So mainly, phospholipids are
going to be the main processes
of cell membrane walls and
organelles membrane walls.
These involve having a small
little lipid bilayer,
meaning that it has a little
bit of a structure in between
two other structures and we’ll get
to be what that is in a minute.
and cholesterol make up the
predominant way in which we
have this bilayer take place.
And you notice that
all three of these
and cholesterols have a
colored component that’s
on one side and a
tail component component
that’s on the other.
And this is very important in how these
molecules will eventually orient
to two different sides
of the membrane.
So let’s get to those now.
If you have a phospholipid and you
place it within water and oil,
the heads of the phospholipids will face
the water and the tails will face the oil,
and we call those tails that
face the oil nonpolar ends.
Polar ends, however, will
face towards the water
and this allows for a natural
barrier that will occur.
But remember that we have a
double layered membrane,
meaning that we have water on
both sides of the membrane.
Therefore, you’re going to have polar
heads facing both of the water sides.
Interestingly, things like
phospholipids oftentimes can
move a little bit
within the wall.
But they usually only move back and forth
or maybe they spin around a little bit,
but they don’t flip flop very often from
one side of the membrane to the other.
Now, why are you partitioning
some of these particulars spaces?
Well, one is that the fluid
inside the cell is often
very different than the
fluid outside of the cell.
We call the fluid within the
cell intracellular fluid
and the fluid outside the
cell extracellular fluid.
That extracellular fluid can be broken
up into two different components.
One is interstitial fluid and the
second is what’s in blood or plasma.
So how to think about those differentiations
is what’s inside the cell’s interstitial
fluid and then everything else is outside
the cell or extracellular fluid.
The fluid that’s directly
around the cell is interstitial
and that which in the blood vessels
themselves is plasma or blood volume.
We’re going to get back to intracellular
and extracellular fluids especially
as we talk through the renal system and
how a person controls blood volume.
But it’s very important at this
portion to think about what
breaks those into those specific
different fluid compartments.
That’s the fluid itself.
Now, we need to also talk about what
particular substances within that fluid
or their constituents that will be
different across that cell membrane.
So inside the cell, we don’t
have as very much sodium.
Maybe in this case, we have 12 millimole
versus extracellular we have 145.
So there’s a differentiation
fluid sodium and
extracellular fluid sodium.
Potassium is the opposite.
It has more of course potassium
inside the cell and outside the cell.
Calcium has more outside the
cell than inside the cell.
Magnesium isn’t as different
across the cell membrane,
but there’s still more in
the extracellular fluid.
In terms of chloride, more
in the extracellular fluid,
phosphate about even,
bicarb, more in the extracellular
fluid than intracellular,
and finally, hydrogen ions
are very similar across
with maybe a bit more in
the extracellular space.
So you think about this, we have
all of these different ions with
different concentrations inside
the cell versus outside the cell.
We have to think about
what that is going to
mean for that cell and
why that might occur.
So there are reasons why you have more
sodium outside the cell than inside
and more potassium inside
the cell than outside,
and this oftentimes will allow
us to develop an electrical
gradient or a chemical gradient
across the cell membrane.
And that’s going to be very, very important
for us to transport various substances.
So we oftentimes use
ions or their electrical
charge as ways in which we
can get other substances
that we really need
inside the cell from the
outside the cell fluid
or extracellular fluid.
In terms of other macronutrients, like
glucose and amino acids and proteins,
in terms of glucose, there’s
almost always more glucose
in the extracellular fluid
With amino acid, it varies a lot between
which ones we are talking about.
In terms of proteins, usually
there are more proteins in
the intracellular fluid than
the extracellular fluid.
So now that we know that there are differences
in the fluid and its constituents,
let’s keep moving on in
talking about cell membranes.
Now, why do we need to have these various
solute differences across the membranes?
Well, one is, is this
allows us to develop
various means of transport
to get things from
one side of the cell membrane all the way
over to the other side of the cell membrane.
The fastest way to do that
is to through a pore.
So pores are the fastest
means in which we can get
across the cell membrane.
Another way to do it is via ion channels
and ion channels are also pretty fast.
The slowest of which though
are via these ATPases.
So ATPases can divide it
into main categories.
The first are pumps and there are
three different types of pumps
and we'll spend the most time
talking about the V-type pump.
And the other type of ATPases
are ABC transporters
and these particular transporters
are very specialized
and we’ll get to a few of
them later on in the course.
The last type of transport
mechanism are solute
carriers and there are a
loads of solute carriers.
In fact, there may be even more than 300
or so different types of solute carriers.
And they can be symporters, meaning that two
things are going in the same direction.
They can be exchangers
in which one thing goes
in one direction and
something goes in the
opposite direction or
they can be uniports and
only go in one direction
with one molecule.
These symporters and antiporters
usually need some sort of gradient
to drive that transport
So again, we have four different
methods of transport:
pores, ion channels, ATPases,
and solute carriers.
All of them have different speeds and
they have various amounts of selectivity.
So what I think we’ll do for the
rest of this particular lecture is
go through some prototypes of these
various types of transporters.
And we’ll go into depth
in one particular one
so that you understand
how that process works.
And then as they come up
throughout the various lectures,
you can apply the same
kind of principle
for pores, ion channels,
ATPases, and solute carriers.