Parts of a cell


Let’s talk a little bit about
the structure of the cell. I’ve done a lot of videos where
we deal with things that go on inside of them, but I
haven’t done one where we just talk about the entire
structure of them. So a good place to start is–
let me just draw the membrane. And the cellular membrane is a
good place to start because this is what separates the cell
from the outside world, and to a large degree, it kind
of defines the cell. It defines it as this very,
very small compartment. That’s where the word,
“cell” came from. So let me label that. Cellular membrane. And all cells have a
cellular membrane. Now if we think about maybe the
most important thing that defines a cell, you’ve probably
seen in the DNA videos and we’re going to talk
about translation and transcription and all of that,
that what defines what a living organism is,
is its DNA. So all cells have DNA
inside of it. And I won’t go into the details
of how DNA defines what an organism is. I’ve done that in some detail
on the DNA videos. But all cells have DNA. This is more of an anatomy of a
cell video than necessarily the function, but we’ll go into
the function because we need to know what these
different parts do. So this is, right here,
this is the DNA. And it’s in its chromatin
form here. There’s also little
proteins here. Not in all organisms, but
we’re going to stick to eukaryotes, and I’ll talk
a little bit about the difference between eukaryotes
and prokaryotes in a second. But we have DNA. As I’ve drawn this cell right
now, this is pretty much any cell, and any animal or plant
or whatever kingdom could look like this. I haven’t drawn a lot
of the details. I’ve just drawn the DNA and
the cellular membrane. Now here’s kind of the first
major division in the living world, or at least from our
point of view, or it seemed obvious, is that some
cells have a membrane around the DNA. So they’ll have a membrane
around the DNA that separates the DNA and the chromatin and
everything that makes up the stuff within the DNA, separates
that from the rest of the cell, and this
is called a nucleus. This is called a nucleus. And I said that’s a major
division because when some people looked at some cells
and they saw a nucleus and other cells and they didn’t see
a nucleus, they said, hey, this is a good way to classify
organisms. So they called the things that had nucleuses,
eukaryotes. These have a nucleus. So as I’ve drawn this cell right
here, it is a eukaryote. Now, if you do not have a
nucleus, you are dealing with a prokaryote. No nucleus. And examples of prokaryotes,
the two big groups of them, are bacteria and Archaea. Now, Archaea are really
interesting. We know very little
about them. They were originally thought to
be types of bacteria, but now people are realizing that
they’re this whole completely other group, and we’ve actually
observed a very small subset of them, so it’s a
very fascinating group. And it actually turns out that,
evolutionarily speaking, you shouldn’t make this division
first. It actually makes more sense to divide
things into eukaryotes, I’ll just write that Euk, bacteria
and Archaea. You don’t want to do this
division first. There are actually three separate
groups that you want to start off with. We’ll talk more about this
in future videos. But if you want to say
who has a nucleus? Well, eukaryotes have a
nucleus by definition. Who does not have a nucleus? Well, the bacteria and the
Archaea do not have nuclei, just like that. But I’m going to focus on
eukaryotes because they tend to be a little bit
more complex. They tend to be larger. And most of what we talk about,
at least in the videos so far, are dealing
with eukaryotes. Eukaryotes include plants,
animals– we’re animals, at least I am– animals and fungi,
and there are other groups within eukaryotes, but
these are the ones that we normally deal with in
our everyday world. But let’s go back to looking
at the anatomy of the cell. So we have our DNA. We know that it gets transcribed
into mRNA, That mRNA leaves the nucleus, and
it gets translated into proteins at the ribosomes. So the ribosomes are these
little complexes that could be floating all over the cell, and
we’ll see in a second that they can also be attached
to these other membrane structures. So this is a ribosome. And if all this talk of DNA
transcription into mRNA and mRNA leaving the nucleus and
traveling to the ribosome to be translated into proteins
makes no sense to you, there are several videos where I
go into that in detail. But what I want to do is just
focus on all of the different parts to kind of give a
big picture of things. So ribosomes are where mRNA that
gets transcribed inside the nucleus from DNA, where it
gets translated into proteins. So you can kind of view them as
the place where information actually turns into proteins,
which then can be used anywhere else in the cell. And these ribosomes, they’re
made up of proteins, and actually they’re made up RNA. And so one question is, well,
where are the pieces of the ribosomes made? Well, some are made by proteins,
which might be made in other ribosomes. But some of it, the mRNA,
ribosomes, you can kind of view them as this big
mess, if you were to look at it in detail. There’s some protein there. And I’m not drawing it that
realistically, but then you have some mRNA tied in with
the protein, and the mRNA isn’t used for actual
information purposes like it normally is when it goes from
DNA to the ribosome. Within the ribosome itself,
ribosomal RNA is actually used as part of the structure. It actually helps the ribosome
function as a ribosome. So it’s actually part
of the ribosome. And all of that gets built in
a part of the nucleus called the nucleolus or the nucleole. Let me write that down. So this right here, this is
interesting right here. This is the nucleolus
or the nucleole. And it isn’t a separate
organelle, and it’s not separated by a membrane, but it
shows up in a microscope. So when people first saw it,
they said, hey, there’s like a bundle there. That must be like the core of
the nucleus or something. But it turns out what it is,
it’s the densely packed– you’ve got DNA and RNA, and
that’s actually where the ribosomal RNA, the stuff that
makes up the ribosomes, is actually produced. But it’s so dense that it shows
up on a microscope, and that’s why people decided to
name it something different. But it is not membrane bound. It’s not an organelle
within an organelle. It’s just densely packed
proteins and ribosomal RNA, and it’s where ribosomal
RNA gets produced. So anyway, we’re at
the ribosome. That’s where proteins
get produced. But if the ribosomes are just
floating around, if they’re free ribosomes, then those
proteins will just– once they’re produced at the
ribosome, they’ll just float around here in the fluid
inside the cell that we call cytosol. But what if we wanted to produce
proteins that are supposed to end up, maybe in the
membrane of the cell, or maybe outside of the
cell itself? Cells produce things that are
used by other cells, that are used by the rest of the body. And here we have to go to
proteins that are attached to this membrane. You can kind of view it
as a bunch of tunnels. Let me see how well
I can draw that. I’m going to draw
it very roughly. You have this thing called the
endoplasmic reticulum. Endoplasmic reticulum. You can just view it as a bunch
of tunnels like that. Endoplasmic reticulum. And they eventually lead to
something called Golgi bodies, named after Mr. Golgi himself. So I’ll do the endoplasmic
reticulum in yellow and I’ll do the Golgi bodies in green
maybe, just like that. I’ll tell you in a second
what they are. So what’s going on? This is kind of a big kind of
stack, or you can kind of view it as a bunch of folded
membranes together. And some ribosomes are actually
attached to this part, which I call the
endoplasmic reticulum. So we have ribosomes attached. Some of them are free, some
of them are attached. And let me write
my labels down. So this right here, and we use
the space over here, this big flap of convoluted
membrane, that’s the endoplasmic reticulum. It’s fun to say. Endoplasmic reticulum. Maybe a good name for a band. Endoplasmic reticulum, and the
parts that have ribosomes attached to it are called the
rough endoplasmic reticulum. Maybe even a better
name for a band. So right here, where I have
attached ribosomes, so these ribosomes are attached right
here, this is the rough endoplasmic reticulum, or
maybe the rough ER. The rough ER, ER standing for
endoplasmic reticulum. And then where we don’t have any
ribosomes attached, that’s the smooth endoplasmic
reticulum. So right here, that is this
smooth endoplasmic reticulum. And I’m going to tell you in a
second what this is, but we can keep following
the membranes. Eventually we get the
Golgi bodies. And what happens is,
I gave you a hint. In our free ribosomes, mRNA gets
there, gets translated into proteins, and the proteins
then just float around in the cytosol. But what if we want proteins
that should end up in membranes or that end up
outside of the cell? And that’s where the endoplasmic
reticulum and the Golgi bodies come into play. Because now what we can do is,
we have mRNA coming outside of the nucleus, and it can attach
to the ribosomes or it can be translated by the ribosomes
in the rough ER. And what happens here is your
mRNA comes here and it’s getting– and I drew that
arrow very small– it’s getting translated on the
outside of the endoplasmic reticulum, but as the protein
gets produced, it’s getting pushed into the inside of the
endoplasmic reticulum. And when I say the inside of the
endoplasmic reticulum, I’m talking about this area. I’m coloring it in. This is the inside of the
endoplasmic reticulum. And so the proteins will
get pushed into the endoplasmic reticulum. The ones that are to be used
outside of the cytosol, outside of the cell or maybe
in the cell’s membrane. So proteins will end up here. That’s why those ribosomes are
on the membrane, because they can translate things that are
outside of the endoplasmic reticulum, but as the proteins
get produced, the amino acid chain ends up inside of it. Let me make a blow-up
of that because I think that’ll be useful. So let me draw– let’s say this
is the membrane of the endoplasmic reticulum
and then you have ribosomes attached to it. Let’s say that’s a ribosome on
the endoplasmic reticulum. And is this is going to be the
rough endoplasmic reticulum. And what you can have is mRNA
coming into one side of it. mRNA can kind of come
in through here. Maybe it’s going in
that direction. It’s getting translated
into the proteins. But then the protein, as the
amino acid chain is built up, will pop out on this end
of the membrane. Remember, this is our membrane
of our endoplasmic reticulum. So even though the mRNA is on
the outside, because the ribosome is attached to
it, the protein can show up on the inside. So once the protein is built,
maybe then it folds all up, you know, a protein is just a
folded chain of amino acids, it can travel through the
endoplasmic reticulum. And it travels through it. It travels through the smooth
endoplasmic reticulum all the way until it gets to
the Golgi bodies. And all sorts of other
things happen. I’m oversimplifying things, but
I just want to give you a sense of what everything
in the cell does. And once the protein travels to
the Golgi bodies and they get ready for traveling outside
of the cell, or maybe traveling to the cell’s
membrane, they’ll actually bud out of the Golgi bodies. So let’s say this same protein,
when it gets to the Golgi bodies– remember it’s
inside of the Golgi body, so let me draw the membrane of the
Golgi body right there. The protein might
end up there. It’s just a big chain
of amino acids. And then it will bud out. So let’s say it looks like that,
and then maybe the next step it will look
something like. It will look like that. And the very next step, you
could imagine, will look something like this, where
it’s actually completely budded out. It’s popped out a little
of the membrane of the Golgi body with it. So now the protein
is surrounded by its own little membrane. So let’s think about
what happened. We had DNA transcribed
into mRNA. mRNA goes to a ribosome
that’s attached to the endoplasmic reticulum. It is translated into a protein
that travels through the endoplasmic reticulum. First, the rough, where
all the ribosomes are, then this smooth. The smooth has other
functions. It also helps produce hormones
and other fatty compounds, but I won’t go into detail there. But it just travels. It connects to the
Golgi bodies. Then the Golgi bodies, the
proteins can bud off and take a little bit of the
membrane with it. And this idea of something being
surrounded by a membrane and just traveling in the cell,
so maybe the protein now looks like this. I’m enlarging it. Maybe the protein is there and
then it took a little bit of the Golgi body membrane
with it. This is called a vesicle. And this right here, let’s add
another one right here. I’m just doing that
to label it. That’s called a vesicle. And a vesicle is really just
a very general term for anything, little small things,
mostly proteins, in the cell that are just floating around,
that are surrounded by their own little mini-membranes. And the reason why this
mini-membrane is useful is now this protein can now
float to the outer membrane of the cell. It can also float to other
parts of the cell. I’m doing a simplification. And then it can merge either
with the membrane of the cell, or it can use this membrane,
this little vesicle membrane it has, to facilitate it getting
out of the cell. You can imagine that it
eventually, you know this thing– let’s say this is the
outer membrane of the cell. And I’m doing a gross
oversimplification. I’m not even drawing
the bilipid layer. But just to have a visual
impression of what it might look like, that’s the vesicle
there, the little protein inside of it, and it gets
closer and closer to the membrane, and then it can
merge with the membrane because it’s made up
of the same stuff. It merges with the membrane,
your protein inside. I switched colors arbitrarily. But now, all of a sudden, once
it merges with the membrane, then the protein can exit the
actual cell, or maybe it can actually embed itself within
this membrane, within the outer cellular membrane, which
I drew very thin, but it has two layers to it. And we’ll talk more
about that. And I could probably make
a whole video on it. So those are– we’ve already
made pretty good progress on showing the anatomy
of the cell. There are a few other things
we can throw in there. There are things called
lysosomes, which exist in animal cells that contain
enzymes in them that help dissolve other things. So if a lysosome attaches to
something else and is able to spew its enzymes into it,
it usually kills it. It usually digests it. So that’s what a
lysosome does. In plants, you have things
called lytic vacuoles, and those are really the same thing
as a lysosome in terms of their function in that
they’re really big vesicles. In fact, in general, a vacuole
is just a big vesicle. It’s just a general term for a
big membrane-bound organelle. Vacuole. And once again, what’s
an organelle? Let me write down that word. Organelle. It’s just a membrane-bound
subunit of a cell. Just like my liver is a subunit
of Sal and it’s an organ, an organelle is
a subunit of a cell. So a vacuole is just a general
term for a membrane-bound organelle that stores stuff
inside of our cells. So a lytic vacuole would be a
vacuole in a plant cell that stores a bunch of enzymes, and
if it were attached to something else it would dissolve
it if it were able to spew its enzymes into
the other thing. Now, there’s several organelles
that we’ve talked about within the context
of respiration and photosynthesis, and I go into
detail in those videos. But we have things called
mitochondria. Mitochondrion cells. And they have inner and outer
membranes, and this is where we produce our energy, where
sugars are turned into ATP. I’ve done detailed
videos on this. These actually contain their own
DNA, and they can actually reproduce on their own, which
makes people believe that they exist– that their ancestors
once existed as independent prokaryotic organisms, that at
some point they just kind of stumbled upon the notion of,
hey, why don’t I live inside of other organisms and kind
of live in symbiosis? So mitochondria, these are
organelles that at one time their ancestors might have been
independent prokaryotes. Mitochondria. That’s where cellular
respiration takes part, and we go into detail on that. And then on the plant cells,
this is only in– well, definitely not in animal cells–
you have chloroplasts, which are a subset of things
called plastids, but chloroplasts are the
most famous. Maybe I should do that in
especially green, green. So we have chloroplasts. And we know they have little
thylakoids in there. This is where photosynthesis
takes place. You have your grana
and all of that. And I do go into detail in the
photosynthesis videos, but it’s good to know. These are other organelles. And just like mitochondria, they
have their own DNA and their own ribosomes. And so the belief is that they
were once independent prokaryotes that learned to live
in symbiosis inside of larger eukaryotic cells. So we’re almost done, really,
with the structure cells. There’s other things we
can throw on here. If we’re dealing with plant
cells or non-animal cells, we’ll have something called a
cell wall, which will give some strength to the
outer membrane. You can kind of view
it that way, or it gives it some rigidity. So you have things called cell
walls, although they’re not necessarily completely rigid. You can almost view them as
balloons that provide just a little bit more rigidity. Things like wood actually have
double cell walls for really hard-core rigidity. So this is a cell wall. This is in non-animals. And in plants it’s made out of
cellulose, not cellulite. That used to confuse me. So this gives extra rigidity
or form to the cellular membrane. And then to actually give the
cell its actual structure, you have these things called
microfilaments, or sometimes actin filaments, and these are
these little pipes that go throughout the cell. These actually help give the
cell its actual structure in three dimensions, and actually,
they can participate in things moving around within
the cell or even the cell moving around itself. And just to be complete and
make sure we’re covering everything, if you watch the
mitosis and meiosis videos, you have these things
called centrioles. I go into detail there. Centrioles that are right
outside of the nucleus. Two centrioles that are at right
angles to each other make up a centrosome, and they
kind of coordinate the microtubules when we start
splitting the cells in mitosis and meiosis. I won’t go into detail there. I’ve done many videos
on that as well. But so far, this is pretty much
everything you need to know– or at least a first
overview– of the structure of the cell. And in one video, we finally
got it all in one place. This is pretty much– I haven’t
delved into detail on everything– on all of the
major pieces of the cell. So hopefully, you have a better
big-picture view of how things are organized
inside of it.

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