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(09-07-11) Lecture 3 Cell Membrane (part 2)
All these compounds that make up the membrane are related in function
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The top part as we mentioned is the polar part of the molecule and the tail gives it
the nonpolar quality
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The other two are polar but
they have a negative charge
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Its structure is a
little different you have a two fatty acid chains and the linkage is a little different to the glycerol
molecule
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Cholesterol is also a molecule with a polar end and a nonpolar end
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This means that it can interact with phopholipid molecules
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It can occur in two shapes: 1
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2
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So this is why It is
important for fatty acids found in the plasma membrane to have two fatty acid tails and not one
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This is telling you that
energetically the most stable position for the membrane is a sphere
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The
advantage of this is that it is one way to study the characteristics of the membrane
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Myelin is just a coating of the nerve cells
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Functionally why is cholesterol more important on the outside than on the
inside
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It means that
things cannot get through the membrane very easily
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What is our normal concentration
of cholesterol in the blood
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But the recent thinking by internist is
that the lower your cholesterol the better
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There is a point that you get too low
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Why is
this? This is because a lot of toxic material go through our body and if our body doesn’t have its full
complement of cholesterol it can create a problem
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We are still talking about phospholipid molecules, lets look at their movement
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The first movement is inflection, which means that the tail flickers
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You rarely see flip-floping of membrane phospholipids moving from one layer to the other layer
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For example, there are
three phospholipids that we talked about: phophotidylserine, Phophatidylethanolamine,
phophatidylcholine
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When you have membranes consisting
of these three phospholipids it tends to be thinner in thickness
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An illustration of how cholesterol interacts with other lipid molecules
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You have two phosopholipid molecules and you have
cholesterol in the middle
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The membrane in a real cell is not normally
homogenously distributed
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On the surface of the membrane there are little islands
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They are called rafts, what constitutes rafts: for one
thing the membrane is a little bit more thickened in that area, so you got sphingomyelin there
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A lot of these rafts are consistently together so you
have a certain group of protein that aggregate together and they stay together for a specific type of
function
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And at other times it can be transient
meaning appearing when needed and disappearing
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Another feature, where does the lipid droplet come from
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They have to be stored some place and they are stored in this granule vesicle called lipid droplets
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The ER has an outer membrane and an internal membrane and
the inside is called the lumen or the cisternae
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So in a lipid bilayer, only one layer of the
ER is budded off
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A lipid droplet is the storage site for
fatty material in a cell
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What are those protein in the bilayer for? They are
enzymes that can be use for biosynthesis, they can transform cholesterol intermediates to other kinds of
intermediate
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I will continue the character of phospholipid molecules
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Asymmetric
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When you have these molecules group together the
environment not just the molecule attract other things in the cell which is not obvious
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So every structure has a function
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What is it? You have a sugar unit inositol,
it is an isomer of glucose and then you have a phosphate bridged to a glycerol molecule and two of the
molecules of glycerol have an actyl group and an actyl group
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This molecule
is very important in signal transduction
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So you have an Phophotidyl inositol molecule with two groups phosporylated
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Second way for signal to be transduced is that Phophotidyl inositol can be
broken up and when it is broken up you will have a molecule called inositol 1,3,5 triphosphate (IP3) so
this is a secondary messenger that will carry information from the outside of the cell to the inside of the
cell
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Outside of the cell you have a lot of sugar units that are bound to the outside of the membrane
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Secondly they can be electrically charged
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There are some cells that if you dissociate them will reassemble, glycolipids are the
major reason why after dissociation these cells can reassemble
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They look different they perform different functions
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)
Integral membrane protein: this means that this protein traverses the membrane and it has three
domain: the extracellular domain the intracellular domain and the transmembrane domain
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The site when a hormome is floating
around, the receptor can recognize the hormone by the extracellular domain
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2
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3
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What is responsible for the anchoring? There is covalent binding of the
polar end of the membrane to the Amino acids around, so it will not flip but it does move though
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)
This one you have the helix inside the hydrophobic environment of the membrane
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)you have lipids
and you a protein directly linked to the lipid of the membrane, so it is not transmembrane at all
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6
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7-8
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This can occur both on the extracelluar and intracellular
side of the membrane
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Give me the name of two types
of noncovalent bonding:1
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) Ionic bonding
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There is a way that you can test these amino acids usually do not exist in linear form
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If you measure the hydropathy index or measure the hydrophobic/hydrophilic
character it is about this number
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You will find that most of the amino
acids that transverse the plasma membrane are mainly hydrophobic amino acids
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If you stretch that protein out and measure the hydrophobicity and
hydrophilicity of that protein, you will find out that part of that multiple helix that transverses the

plasma membrane are made up of hydrophobic amino acids
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The interesting thing about aqua porin it is that it can open up and close
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The important thing is that when the channel opens up it is made up of two alpha helices in
which outside of these helices it is hydrophobic and is able to sit comfortably in the membrane
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The aqua porin can sit comfortably and when necessary it can allow water molecules to
pass through
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The Beta Barrel has multiple strands of helicies mashed together
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So that when you
have glucose trying to pass through you have to allow this specific passage
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You have integral membrane proteins that on the outside contain sugar
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Because gut has a lot of a very hostile
environment so with a lot of glycosylation it protect the cells

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