Hello and welcome again to one more of
our series on physiology concepts. Today
we are going to talk about the
transportation across the plasma
membrane. And I want to remind you that
the cell is surrounded by a membrane
that is made up of two layers of
phospholipids. And we're going to see how
substances are moving in and out across
this plasma membrane. So there are two
ways by which things move in and out of
the cell. It's either by a passive process
or an active process. What are the two
major differences over here that we have
to understand? First, in the passive
process no energy is required. There is
no ATP that's going to be used up by the
cells. In active transport we need to use
ATP. In the passive, we are moving
substances from one side of the cell
that has the greater concentration to
the lower concentration. So we are moving
substances down a concentration gradient.
Over here we are doing the exact
opposite. We are moving substances from
an area of lower concentration to higher
concentration which is very difficult to
do. This is why we will need ATP. These
are the mechanisms of passive
transport filtration,
diffusion, two types of diffusion and
osmosis, and these are the mechanism of
active transport which are going to be
discussed in another presentation. So for
today we are discussing passive
transport mechanism. This is the first
one.
Filtration. What happens in filtration is
water in a certain area is going to
carry some of the dissolve solute and
move them out, force them
out. We can see this happens very often
in the kidney where we are trying to
take substances out of the blood and
filter them out to the kidney. These are
waste products and we want to get rid of
them. Now what is the force that is going
to push the water and the dissolved
substances out? This is hydrostatic
pressure. Very important. So hydrostatic
pressure is the force that is exerted on
this membrane and it is due to the
volume of water that is in this area
here. This is a small capillary blood
vessel. So the water present here creates
a force which we call the hydrostatic
pressure and it will push the fluid out.
Remember that the plasma membrane is
very thin and it will always keep larger
particles in. So, as from the previous
slide, red blood cells and bigger
particles are never going to be moved
out. So where does this come from? Really,
the volume of fluid here or the blood
here
is represented by the blood pressure. So
the greater the blood pressure, the more
filtration will occur. The greater the
hydrostatic pressure, and visa versa.
So remember, this is the hydrostatic
pressure, the pressure created by a
volume of water. The greater the
hydrostatic pressure, the greater the
filtration. So if a person has low blood
pressure, if they are bleeding and they
lost some blood, the filtration across
the kidney is going to be far less than
the normal. And as soon as the paramedics
come to this person they have to
administer fluids in order to increase
the blood pressure, increase the blood
volume, bring the heart
hydrostatic pressure back to normal so
this person person can filter waste out
from the blood to the kidneys. The second
mechanism is diffusion and we have two
types of diffusion, the simple and the
facilitated. In the simple diffusion,
simple means it's unassisted. I don't
need any help. So if you look here, this
is the cell membrane or the plasma
membrane. It's a bilayer of lipids. And
outside the cell I have a higher
concentration of solute. So they will
simply diffuse from an area of higher
concentration to an area of lower
concentration inside. Now, what substances
can move through this process? Either
they have to be very small to diffuse
through, or they have to be lipid soluble because remember,
lipids can pass through lipids, but if
they're not lipid soluble they won't be
able to pass. Famous examples that pass
into and out of the cell by simple
diffusion are oxygen and carbon dioxide.
So, oxygen in the blood outside the cell
can move in to give the cell the
nutrition. Carbon dioxide that
accumulates inside the cell can move out
again so that it can be removed as waste.
Now do all substances move at the same
rate? No. So there is a - there are certain
factors that will affect the rate of
diffusion whether they diffuse faster or
slower. Increasing the temperature will
increase diffusion. Larger molecules will
be very slow to diffuse. If we increase
the concentration there's more diffusion.
If I have a surface area that has a
bigger membrane, definitely substances
can diffuse faster. And the more
permeable my substance,
the more my rate of diffusion. In
facilitated diffusion, I need a helper.
I need a facilitator. And these helpers
or facilitators are actually plasma
proteins that are in the plasma membrane
itself.
We call them carrier proteins and so
we're going to look at carrier-mediated
transport. Some of those proteins are
very specific for specific substances.
And we use this in large molecules like
moving amino acids in and out of the
cells, and moving glucose in and out of
the cells. We're going to look at glucose
because this is -- I'm sorry -- this is first
how facilitated diffusion occurs. This is
my carrier protein. Higher concentration
of the substance. The solute is going to
move through my carrier. It's going to
attach to a specific site which we call
the binding site or the active site. The
protein is going to change its shape and
sort of squeeze my solute in and release
it on the inner side of the cell. So this
is how facilitated diffusion works. We're
looking at a specific carrier here or
transporter, and this is very important
type of transportation that occurs
across our cells. The glucose transporter,
glute or glucose transporter, is a
specific protein that is in the plasma
membrane of the cells, and it will
specifically transport glucose from
outside, from the blood into the cell. So
the cells can use it to make ATP or the
cells can store it if they don't need it
immediately. Now one thing to remember is
there is a certain
number of those glucose transporters. And
if we have too much glucose, more than
the number of transporters, the cell is
not going to be able to transport any
more glucose molecules and it's going to
be saturated and the rate of diffusion
of glucose inside is going to decrease.
So, we have a concept here that we call
transport maximum. It's because I have a
limited number of transporters. I'm going
to transport a maximum number of glucose
molecules. The rest won't be able to come
in. They will move from the blood into
the urine and be excreted outside the
body. Types of membrane carriers or
transporters. A uniport is a protein
that will allow one solute only to move
in one direction, either from inside out
or from outside in. A symport has the
ability of transporting two substances
at the same time in the same direction.
So this is simultaneous. I can move
those two substances together in the
same direction. An antiport has the
ability of moving two substances in two
different directions. So this solute is
leaving at the same time this solute is
coming in.
The third mechanism of passive transport
is osmosis. And in osmosis we are simply
talking about diffusion of water. So
again water will always diffuse from an
area of higher concentration of water to
an area of lower concentration of water.
So if we look over here. If this is my
membrane, my semi permeable membrane, I
have a higher concentration of water and
the lowers of water will move from this
side to that side. So we see that the
water level is going to increase. What
will keep water from going back? Water
here is attracted by those solutes. So
those solutes exert a force and this
force will keep pulling water into that
region. So we are going to concentrate on
this concept and this is called osmotic
pressure. So osmotic pressure is the
pressure that is exerted by the solute
to pull the water towards them, and this
is very important.
Now this osmotic pressure depends on the
osmolarity, the concentration of solutes
over here. So if I increase the solute, if
I come and add some sodium to this
compartment, I am going to increase the
osmotic pressure. I'm going to increase
the pull of water. I'm going to get some
more water and this level of water is
going to increase.
Aquaporins. This is to remind you that
movement of water is not simply through
osmosis. It can also occur through
specific plasma membrane channels. These
are proteins that we call aquaporins and
they will again allow water to move from
an area of higher concentration to lower
concentration. We need to look at these
two pressures because we've already
mentioned these and see how they work
hand-in-hand.
Remember, hydrostatic pressure belongs to
the volume of water over here and it
would want to push the water out. Osmotic
pressure depends on the concentration of
solutes inside here and it will want to
pull the water in. So what is the result?
Is water going to move out or is
going to be pulled in? It depends on who
is stronger. The greater of these is
going to be the one that is going to
either pull the water or push it out.
Since we are talking about osmosis we're
going to talk about osmolarity. So what
is osmolarity? It is actually the
concentration of solutes in a solution.
And from this concept we need to talk
about tonicity.
What is tonicity? It's the ability of a
solution to change the shape or the tone --
this is where this word comes from --
of a cell by changing the volume of
water inside this cell. So we're going to
look at three types of solutions, the
isotonic, hypotonic and hypertonic. Cells
can be surrounded by one of these and
how is water going to move in or out of
the cells? It depends on this very
important rule to remember. Water will
always follow the salt. So let's look at
an isotonic solution. Isotonic solution
means this solution surrounding the
cells has exactly the same concentration
of solutes as inside the cell so there
is no net movement of water. No water
moving in or out of the cell. We call
this solution isotonic. And common
solutions that we give to patients are
normal saline point nine percent when I
don't want to change the shape of their
cells this is what we administer. In
hypotonic solution the solution
surrounding the cells here is hypotonic,
means the number of solute is less. I
have more solutes inside here so water
will follow the salt. So water is going
to be pulled into those cells. They might
enlarge and swell and sometimes they
might burst and this is an adverse
effect. They might lyse and die so we
have to be very careful ever with giving
hypotonic solutions. The last one and the
last of our presentation here is
hypertonic solutions. If these
cells are surrounded by a hypertonic
solution where solutes are at higher
concentration over here, water will
follow the salt. Water will be sucked out
of those cells. These cells might shrivel
and crenate and again die. So this
completes our presentation on the
passive membrane transport and thank you.
for joining us.