So we have a great understanding now of membrane structure and some
of the things that we find in the membrane which include proteins.
Many of these proteins are involved in transport
across cell membranes which is the topic of this lecture.
So in this lecture, we will be differentiating
between three mechanisms of passsive transport,
that is transport that does not require energy.
By the end of the lecture, you should be able to diagram
the association between primary active transport
and secondary active transport.
And in addition, you should be able to distinguish
between three different modes of bulk passage.
So we'll begin by exploring what we mean by passive transport.
Passive transport is mainly dependent on concentration gradients.
So for example, an area of high concentration
and an area of low concentration,
we can see movement from high concentration to low concentration
just based on movement of particles in the air, atomic movement.
Remember when it's warmer, things move faster,
when it's cooler, things move slower.
So if I were to spray some perfume in this corner of the room,
eventually the people over in that corner of the room
will smell the perfume. This is simple diffusion.
Now when we have simple diffusion occuring,
there's no energy required.
It would take some energy however
if we wanted to put the perfume back in the bottle.
So that would be active transport.
So passive transport requires absolutely no energy.
Things will diffuse through the environment
or throughout the cell.
Now if we have a cell membrane in the way,
can things pass through that cell membrane?
And the question comes with
what can actually pass through the cell membrane?
We already know the middle of the cell membrane,
the sandwich. We have hydrophilic outer edges
and a large hydrophobic lipid friendly volume in the middle,
which is fairly unfriendly if you are a polar molecule.
So non-polar molecules can actually squeeze between
the hydrophobic heads because they're quite small, very thin layer.
And it can make its way if they're small enough,
squeeze their way between the lipids.
So we'll see often that lipid based or hydrophobic molecules
can pass through the cell membrane.
Things like steroid hormones will pass through a cell membrane.
However, things that are large and polar
cannot pass through a cell membrane.
So in passive transport, we move things
from high concentration to low concentration
but what if those molecules are indeed large
or hydrophilic molecules?
So hydrophilic molecules need a passage to pass through
in order to get into the cell.
We actually need to provide an aqueous passage.
So let's take a look at this a little bit closer.
In order to facilitate the diffusion of
larger or polar molecules through the cell membrane,
we need to have either a channel protein or a carrier protein.
Channel proteins act to form, just like they sound, a channel.
And still this is passive transport.
We've got diffusion down the concentration gradient
from high to low concentration
but it's just facilitated by a channel protein.
This interior column of a channel protein is aqueous
continuous with the external environment
and the internal environment of the cell. And so
hydrophilic molecules can pass through that passage quite easily.
These channels can either be open or closed. Sometimes they
open passively but other times we require a signal molecule
or a gated channel, requires a signal molecule.
So signal comes along, binds to it, causes it to open.
When that signal molecule falls off, the channel closes and we can
no longer move something even down its concentration gradient
because there's no channel for it.
Then we have the idea of carrier proteins.
Carrier proteins are similar to channel proteins
in that they involved movement down the concentration gradient.
So we go from high to low concentration
just like we do with the channel protein.
The only real difference here is that we have a
confirmational change in that carrier protein,
such that when the ligand, the thing that
wants to go in the cell binds to it,
that triggers opening of the protein and
allowing it to carry the molecule through and close.
So moving down the concentration gradient, you've got a
fair amount of pressure from molecules on the outside here
pushing that carrier proteins full when the ligand binds,
the molecule wants to enter binds
that causes a confirmational change in the carrier protein
and it is carried through the membrane
and dropped off in the inside of the cell.
This still requires no energy because
we're moving down the concentration gradient
from an area of high concentration outside the cell
to an area of lower concentration inside the cells,
so no energy required.
These two mechanisms, whether we're talking about
channel proteins or carrier proteins,
allow membranes to be selectively permeable.
And it's pretty cool if you think about it
because now the protein can choose
where to put channels in the membrane
or where to put carrier proteins in the membrane.
If we want, say sodium, to move in and out of the cell,
and we want lots of it to move in and out of the cell,
those are gated channels. But we could put
lots and lots of those gates into the membrane,
so that we can get lots and lots of sodium to move
in or out of the cell, depending on the concentration gradient.
So this is one of the ways that cells regulate
how they're functioning.
So proteins of course come from DNA.
They're made by ribosomes.
And we use that cell membrane transport system,
the endomembrane system that we've introduced previously
in order to transfer these molecules,
the proteins to the cell membrane
and allow additional transports.
So selectively permeable membranes
are what we see in cell membranes.