- [Voiceover] All living things are made of cells.
And humans and all multicellular organisms
are made of a type of cell
called eukaryotic cells.
But what is it that makes a cell eukaryotic?
To answer that question, let's look at
the two major types of cells.
On the left is a prokaryotic cell,
and on the right is a diagram of a eukaryotic cell.
So let's define eukaryotic cells as how
they're different than prokaryotic cells.
So usually, eukaryotic cells are much larger
than prokaryotes, and they're also,
eukaryotic cells are also found
in multicellular organisms,
although there are single cellular eukaryotes
such as certain protists.
But what are the defining characteristics
that make a cell eukaryotic?
The most important thing that eukaryotic cells
possess that prokaryotes don't
is the idea of compartmentalization.
This is kind of a big word,
but essentially, what it means is
that the cell is divided into different compartments.
And we can see some of these compartments
drawn here on the right in our eukaryotic cell,
whereas our prokaryotic cell doesn't appear
to have many different compartments.
But why is it that a eukaryotic cell
has lots of different compartments within it?
Well, I kind of think of it as rooms in a house.
So if you have lots of different rooms in a house,
each room can do something different.
So you could have a living room, and a kitchen,
and a dining room, and a bathroom, and a bedroom.
And they all have a different function.
Prokaryotic cells then would be like
a one-room house where everything
is in a single room.
And you could see that this single room
has to be able to do everything.
So the compartmentalization of the eukaryotic cell
into different compartments or kind of rooms
allows each different compartment to have
a different function.
And this is really important in the complex
needs of the cells.
The name for these compartments
in the eukaryotic cell is organelles.
Just like your body has different organs
that have different functions,
your heart pumps blood,
your lungs take in oxygen,
different organelles in the cell
have different functions.
And these organelles are separated
from other parts of the cell by membranes.
And the name "eukaryotic" comes
from one of these organelles.
The word "eu" means "well" in Greek,
and "karyote" means "kernel" or "nut."
So when scientists originally looked
at eukaryotic cells, they saw a very well-formed
circular object in the center that looked
kind of like a nut.
And so, they called these cells "eukaryotes"
because they had a well-formed nut
in the center of the cell.
This nut is what we now know as the nucleus.
The nucleus is a circular membrane-bound
organelle that contains all of the genetic material
in the cell.
So all of your chromosomes are inside the nucleus.
This separates them from the outside of the cell.
The nucleus is the defining organelle
of eukaryotic cells.
In contrast, prokaryotic cells have
their genetic material as well as everything else
in the cell just kind of floating around
all in one big space.
And so, in prokaryotes, to divide and become
new cells, all they have to do is make two copies
of everything, and then split down the middle
in a process called binary fission.
But in eukaryotes, we can see that
because the cell has all of these different
compartments or organelles, there is a more
complex way to divide into a new cell.
And the process that eukaryotic cells use
to divide is called mitosis.
So these are the defining characteristics
of eukaryotic cells that sets them apart
from prokaryotic cells.
They're compartmentalized into different
membrane-bound organelles,
the most important of which is called the nucleus,
which is responsible for the name "eukaryotic."
And finally, they divide by the process of mitosis.
Let's go through some of the other important
organelles within eukaryotic cells.
So first, let's go back to the nucleus.
And the nucleus, I kind of think of as
the control center of the cell
because that's where all of the DNA is,
and where DNA is transcribed into mRNA.
Probably the next most important organelle
in the body are called mitochondria.
Mitochondria, you can think of
as the cell's power plant
because mitochondria is the site of cellular
respiration where glucose is converted into ATP
to provide the energy for everywhere else in the cell.
The next organelle we'll look at is called
the endoplasmic reticulum.
And so surrounding the nucleus are
dozens and dozens of folds of membranes
in this really complex pattern.
But the unique thing about it,
inside these folded membranes
is it's all continuous, so it all encloses one
single compartment, even though the membrane
is folded back and forth on itself
lots and lots of times.
And the endoplasmic reticulum is primarily
the site of protein synthesis.
mRNA, which is transcribed from DNA
in the nucleus, is translated
at the endoplasmic reticulum.
So I think of the endoplasmic reticulum
as the factory of the cell
where proteins are produced.
After these proteins are produced,
they are sent to another organelle that also has
kind of a lot of folded membrane structure,
but it's not a continuous compartment
like the endoplasmic reticulum.
And this organelle is called the Golgi apparatus.
And the Golgi apparatus is kind of like
the mail room of the cell.
So proteins that are produced
in the endoplasmic reticulum
are sent to the Golgi apparatus,
and the Golgi apparatus is able to send
those proteins to other parts of the cell,
depending on where that protein needs to go.
So if the protein needs to be secreted by the cell,
the Golgi apparatus sends it to the outer membrane
of the cell and releases it.
If it needs to go to a different organelle,
the Golgi apparatus can send those proteins,
as well as other biological macromolecules,
to those sites as well.
The last two organelles to mention
are the lysosome and the peroxisome.
And cells can have multiple ones of each of these,
but I mentioned them together because they both
have a similar structure.
They're kind of a spherical organelle
enclosed by a single plasma membrane.
And the environment inside the lysosomes
and the peroxisomes is very different
from the environment outside in the cell's cytosol.
And so the lysosomes and peroxisomes
are kind of like recycling centers in the cell.
That's because when cellular components
are no longer functional and need to be
broken down, they're trafficked to the lysosome
where they are broken down into their more
basic components that can be reused to rebuild
new proteins or other macromolecules.
The peroxisome has a similar function
to the lysosome, but it's a little bit different
because in the peroxisome is the site where
reactive oxygen species like peroxides
are reduced into nontoxic forms.
So these are the main organelles
of the eukaryotic cell, which we remember
is defined by the compartmentalization of the cell
into different membrane-bound organelles.