Cell membrane introduction | Cells | MCAT | Khan Academy
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Cell membrane introduction | Cells | MCAT | Khan Academy

So when you go
swimming or showering, have you ever wondered why
don’t your cells in your body fill up with water or
why don’t the substances in your cells leak
into the pool? Well, the reason is
because we actually have a very important
structure that prevents this from happening. This is what we call
the cell membrane. The cell membrane is what’s
on the outside of a cell. So if we have a very
basic picture of a cell here with a little
nucleus on the inside, this pink outside layer is
what we call the cell membrane. The cell membrane
can protect our cell from the outside
environment, and it can determine what can
enter and leave our cell. This is a property that
we call semi-permeability. It is somewhat permeable. Somethings can enter,
while other things cannot. So since this is such
an important part of our cell– in fact,
it’s one of the reasons why we can actually
survive in the world. So what actually makes
up this structure? Well, the main building
block of a cell membrane are what we call phospholipids. There are other substances
that make up our cell membrane, but the most important building
block are phospholipids. And so phospholipids have
three major components. The first is a
phosphate head group. The second is a
glycerol backbone, and the third are
two fatty acid tails. So the way we draw this is
we give the phosphate head group kind of like a head. It’s a circle. And two fatty acid
tails hang down from it, kind of like
strings on a balloon. So the way I kind of remember
this is a phospholipid looks like a balloon,
but with two strings. Now, where’s our
glycerol backbone? Well, our glycerol backbone is
actually what it sounds like. It’s what holds the fatty acid
tails to our phosphate head. It’s the backbone
of this molecule. So it’s usually not
drawn in the picture. But just remember
that it’s there, and it holds our
two fatty acid tails to our phosphate head group. So this structure actually has
a very interesting property. Up here, this head group is
actually hydrophilic or polar. So hydrophilic means
that it’s water loving. This phosphate
head group will do whatever it can to get to water. It loves water. But these fatty acid
tails, because they’re very, very long carbon
chains, this is hydrophobic. I remember hydrophobic because
a phobic or phobia is fearing. So hydro is water, so
it’s water fearing. These two fatty acids
will do whatever it can to get away from water. A molecule that has
both of these things together is what we call
and amphipathic molecule. It means that the molecule
has a hydrophobic section and a hydrophilic section. So in water, what would this do? So let’s say we put a ton
of these molecules in water. Once in water, the
hydrophobic heads want to be as close
to water as possible. But the tails don’t. So what will happen
is these phosphate groups are going to cluster
together while the tails try to shield themselves
away from water. But since this is a
substance that’s in water, water’s going to
be down here, too. So this will actually form
a really unique structure, because the fatty
acid tails are going to start grouping like this. And the phospholipids are
going to be kind of upside down so that the phosphate head
groups can be close to water, while this inside section
can be hydrophobic and away from water. This is what we call a
phospholipid bilayer. This is the basic structure
of a cell membrane. And like we mentioned,
this inside section is going to be hydrophobic. So now we have the structure
that looks kind of like this. We call this our
phospholipid bilayer, or lipid bilayer for short. But doesn’t this section here
also interact with water? How can this
structure be like this if this section here
still touches water? And we know that
the fatty acid tails don’t want to touch water. Well, in a cell in real life. What actually
happens is we end up with the structure that
forms a circle like this. Now, this is a fairly
crudely drawn picture. In a cell, this wall
is actually pretty thin compared to the entire body. So you’ll notice
that this water here doesn’t become a
problem anymore, because in our actual cells,
water can be on the outside and on the inside. And no matter where this
cell membrane touches water, it’s always going to be the
phosphate head groups that are hydrophilic that
are seeking out water. And inside the cell
membrane, we actually have a hydrophobic section. So moving on to new picture, we
mentioned before that the cell membrane is
semi-permeable, and we’re going to explore that
a little bit more. So I’ve taken the
liberty of pre drawing a very long picture
of a cell membrane. So as we mentioned,
the cell membrane is actually a sphere
that surrounds our cell. For the sake of
this lesson, we’re going to draw it out
in a straight line. And we’re going to say that this
can be the outside environment or the extracellular,
and this can be the inside or
the intracellular. So you’ll notice that
the cell membrane has these phospholipids packed
really closely together, so usually small molecules are
what can pass through the cell. Another property of the cell
membrane that we’ve discussed is that this inside
section right here is really hydrophobic. So generally small,
nonpolar molecules can pass through
our cell membrane. This is what we call
passive diffusion. So what is a good example of
a small nonpolar molecule? Well, the most common type
of small nonpolar molecule tend to be gasses, things
like, O2, for example, or CO2. These are things that
surround us every single day. And our cell, in a sense,
breathes these molecules in and out of our cell. So gases can very easily pass
through our cell membrane, and it’s very fast. They are small and
they are nonpolar. So what else does our cell
interact with every single day? Well, the most
common one is water. So water’s actually a pretty
small molecule, and it’s polar. So something else that’s
similar to water is ethanol. This is like alcohol
that we can drink. So how do these interact
with our cell membrane? Well, we said that the cell
membrane likes small molecules, so these can actually pass
through our cell membrane. But our cell membrane
prefers nonpolar molecules. So these are actually going
to pass through really slowly, and they can pass
through because they’re so tiny that they kind of
sneak by, but pretty slowly, because this very hydrophobic
region is still not going to like having
the water in there. So if we have small polar
molecules, what about something that is a large but
nonpolar, like benzene? Benzene can actually pass
through our cell membrane. Even though it’s
large, it’s nonpolar. So it’s going to
get along really well with that hydrophobic
region in our cell membrane, but it’s going to
pass very slowly. Now, as a little
bit of a fun fact, benzene used to be used in labs
for students and researchers to wash their hands. Scientists actually
found out that benzene can pass through
our cell membrane and cause harm to ourselves. What about something
that is large and polar? Well, a molecule like this
would be sugar or glucose. Glucose actually cannot
pass through our cell. It’s large and it’s polar. It’s the complete opposite of
what the cell membrane allows to pass through the cell. So glucose will have to be
absorbed by ourselves through other means, but it cannot
pass through the cell membrane. What about charged molecules? These are also all
over the place. What’s an example of
a charged molecule? Well, something like a
chloride ion, a sodium ion, or any sort of ion. Another pretty common
charged molecule are actually amino acids. And since these are charged,
they’re so incredibly polar or charged that they
also cannot pass through. So, in summary, our cell
membrane protects our cells and determines what enters
and leaves, a property that we call semi-permeability. And this cell
membrane is made up of a whole bunch of
phospholipids put together. Since our cell membrane has a
very large hydrophobic region, it prefers nonpolar molecules. And since these phospholipids
are packed so closely together, our cell membrane also
prefer small molecules to pass through. So our cell membrane
is semi-permeable, allowing, generally, small
and nonpolar molecules to pass through
the cell membrane.

About James Carlton

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90 thoughts on “Cell membrane introduction | Cells | MCAT | Khan Academy

  1. Instead of saying that it's semi-permeable, the proper term that should be used is that it is selectively permeable. Otherwise it's a great video!

  2. just lovely. so much better than some book crap that just rambles on about jibberjabber that nobody understands. i think this explanation is better than what's in my textbook. got to love this.

  3. man its not bad but this is rlly basic in neeeed the compositions and the chemichal formulas of the lipids, that is good for high school but not for biology in university

  4. This video helps me to understand the cell membrane easily than my own teacher explaination. Thanks a lot.

  5. My teacher has a really strong accent so I can't understand her as well, but this video cleared everything up 100%. Thanks so much!

  6. why does the phospholipid bilayer only allow non-polar molecules to pass through? I understand its because it's hydrophobic…but how does that link to it being able to diffuse quickly???

  7. Sir, thank you for your video. Since I'm studying for my Biology exam, would you mind answering some of my questions? They are: Can a lipid-soluble molecule pass through the proteins? A phospholipid has a hydrophilic head, the head consists of a phosphate which occupies a little space, but also a glycerol? I've been spending a whole day to think isn't a glycerol hydrophobic, and a phosphate is hydrophilic but a phosphate only occupies a little space.

  8. The key point at 3:50 is that *the water is also hydrophilic*. Water likes to touch other water (water molecules are attracted to each other but not attracted to fatty tails). This is the main reason why the water would prefer to escape from inside the layer.


  10. how does O2 and CO2 pass through cell membrane . as you said they are non polar and it can pass through fatty acid easily but before it reach fatty acid it has to cross the phosphate head . so i don't understand how non polar molecule cross the phosphate head . plz answer @all
    and thanks a lot for making such great videos @khan academy

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  12. I don't think I have ever known enough about cells to wonder why they don't fill up with water whenever I shower or go swimming. I mean seriously is that a thing, who wonders about that?

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