- [Voiceover] We have other videos where we talk
about how small molecules, or ions,
might be able to go through a cell's membrane
in different ways, whether actively or passively,
maybe facilitated in some way.
What we wanna talk about in this video
is how we can do this for larger things.
We're gonna focus on here is bulk,
bulk transport.
Transport.
So this first example,
you could imagine this cell with this mauve
or purple-colored membrane is engulfing
this big green thing which is
maybe a bacteria or something.
And so you see that the membrane...
Let me make it very clear.
This is inside.
This is inside the cell.
This is outside.
Outside the cell.
And you can see the cellular membrane
starts to wrap around this,
I guess we can think of this a bacteria,
then it fully wraps around and then
that membrane that was wrapping around
the bacteria pinches off and now the bacteria
is inside of the cell, and it's wrapped
by this membrane.
And this process where you're engulfing these large things,
we call this Phagocytosis.
So this is phago...
Phagocytosis.
And the prefix I guess you'd say, phago
comes from the Greek for "to eat".
So this is literally about cell eating.
And in many cases, this thing that is now in here,
you could view this as the cell's food,
this compartment that is holding this,
in this case bacteria, is gonna transport it
maybe to a lysosome, so it can be processed
and digested in some way.
We would call this big compartment,
this membrane-bound compartment,
we would call this a food vacuole.
Food.
Food vacuole.
Now this scenario down here is similar but different.
Over here I have the cell
which is I see part of its membrane
and it's in magenta right over here.
We can see The Phospholipid Bilayer .
That's why I drew two lines for the membrane.
And instead of engulfing a large particle or bacteria,
it's just engulfing some fluid.
It's engulfing some fluid, so you see it's starting
to wrap around this section of fluid,
wraps even more around this section of fluid,
and then the membrane that was around it
completely pinches off and goes into the cell.
And I'm drawing all of these things in two dimension,
but this would actually be happening in three dimension.
So this wouldn't just be a circle,
this would right over here would be a sphere.
And this thing that has been pinched off
and is now inside the cell, we call this a vesicle,
which is just a general term for these membrane-bound
compartments inside of cells.
And this process where the cell has essentially drunk
a bunch of fluid and the stuff
that happens to be in the fluid,
we call this Pinocytosis.
Pinocytosis.
And pino comes from the Greek word "to drink".
And I'm always fascinated by word roots.
And I'm not a linguistic expert here,
but it's neat because even in languages
I'm familiar with like Hindi and Urdu,
the word pina means "to drink",
so maybe it's even related to the word pani
which is in those words in those languages.
I know all of these have a shared linguistic root,
so it's always fascinating to see
these linguistic connections.
So this is Pinocytosis where the cell is drinking
so to speak, but it's also getting the other stuff
that's in that fluid.
This is Phagocytosis, the cell is eating.
And these are both special cases
of I guess the more general term
of engulfing in this way
which is called Endocytosis.
Endocytosis.
So Phagocytosis is a form of Endocytosis,
and Pinocytosis is a form of Endocytosis.
Now the next question you might say is:
OK I can get that this happens,
this can be observed under a microscope,
but how does it happen?
How does the cell wrap around and pinch around?
And like I say in a lot of videos,
people think that we understand some of it,
but this is not fully fully understood.
There is views that well the cytoskeleton
must be involved in some way.
It has to create space here for this thing
to be able to pinch off and move in that direction.
It maybe will help actually the cell's membrane wrap around
in some way, but these are all areas of active research.
How does this Endocytosis actually occur?
How does the cell know what to consume
I guess you could say, and then how does the membrane
actually behave in this way to do it,
to do it well?