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MCAT
Course: MCAT > Unit 6
Lesson 1: Cell membrane overviewCell membrane fluidity
Cell membrane fluidity is the property of the cell membrane that allows it to adapt its shape and movement to different conditions. Three key factors influence cell membrane fluidity: temperature, cholesterol, and the kind of fatty acids in the phospholipids that form the cell membrane. These factors alter the space and interaction between the phospholipids, and how this affects the fluidity of the cell membrane. Created by William Tsai.
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- Since cholesterol acts as a "buffer", does it only affect fluidity when temperature is also in play (at low/high temperatures)? So in normal temperature conditions, would cholesterol have no affect on the fluidity?(13 votes)
- Cholesterol still effects membrane fluidity at normal temperature. Think of it sort of like a gradient. At low T's, cholesterol increases fluidity by preventing overly tight packing of the phospholipids . At higher T's, cholesterol decreases fluidity by pulling the phospholipids closer together so at medium T's, cholesterol is keeping the bilayer together but also preventing the phospholipids from overpacking. It is essentially playing both roles at medium temperatures.(2 votes)
- Can somebody give a example of a medical situation that the doctor could intervene in membrane fluidity?(11 votes)
- Spur cell anemia (acanthocyte formation) would be the overproduction of cholesterol in RBCs. This high concentration of cholesterol would cause increased rigidity leading to a rupture of the hemeglobin. An overactive spleen would be the pathology of this anemia. So the treatment would involve regulation of the splenic activity and not the fluidity of the membrane directly.(3 votes)
- What affects how much saturated fat and how much unsaturated fat the phospholipid bilayer is composed of?(11 votes)
- Maybe you are not right. Think about the case in which you are genetically programed efficiently but you eat only saturated fats and lots of trans fats. Well, your body will become very rigid, you can't get enough nutrients through the cells, and you will die pretty soon. So, I think you should be able to modify and training the cells to work efficiently and in balance. I think that unsaturated fats can't be more than 10% in the total membrane, for a lot of reasons and naturally divine occurred limitations: temperature, blood density, oxygenation, etc. For example, high activity levels, diets and environment influences a lot.(2 votes)
- How does cholesterol decrease the fluidity in lipid rafts? I thought that the glycosphingolipids in the rafts are packed tightly together so shouldn't cholesterol increase the distance between these lipids thus increasing the fluidity?(5 votes)
- what exactly is fluidity ?(3 votes)
- What I got from the video was that the fluid nature of the lipid bilayer means that the molecules within the layer such as the phospholipids can move around as freely as they want. If the temperature is low, then the phospholipids will not be able to move around as much and will be in its crystallized state. Therefore, fluidity just means movement.(4 votes)
- So cis-unsaturated fats increase fluidity because they cause these kinks or bends. Do trans-unsaturated fats also increase fluidity? To my knowledge, they do not create the same kinks/bends as cis configuration does.(4 votes)
- Trans-unsaturated fats will not have kinks in them, and will therefore not significantly affect membrane fluidity. However, it is interesting to note that nearly all unsaturated biological fatty acids have cis double bonds in their tails; trans-unsaturated fats are extremely rare in living organisms. :)(2 votes)
- Why do the phospholipids move towards the cholesterol when its too warm?(4 votes)
- Cholesterol has an OH group on one of its ends. Since the oxygen in the OH group is highly electronegative, it hogs all of the electrons and a positive charge is created on the hydrogen side of the group. That side then forms a hydrogen bond with the negatively charged oxygen on the phosphate group.
The rest of the cholesterol molecule is made up of carbon and hydrogen, so it is non-polar. It aligns itself with the non-polar, hydrophobic lipid tails of the phospholipids. It attracts the lipid tails through Van der Waal forces (London dispersion forces).
Because of these forces of attraction, phospholipids tend to stay around cholesterol molecules when it is warm (and in other temperatures). So, they stop nearby phospholipids from moving about, thereby reducing the fluidity of the membrane.(1 vote)
- Is it the double bond in the unsaturated fatty acid that gives it the kink or bend.?(2 votes)
- Yup! a cis (same side) double bond will create a "kink" in the carbon chain(5 votes)
- I understand how and why temperature changes effect fluidity of phospholipids. I also understand that cholesterol counteracts the change in fluidity. However, the cholesterol, as you mention, acts as a buffer, which implies that as temperature increases, membrane fluidity still has a net increase in fluidity, correct? In other words, do cholesterol interactions ever actually have such an effect that an increase in temperature decreases membrane fluidity overall? Thanks!(3 votes)
- Don't overthink this one. In a hypothetical cell membrane where cholesterol doesn't exist, temperature increase and decrease will increase and decrease fluidity, respectively. Now, if you add cholesterol to a cell membrane, the reverse is true. Also, cis-fatty acids will increase membrane fluidity as well.(0 votes)
- How do steroids maintain the fluidity of a membrane?(2 votes)
Video transcript
Why doesn't our cell membrane
fall apart when it's too hot, or why doesn't our cell membrane
freeze when it gets too cold? Well, our cell membrane actually
has a very unique property called membrane fluidity. Now, a lot of different factors
can affect membrane fluidity. But the three most
important ones that we're going
to focus on today are, number one, temperature,
number two, cholesterol, and number three, which is
whether we have unsaturated or saturated fatty acids. Now, just to quickly
to remind us, the building blocks
of a cell membrane are what we call phospholipids. And it looks like this. There's a phosphate
head group that's represented by a circle
and two fatty acid chains, kind of like strings
hanging below. And in this video, we're
actually really going to focus in on the impact
of phospholipids in our cell membrane. So the first thing we're
going to start off with is temperature. We have low temperature,
and obviously we have high temperature. So let's pretend that
our cell membrane is only made up of phospholipids. What do you think
our cell membrane's going to look like
at low temperatures? Since the temperature is
low, our phospholipids are actually going to
start clustering together really closely,
kind of like that. And the reason why is because
these phospholipids are at low temperature, which means
they don't have a lot of energy to move around a lot. So they're going to huddle
really close together. At extremely low
temperatures, we actually call this a crystallized state. And since they're
huddled so close together and they don't have a lot
of energy to move around, the fluidity is
actually pretty low. So as the temperature
decreases, the fluidity of the cell membrane
also decreases. What happens at
high temperatures? Well, at high temperatures,
our phospholipids have a little more energy. So they're going to move
around a little bit more and cause themselves
to have more of a distance between each
other, kind of like that. So you'll notice that the
distance between phospholipids is now much greater than
what it was over here, at low temperatures,
which is very, very small. So this increased distance
allows our fluidity to increase, because
there's much more room for the cell
membrane to move around. So as the temperature
increases, our membrane fluidity also increases. What happens when
we add cholesterol? Well, at low temperatures,
our phospholipids still tend to cluster pretty
closely together. But occasionally, something
really interesting happens, which is when
cholesterol actually inserts itself between the
phospholipids, like this. And it doesn't do this for
every single phospholipid, but it'll occasionally insert
itself into the membrane. The same goes for
the phospholipids that are underneath. And you'll notice that the
membrane doesn't always have to line up in the sense
that the phospholipids can actually be in the
same place as the ones above or in a slightly
different place. In some membranes
there's more cholesterol, and in others there is less. But the presence of
cholesterol itself does something really unique. And what that is, is
it actually increases the distance between some
of the phospholipids. And like we've talked about
for the high temperatures, as the distance between the
phospholipids increases, the fluidity can also increase. What happens at high
temperatures with cholesterol? At high temperatures,
our phospholipids are already pretty far apart,
just like the above picture. But just like before,
the cholesterol will insert itself into the
membrane at random places. And what this will
actually do is it will cause the phospholipids
to pull themselves closer together, because
they kind of want to attach to that cholesterol. So now there's
more stuff inserted throughout the membrane, and so
the molecules in the membrane are now closer together. So the fluidity
actually decreases. So cholesterol is actually
really interesting, because at low temperatures,
the fluidity will increase. And at high temperatures,
the fluidity will decrease. You can kind of think
about cholesterol like a buffer, kind
of like in chemistry. It allows our cell
membrane to remain at a fairly stable and
normal level of fluidity. When the temperature
gets too low, the fluidity will
increase a little. And when the temperature
gets too high, the fluidity will decrease. So moving on to our
third one, which is the presence of saturated
versus unsaturated fats, we're going to go ahead and make
a new canvas to give ourselves a little bit of room. So in number three, we're
comparing the presence of saturated versus
unsaturated fats. And when we're talking about
saturated versus unsaturated, we're talking about the
fatty acid chains that are hanging below our
phosphate head group. So just to remind
us from chemistry, a saturated fatty acid
can be represented like this, where every angle
or pointy end is a carbon. In the case of an
unsaturated fatty acid, it can look pretty different,
because an unsaturated fatty acid means that we
have some double bonds. So let's say we have two
double bonds like that. By themselves, it doesn't
seem to be anything special. Granted, they look different. But how will these interact
with multiple fatty acids next to them? So in the case of
a saturated one and in the case of
an unsaturated one, our molecule will still
have some double bonds. And what's really
unique is you'll notice that in the
saturated fatty acid, these two fatty acid chains
stack together really neatly, kind of like Legos. But in our unsaturated
fatty acid, these two don't really
stack together that neatly. How will this affect
our membrane fluidity? Well, for the sake of this
particular explanation, we're going to draw the
saturated fatty acid chains as straight
lines, like this and just because we're
trying to represent the fact that these straight
lines stack together really well. So what's going to happen is
they'll stack pretty closely together, and so will
the ones underneath. And since the distances between
the molecules is pretty small, our fluidity is
actually pretty low. So what do you think will happen
with our unsaturated fatty acids? Well, you'll notice that
there's a little bit of a bend now in these fatty acid chains. So I'm actually going to
represent the phospholipid with a little bend in it. And these might occur
at different places, or they might have both
of them being bent. But you'll notice that
I'm unable to draw these phospholipids
as closely together. There becomes more distance
between these phospholipids because of this unsaturated
bend in our phospholipids. So since there's more distance
between our phospholipids, the fluidity increases. Just to quickly sum up, today
we learned the three factors that can affect
membrane fluidity, the first being temperature. As temperature increases,
fluidity also increases. The second is cholesterol. And cholesterol
acts as a buffer, increasing fluidity
at low temperatures and decreasing fluidity
at high temperatures. And the last are
unsaturated fatty acids in our phospholipid. When we increase the
amount of unsaturated fatty acids in our cell membrane,
the fluidity also increases.