acids.

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And that's important to know.  So a
triglycerides is glycerol onto which

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three fatty acid chains
have been added.

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And it's the fatty acid chains that
are hydrophobic.

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So triglycerides are a very common

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type of lipid.
Now, after I just told you,

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ah, it's fixed.  After I just told
you that lipids were

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all hydrophobic.
I'm now going to tell you that some

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lipids are amphipathic.
And they're amphipathic,

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so amphipathic means that they have
both polar plus nonpolar components.

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And one of the most important
amphipathic molecules in the lipid

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field is a phospholipid.
So this is a molecule that's got

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00:17:02,000 --> 00:17:08,000
these fatty acid long carbon chains
on one end and on the other end has

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got a very polar phosphate group.
OK?  And here's phosphate with

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00:17:14,000 --> 00:17:20,000
something called choline added onto
it.  So this is phosphatidylcholine.

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And phospholipids, and I spelt
amphipathic incorrectly.

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00:17:26,000 --> 00:17:32,000
Phospholipids are essential for
formation of the lipid bilayer

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00:17:32,000 --> 00:17:38,000
membranes.  And the reason they are
is because of this dual charged and

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00:17:38,000 --> 00:17:44,000
noncharged portion.
So here is a diagram that I showed

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00:17:44,000 --> 00:17:50,000
you last time that was not as well
labeled.  This is a diagram of a

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00:17:50,000 --> 00:17:56,000
lipid bilayer,
so one of the membranes that

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00:17:56,000 --> 00:18:02,000
surrounds your cells and
your organelles.

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00:18:02,000 --> 00:18:07,000
Facing out onto the water of the
cell are the hydrophilic heads of

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00:18:07,000 --> 00:18:12,000
this phospholipids,
the charged heads.  And then facing

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00:18:12,000 --> 00:18:17,000
one another are two rows,
OK, of these hydrophobic fatty acid

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00:18:17,000 --> 00:18:22,000
tails.  So the bilayer is because
you have one layer of these

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00:18:22,000 --> 00:18:27,000
molecules with the hydrophilic heads
facing the water,

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00:18:27,000 --> 00:18:32,000
the hydrophobic tails pointing in.
And those interact with another

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00:18:32,000 --> 00:18:36,000
layer where the hydrophilic heads
are pointing to the water and the

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00:18:36,000 --> 00:18:40,000
hydrophobic tails are pointing
inwards.  The hydrophobic tails

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00:18:40,000 --> 00:18:45,000
interact with one another and you
get this very stable lipid bilayer.

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00:18:45,000 --> 00:18:49,000
OK.  It's very cool.  That's how
things work, and they work very

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00:18:49,000 --> 00:18:53,000
easily.  And you can get them to be
synthesized in a test-tube very

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00:18:53,000 --> 00:18:58,000
easily.  Here is another one.
Saturated versus unsaturated fats.

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00:18:58,000 --> 00:19:08,000
Unsat, OK, as an unsaturated.

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00:19:08,000 --> 00:19:14,000
In saturated fats, the fatty acid
side chain contains no double bounds.

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00:19:14,000 --> 00:19:20,000
OK?  So it saturated four hydrogens.
In unsaturated fats, there are some

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00:19:20,000 --> 00:19:26,000
double bonds that would allow one to
add more hydrogens if they

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00:19:26,000 --> 00:19:32,000
were broken.  OK?
So saturated versus unsaturated fats.

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00:19:32,000 --> 00:19:38,000
So saturated fats are the things
you find in butter and in hard fats.

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00:19:38,000 --> 00:19:44,000
And they're supposed to be bad for
you.  OK?  So saturated fats have

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00:19:44,000 --> 00:19:50,000
been implicated in raising something
called low density lipoproteins

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00:19:50,000 --> 00:19:56,000
which are involved in various
aspects of cell function that are

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00:19:56,000 --> 00:20:03,000
believed to lead to heart disease.
But it turns out that there's more

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00:20:03,000 --> 00:20:09,000
to it than that.
So if you've ever been really bored

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00:20:09,000 --> 00:20:15,000
and you've read the ingredients on
the box of cookies that you've eaten,

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00:20:15,000 --> 00:20:21,000
you may see something that says
partially hydrogenated vegetable oil.

219
00:20:21,000 --> 00:20:27,000
Yeah?  OK.  Now,
your vegetable oil is rather

220
00:20:27,000 --> 00:20:32,000
unsaturated.  OK?
It's got a lot of double bonds.

221
00:20:32,000 --> 00:20:37,000
And one of the reasons that it is
liquid is because these double bonds

222
00:20:37,000 --> 00:20:42,000
are there and you don't get the
chains of the lipid packing very

223
00:20:42,000 --> 00:20:46,000
tightly and it's a liquid.
But you can take your vegetable oil

224
00:20:46,000 --> 00:20:51,000
with lots of unsaturated bonds and
you can partially saturate it.

225
00:20:51,000 --> 00:20:56,000
And you can get out of that two
kinds of partially saturated fat,

226
00:20:56,000 --> 00:21:01,000
partially hydrogenated fat.
You can get this stuff called

227
00:21:01,000 --> 00:21:05,000
transunsaturated fat.
Now, if you look at this diagram,

228
00:21:05,000 --> 00:21:10,000
you will see these fatty acid side
chains are in nice order here.

229
00:21:10,000 --> 00:21:14,000
OK?  And they are because the
hydrogens that are across from the

230
00:21:14,000 --> 00:21:18,000
double bonds are intrans from one
another.  They're kind of diagonally

231
00:21:18,000 --> 00:21:23,000
opposite from one another.
And what that does is to allow

232
00:21:23,000 --> 00:21:27,000
these side chains to pack very
tightly, and you get a very

233
00:21:27,000 --> 00:21:32,000
hard fat that way.
Somehow this partially hydrogenated

234
00:21:32,000 --> 00:21:36,000
stuff is very bad for you.
It raises your LDL, it reduces this

235
00:21:36,000 --> 00:21:40,000
stuff called high density
lipoprotein, which is good for you,

236
00:21:40,000 --> 00:21:44,000
and it increases your heart disease
risk.  Interestingly,

237
00:21:44,000 --> 00:21:48,000
you do not find these types of
molecules in nature.

238
00:21:48,000 --> 00:21:52,000
You only find them when you
hydrogenate your vegetable oil and

239
00:21:52,000 --> 00:21:56,000
make your margarine or make the
stuff that McDonald's cooks its

240
00:21:56,000 --> 00:22:00,000
French fries in, OK, and
its chicken nuggets.

241
00:22:00,000 --> 00:22:04,000
So a couple of years ago there was
so much issue about this that

242
00:22:04,000 --> 00:22:09,000
McDonald's pledged to find a
different vegetable oil in which to

243
00:22:09,000 --> 00:22:13,000
fry its fries.
And it's still searching,

244
00:22:13,000 --> 00:22:18,000
apparently, because there's some
issue with taste being compromised

245
00:22:18,000 --> 00:22:22,000
as you go to healthier fats.
OK.  Now, in contrast to the

246
00:22:22,000 --> 00:22:27,000
transunsaturated fats there's this
sysunsaturated fat also.

247
00:22:27,000 --> 00:22:30,000
And if you look here,
you can see that these side chains

248
00:22:30,000 --> 00:22:34,000
are kinked.  And the reason they're
kinked is that the hydrogens across

249
00:22:34,000 --> 00:22:38,000
the double bond are on the same side
of the molecule as one another.

250
00:22:38,000 --> 00:22:42,000
That has the effect of not allowing
the side chains to pack tightly.

251
00:22:42,000 --> 00:22:46,000
So you get side chains floating all
over the place,

252
00:22:46,000 --> 00:22:50,000
and that changes the energy of the
fat so that it is a liquid rather

253
00:22:50,000 --> 00:22:54,000
than a solid.  And these are much
better for you,

254
00:22:54,000 --> 00:22:58,000
these sysunsaturated fats.
And you find them in lots of

255
00:22:58,000 --> 00:23:03,000
vegetable oils and vegetable fats.
OK.  Here's another one.

256
00:23:03,000 --> 00:23:10,000
Here's our joke for the day.
OK.  Steroids.  Steroids are a

257
00:23:10,000 --> 00:23:17,000
lipid.  They are derived from,
OK, so let me just say for this

258
00:23:17,000 --> 00:23:24,000
unsaturated.  I mentioned sys and
transunsaturated fats.

259
00:23:24,000 --> 00:23:32,000
I'm going to mention steroid
hormones.

260
00:23:32,000 --> 00:23:35,000
There are a few of them.
Steroids are not all hormones,

261
00:23:35,000 --> 00:23:38,000
although cholesterol, we don't think
of cholesterol as a hormone,

262
00:23:38,000 --> 00:23:42,000
although, actually, I think it
really is.  Vitamin D is a vitamin

263
00:23:42,000 --> 00:23:45,000
that works together with other
molecules.  Cortisol and

264
00:23:45,000 --> 00:23:49,000
testosterone are classically thought
of as hormones.

265
00:23:49,000 --> 00:23:52,000
Very small amounts of them will
change body function in profound

266
00:23:52,000 --> 00:23:56,000
ways.  And we'll have more to say
about testosterone later

267
00:23:56,000 --> 00:24:00,000
on in the course.
The androgens that lead to people

268
00:24:00,000 --> 00:24:05,000
who have large green muscles are a
class that are related to

269
00:24:05,000 --> 00:24:11,000
testosterone, androsterone and so on,
but they're all the structures with

270
00:24:11,000 --> 00:24:16,000
these four rings in this
characteristic way are all

271
00:24:16,000 --> 00:24:21,000
characteristic of a steroid.
All right.  So that is all I have

272
00:24:21,000 --> 00:24:27,000
to say about lipids.
Let's move onto either number two

273
00:24:27,000 --> 00:24:34,000
here, carbohydrates.
So carbohydrates have as their --

274
00:24:34,000 --> 00:24:45,000
OK.  Carbohydrates.

275
00:24:45,000 --> 00:24:52,000
Also classically known as sugars.

276
00:24:52,000 --> 00:24:57,000
And because of that there are a
couple of functions of carbohydrates.

277
00:24:57,000 --> 00:25:02,000
They are the major source of
short-term energy.

278
00:25:02,000 --> 00:25:07,000
If you need short-term energy you
use carbohydrates.

279
00:25:07,000 --> 00:25:12,000
They're also a carbon source,
so we'll talk extensively about

280
00:25:12,000 --> 00:25:17,000
building up molecules.
And you need carbons to do that.

281
00:25:17,000 --> 00:25:22,000
Carbohydrates serve as a carbon
source for building other molecules.

282
00:25:22,000 --> 00:25:27,000
And, as I'll mention briefly, they
also can convey information.

283
00:25:27,000 --> 00:25:31,000
Carbohydrates have,
as their chemical principle,

284
00:25:31,000 --> 00:25:35,000
so we can find a chemical principle
that unifies them,

285
00:25:35,000 --> 00:25:40,000
they all contain carbons that are
joined to a hydrogen and to a

286
00:25:40,000 --> 00:25:44,000
hydroxyl, and obviously to other
things as well.

287
00:25:44,000 --> 00:25:49,000
So you can loosely give
carbohydrates the formula of CH2O to

288
00:25:49,000 --> 00:25:53,000
the N.  Now, the monomer,
now we can actually, with lipids

289
00:25:53,000 --> 00:25:57,000
it's very hard to talk about a
monomer and a polymer.  In

290
00:25:57,000 --> 00:26:02,000
carbohydrates one can.
And one can talk about the monomer,

291
00:26:02,000 --> 00:26:08,000
which is called a monosaccharide
which polymerizes to form a

292
00:26:08,000 --> 00:26:14,000
polysaccharide.
And they do so.

293
00:26:14,000 --> 00:26:19,000
One can also get,
let me just write it here,

294
00:26:19,000 --> 00:26:25,000
one can also get disaccharides where
disaccharides are two and

295
00:26:25,000 --> 00:26:31,000
polysaccharides are really
more than two.

296
00:26:31,000 --> 00:26:36,000
They're usually more than ten or so,
but for our purposes more than two

297
00:26:36,000 --> 00:26:42,000
is fine.  OK.  So in order to form a
polymer of carbohydrates,

298
00:26:42,000 --> 00:26:48,000
one forms a glycosidic linkage or
glycosidic bond.

299
00:26:48,000 --> 00:26:54,000
So it's a covalent bond where one
has --

300
00:26:54,000 --> 00:27:09,000
Two carbohydrates or two

301
00:27:09,000 --> 00:27:17,000
monosaccharides interacting with one
another through a condensation

302
00:27:17,000 --> 00:27:26,000
reaction, classical condensation
reaction with the elimination of

303
00:27:26,000 --> 00:27:35,000
water to give you, excuse
me, yeah, no.

304
00:27:35,000 --> 00:27:40,000
I wrote this down incorrectly in my
notes.  Apologies.

305
00:27:40,000 --> 00:27:46,000
OK.  To give you a glycosidic
linkage.  OK?  So this is a

306
00:27:46,000 --> 00:27:51,000
classical condensation reaction.
Now, polysaccharides or

307
00:27:51,000 --> 00:27:57,000
carbohydrates are interesting.
They can either be rings or they

308
00:27:57,000 --> 00:28:01,000
can be linear.
And they can also be modified.

309
00:28:01,000 --> 00:28:05,000
So like the lipids where you could
get phospholipids,

310
00:28:05,000 --> 00:28:09,000
carbohydrates can have various extra
things added onto them that changes

311
00:28:09,000 --> 00:28:13,000
their properties.
Let's take a look at a few of them.

312
00:28:13,000 --> 00:28:17,000
Here are some monosaccharides.  And
the ones that are super important

313
00:28:17,000 --> 00:28:21,000
for you to know are these two down
here.  OK?  And you should know the

314
00:28:21,000 --> 00:28:25,000
structures of these.
This is ribose and deoxyribose.

315
00:28:25,000 --> 00:28:29,000
And the reason for this will become
clear when I move onto the next

316
00:28:29,000 --> 00:28:33,000
class of macromolecules,
DNA and RNA, because those sugars

317
00:28:33,000 --> 00:28:37,000
are part of DNA and RNA.
OK?  So these ribose and deoxyribose

318
00:28:37,000 --> 00:28:42,000
are five carbon rings.
You can see they're not quite a

319
00:28:42,000 --> 00:28:47,000
five, they're not a five carbon ring.
They are five carbon sugars.

320
00:28:47,000 --> 00:28:52,000
And this is an isomer of one of the
forms that this five carbon sugar

321
00:28:52,000 --> 00:28:57,000
can take.  OK.
So some of you may be lactose

322
00:28:57,000 --> 00:29:02,000
intolerant and might not be able,
in fact, to enjoy digging in and

323
00:29:02,000 --> 00:29:07,000
enjoying a bowl of creamy
delicious ice cream.

324
00:29:07,000 --> 00:29:11,000
So, in fact, the reason,
I didn't make that up, though the

325
00:29:11,000 --> 00:29:15,000
reason you cannot is because you
need a specific protein,

326
00:29:15,000 --> 00:29:19,000
a specific enzyme that we'll talk
about, not the specific enzyme,

327
00:29:19,000 --> 00:29:23,000
but we'll talk about the notion of
enzymes in a couple of lectures that

328
00:29:23,000 --> 00:29:28,000
breaks a bond between the
disaccharide that is lactose.

329
00:29:28,000 --> 00:29:33,000
So lactose is a disaccharide that is
a galactose joined together with a

330
00:29:33,000 --> 00:29:38,000
glucose.  OK.  So these are six
carbon sugars.

331
00:29:38,000 --> 00:29:43,000
And there's a bond between the two
of them that you have to break in

332
00:29:43,000 --> 00:29:49,000
order to digest the lactose from
milk.  And if you cannot then this

333
00:29:49,000 --> 00:29:54,000
lactose causes you digestive issues.
Here on the top is sucrose which is

334
00:29:54,000 --> 00:30:00,000
a dimer, a disaccharide,
a glucose joined to a fructose.

335
00:30:00,000 --> 00:30:04,000
OK.  So here's an interesting
question.  I don't know when you

336
00:30:04,000 --> 00:30:09,000
were little, but when I was little I
used to try and eat grass.

337
00:30:09,000 --> 00:30:13,000
And, clearly, that doesn't work
very well.  Humans cannot eat grass.

338
00:30:13,000 --> 00:30:18,000
Whereas, cows and many animals can
survive just very well on a diet of

339
00:30:18,000 --> 00:30:22,000
just plants.  And that is because we
cannot deal with this carbohydrate

340
00:30:22,000 --> 00:30:27,000
in plants called cellulose.
Now, interestingly,

341
00:30:27,000 --> 00:30:31,000
we can digest other plant
carbohydrates,

342
00:30:31,000 --> 00:30:36,000
particularly starch or amylose which
is what most of the carbohydrates

343
00:30:36,000 --> 00:30:41,000
that we eat come from.
The reason for this is interesting,

344
00:30:41,000 --> 00:30:45,000
and it has to do with these
glycosidic linkages.

345
00:30:45,000 --> 00:30:50,000
OK.  So here we have a linkage
between glucose,

346
00:30:50,000 --> 00:30:55,000
two glucoses that give something
called maltose.

347
00:30:55,000 --> 00:30:59,000
And maltose is part of the starch
carbohydrate, or the amylose

348
00:30:59,000 --> 00:31:04,000
carbohydrates.
And what you should know here about

349
00:31:04,000 --> 00:31:09,000
these chains, these circles,
and you should remember from old

350
00:31:09,000 --> 00:31:14,000
chemistry as if there is a bold part
in the front of this ring then it is

351
00:31:14,000 --> 00:31:18,000
coming out of the board towards you.
OK.  So here you have these two

352
00:31:18,000 --> 00:31:23,000
glucoses linked such that a bond
joining them is coming out of the

353
00:31:23,000 --> 00:31:28,000
board towards you.
Now, conversely,

354
00:31:28,000 --> 00:31:33,000
and this is called an alpha 1,
4 bond where the bonds coming out of

355
00:31:33,000 --> 00:31:38,000
the board towards you in cellobiose,
which is a precursor for cellulose,

356
00:31:38,000 --> 00:31:43,000
the grass carbohydrate,
the bond goes back, OK, back away

357
00:31:43,000 --> 00:31:48,000
from the rings.
And that's called a beta 1,

358
00:31:48,000 --> 00:31:53,000
4 linkage.  And we can break this
alpha 1, 4 linkage but not this beta

359
00:31:53,000 --> 00:31:59,000
1, 4 linkage.  And I'll come back to
this when we talk about enzymes.

360
00:31:59,000 --> 00:32:03,000
We do not have the correct enzymes
to do so.  And finally with respect

361
00:32:03,000 --> 00:32:07,000
to carbohydrates,
I mentioned that they were sources

362
00:32:07,000 --> 00:32:11,000
of information.
One of the interesting sources of

363
00:32:11,000 --> 00:32:15,000
information from carbohydrates is in
your blood cells.

364
00:32:15,000 --> 00:32:19,000
So your blood group is as a result
of adding specific chains of

365
00:32:19,000 --> 00:32:23,000
specific carbohydrates,
specific monosaccharides,

366
00:32:23,000 --> 00:32:27,000
and a whole bunch of them,
onto particular red blood cell

367
00:32:27,000 --> 00:32:32,000
surface proteins.
So if you're an A blood group,

368
00:32:32,000 --> 00:32:37,000
you have a series of specific
carbohydrates added on in order to

369
00:32:37,000 --> 00:32:42,000
one of these cell surface proteins.
If you're a B blood group, you also

370
00:32:42,000 --> 00:32:46,000
have this carbohydrate chain but the
specific carbohydrates,

371
00:32:46,000 --> 00:32:51,000
the specific monosaccharides are
different.  All right.

372
00:32:51,000 --> 00:32:56,000
So let's move onto the reason that
you're all really sitting here.

373
00:32:56,000 --> 00:33:01,000
So, really, the reason that you're
all sitting here is because of this

374
00:33:01,000 --> 00:33:05,000
next group of macromolecules.
And I don't mean that that's the

375
00:33:05,000 --> 00:33:09,000
reason, you know,
you're alive, that you exist in life.

376
00:33:09,000 --> 00:33:12,000
I mean that's the reason you're
talking 7.013.

377
00:33:12,000 --> 00:33:16,000
Because nucleic acids are the thing
that have, and the understanding of

378
00:33:16,000 --> 00:33:20,000
nucleic acids is what has
revolutionized biology so that --

379
00:33:20,000 --> 00:33:28,000
So that everyone in every discipline

380
00:33:28,000 --> 00:33:33,000
at MIT has some association,
has some way of using the

381
00:33:33,000 --> 00:33:39,000
information from nucleic acids to
their advantage and in their careers.

382
00:33:39,000 --> 00:33:44,000
OK.  So nucleic acids,
as I'll discuss, do serve as an

383
00:33:44,000 --> 00:33:49,000
energy source or as a way of storing
energy, but most important is their

384
00:33:49,000 --> 00:33:54,000
function, and that's a super
important function.

385
00:33:54,000 --> 00:34:00,000
But the reason you're talking 7.
13 is not because of that.

386
00:34:00,000 --> 00:34:07,000
The reason you are is because of the
information content that nucleic

387
00:34:07,000 --> 00:34:14,000
acids are able to carry and to
perpetuate from one generation to

388
00:34:14,000 --> 00:34:21,000
another.  So, really,
what they are involved in is

389
00:34:21,000 --> 00:34:29,000
information storage and
transfer.  All right.

390
00:34:29,000 --> 00:34:35,000
So, again, as for carbohydrates,
we can talk about monomers and we

391
00:34:35,000 --> 00:34:41,000
can talk about polymers.
The monomers for nucleic acids is a

392
00:34:41,000 --> 00:34:47,000
nucleotide.  Oh,
OK.  Enough air play.

393
00:34:47,000 --> 00:34:53,000
Watson and Crick get a lot of air
play, so that's enough.

394
00:34:53,000 --> 00:35:00,000
You can go and look at their
picture at your leisure.  OK.

395
00:35:00,000 --> 00:35:03,000
So I've taken this from your book
and I'm going to draw some stuff on

396
00:35:03,000 --> 00:35:07,000
the board.  I want to point out that
in the new edition of your book,

397
00:35:07,000 --> 00:35:11,000
you will find there is a mistake in
the structure of your nucleotides.

398
00:35:11,000 --> 00:35:15,000
That was true in the last edition.
I wrote to the senior author.  I

399
00:35:15,000 --> 00:35:19,000
said there is a problem with the
sugar structure in the nucleic acids.

400
00:35:19,000 --> 00:35:23,000
Would you please change it for the
next edition?  And he said

401
00:35:23,000 --> 00:35:27,000
absolutely we will.
And the new edition came out and

402
00:35:27,000 --> 00:35:30,000
there we are.  OK?
So we'll have to,

403
00:35:30,000 --> 00:35:34,000
so we're going to deal with that,
but I think we'll get you all free

404
00:35:34,000 --> 00:35:38,000
copies, or your successors free
copies of the next edition of the

405
00:35:38,000 --> 00:35:42,000
book or something to make up for
this.  But, actually,

406
00:35:42,000 --> 00:35:46,000
it's a good exercise for you because
you can learn the proper structure

407
00:35:46,000 --> 00:35:50,000
of the sugar.  So what is a
nucleotide?  OK.

408
00:35:50,000 --> 00:35:54,000
A nucleotide, as you gather,
has a sugar associated with it.

409
00:35:54,000 --> 00:35:58,000
It has, the sugar it has associated
it with is either ribose

410
00:35:58,000 --> 00:36:02,000
or deoxyribose.
This sugar is joined to a phosphate

411
00:36:02,000 --> 00:36:07,000
group.  And the sugar is also joined
to something called a base.

412
00:36:07,000 --> 00:36:12,000
Now, the sugar is a five carbon
ring.  I'll come back to this slide

413
00:36:12,000 --> 00:36:17,000
in a moment and we'll talk about the
bottom part of it.

414
00:36:17,000 --> 00:36:22,000
The sugar is a five carbon ring.
OK?  Here it is.  It's actually a

415
00:36:22,000 --> 00:36:27,000
four carbon ring with an oxygen and
there's another carbon

416
00:36:27,000 --> 00:36:32,000
sticking out.
And what they've done in your book

417
00:36:32,000 --> 00:36:36,000
is to forget about this carbon up
here and turn this oxygen here into

418
00:36:36,000 --> 00:36:40,000
a carbon.  So it's kind of weird.
Later in the book the structure is

419
00:36:40,000 --> 00:36:44,000
correct, but you should bear this in
mind.  I've pointed this out in your

420
00:36:44,000 --> 00:36:49,000
PowerPoint handouts.
OK.  So what's really important,

421
00:36:49,000 --> 00:36:53,000
there are two things about
nucleotides that are really

422
00:36:53,000 --> 00:36:57,000
important.  One is this phosphate
and sugar because that is the way

423
00:36:57,000 --> 00:37:02,000
nucleotides polymerize
with one another.

424
00:37:02,000 --> 00:37:07,000
And the other is the base which is
the part of the molecule that gives

425
00:37:07,000 --> 00:37:13,000
rise to the information.
Actually, let me just stop this and

426
00:37:13,000 --> 00:37:18,000
write the bases here.
So let me tell you the names of the

427
00:37:18,000 --> 00:37:24,000
bases.  And you need to know these.
There are four of them.  Five

428
00:37:24,000 --> 00:37:30,000
really.
There is adenine,

429
00:37:30,000 --> 00:37:37,000
guanine, cytosine and thymine.
And then there's also something

430
00:37:37,000 --> 00:37:45,000
that's an alternate to thymine
called uracil.

431
00:37:45,000 --> 00:37:52,000
And they are abbreviated by their
first letters.

432
00:37:52,000 --> 00:38:00,000
OK?  So A, G, C and T and U.
And you need to know these letters.

433
00:38:00,000 --> 00:38:07,000
And you need to know what they stand
for.  All right.

434
00:38:07,000 --> 00:38:16,000
So let me, in the last two minutes,

435
00:38:16,000 --> 00:38:22,000
tell you about some properties.
Actually, let me tell you about the

436
00:38:22,000 --> 00:38:28,000
bond that forms the nucleic acid
polymer.  And then I'll finish this

437
00:38:28,000 --> 00:38:34,000
off next lecture.
The bond that forms the polymer is

438
00:38:34,000 --> 00:38:44,000
called a phosphodiester bond.

439
00:38:44,000 --> 00:38:48,000
And it forms by the reaction of the
phosphate and the sugar with one

440
00:38:48,000 --> 00:38:53,000
another, or groups on the phosphate
and the sugar with one another.

441
00:38:53,000 --> 00:38:58,000
The base has got nothing to do with
this polymerization.

442
00:38:58,000 --> 00:39:03,000
So we have here a phosphate with
some oxygens that's joined onto the

443
00:39:03,000 --> 00:39:09,000
sugar.  So I'm abbreviating the
phosphate P, I'm abbreviating the

444
00:39:09,000 --> 00:39:15,000
sugar S, and I'm abbreviating the
base B.  OK?  And the sugar is

445
00:39:15,000 --> 00:39:21,000
attached to a base.
And the sugar has, in one part of

446
00:39:21,000 --> 00:39:27,000
it, a reactive hydroxyl group.
And the phosphate has some reactive

447
00:39:27,000 --> 00:39:31,000
oxygen groups.
And let's not worry too much about

448
00:39:31,000 --> 00:39:35,000
bonds and double bonds
and things for now.

449
00:39:35,000 --> 00:39:51,000
OK.  And I'm not worrying about

450
00:39:51,000 --> 00:39:55,000
charges on oxygens and things either.
What happens,

451
00:39:55,000 --> 00:39:59,000
basically, is that there is an
interaction between this hydroxyl

452
00:39:59,000 --> 00:40:03,000
group and the phosphate group,
OK, or one of the oxygens on the

453
00:40:03,000 --> 00:40:07,000
phosphate group.
And you get out of this a linkage

454
00:40:07,000 --> 00:40:26,000
that goes --

455
00:40:26,000 --> 00:40:31,000
OK.  Where you get between the sugar
and the phosphate this

456
00:40:31,000 --> 00:40:37,000
phosphodiester linkage.
OK?  An OPO linkage that joins the

457
00:40:37,000 --> 00:40:42,000
two sugars to one another through a
phosphate molecule.

458
00:40:42,000 --> 00:40:48,000
This is your phosphodiester bond.
And I want to point out that the

459
00:40:48,000 --> 00:40:53,000
bases, which I will tell you next
time, are the part of the molecule

460
00:40:53,000 --> 00:40:59,000
that carries the information,
have got nothing to do with the

461
00:40:59,000 --> 00:41:04,000
polymerization of nucleic acids.
So I'm going to continue with this

462
00:41:04,000 --> 00:41:08,000
next time and briefly finish nucleic
acids and then move

463
00:41:08,000 --> 00:41:11,000
onto the proteins.