Main content
Course: MCAT > Unit 6
Lesson 1: Cell membrane overviewMembrane dynamics
Let's explore some of the ways that the structure of the fluid mosaic model influences its function by focusing on the movement of phospholipids. Learn about the different types of diffusion, including transbilayer (flip-flop) and lateral, and the role of catalysts like flippase, floppase, and scramblase in these processes. By William Tsai.
Want to join the conversation?
- Who came up with the names Flippase and Floppase? They're quite amusing names. Usually the names for these things are complex and hard to remember, but I think I'll remember these two.(33 votes)
- why do phospholipids move from one leaflet to another(5 votes)
- From what I've learned, this is actually not entirely correct? I took a cell biology class last spring semester. At2:03the speaker mentions transbilayer diffusion and how that is 'flip-flopping' but my cell biology professor taught us that phospholipids do NOT ever readily flip flop without ATP in the case of flippase or floppase or without being 'switched' via scramblase. Who is wrong?(3 votes)
- You probably don't need this question answered anymore, but the video is right. Membrane phospholipids can switch from the outer to inner membranes (or vice versa), but it is a very slow process. I learned it as transverse diffusion in my introductory biochem course.(2 votes)
- how does the membrane get bigger (lets say before cell mitosis ) ?(2 votes)
- The cells needs to make more phospholipds, cholesterol and membrane proteins to expand and then send these to the cell plasma membrane. Here is a useful picture, and I will try to explain it.
http://www.ncbi.nlm.nih.gov/books/NBK26892/figure/A2412/?report=objectonly
These are synthesized in the cell using the endosome system (endoplasmic reticulum, Golgi complex). The newly produced phospholipds, cholesterol and membrane proteins are packed into a vesicle, think of this like a shipping container. The wall of the vesicle are made of the same lipid bilayer mix needed to expand the cell's membrane. The vesicle travels to the surface and fuses to the existing cell membrane, which causes it to expand.(3 votes)
- what is the use of the movement of the cell membrane?(3 votes)
- What hormones will affect membrane dynamics?(2 votes)
- would the ATP --> ADP + Pi conversion of floppase be coming from the intra cellular side? if it's going from inner leaflet to out leaflet? Also why doesn't scramblase need any energy for moving across the membrane if it's not going down a concentration gradient(1 vote)
- how do double bonds in the phospholipid tails affect the fluidity of the phospholipid layers?(1 vote)
- why is this movement from one leaflet to another called DIFFUSION?(1 vote)
- When you talk about movements being slow and fast, can you please give us some idea of what you mean by that? Are we talking milliseconds, seconds, minutes, or what?(1 vote)
Video transcript
In this video, we're going
to explore a little bit about membrane dynamics. So we know that in our fluid
mosaic model of our cell, everything in the cell
membrane moves around. So our cholesterol moves
around, and our phospholipids move around, and our
proteins all move around. But in this video,
we're actually going to focus in on
our phospholipids. So over here, I've pre-drawn a
picture of our cell membrane. And you notice that these
phospholipids are really tightly packed together. So how do these lipids,
these phospholipids, actually move in
our cell membrane? Well, before we get into
answering that question, we're just going to quickly
label our cell membrane. So out here we have
our extracellular. This is the outside of our cell. And in here we have
our intracellular, or the inside of the cell. And there's an
important distinction that we have to make between
intra and intercellular. Intracellular is the
inside of the cell, while intercellular
is between cells. I like to remember this
by thinking about the word intercontinental. That means between continents. So intercellular must
mean between cells. We sometimes call
the phospholipids that border the
extracellular environment as the outer leaflet. And we call the
phospholipids that border the inside, or the
intracellular environment, as the inner leaflet. And you'll notice that this
cell membrane that we've drawn is very basic. We've taken out all the
cholesterol, the proteins, and all of the other stuff
that makes up the cell, because in this
video we're actually going to focus in on the
phospholipids themselves. So the first type of
movement is uncatalyzed. This means that there's
no need for a catalyst. So one type is we
can actually have a phospholipid on
the outer leaflet move onto the inner leaflet. Or we could have it in reverse. We can have something on
the inner leaflet move to the outer leaflet. We call this
transbilayer diffusion. And this actually
has a nickname, which we call flip-flop. And this type of
movement is really slow. There's no catalyst. And you're trying to move
a phospholipid from one leaflet to the other. So this process is very slow. And it doesn't
happen that often. Now, there's another
type of movement where we can have a phospholipid
like this actually move side to side. And this is what we
call lateral diffusion. And just to give a bird's-eye
view of what this actually looks like, if we actually have
the head of our phospholipid, the movement is not
just from side to side, but it goes all around
the cell membrane. So if we were to look
at this from the top, this phospholipid can
move in any direction. It's what we call
lateral diffusion. And since we're not actually
switching between leaflets in this type of movement,
this is actually pretty fast, and it happens a lot
in our cell membrane. So if we have
uncatalyzed movement, naturally, we will have
catalyzed movement. And since this
movement is catalyzed, we're going to need a catalyst. And in this case, our
catalyst will be a protein. The first one we're
going to talk about is we can have something
on the outer leaflet, like this, actually
flipped to the inside, kind of like our
transbilayer diffusion. But this time, it's aided
by a protein-- a catalyst. And not only so,
this process actually uses ATP, which we call
adenosine triphosphate. Just to remind us,
ATP breaks down into adenosine diphosphate,
ADP, with the phosphorus. And this provides energy
for this reaction to happen. This catalyst that we
use, or this protein, is actually called flippase. And this is pretty fast compared
to our transbilayer diffusion. And the next one we have,
again, also uses a catalyst. So we have another protein. And this particular
protein is called floppase. And floppase also uses ATP. Now what floppase
does is it actually brings a phospholipid
on the inner leaflet to the outer leaflet. So it does the opposite
of what flippase does. And again, this is
also pretty fast, because it uses a catalyst. Now the last one
that is catalyzed does something
really interesting. It actually brings
a phospholipid from the inner leaflet
to the outer leaflet and one from the outer
leaflet to the inner leaflet. And this is what
we call scramblase. And this actually
does not need ATP. It doesn't require that
extra input of energy. And again, because it is
catalyzed, it is pretty fast. So in summary,
these are the ways that allow our phospholipids to
actually move around our cell membrane and create this
amazing, moving, fluid cell membrane structure that we
call the fluid mosaic model.