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Course: MCAT > Unit 6
Lesson 1: Cell membrane overviewPhospholipid structure
Phospholipids are molecules that form the cell membrane. They consist of a polar phosphate head group and two nonpolar fatty acid tails joined by a glycerol backbone. The phosphate group can link with different molecules, such as serine or choline, to generate diverse kinds of phospholipids. The fatty acid tails can have cis or trans double bonds, which influence the membrane fluidity. The molecular details of phospholipids and their variations can be drawn as shown. Created by William Tsai.
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- video quality is very low - even in full screen mode. Would very much like a higher quality video - it's challenging to see what the pen is writing as the opacity is very, very low.(60 votes)
- Watching this video on smaller screens is a lot easier to see compare to full screen mode. I find that watching on my phone instead of a PC, it is easier to see the writing(1 vote)
- this is a feedback please provide better graphic for the video(12 votes)
- What are some examples of phospholipids besides semi-permeable cell membranes?(3 votes)
- This may surprise you, but the class of lipids that phospholipids fall under are really only found, in their natural biological state, as the major components of cell membranes. There are different types of phospholipids that make up substances such as lecithin and cephalin, but the majority serve as some part of a cell's (plant or animal) membrane.
Now, phospholipids have been produced commercially, and are used in nanotechnology and materials science. In this artificial state they have also been used in medical drugs, food (as emulsifiers), and cosmetics.(14 votes)
- Can there be 3 fatty acid tails? or only 2(3 votes)
- No, a phosphlipid cannot have 3 tails because the 3 carbons of the glycerol backbone is used up: two by the fatty acids and one by the phosophocholine group. There are lipids with three chains (no phospho group) such as triacylglyceride (TAG).(10 votes)
- at1:15why does the Glycerol have two OH and one HO?(2 votes)
- OH and HO are the same! They both denote an oxygen bonded to a hydrogen. All of these oxygens in the glycerol are then also bonded to a carbon. Since the oxygen is the one bonding with the carbon it is next to (and not the hydrogen), it is standard to write the oxygen connecting to the carbon.
Therefore, if the OH is on the left of a bond when drawing a chemical structure, it will be written as HO instead. This clarifies that the O is what is bonded to the C.(3 votes)
- this video is poor quality. on a black background. can barely see color.(3 votes)
- why phospholipids form a thin layer on surface of a aqueous solution?(2 votes)
- It's a lower free energy state for the system for water to be associating with water, excluding the hydrophobic fatty acid tails. The phosphate heads, which are soluble, stick down into the water.
Having the hydrophobic tails stick out on the surface is one way to exclude them.
Another way is for micelles to form with the phospate heads pointing out into the water.
Yet another way is for phospholipid bilayer enclosures to form.(3 votes)
- How is the phosphate head polar when its has a negative charge?(2 votes)
- This video was really hard to see / not conducive to the great lesson he was trying to show :((2 votes)
- guys is glycerol backbone polar or non polar(1 vote)
- the glycerol is non polar. To be a polar group there needs to be a electronegative charge on one side of the molecule. Looking at the structure of the glycerol there are three alcohol groups(OH) on alternating sides not producing the electronegative field needed on any particular side to be polar. Further once the glycol becomes the backbone in the phospholipid the actual ester bonds though more electronegative than a carbon to carbon bond do not make the molecule polar. The phosphate group is polar because the oxygen atoms, namely one having only one bond when to fill its octet it needs two bonds( ei another electron to be happy), has a much stronger electron pull than any other part of the molecule.(3 votes)
Video transcript
In this video, we're
going to actually explore in detail the structure
of phospholipids in our cell membrane. Just to briefly remind
us, our phospholipid is often drawn like this. It has that polar
phosphate head group, and it has two
fatty acid chains. And all of this is held
together by glycerol backbone. But what does that really mean? What dose is picture actually
look like down to a molecule? Well, let's talk about the
first one, the fatty acid. And just to remind
us, there are actually two fatty acids-- times 2. You can see that there are
two tails hanging down here. So our fatty acid is
basically a carboxylic acid attached to a very
long carbon chain. And so our carboxylic
acid is like this. It has a double bonded
O and a hydroxyl group. And it has that really
long carbon chain which we're just going
to call an R-group. The next one is our
glycerol backbone, and glycerol is a
pretty basic structure. It looks like this. It has three carbons attached
to three hydroxyl groups-- three alcohol groups. And there's only one glycerol
in each phospholipid. a The last one is our phosphate
group, that big polar head group that we talk about. And just like you
would think, there's a phosphorus in a
phosphate group, and there are four
oxygens attached to it. Now, what does this actually
look like all put together? Just for the sake of
time, I've pre-drawn a picture of all
this put together. It looks like this. So you can see that we have our
two fatty acid chains attached through an ester bond
with our glycerol attached through another ester bond
with our phosphate group. Now, you'll notice that
one of the negative oxygens is missing, and
it's been replaced with a hydroxyl group--
an alcohol group. And that's what this
is in the orange. Well, that's
because in our cell, a phospholipid actually
looks like this. The negative oxygen actually
picks up the hydrogen and becomes an alcohol group. Now, a phospholipid molecule
that looks like this is actually pretty rare in our
cell membrane, and the reason why is because
phospholipids can occur. And the reason why is because
this molecule could actually bond with several
different molecules, giving a really diverse
set of phospholipids. And again, for the sake of time,
I've pre-drawn these molecules, and unless you're a researcher
who really, really loves the cell membrane,
you probably won't need to know this by heart,
because these structures get a little complicated. But it's still good
idea to get acquainted to what they kind of look like. So there's serine, choline,
ethanolomine, inositol, and glycerol. And you'll notice
that I've also drawn these particular special
hydroxyl groups in orange, and the reason why is
because these hydroxyl groups in orange, from serine,
choline, and so on, can actually bond to our
phosphate group through what we call a phosphoester bond. Now, what would this actually
look like in a real molecule? What would it look
like if serine actually bonded with this phospholipid? Well, we're going to transition
briefly into another slide, just because I'm
running out of space. And you'll notice that there
are five different phospholipids that they can actually occur. There's phosphatidylserine,
phosphatidylcholine, phosphatidtylethanolomine,
phosphatidylinositol, and diphosphatidylgylcerol,
also known as cardiolipin. And you'll notice
that in this last one, there are actually two
phosphatidyl p groups that actually bond to
a middle gylcerol. And again, unless you're someone
who really researches the cell membrane, you
probably don't need to know these
structures by heart, but what we do need to know
is that the phospholipids in our cell membrane are
actually very, very diverse, and there are several
different forms they can take. So if we were to
take a look in detail at the phospholipids that
make up our cell membrane, we would actually
find all of these scattered throughout
the membrane. Now, we're just going to go
back to our original picture. Just to remind us, this is
again our nonpolar side, and this side in
the yellow is polar. And so if we were to match
up our general picture of a phospholipid to the
picture that we've drawn here, it would actually
look like this. This is our polar
head group, and we have two fatty acids here. And again, you'll notice that
the glycerol group isn't really drawn in, because that's what
holds everything together. And just to wrap up, we need
to talk about one brief thing. So we have our
phospholipids like this. Now, this so-called
R-group is made up of a really long
chain of carbons. Now, in many cases,
these carbons can actually form double
bonds with each other, like a lot of
different carbons do. And remember that double
bonds occur in the form of cis and occur in the form of trans. So a cis bond in chemistry is
when we have a double bonded carbon and we have the
carbons on each side being on the same side, while
in the trans bond, our carbons are
on opposite sides. Again, these are hydrogens. And if we were to zoom out
of this detailed molecule, in the case of
trans, our fatty acid would just be pretty
straight, like that. But in the case of cis, we
can actually create a kink, because this bend from our cis
bond actually gives it a kink. And this actually has
a lot of significance when we talk about the
fluidity of a cell membrane. So in summary, our
phospholipids are made up of three major things-- fatty
acids, glycerol, and phosphate. And these three things
actually looks like this if we were to draw out
a detailed molecule. And not only so, but
there's an OH group on this polar phosphate
group that actually can bond with several different
types of molecules, producing a really, really
diverse set of phospholipids that make up our cell membrane.