So let's switch gears and talk about
the fluidity of the membrane.
So there are several structures as we
said, embedded throughout the membrane
and a lot of these structures need to be mobile,
they need to be able to move around the cell.
And so the dynamic flow of the
membrane allows for this to happen.
As well, the membrane proteins and the lipids
within it usually stay in their half of the bilayer.
So if you think about the inner half of the
bilayer versus the outer half of the bilayer
the membrane proteins are mostly moving
lateral although some flipping can occur.
As well, emebedded within our
membranes, we have cholesterol.
Cholesterol is actually there to stabilize the
membrane and make it so that it is not too fluid.
So while we do want our
membranes to be dynamic,
we also need it to have
some type of structural integrity
and the cholesterol acts as
kind of like the support beams
in our membranes so that there is something
there that makes it have a little bit of rigidity.
Now, the purpose of the plasma membrane is of course
to separate our external and internal environment
but because of the structure
of the plasma membrane
where you have the phosphate
heads and the fatty acid tails,
it is selectively permeable.
It's selectively permeable because everything
cannot pass through the fatty acid tails,
they almost act as like a gatekeeper.
So, it is permeable to small,
non-polar, uncharged molecules
because those things
can pass through the gate.
They can pass through fatty acid
tails because they do not have a charge
and they're not polarized in any manner.
But if you're trying to get
something across the membrane
whether it's getting
it in or getting it out,
it usually requires some type of transmembrane
protein that can act as a channel or transporter
in order to allow these
substances to move across this gate
or fatty acid tails that block everything.
And as well, sometimes things are so big
that the only way they can actually get in
is through vesicular transport where
it is brought in through a vesicle
or it is released from
the cell through a vesicle,
and we'll talk about that a little later.
So when it comes to movement
across the plasma membrane,
it usually involves a
So concentration gradient is the
difference in concentration of a chemical
between one side of the plasma membrane
and the other side of the plasma membrane.
So think about it from the standpoint of:
one side might be really crowded
while the other side is less crowded
and things are gonna try to move away from
being really crowded to less crowded.
The same way you would do, let's say in any
situation where you're in a really crowded room
and there's an opportunity
to move away from the crowd.
As well as concentration gradients,
we also have electrical gradients.
This is especially important in cells and the
nervous system and in the muscular system
which we will talk about later.
And if you put those two together, you can also have
a gradient known as an electrochemical gradient
which becomes important when we
talk about cellular respiration.
So in order to move substances across the cell,
there's usually two ways that this is done.
It can be a passive process where things
just move down their concentration gradient.
Examples of passive processes
include simple diffusion,
where non-polar, uncharged substances can
move passively through the plasma membrane
down their concentration gradient
without any help from anything else.
However, if you're trying to
move things like water or ions,
you might need a little bit of help.
In this we use facilitated diffusion.
Facilitated diffusion involves the use
of channels and transport proteins
in order to move things down
the concentration gradient.
In both of these processes, it's passive
because there's no energy required.
The same way, if you put a ball at the top of the
hill and let it go without actually pushing it,
it will roll down the hill on its own.
These processes are the same way.
A third passive process is osmosis.
Osmosis is specific for the movement
of liquid across the plasma membrane.
And in this, it's also a passive process
and involves movement of liquid
from a higher liquid concentration
to a lower liquid concentration.
Active processes involve energy input in order to move
things from one side of the membrane to the other.
There are multiple types of active processes
including primary and secondary transport
as well as vesicular transport.
In both of these, we are usually moving
something against their concentration gradient.