Hi. It's Mr. Andersen and welcome to biology essentials video number 16. This
is on transport across a cell membrane. If you haven't watched the video on cell membranes,
so the parts of the cell membrane like the phospholipids, the glycolipids, the proteins,
cholesterol. If you haven't watched that make sure you do that first because I'm assuming
that you know all the parts of a cell membrane as we talk about transport. So imagine right
here we've got on this side, we've got a, some gas. It's in a container. But it's locked
within that container. So it's sealed within the container. So if I were to open up that
container on one side what's going to happen? Well these molecules are moving around. They're
constantly bumping off of each other. And so when you open up a new space on this side,
they're simply going to move into that. Now how much energy does that require? It requires
no energy at all. They're just randomly moving around. They're going to randomly into that
space. Likewise, if I remove the container on the outside, what's going to happen? They're
going to randomly keep moving. And so that process, that random movement is something
called diffusion. Now if we want to move those molecules in the other direction, we can do
that in a cell. And lots of times we have to do that in the cell. But it's usually going
to require energy to do that. And so when you do that we're going to cash in some ATP
and that's called active transport. And so to summarize what I'm going to talk about
in this podcast, there are two forms of transport. We have passive and then active. And so the
greatest form of passive transport that I'll talk about or the most common is going to
be called diffusion. Diffusion is just that random movement of particles. It's super important
because that's how you get oxygen into your body and that's how you get rid of things
that we don't need like carbon dioxide. A specific type of diffusion is osmosis. And
osmosis is simply diffusion of water across a semipermeable membrane. That has a huge
impact on cells because if they are in a hypertonic, hypo or isotonic environment they're either
going to lose, gain or nothing is going to happen to the water inside them, according
to osmosis. And so that's something that we have to battle. But we can also use to our
advantage. A specific type of passive transport is called facilitated diffusion. It's just
like diffusion but we need to use proteins to actually move the material across. These
things, passive transport require no energy. And so active transport is where we need to
cash in, remember, a little bit of ATP to move things across their gradient or against
their gradient. What that means to move against your gradient is to move where you don't want
to go. And so proteins and ATP are required to do active transport. The most famous type
of active transport is the sodium-potassium pump. I'll talk about that and its importance
in maintaining a gradient on nerve cells. And then on the large scale, a large scale
form of active transport are both endocytosis and then exocytosis. So that's just not moving
a few molecules. It's move big particles. Even organisms across a membrane. And so let's
get started. First type is going to be called diffusion remember. Diffusion requires no
energy. It's just molecules moving around randomly and then filling in a space. And
so in this diagram right up here, I've got two gases. We'll call this gas A and then
gas B, which is going to be a little bit darker. They're separated by a wall. And so these
particles are randomly moving around. If I remove that barrier and then check back on
it a little bit later, we're going to find that each of those molecules have spread up
according to their gradient. In other words the gray is going to move in this direction
and the black is going to move in that direction. And so that would be called moving with their
gradient, or along their gradient. Now it's not just a linear path. You can see right
here that it's going to be a random bounce that whole time. Where's this play out inside
our body? Well these are the alveoli. Alveoli are going to be in the lungs. And so our lungs
are one way. In other words, you breathe in air. It eventually goes all the way down to
the level of the alveoli, which are these small sacks of really thin cells. And then
we're just simply going to have diffusion across that gradient. You have a lot of oxygen
when you breathe in in the alveolia And so that's going to flow right into the capillary
beds. And likewise we have a lot of carbon dioxide in our capillary beds and that's going
to flow back into the alveoli. And so that requires no energy. And so when I go like
that and take a big breath, that oxygen is going into my alveoli, it's going into my
blood supply. And in fact it's moving into the cells of my body according to diffusion.
It requires no energy. Likewise, when I breath out, that carbon dioxide is coming out through
a process of diffusion as well. How much energy does that require? None. Let's go to the next
one. A specific type of diffusion is called osmosis. So osmosis, if we were to define
it is the diffusion of water across a semipermeable membrane. And so this the U tube experiment.
In the U tube experiment what we have are two different concentrations of water. Let's
think of this as sugar water. And this as less sugary water. Now the sugar can't move
across the membrane, but the water can. And so where's the water going to move here? The
water over time is going to move from an area of high water concentration to low water concentration.
And so if you were to watch this U tube experiment, you'd see that on this side the water is mysteriously
rising up. Because you can't see the sugar that's dissolved inside the water. But it's
going to do that until the concentration on either side of that semipermeable membrane
is going to be the same. In other words, the ratio of water to the sugar molecules is exactly
the same. And that's why when you throw salt on a slug, the slug is going to shrivel up.
And the reason why, let me get some water, is that the water is going to be from an area
of high water concentration inside the slug to low water concentration on that salty area
on its surface. Where does that play out as far as humans go? Well this is a red blood
cell. And so red blood cell is surrounded by plasma. And the concentration of the plasma
is the same as the red blood cell. And the reason why is that we're going to have water
flowing in and water flowing out. In other words it's at equilibrium. But we're not going
to have it go radically in one direction or the other. If you were to inject salt water
into our blood, what would happen to it? Well if you think about that, there's going to
be salt water out here. So there's going to be a lower concentration of water outside
of the blood cells and so the water is going to flow out. And that's going to cause the
blood cells to shrivel up. Likewise, if you were to inject distilled water into blood,
what's going to happen? Now we actually have more water outside the blood cell. So the
concentration of water is greater out here. It's going to flow into the blood. And it's
actually going to lyse the cell. It's going to explode the cell. And so if you're surrounded
by a liquid that has a higher solute concentration, we call that hypertonic. If it's lower we
call that hypotonic. And then eventually when we reach equilibrium, we call that isotonic.
But it's the movement of water across a semipermeable membrane. What's the semipermeable membrane?
In this case it's the cell membrane. It's that membrane that surrounds all living things.
An example of diffusion where we still don't require energy but we do require protein is
something called facilitated diffusion. So an example of this could be if we're moving
it looks like sugar molecules right here. So sugar molecules like this, but we're moving
it through a protein or we're moving it through a protein that has a different confirmation.
Confirmation is the shape of the protein. It's still moving along its gradient. In other
words if you look up here, we have a greater concentration of sugar, greater concentration
of this molecule up here. It's still moving along its gradient. In other words, from a
high concentration to a low concentration. But since it's requiring a protein to do that
we call that facilitated diffusion. An example of that, I made a little animation here, we
can use something called the glucose transport. That's a glut, I love that word. So the glucose
transport protein is going to sit right within that phospholipid bylayer. So it's a protein
inside here. Now we've got our glucose out here. And if you think about it, its gradient
is in the top to the bottom. In other words we have more glucose on the top then we do
on the bottom. But it can't move through. Glucose is too large to move through this
membrane. And so as it randomly moves along, there will eventually be a connection. So
we make a connection right here, there's a chemical connection or a bond right here.
That causes a conformational change in the glut, and conformational change in this glucose
transport protein. And so what that means is it's simply going to change its shape.
As it changes its shape, then it's going to force that glucose in this direction. And
so the glucose is still moving around randomly, but since we're using this protein to do that,
then we call that facilitated diffusion. It's still moving along its gradient. And it's
going to be keep moving it along its gradient until it hits another one. And it's going
to move in that direction. Now if you think about it, what if we want to move the glucose
in the opposite direction? What if we want to make the glucose, instead of moving from
high concentration to low, from a low concentration to a high? Where might we see that might be
during in the, for example the lining of your small intestine. We've got a lot of glucose
in the cells inside there, but maybe not a lot of glucose inside the small intestine.
Let's say we want to move it in the other direction. Well then we could tap something
call co-transport. So we could use for example, sodium out here. And as sodium flows in this
direction, we could carry the glucose in the other direction. But right now I'm hinting
act the next form of transport and that's called active transport. Active transport
requires energy. And so the most famous of all active transport proteins is probably
called the sodium-potassium pump. And sodium-potassium pump looks just like this. It's a protein.
But what it's going to do is it's going to move sodium outside of the cell and it's going
to move potassium inside the cell. And if you think about it, we're moving sodium out
here. And if you look at it, there's actually more sodium already out here. And we're moving
potassium in the other direction. There's more potassium here on the inside. And so
to do that we have to use ATP. And so you can see right here that we have adenosine
triphosphate that's attaching a phosphate to the sodium-potassium pump. And as it does
that it causes a change. It's causing a change in the shape which is moving the sodium to
the outside. It's moving the potassium to the inside. And then we have to use more ATP
to do that. And so, it's a constant supply of energy required to maintain that sodium-potassium
pump. But all the nerves inside our body use a sodium-potassium pump. And lots of cells
inside our body use the sodium-potassium pump to keep that correct balance of sodium on
the outside and then potassium on the inside. But that's called active transport. And it
requires 1 ATP to move every 3 sodium ions and every 2 potassium ions to the inside.
A big scale movement across a membrane is called endocytosis and exocytosis. And so
endocytosis means moving cells inside. So when would you do that? Well this right here
is a phagocyte. A phagocyte is going to be a white blood cell that is going to move around
and it's going to eat invading cells. And so if you think of all of these green bubbles
out here as bacteria, we want to destroy the bacteria. And so we have these phagocytes
and what they'll do is they'll actually fold their membrane in. So they'll fold the membrane
in and as they do that they create a sphere. It's called a phagosome. And that phagosome
is going to contain all of these invaders, these pathogens inside it. And so it's not
just a few molecules. We're talking about a lot of material. There's even liquid in
here as well. So that phagosome will move to the inside of the cell. It'll then attach
to a lysosome and make something called a phagolysosome. And what happens there, well,
this digestive enzymes are going to pour into this phagosome. It's going to digest the material
on the inside. We can then make antibodies based on the shape of that after it reaches
the nucleus. But since we're taking in a large amount of material, that's endocytosis. Does
this take energy? Of course. Yeah. We're going to move it against its gradient. So we also
going to move this membrane so that requires ATP to do that. So it's a form of active transport.
And then finally exocytosis is simply moving in the opposite direction. And so a great
example of that you're probably familiar with, this is a nerve signal moving in this direction.
In other words we have an action potential moving in this direction. And so we have to
send that signal across the synapse which is going to be this gap between two different
neurons. To do that we use what are called neurotransmitters. Neurotransmitters are these
molecules that are moving across that synapse to the other side. And then they're going
to start an action potential on the other side. And so that nerve signal can keep moving
in this direction. But to do that we have to release a lot of neurotransmitters. And
so that process is called exocytosis, or the release of large amounts of material. Those
will go across and they'll open up these gated channels on the other side. And since we're
moving a lot of material that's called exocytosis. And so again in summary, if you're not adding
energy it's called passive transport. If you are it's called active transport. But both
of these are ways to move materials across the cell membrane. And I hope that's helpful.