Cell membrane proteins | Cells | MCAT | Khan Academy


In this video, we’re going
to explore membrane proteins. Did you know that
the cell membrane can be composed of
up to 75% protein? So most cell membranes have
about 50% or less protein, and the proteins are there
because the cell membrane uses proteins for pretty
much everything that it does– all of
these cell membrane processes that it performs. So just to remind us what a
cell membrane actually is, a cell membrane is made
up of little things that look like this, which
are called phospholipids. And they come together and form
what we call a lipid bilayer. So over here, I’ve
pre-drawn a lipid bilayer. And it’ll look
something like this. It’ll be made up a lot of
these small phospholipids that we’ve drawn above, and
it’ll make up our bilayer. So you can see
that there are two layers of these phospholipids. Now, there’s two major types of
proteins in the cell membrane. The first can look
something like this. And this can appear anywhere
in the cell membrane, and there are
usually quite a few of these throughout
the entire cell. So this is what we call
an integral protein. You’ll notice that it’s
called an integral protein, because you can think of
it like it’s integrated throughout the entire membrane. Another type of protein
that we might encounter might appear on top
of the membrane. Occasionally, it might be
slightly into the membrane, and it can also rest on
top of integral proteins. And this we call
peripheral proteins. And the reason why we call
it a peripheral protein is because it’s on the
peripheral, or the outside, of the cell membrane. The difference between
peripheral and integral proteins is that integral
proteins are really stuck inside the cell membrane. As you can see in this
picture, the integral protein is really inside the
membrane, and as a result, it will be very
difficult to remove. Peripheral proteins kind of
attach and remove themselves from the cell membrane
or from other proteins. They generally are there for
different cell processes, so for example, a hormone
might be a peripheral protein, and it might attach to the cell,
make the cell do something, and then leave. Peripheral proteins
can also exist inside the cell on
the cell membrane. Another type of protein
is extremely rare, and it can appear inside
the cell membrane like that. And we call this a
lipid-bound protein. Why might you
think a lipid bound protein is so difficult
to find, so rare? Well, the reason why is
because proteins are there to interact with the outside
environment, and lipid bound proteins are
stuck on the interior of the cell membrane itself. So it can really
interact with the outside of the cell or the inside the
cell, so it doesn’t really serve a big function in
terms of the cell membrane performing its duties. We’re going to spend a
little bit of time talking about two types of
integral proteins that are extremely important,
because these two proteins are found all over
the cell, and they help the cell maintain
homeostasis, or balance. The first type can look
something like this. Again, this is an
integral protein. What do you think this
protein might be used for? This isn’t two proteins. It’s actually one protein
with a hole through it. Well, this protein
is actually used to allow things to
pass through the cell. We call this a channel
protein, and like the name kind of implies, there’s
a channel, or hole, inside the protein that
lets things pass through. So for example, if there
is some sort of ion– let’s say this is an
Na+ ion, a sodium ion, this is outside the cell. And the cell at
this point really needs these sodium
ions to perform a really important process. So what the channel
proteins do is they’ll allow these
outside extracellular ions into the cell. And normally, these
sodium ions wouldn’t be able to pass through the cell
membrane just by themselves. These channel proteins
allow our bodies to take in different materials
from the outside environment into our cells. What they can also do is
they can also do the reverse. So let’s say your cell
has way too much sodium, and it needs to get rid of it. So channel proteins can
start pumping these out. Channel proteins generally
don’t require energy, so there’s no energy needed. Sometimes we call energy ATP. And another thing that’s
special about channel proteins is you’ll notice that it will
go with the concentration gradient. So out here, there’s a lot, and
inside, there’s very little. So it’ll pump from where
there’s a lot of sodium into where there’s very little. So it’ll go what we call down
a concentration gradient. The second type of very
important integral protein is called a carrier protein. And like the name implies,
it carries substances into the cell. I kind of picture it like a
baseball glove, like this. So if there’s a molecule
that’s outside the cell and the cell actually needs
this molecule– so what the carrier protein will
do is it’ll actually protect this substance so that
it can enter the cell safely. It can also do this in reverse. It can take something
inside the cell and pump it outside the cell. And this type of protein
is really important, because unlike channel
proteins, carrier proteins can go against the
concentration gradient. And this is really important,
because say your cell has a lot of chloride ions,
and your body needs more to perform a certain process. So what your body can do is it
can bring more chloride ions into your cell, even
though your cell already has a lot of chloride ions. So carrier proteins can
sometimes use energy or ATP. Finally, there is
a type of protein that can exist on any of
these that we’ve drawn here, and this is what we
call a glycoprotein. So what a glycoprotein
would look like is there’ll be a chain of
sugars attached to a protein, and it can be on integral
proteins, peripheral proteins, channel proteins. Glycoproteins, you’ll notice,
have the prefix glyco, which means sugar. And basically, it’s
just sugar plus protein. And the purpose of glycoproteins
is that it’s used in signaling. So it allows a cell to
recognize another cell. So in summary, in this
picture that we have drawn out of a cell membrane and
several different proteins, we have two main
classes of proteins. We have peripheral
proteins, which are on the outside of the
cell, and they’re really easy to remove. We have our integral proteins,
which are stuck inside the cell and really tough to remove. We have our lipid
bound proteins. We have channel proteins,
which allow things to move through the cell by
its concentration gradient, and it doesn’t require energy,
and it doesn’t require ATP. We have our carrier
proteins, which are kind of like
a baseball glove. It can take in a
particular molecule and let it out inside the cell,
or it can do it in reverse. And these can sometimes
use ATP, and what’s special is they can go against the
concentration gradient. And finally, we
have glycoproteins, which really can be any of the
proteins that we’ve drawn out. It’s a sugar plus a protein, and
it participates in signaling, so cells can
recognize each other.

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28 thoughts on “Cell membrane proteins | Cells | MCAT | Khan Academy

  1. Dammnn his method of explanation is so smooth and understanding. Every word he says just just gets in my head within seconds!
    He's truly my carrier protein. haha

  2. It means thats cell membrane contains 6 types of protiens ? Im little bit confuse plz answer me.U explained very well โฃ๏ธ

  3. Great video!!! But I don't think carbonhydrates are also located at the interior side of the membrane, or am I wrong? It wouldn't make sense to locate carbonhydrates on this side, since there are no cells to interact with

  4. Integral membrane proteins are not always integrated throughout the entire membrane. There are 2 classes of integral membrane proteins a) Monotopic and b) Polytopic. Monotopic only interact with one side of the leaflet(one side of the phospholipid bilayer). Polytopic integral membrane proteins are ones that pass the entire phospholipid bilayer like the one you drew. Thanks!

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