Welcome once more to our series of
physiology concepts. We just finished
talking about passive membrane transport.
Now we are going to look at the active
membrane transport. So this is a reminder
of what happens in active membrane
transport. Energy is required. The cell
has to use some of its ATP. Why is this?
Because we are trying to move substances
against their concentration gradient,
which means I'm moving substances from
lower to higher concentration which is
the opposite of the norm. This is why the
cell has to use ATP. We are going to look
at the different mechanisms, the ionic
pumps first and then we are going to
look at the vesicular transport. We're
going to look at the mechanism of
exocytosis, endocytosis with
it's two parts of phagocytosis, and pinot
cytosis. So this is the ionic pump. This
is the biggest and a very important
example. Here we are looking at the
sodium potassium pump. So where is this
pump located? It's actually quite located
on the plasma membrane. As you can
see, my lipid bilayer here. Before talking
about this pump I need to remind you
that in the extracellular fluid
sodium is always in higher concentration.
So I have a higher concentration of
sodium over here. In the intracellular
fluid, I always have a higher
concentration of potassium. So let's see
what the pump is doing. The pump is going
to transport three sodium ions from
inside to outside which means I am
moving them against the concentration
gradient from an area of lower
concentration to higher concentration.
In order to do this the pump has to use
ATP. So the three sodium molecules are
going to move into the pump and
eventually be ejected outside to the
extracellular fluid. At the same time the
pump is going to take two potassium ions
and bring them inside the cell. And this
is why as I keep reminding you we have
to use ATP. Now this pump is very active
in certain areas of the body where
excitation occurs along nerves and
muscle cells. When a signal comes to a
nerve or a muscle cell, sodium will go in
the cell. Potassium will leave the cell
in a certain sequence and we have to
bring the normal concentration back
again.
Who does this? Our sodium potassium pump.
So as a reminder, the sodium potassium
pump will take three sodium ions and
move them outside the cell, bring two
potassium ions inside the cell. This is
to make sure that sodium concentration
is always going to be higher in the
extracellular fluid and potassium is
going to be higher in the intracellular
fluid. The second mechanism is the
vesicular transport. So we are moving
substances in vesicles, in small
containers. Large particles move by
vesicular transport. And we have two
different types. In exocytosis we
removing particles from inside the cells
out. In endocytosis we are bringing
substances from outside the cell in. And
we are going to talk about phagocytosis
which is cell eating and pinocytosis
which is cells drinking.
This is how exocytosis occurs. So if this
cell is secreting something to the
outside, commonly enzymes, commonly
different proteins have to go outside
the cell. How does the cell move those
substances? Hormones as well is one
important example. It's going to move those
substances in a vesicle and we see that
the vesicle will eventually get very
close to the plasma membrane linked to
it. It opens and now exocytosis occurs. If
the cell is trying to take something in--
it's a large particle -- again, part of the
plasma membrane builds a vesicle and
this vesicle is transported in. The two
entities of endocytosis. Phagocytosis is
cell eating and this is a very important
concept by which our immune cells
destroy pathogens. So if this is one of
our body cells and this is a pathogen
like a small microbe, there's going to be
extension from the plasma membrane that
will make of make sort of pseudopod
surrounding the pathogen. It's going to
pull it in and it made a vesicle, as you
can see here. So this is my vesicular
transport. This vesicle is now going to
be attacked by the lysosome. It's going to
break down and then the remains, the
residual material is going to be removed
or ejected out of the cell by exocytosis.
So you can see endocytosis
occurring on this side. Exocytosis
occurring on the other side. And again,
this is a concept that is used by our
immune cells every and each hour of the
day.
Pinocytosis is cell drinking. So the
cells, most of the body cells will take
small droplets of water from the
exacellular fluid with some of the dissolved
substances and bring them in the cell
in a vesicle. Why do they do this? It's
the cell wanting to know what exactly
surrounds it. So the cell is aware of the
surroundings and this occurs in most of
our human body cells. And this concludes
our discussion on active membrane
transport and thank you for joining us.