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PROFESSOR: Hi everyone.

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Welcome to the biochemistry
help sessions.

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This is the covalent bonds,
chirality, electronegativity,

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and hydrogen bonds module.

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In this help session, I'll be
going over the first practice

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problem in this module.

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If you have not had a chance
to look over this problem

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yourself, please go back, look
over the problem, try it, and

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then come back and we'll
go through it together.

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Now that you've had a chance to
look at the problem, let's

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get started.

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First of all, what
is this molecule?

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To me, this molecule looks
like it's a sugar.

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It's a polysaccharide, to be
exact, and it's actually a

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chain of glucose molecules.

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So in this problem, the first
part of the problem asks us to

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name each circled bond
in this molecule.

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So the first circled bond is
right here, and this is a

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carbon to carbon bond.

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Now, the first thing that we can
notice about this bond is

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that it's a carbon to carbon
bond, and we know that carbon

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only forms covalent bonds
in biological systems.

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So the first thing we can say
is that this bond is a

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covalent bond.

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Second, we need to decide
if this bond is

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polar or non polar.

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And the way we decide that
is we look at the

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electronegativity of
the atoms involved.

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In this bond we have
two carbons.

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And so since they're the same
atoms, we know that they have

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the same electronegativity.

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And bonds between two atoms of
the same electronegativity are

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non polar bonds.

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So we know that this carbon
to carbon bond is

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covalent and non polar.

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Now we'll look at the
second bond circled.

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This bond is a carbon
to oxygen bond.

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It is similar to the first
bond in that it involves

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carbon, so we can say that
it is a covalent bond.

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What's different about this
bond, however, is that it is a

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carbon bound to an oxygen.

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And when you look at the
electronegativities of carbon

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and oxygen, you find that
oxygen is much more

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electronegative than carbon.

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So in a carbon to oxygen bond,
you actually have oxygen is

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greedy, and it's going to take
the electrons closer to itself

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and not share them equally
with the carbon.

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So in this kind of bond,
oxygen will be slightly

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negative and carbon will be
slightly positive, because the

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electrons are attracted
closer to the oxygen.

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In a bond where you have a
slightly negative side and a

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slightly positive side, we
call this a polar bond.

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So we can say that a carbon to
oxygen bond is a polar bond.

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So in this first part, we
learned that the difference

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between a covalent and
a non polar and a

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covalent polar bond.

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The second part of the problem,
part B, asks if this

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sugar molecule is soluble
in water.

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So first you need to know
whether this molecule is polar

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or non polar.

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And since we know that it has
carbon to oxygen bonds, we

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know that it has polar bonds,
so it is a polar molecule.

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And what do we know
about water?

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We know water is an oxygen
bound to two hydrogens.

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And since it has this very
electronegative oxygen, we

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know that it is also
a polar molecule.

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And an easy rule of thumb is
that like dissolves like.

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So since these are both polar
molecules, sugar dissolves

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very easily in water.

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If you had a fat, on the other
hand, which is a non polar

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molecule, it would not
dissolve in water.

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That's why when you put
oil and water, you

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don't see it dissolve.

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So now that we understand why
sugar dissolves very easily in

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water, we'll move into the third
part of the problem,

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where they ask what types of
bonds can form between these

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long polymers.

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So looking at these polymers,
the first thing that we can

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notice is that there are these
oxygens and hydrogens on each

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of the polymers.

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And looking at these, we can see
how you can have hydrogen

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bonding, which we learned about
in class, between the

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oxygen of one sugar molecule
to the hydrogen of another

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sugar molecule, thus forming
hydrogen bonding networks

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between the sugar molecules.

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And in this way, they can
interact with each other.

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So in this help session we have
covered the difference

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between a covalent non
polar bond and a

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covalent polar bond.

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We have covered why water can
dissolve sugar very easily,

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and what kind of bonds
can form between

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these two sugar molecules.

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Thank you.