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.