- [Professor] Cell biology,
this lecture looks at
what is a cell made of
and how does the cell maintain itself?
How does it function on
an independent level,
because as we know, all living things
are made of cells and all cells
have the ability to self replicate
from one generation to the next.
So it's important that we look at
all the ins and outs of them.
So that brings us to cell theory.
The theory is that all living
things are made of cells.
That is our definition of life,
that from the simplest
of cells, like bacteria
and certain protists, even fungi,
to the more complex organisms
of animal and plant life,
everything is made of cells,
and cells are the smallest unit
that can function independently.
We also know that all cells
come from pre-existing cells,
and so, the first thing
I want to briefly discuss
is more of a side note, but
it's to help you understand
why we came up with cell theory
and why we still are in the
dark about a lot of things.
It's because, remember the first step
of the scientific method is what?
(students chattering)
Observation, and until we
have the capacity to observe
the very very small, we really
don't know what's going on.
Now, we do have very very
high powered microscopes,
but there's two main
divisions of microscopes
that are used today.
There's a wide variety,
but two main divisions.
The first are called light microscopes.
These have the advantage
of being able to see
live cells in action,
but the problem that,
the disadvantage that we have with these
is that the resolution is
not as good as it could be.
We're looking at things that
are a couple of micrometers.
That's a millionth of a meter,
okay, so very very small.
Micrometer units are very very small,
and that's why we can't see
them with the naked eye,
but we can if we magnify it
through the use of these microscopes.
Now the more powerful microscopes
called electron microscopes give us
a much higher resolution,
but there's a trade off.
The higher the resolution,
such as with the electron microscopes,
we have to hold the cells still.
We usually have to coat them
with some type of metal.
In a sense, we kill them.
So we can't see live cells with
these high-powered microscopes.
We can see snapshots of what's
going on inside of them,
but we really can't see a
lot of the inner workings
and what's going on.
So it's the equivalent of
being in the nosebleed section
and watching a football
game and being like,
"I see people moving down there,
"but I'm not exactly sure
where the ball is or anything."
versus taking snapshots of high resolution
of the quarterback carrying the ball,
but you don't see where he's going.
You just see this, you know,
quarterback holding the
ball and you're like,
"Oh, there's the ball, but
what does the ball do?"
Between the two, we get a good
picture of what's going on,
but there's still more to learn.
There's still quite a bit to learn
about what's going on inside of a cell.
The reason for that is because,
even with our most high-powered
electron microscopes,
we can only see snapshots
of proteins and organelles
and whatnot of the cell
and what they're doing.
We're still not fully at the capability
of seeing individual atoms.
We can see a few molecules, like water,
with these high high
high-powered microscopes,
but we're still not quite at the point
of being able to see the atoms.
So this kind of gives
you an idea as far as
what microscopes allow us to do,
to observe the very very small.
All right, let's talk about
what the unifying concept of all cells are
because, since all living
things are made of cells,
there are a couple of facts
that are common in all cells.
So this is one of the first
things I'll test you on
is what do all cells have in common.
All right, now, we talked
about in the last lecture,
about some of the organic molecules,
and that's what you're gonna see here
is that, as the organic
molecules come together
to form larger structure,
what's the next level of organization
that we learned back in Lecture One?
Molecules come together to form what?
You have atoms, molecules, and then what?
(students mumbling)
Organelles, all right, organelles.
So, that's what we're gonna
mainly learn about today
is what are the organelles of a cell,
'cause as we learned,
organelles are to the cell
as organs are to an organism,
meaning our heart is an organ.
Has a very particular job,
very specific function.
So is an organelle.
It plays a specific role within the cell
to keep the cell alive.
So all of the organelles
that combine together
ultimately give the cell
its overall function.
All right, so one of the first organelles
is the cell membrane.
You just learned in this last lecture,
what is the cell membrane mostly made of?
What's the organic material
that mostly makes up the cell membrane?
It's that fat layer that
surrounds all cells.
Phospholipids, good, which are
the lipids that have the fatty acid tails
which are hydrophobic,
but the polar phosphate charged heads
which are hydrophilic.
As such, they create this fat barrier
that surround the cells.
We call this the cell membrane.
It's what keeps the external
and internal environment separate.
This is how a cell is able
to maintain homeostasis,
by regulating traffic in
and out of this membrane.
So, cells maintain homeostasis
by bringing things in,
bringing things out,
keeping things out, doing all the things
it needs to do to maintain the
proper internal environment.
That's why it's so important.
The cell membrane is not
just made of phospholipids,
although that is one of
the major components of it.
We'll learn a little bit later on,
probably Thursday, about what
are the other components.
Now, there's an environment inside of here
that's mostly water, but
we don't just call it water
because there's water on the inside.
So how do we differentiate
between the two?
We call the inside of
the cell the cytoplasm.
So the cytoplasm is essentially
the internal fluid environment,
and though it's mostly water,
there's a lot going on in here.
There's a lot of just
mechanics of proteins
and metabolism and all sorts of stuff.
So, though we look at cells
and they may look somewhat clear,
it's just because we cannot see
with the live microscope, the resolution,
but let me jump ahead and
show you a couple things.
When you look at the inside of a cell,
it's not just a liquid environment.
I mean, there are these structures
that are just amazingly complex.
Here's just some example
of some of the inner workings
using a very high-powered
electron microscope.
So, though we may look at a cell,
here's a plant cell with
the light microscope,
but if we look deeper with, lets say,
an electron microscope, we
can see structures like this,
which are very highly
complex and organized.
So, cytoplasm, the fluid
environment inside of a cell.
All right, so all cells
have a cell membrane.
All cells have a cytoplasm.
Now let's talk about the
other four components.
All cells have at their
core genetic information,
which we call DNA.
Now it's not structured
necessarily the same way.
All DNA is a double helix,
however, bacteria, they
have this large ring of DNA
whereas we have chromosomes.
So, the way in which we organize our DNA,
the bacteria loop their DNA around
and create this one large
circular double helix structure.
We have linear pieces
of double-stranded DNA
that make up our chromosomes.
So that's really the main difference.
We all have DNA, but not
all cells have chromosomes.
So there's a big difference
between how we organize our DNA
from one species to another.
So we all have DNA.
Well, as we mentioned in the last lecture,
DNA actually is used to make the proteins,
but before the proteins are made,
we make a copy of it.
It's like copying that
page out of the recipe book
that you want for that day,
and that produces a what?
It produces an RNA.
So all cells have an RNA or
not an RNA, they have RNA.
If the cell wants to make
proteins, which all cells do,
they first must copy that
portion of the DNA into RNA.
Okay, well, the RNA again,
is still just a template.
It's still just a recipe.
It's not the actual protein.
So what the cells need to do
is to take that information
and create a protein.
And we learned in the last
lecture how proteins are made.
What are the monomers of the proteins?
- [Student] Amino acids.
- [Professor] Amino acids
and how are they assembled together?
(student mumbling)
They are forming polypeptide chains.
So, how do we know what
order to put them in?
(student mumbling)
According to the RNA
which, according to the DNA
is the template from that.
These are just copies of one another.
The RNA is just a copy.
So what actually assembles them together,
because all cells have DNA,
all cells have RNA and
all cells make proteins,
but you actually have to
manufacture the proteins.
Well, here's where we get into
our second organelle called ribosomes.
All cells have ribosomes,
a combination of both
protein and nucleic acid.
So what is a ribosome's
function as an organelle?
Well, it's an enzyme which
undergoes dehydration synthesis
by covalently bonding amino acids together
into a polypeptide chain.
So what is the role of
ribosome, to make proteins.
So how do cells make proteins?
They essentially copy the DNA to RNA,
and then the ribosome reads the RNA,
covalently bonding the amino acids
in the pre-established sequence.
This right here is gonna
be all of Lecture 10.
We're gonna spend an entire lecture
going through how that works.
How do you copy the DNA to RNA?
How do you make a protein from that RNA
by using the ribosomes?
For now, you just need to
know these six components.
Now, cells ultimately are
restricted in their size
because the larger they are,
the harder it is for them to be able
to maintain homeostasis,
but there is a few
tricks that cells can do
to get around this because,
as their volume increases,
they have that much more area
that they have to maintain.
So, some of the tricks that cells can do
to ultimately be very very long or whatnot
like your muscle cells.
Your muscle cells actually span
the whole length of your muscle.
So if you look at your muscle as an organ,
the individual cells that form
all the fibers that make up your organ,
are the length of your organ.
That's how long those cells
are, one continuous cell,
but they're very flat in nature.
So even though they're very long,
they have a lot of surface area
which allows them to exchange nutrients
with their environment.
So they have this flattened
or cylindrical shape.
Another example of this, to be able to
increase the surface
area are in your lungs.
Your alveoli, which are the
small cells in your lungs
that are surrounded by capillaries
and this is where oxygen and
carbon dioxide exchange occur,
due to the flat nature of those cells,
then the oxygen and carbon dioxide
can go through the cytoplasm
of the cell fairly rapidly.
And this is necessary in
order for this exchange
to be able to occur.
In fact, if you look at the
total amount of surface area
compacted into your lungs
due to these flattened cells,
it's about the size of
half of a tennis court.
That's how much surface area you have
just inside your lungs alone.
And that's essential to be able to get
enough exchange of
oxygen and carbon dioxide
with the atmosphere to do so.
Another concept that cells can use are
to actually project portions
of their cell membrane,
almost like finger-like extensions
to increase their surface area
without increasing their volume very much.
And you see this in cells
like in your intestines
where they have these protrusions
that increase the overall surface area,
but these little bumps right there,
that actually increases the
surface area of the cell
so that you can get more
and more absorption.
So how much of a difference
does that make in your intestines?
Well, if you were to take
a 16-square-foot section
of your intestines and look
at the actual surface area
of food absorption, it's
more like 2200 square feet.
That's the difference between having
just flat cells that have no extensions
and these tiny little,
finger-like extensions
that increase the surface area.
I mean, 16 square feet
to 2200 square feet,
it's kind of a no brainer on
that on why it's necessary.
So, those are some of the tricks
that the cells can do to
change their cell membrane
without really getting bigger
in terms of their volume.
But still, they can't get too big.
Otherwise, they start having issues
of taking out the
garbage, getting nutrients
and taking care of themselves.
All right, so let's talk
about the cell membrane
as its first organelle.
We talked back in the last lecture
about phospholipids.
Those are the glycerol
and fatty acid tails
with the phosphate head,
so that a big portion
of them are hydrophobic,
but the head is hydrophilic.
So they orient themselves
to where they form
this very fluid fat layer,
where the phospholipids are on the outside
and the inside of the
cell, you know, facing,
and then the tails are
sandwiched in between,
creating this hydrophobic layer
in which the water is
typically impervious to,
although there are, there is some leakage
as we'll show in the next lecture.
Now it's not just made of phospholipids.
We call the cell membrane a fluid mosaic.
The fluid part comes about
because the phospholipids are free to move
within that bilayer.
They're not covalently
bonded to each other.
There's not even hydrogen
bond going on there.
The main thing that holds them together is
the fact that these phospholipids
will not pop out of the membrane
because to do so would expose
those tails to the hydrophilic
or the water environment,
and they don't like that.
So they stay together in this fashion.
Yes, you can burst the
membrane by busting it open,
but that usually destroys the cell.
But it's not just made of phospholipids.
What's a mosaic?
Like, you know, you go into
a church, what's a mosaic?
(student mumbling)
Okay, a bunch of different
pieces put together
to create some art form.
It's got different-colored glasses.
You might have different
tiles coming together.
The same concept, the reason why we call
the cell membrane a fluid mosaic is
because it's not just
made of phospholipids.
It's made of a myriad
of different proteins,
each with different functions.
And so, if you really
look at the membrane,
it doesn't look like this.
It looks more like this.
There is all sorts of stuff
going on inside the membrane.
You've got these purple proteins,
which just represents a
variety of different proteins.
You've got other proteins that
are just beneath the surface.
You've got even cholesterol in here.
There's a wide range of organic molecules.
Those green groups are the carbohydrates.
These are things like oligosaccharides.
Remember, we talked
about oligosaccharides.
What are they used for?
It wasn't that long ago.
(student mumbling)
Cell recognition, good.
So these oligosaccharides are attached
to the surface of your cells
like your red blood cells
and your other cells
and tissues in your body,
and that's what gives your
body its cell recognition.
So you have almost every
carbohydrate group here.
You've got fats, you've got proteins,
you've got carbohydrates.
This is a very dynamic organelle
that plays a very specific role
of maintaining homeostasis in the cell.
You've got other proteins,
which form channels to allow things
in and out of the membrane.
So this is one of the first
concepts you gotta know
about the cell membrane.
One of the ways in which
it maintains homeostasis
is by selective passage
through the membrane.
It selectively allows things to come in,
selectively allows things to come out,
prevents things from coming in,
prevents things from going out.
So it acts very much
like this building does.
It maintains an internal environment
by heating it up and cooling it down,
depending upon what the
atmosphere is like outside
and the weather and whatnot.
It provides structure and stability.
Certain things are restricted
from coning in and out of the classrooms
and doors, you know, larger vehicles
and things that have no need to come
into this classroom and the like.
So just like a building regulates
the homeostatic environment inside,
so does the cell membrane.
Its main job is to do that.
All right, now, let's look at
the different types of cells.
There are two main divisions of cells
that all life can be categorized into.
You have what we call prokaryotic cells,
which are the more
primitive types of cells
and then you have eukaryotic cells,
which are the more advanced type of cells.
That's what you and I are made up of.
Well, within those groups,
we've got different domains,
and we'll talk more about those
at the end of the semester
when we look at species.
But, there are two types
of prokaryotic cells
or two domains, the bacteria,
which everyone is familiar with
and another group of prokaryotic
cells called archaea.
These typically live in
extreme environments.
So we don't really have
a lot of association
with these types of prokaryotic cells.
But, what makes a prokaryotic
cell a prokaryotic cell?
Well, they're primitive,
which means that they really don't
have much to them.
If you look inside a prokaryotic cell,
there's not much there by
comparison to a eukaryotic cell.
They do have the fundamentals though.
They've got a cytoplasm.
They have a cell membrane.
They have ribosomes, they have DNA,
they have RNA and they have proteins.
Those are the fundamentals.
Now, in some cases,
these can have organelles
that you and I even don't have.
Even though they're very simple cells,
they have certain organelles
that allow them to function.
One of those is what we call a cell wall.
Now, a lotta times people confuse
the cell wall and the cell membrane.
They are distinct organelles.
All cells have a cell membrane,
which is made up of phospholipid bilayer
and maintains homeostasis.
That's the universal
process of that organelle.
So what's the cell wall for?
Well, unlike what the name suggests,
it's not an impermeable barrier.
It's actually very porous.
If you look at the cell wall,
it looks more like this.
It's all these fibers of organic molecules
that are strung together
to form a latticework
on top of the cell membrane,
but it's very porous.
And so, it allows things in and out.
So what is the main
purpose of the cell wall?
Well, it's primarily to
keep the shape of the cells
and to maintain structure.
A lotta times, these
cells that have cell walls
are in environments where they need
to protect their cells substantially,
like plants and protists
and bacteria and the like,
and fungi and whatnot.
These are organisms that have cell walls
made up of a variety of different things.
Some are made of carbohydrates.
Some are made of both
protein and carbohydrate.
There's a variety of material.
For example, plants make their
cell wall out of cellulose.
We talked about that in the last lecture.
Fungi make their cell wall out of chitin,
a just, a different
configuration of a carbohydrate,
which is a little more tough
than even cellulose is.
Bacteria, they actually combine
protein and carbohydrate together
into a substance called peptidoglycan.
The peptid has to do
with the peptide bonds
with the amino acids.
Glycan has to do with
the glycogen and whatnot
that is part of it as well.
So that's where it gets its name.
So, make sure you
understand that a cell wall
does not replace the cell membrane.
All cells have a cell membrane
where they've made up
this phospholipid bilayer
They have proteins.
This is the organelle that
maintains homeostasis.
This is what regulates what
comes in and out of the cell.
The cell wall merely provides
stability, structure,
strength to the cell.
Plants, they can't move, so the only way
they can protect themselves is
by creating this very tough
cell wall around their cells.
Fungi live in extreme environments,
and so, they need that cell wall
to help maintain differences
in overall pressure
and chemical and whatnot.
Bacteria are no different.
They live in extreme environments as well,
where cells would normally break apart.
And they typically have to have
this hard outer coating as well.
And as you can see,
there are many bacteria
that have various shapes.
Some are rod, some are spherical,
some are even spiral in nature.
And that's done by the cell wall.
That's what gives the cell
its overall cell shape.
So that's another organelle.
Now not all organisms have that.
For example, animals don't.
You might think, well, don't some animals
create an exoskeleton?
Yeah, they do, made of chitin,
but their cells, their actual cells
that are protected by that exoskeleton
don't have cell walls around them.
They're just like you and I.
So, though animals and whatnot can create
a very tough outer shell,
the same principle applies.
We don't have a cell
wall around our cells,
around our individual cells.
Now notice the complexity of
the animal and plant and whatnot.
These are the eukaryotic cells,
the more advanced cells
that typically have
lots and lots of different
organelles to them.
In fact, when we study
these organelles today,
you're gonna see that, really,
bacteria have little to none of them
and we have all sorts of
different types of organelles.
Now the arrangement and
the types of organelles
that a cell has ultimately determines
what the cell's going to do.
There are some cells in your body
that are more abundant in mitochondria
because they're the
ones that are undergoing
most of the metabolism.
There's other cells in your body
that have a higher abundance of,
maybe, internal
cytoskeleton, like your skin
because they form more
of a protective layer.
So depending upon the
composition of the organelles
ultimately tells you what
the cell is going to do.
So how do our cells keep their shape?
Animal cells actually keep certain shapes.
How do we make flat cells for our muscles
and flat cells for our lungs
or maybe cuboidal cells for our intestines
with all these finger-like projections?
Well, it comes down to an
internal scaffolding of proteins,
which we call the cytoskeleton.
That'll be the last organelle
we talk about today.
That's how animals keep their cell shape.
In fact, you can see here,
this is an electron-scanning
micrograph of a cell.
You can see a wide variety
of different organelles.
Here's the nucleus,
which takes up a huge portion
of the cell and the like.
So here's just kind of, this shows those
inner fibers or proteins that kinda
keep the shape of the cell.
That's how animal cells keep their shape
because we don't have a cell wall.
Plant cells, notice there's
several different other organelles.
They have this huge
organelle called a vacuole,
which is kind of a storage container.
I'm just giving you an overview.
We're gonna go into these in detail here.
They've got other
organelles in certain cells,
in the leave cells and whatnot.
These organelles play a key
role for biology in life
because they're the ones that are actually
capturing the sunlight and turning it
into sugars and fats and proteins.
These are the chloroplasts
that you only find in
photosynthetic organisms.
They're not in animal cells.
They're not in fungi.
They're not in a lot
of different organisms.
Notice the two layers here as well.
Plant cells as well have a
cell membrane and a cell wall.
So there's a lot of complexity
in each one of these cells
that we're gonna go through.
All right, so let's talk first about
some of the interactions
that cells have with one another
due to their cell membranes and cell walls
because this is another
function of these organelles.
Cell membranes don't just regulate
internal and external environments.
They also provide attachments
so that you and I, made
of trillions of cells,
can form these more complex structures
of what you and I are.
More simple organisms typically don't have
the complexity that we have
in terms of how the cells get put together
and attached to each other.
Now plants, they don't necessarily use
their cell membrane to
attach to one another.
They use their cell wall.
So here's another example
of what a cell wall can do.
It provides attachments so that the cells
literally can fuse their
cell walls together
and form these very very
rigid tough structures.
That's why plants are so resilient,
because they have individual cell walls,
but they're fused together.
They create these larger structures.
One of the interesting
things about these is
that they can create these little pores
that allow the cells to share nutrients
and communicate with one another,
and this is key for the plant's ability
to transport water from the roots
through its vascular system.
So these are called plasmodesma
or the plural is called plasmodesmata.
That will show up possibly
on one of your questions.
Others might get a different version
looking at animal cell connections,
but you gotta know this as well.
So plasmodesmata are the connections
that cells make with one another
so that they can share nutrients,
that they can siphon water
through their vascular
structure and the like.
There's another role as well.
On the surface of flowers
where they secrete nectars
that attract pollinators
like bees and other insects,
the plasmodesmata serve that purpose too.
So the plant can actually make sugars
and then secrete them,
and then the insects are attracted to that
and ultimately get the
nectar through those pores
as it comes up to the surface of the cells
where the flowers are at.
All right, now how do you
and I and other animals
make those connections 'cause
we don't have a cell wall?
So how do we make connections?
Well, there's three main
types of connections
that hold our tissues together.
The first one is actually
the second on that list.
It's called anchoring
or adhering junctions.
These are what the
majority of your cells do.
These are loose junctions
that kind of like,
if you've got a jacket
and you button it up.
There's still some flexibility there.
There's still some gaps and
pores between the buttons.
It provides a connection,
but it's not like airtight so to speak,
like zipping up a jacket will be.
That's another type of junction.
So, where does this apply?
Well look at your skin,
grab a piece of it.
Notice how pliable it is.
If it weren't that pliable,
it wouldn't be that pliable
if it didn't have these loose junctions
that allow you to, you
know, move your skin around
on top of it.
So the tissues aren't cemented together.
There's some flexibility there
that allow them to remain connected,
but still be able to
be pliable and movable.
Now, in some areas, you really do need
the cells to zip up by
having these tight junctions.
So what are making all of these junctions?
It's really just the
proteins in the membrane.
Remember I told you how
there are various proteins
within the membranes of the cells.
There are various types of proteins
that perform these types of junctions
or these buttoning up or zipping up.
So if you look at tight junctions,
it's kind of like zipping your jacket up.
You get this almost impermeable barrier
with no gaps in it whatsoever.
So where will we typically find these?
Well, we typically find
them in areas in our body
where we need a lotta filtration,
like our intestines and our kidneys.
The reason for that is
because as nutrients
move across the cells,
we don't want it leaking
through in between the cells.
That would happen if we just
had these button-like junctions
or these anchoring junctions.
So we have to zip the cells up
so that there's no pores
or no gaps in between them,
so that the intestines only absorb
that which they need and
bring it into our bloodstream.
All the rest just kinda
goes by the wayside.
The same thing is true for your kidneys.
They have these tight junctions
to prevent things leaking through
because it's a filtration
organ amongst other things.
So, anchoring are the loose junctions.
Tight junctions are
the ones where you find
that the cells need to
filter a lot of things out,
like in your intestines or your kidneys.
And then the last one
are called gap junctions.
These are proteins that
fuse between two cells
that provide a pore, very
much like the plasmodesmata,
but we don't call them plasmodesmata.
We just call them gap junctions.
So, whereas plants have these pores
because of their cell walls fusing,
animals can have these same types
of instantaneous communication
and sharing of nutrients
by having these gap junctions.
So, where would we need these?
Well, one main area is your heart muscles.
Because of the nature
of how your heart works,
it needs to be able to share
and redistribute the calcium
amongst all the muscles in
that area at the same time.
Otherwise, your heart
would not be able to beat
and have those differential pressures
for your cardiovascular system.
So these pores allow
instantaneous communication between cells.
That's their main role
as far as communication
from one cell to the next,
'cause if you don't have these pores,
then things have to go
through the membrane,
through the space in between
and through this membrane.
It takes a lot longer than just being able
to travel through a nice little channel.
Most organelles that
we're gonna talk about
are what we call
membrane-bound organelles.
What do we mean by
membrane-bound organelles?
Well, you know that the cell membrane
is made up of the phospholipid bilayer.
So are these organelles.
So when we say membrane-bound organelle,
we mean they're made up of the
same fundamental composition
that the cell membrane's made up of.
They're made up of a phospholipid bilayer,
in many cases multiple layers,
and they've got proteins in there.
So they're just as complex
as the cell membrane.
So when we say membrane-bound,
we literally mean the same material
that makes up the cell membrane,
being phospholipids and
proteins and the like.
Now, not all organelles work together
like this group I'm
gonna talk about first,
but because this group of
organelles works together
in tandem, almost like an assembly line,
we have a specific name for this group.
We call it the endomembrane system.
So the endomembrane
system just talks about
the membrane organelles that are
inside the cell endomembrane
and how they work together.
They are distinct, individual organelles.
So there are four in total here.
There's the nucleus, the
rough ER, the smooth ER,
the Golgi apparatus.
These are the four that belong
to the endomembrane system.
So I'm gonna start with
the nucleus obviously,
and then show you how they work together.
All organelles work to keep
the cell healthy and alive
and doing its thing, but
these have a special place
because of its assembly-line nature,
which is why we call it
the endomembrane system.
Okay, so let's start with the nucleus.
I told you that this is one
of the unique characteristics
of eukaryotic cells.
Prokaryotic cells do not have a nucleus.
Now sometimes we call
it the nuclear envelope
because it's a, not only just
a single membrane system,
but a double membrane system.
It has twice the membrane
that the cell membrane has.
Look it here, it doesn't
just have one layer
of phospholipids, it has
two layers of phospholipids.
Now here's an example of
what I mentioned last time
as a quaternary protein structure.
Notice you've got all these
little individual tertiary structures
all coming together to
form this large pore.
So this is an example of
a quaternary structure
where you have all these
different polypeptide chains
folded individually and then collectively,
they make this huge
pore, this huge protein.
All right, so why is the
nucleus just so protected?
Why does it have two membranes
when the cell membrane itself
is only one layer thick?
Well, it's because, this
is the heart of the cell.
The nucleus contains all
of the genetic information
that a cell needs to be able
to maintain homeostasis.
Imagine, some people
might not like this fact,
but imagine if all of the
secrets and all of the data
that our government had would
be available to everybody.
Some of you might argue,
yeah, that's a good thing.
However, there are certain
secrets and other information
that needs to be very well protected,
kinda like in the Pentagon
and other secure areas.
And only certain things have access to it.
Well, the same thing is true for this
because if everything
had access to the DNA,
it would get destroyed,
let's just be honest.
If we all had access
to all the information,
you know, or whatnot, I mean, I don't know
about free information, but anyway,
I'm done with that analogy.
So ultimately, if everything
had access to the DNA,
our DNA would actually be destroyed.
So ultimately, it's regulated.
Access to that DNA is
regulated by these pores.
That's why it's got this
double-protected layer
to prevent things from coming in and out
that don't belong there.
So we, because of the nature of how
we copy our DNA to RNA and how we use that
have this huge protective layer.
Now there is access in and out,
but it's very restricted access,
only a few set of
molecules actually can go
in and out of the nucleus
to access that genetic information, okay.
So that's why you see all
these pores around here
is because it protects the DNA
and it regulates access in and out.
Here's a very high-powered magnification
showing you some of these gaps or pores
that these quaternary protein structures
regulate access in and out of.
So what is the nuclear envelope's job?
To protect the DNA and regulate
access to the information.
That's pretty much its job.
Now bacteria, like prokaryotic cells,
why therefore do they not have nucleus,
you know, for like, well,
they don't protect their DNA.
It's just the nature of
how they use their DNA.
They have other protective measures,
but they don't need a nucleus.
So the other DNA is exposed
to all these other things,
they're much simpler.
They don't have the
complexity that you and I have
and, due to that fact,
they really don't need
to house and protect their DNA
because of how they function.
All right.
So how do you get the
information to the cell
so that it can do what it needs to do?
Well, this references what
we went over last time
with the organic molecules,
how DNA and RNA are
related to one another.
The DNA always stays in the nucleus.
So how do you get that
information out of the nucleus
if it's just nice and cozy
and protected in there?
You make a copy of it.
That's where RNA comes in.
So, in order for the cell to
be able to make a protein,
as they always do because they need that
to be able to function,
they'll say, "Hey, I want
to make some insulin."
"Hey, I want to make some glucagon."
"Hey, I want to make some hemoglobin."
So they find the information inside here.
They copy it to RNA,
and then the RNA leaves the nucleus
out into the cytoplasm,
and the DNA's like, I
did my job, you know.
It gave the information that it needs.
So when the RNA leaves the nucleus,
then where does it go?
Well, the next organelle in the row
in this endomembrane system is called
the rough endoplasmic reticulum
or we shorthand it and
call it the rough ER.
So what's the endoplasmic reticulum?
Well, it's a two part organelle.
It's got two main pieces to it
that each have respective jobs.
Now, when you look under a
scanning electron micrograph,
the reason why we call it rough ER
or rough endoplasmic reticulum is
'cause it's got all
these little bumps on it.
So when you look under this
high-powered electron micrograph,
you see all these little bumps.
Those bumps are ribosomes.
Now we've already learned
what ribosomes do.
What do they do?
(students mumbling)
They make proteins. So guess what?
When the RNA leaves the nucleus,
it goes to the rough ER
where the proteins are at.
The proteins then read the RNA
and know what order to
put the amino acids in.
And, as we learned in the last lecture,
the order of the amino acids predetermines
how the proteins are going to fold
into their tertiary structure
or quaternary structures,
based upon how many
polypeptide chains there are.
So whenever a cell
wants to make a protein,
it copies the DNA in the nucleus to RNA.
The RNA leaves the nucleus,
comes to the rough ER,
where the ribosomes
manufacture the proteins.
So now what do we do with the protein?
Well, the rough ER needs
to stay right there.
It can't really move, it
can't do a lotta stuff.
It just is designed to make the proteins,
but the second half of
the endoplasmic reticulum
is called the smooth ER.
Why do you think it's called smooth ER?
- [Student] It's smooth.
- [Professor] 'Cause it looks smooth.
So what do you think it doesn't have?
It doesn't have ribosomes.
It looks smooth because there are
no little or large
organelles called ribosomes
that are attached to it.
So what's it's job?
It's not to make proteins
'cause it doesn't have any ribosomes.
Its job is to package the proteins up.
So, the main job of the smooth ER
is to manufacture phospholipids
so that it can package up the proteins
into these, what we call, vesicles,
just these little
compartments of phospholipids
that will surround the protein
and be able to transport this.
There's another job,
however, of the smooth ER.
When cells grow, they need to increase
the amount of phospholipids
in their cell membrane.
And the only way in which they can do that
is to manufacture the phospholipids.
That's where the smooth ER comes in.
So the smooth ER doesn't
just package up proteins.
If the cell needs to enlarge,
then it needs to manufacture
more phospholipids
so that it can grow.
So this has its role, the
smooth ER has its role,
not only in packaging proteins,
but also making more material
for the cell to be able to grow,
making more of the membrane.
So that's why we say synthesize lipids.
All right, and if it
didn't synthesize lipids,
every time the smooth ER buds off
a portion of itself, it would start
getting smaller and smaller over time.
So as it packages up the proteins
and sends off a portion of itself,
it needs to renew itself, so to speak.
It needs to make more lipids.
It usually does this
from the triglycerides
because, remember,
triglycerides and phospholipids
are almost exactly the same.
It has just to detach a fatty acid tail
and add a phosphate
group and there you go.
There's a phospholipid.
All right, so then, why does
it heed to package it up?
Where does it need to send it to?
Well, the last area of
the endomembrane system
is called the Golgi apparatus.
Golgi, the reason why it's capitalized is
it's the proper name,
the name of the scientist
who discovered it.
Hence, it's named after him.
So the Golgi apparatus
is a processing center.
It's a modification center.
Now let me ask you,
how does a protein need
to be modified, why?
Well, let's come back to here.
Proteins, when they're
first made in the ribosome,
are just amino acids strung together
and then folded into a tertiary structure,
but in some cases, like for example,
this glycoprotein, as we call it,
because it's got oligosaccharide
plus the protein,
so we call it a
glycoprotein, like glycogen.
The recognition proteins here,
like for your red blood cells,
they're not made right
away in the ribosomes.
The protein part's made in the ribosome
and here is where the
Golgi apparatus comes in.
The Golgi apparatus makes
modifications to the protein
for its final stage.
So in the case of this type of protein,
the Golgi apparatus
attaches the oligosaccharide
to the protein and then
sends it off to the membrane,
and now you've got your
recognition protein.
You're now blood type A or
blood type B or whatnot.
So that's what the Golgi apparatus does.
It can reshape the protein,
fold It a little differently,
attach something onto it
like this oligosaccharide.
It's a processing center.
Let's see.
So it may look smooth, but there are
lots and lots of proteins
in the Golgi apparatus
doing these modifications,
attaching the oligosaccharides to it,
restructuring the protein.
Sometimes, proteins are not quite ready
to work in their environment.
Some proteins need to work
in very acidic environments.
So, in order for them
to be able to survive
that high hydrogen ion concentration,
the protein needs to be
a little restructured
so that it doesn't break apart in that PH.
That's also the Golgi apparatus' job.
So it' not just attaching things to it.
It can also make
modifications to the protein's
overall tertiary structure
so that it can function
better in its environment,
whether it's an acidic,
a basic, or alkaline
or neutral PH, whatever the case may be.
Now these other organelles do
work with other organelles.
They do work with each other,
but not necessarily in a linear fashion.
There is cross talk and exchange
between organelles depending
upon what's going on,
but for the rest of these,
we're just gonna talk
about hem individually
as their respective functions.
So we don't have any more
systems to go through,
but there is a lot of
complexity to these organelles.
So play close attention,
especially when we come to lysosomes here.
There's about four different facts
that I want you to know about lysosomes,
because it has one main job,
but there's many applications to that job.
So what are lysosomes?
Well, if you think of the word lys,
what did we say that means?
Like hydrolysis, hydrolysis,
what does lys mean?
- [Student] Don't make
me think, I'm just gonna
record what you're saying.
- [Professor] What does
lys mean, to break apart,
all right, come on.
So, lys means to break apart.
So lysosomes, these are
the digestive organelles of the cell.
They contain an acidic environment,
a PH of about 2.4 or so.
It might be more like 2.7,
but it's very acidic, okay.
So what happens is, when
the cell eats something,
or let's say an organelle gets worn out,
like a mitochondria gets worn out
and the cell just wants to recycle
the organic molecules that made it up.
That's where the lysosomes come in.
They will fuse with that matter,
and inside of the lysosomes,
you have a variety of enzymes,
each which have specific jobs
to break down carbohydrates,
lipids, proteins and the like.
So, because it's got so
many different enzymes,
it pretty much can digest
most organic molecules.
And the reason why it
contains an acidic environment
is because, if my chance these lysosomes,
it's not just the acid that
helps break up the tissues,
although that helps.
If the lysosome actually bursts open
and releases its enzymes,
remember what happens when
you change the PH of a protein
or the PH surrounding a protein?
What happens to the protein?
- [Student] It unravels.
- [Professor] It unravels.
We call this denaturation,
and that's a good thing
because, if these lysosomes
were to burst open,
which they occasionally do,
and the enzymes remained active,
they would start chewing
up everything in the cell.
You don't want that to happen.
So it actually deactivates the enzymes
if by chance the lysosomes
accidentally burst open.
So it's a two-fold purpose.
The PH helps in the digestion process
because it helps break the covalent bonds
and aids in that lysine
process like the hydrolysis.
But, it's also a protective measure
so that the enzymes don't eat the cell up
if the lysosome actually gets destroyed.
Okay, now let's talk about eating the cell
because there are occasions
where the lysosomes will destroy the cell,
but it's actually a good thing,
and this is where we get into cancer
because, when a cell starts
having too many problems,
it can literally self destruct.
We call this apoptosis, which is
called programmed cell death,
A-P-O-P-T-O-S-I-S, A-P-O-P-T-O-S-I-S.
I don't have any markers today.
Otherwise, I'd write it on the board.
So, apoptosis is programmed cell death.
Why is that essential?
Well, in the cases of cancer,
where your cells are
growing out of control,
normally your body is able to say,
"Hey, you're messed up, kill yourself."
And normally, the cells will,
but in some cases where the cells have
malfunctioned in a certain way,
they don't respond to
your body's immune system
telling them to shut down
and to kill themselves.
And that's when the cancer starts growing
and spreading and having
all sorts of issues.
So, this is a default mechanism
that's absolutely necessary
for the health of your system
when the cells start becoming problematic.
It's also part of your
development process.
If you look at your hand,
the reason why we don't have
webbing between our fingers
is because of this programmed cell death.
As you develop as a fetus,
your hand grows like this
and then the cells die in between.
Same thing for your toes,
same thing for your brain,
you make three times the number of neurons
that you actually need.
Two thirds of them die through apoptosis.
Now you don't get smarter
if they don't die.
You will actually die because
you have too many neurons.
Your brain gets too big.
And now, there's one more
thing about lysosomes
that you should know.
They're not just a digestive organ now,
but when problems, genetic problems arise
because the enzyme is not made properly,
this leads to various diseases.
You may have heard of this one before.
It's called Tay-Sachs disease.
The child usually dies at about age three
because what ends up happening is,
if you have this disease,
there is a defective enzyme
that doesn't break down certain lipids
that surround your nerve
cells in your brain.
Well, as those lipids accumulate,
the nerves essentially die and you die.
So, this disease, there's no cure for it.
Ultimately, it causes cell death
because it can't break down
certain fats in your brain.
And that has to do with your lysosomes.
So, pretty important organelle there,
they're important for
digestion, cleaning up
worn out, old organelles
like your mitochondria
when they get damaged or
whatnot, it'll absorb it.
And here's where the Golgi apparatus
actually makes lysosomes
because it'll take the
proteins that are made
from the rough ER, it'll modify them
so that they can survive in
a very acidic environment,
and then, it creates those lysosomes
that will chew up things
that the cell takes in.
It'll recycle old organelles in the cell.
So these are important
organelles in any cell.
All right, vacuoles, vacuoles are kind of
like lysosomes for the plants,
but you don't really find
huge vacuoles in animal cells.
So plants have vacuoles.
They're mainly storage containers,
but they also serve to
break molecules down.
Now, for testing purposes,
I'm gonna keep it simple so that you don't
get confused between the two,
and we'll just say that
they're storage containers
'cause that's one of their main jobs.
So vacuoles store nutrients, store sugars,
store water, store what
the cells typically need
to be able to undergo photosynthesis
and to store the products
of photosynthesis.
So as you can see, these
are membrane-bound vesicles
that take up a lot of space
in the cell, look at that.
I mean, that's a huge amount
of space for the cell.
Here's the nucleus, there's a vacuole.
So they take up a lot of space.
Let me show you another picture.
There it is.
Look how much space these vacuoles take up
in the plant cell.
So vacuoles are the storage containers
that pretty much store water
'cause you need a lot of
water for photosynthesis.
So these cells need to be able to store
and have a reservoir of water
as well as nutrients and supplies.
All right, there is some degradation.
There are some enzymes, so they're similar
to lysosomes in that regard, but yeah.
Now, peroxisomes, this one's a fun one.
Shoot, I don't have my video with me.
Well, I'll get my video for next time.
Peroxisomes are similar
to lysosomes as well.
You're like, "Man, all
these different things.
"Why don't they just have
one large organelle?"
well, because though they
have similar functions,
they do have their respective roles.
Peroxisomes do break
down certain molecules,
but unlike lysosomes, they're not
the digestive organelle of the cell.
So what is their main job?
Well, the peroxisome
actually gets its name
because of one of its major functions
amongst the many that it does.
One of those is to break
down hydrogen peroxide,
hence the word peroxisomes.
How does it do this?
Well, and why does it do this?
Why does it need to?
Your cells actually
produce hydrogen peroxide
when you undergo metabolism.
When your body is using
oxygen and water and whatnot
to be able to break down sugars
and get energy from them,
this is a toxic byproduct,
hydrogen peroxide.
Now, a lot of you know that
you buy those in brown bottles.
The reason why it's in brown bottles is
because light will actually
break down hydrogen peroxide
back into oxygen and water,
but in the inner workings of your body,
the light can't reach those
nor can it break it down fast enough.
So these organelles have enzymes
that immediately turn hydrogen peroxide
back into oxygen and water.
Now, next time when we meet,
I'll bring a video to show you
that we just did in
the lab a few days ago,
where the liver of an organism,
specifically this cow liver,
but it's in any animal.
The liver has so many peroxisomes in it
that when you put it into a solution
of hydrogen peroxide, it turns it
into oxygen and water so fast
that it literally bubbles over,
heating up and spewing
oxygen and water out of it.
So I'll show you that video.
It's like a side-by-side video
of a denatured liver, a cooked liver
and one that's normally functioning.
It's just amazing to watch.
So, hydrogen peroxide by itself
can actually disrupt a lot
of the cell's machinery.
And so, it needs to immediately
be rendered harmless by turning it
back into oxygen and water.
And that's one of the
main jobs of peroxisomes.
Though peroxisomes do
undergo some metabolism,
even lipid metabolism, which
brings us to another disease
called adrenoleukodystrophy. (laughing)
I've been teaching for over 10 years.
It took me about two to
be able to say that word
without tripping over it.
So adrenoleukodystrophy,
if you think of dystrophy,
like muscular dystrophy, that's really
what this disease is all about is
instead of causing neurons to die,
and therefore causing death,
it usually affects the neurons
that control your limbs.
We call them somatic neurons,
like your skeletal muscle system.
And when these lipids
don't get broken down
due to a malfunctioning
enzyme and peroxisomes,
then it usually affects the nervous system
that affects your limbs, and
hence the muscular dystrophy.
You can't, you lose
control over your limbs.
So it's not as life threatening,
but it is still a very big problem.
So in this situation, this
is due to a defective enzyme
in the peroxisome in
your cells in your brain
versus the lysosomes, which
is the Tay-Sachs disease.
The reason why I mention these now is
because we're gonna come
back to them in Lecture 10
when we start talking about
inheritance of genetic disorders.
We're gonna talk about Tay-Sachs disease.
We're gonna talk about
adrenoleukodystrophy.
We're gonna talk about a
variety of genetic disorders.
So it just gives you a prelude to
really what it is that
we're gonna be studying.
So lysosomes, vacuoles and peroxisomes are
also membrane-bound organelles.
They have a phospholipid bilayer to them,
and depending upon which
enzymes they have in them,
pretty much tells you what they do.
Lysosomes have the digestive enzymes
that break up most organic molecules.
Peroxisomes have the enzymes
that break down hydrogen peroxide
to prevent your cells from
having this interference
due to this byproduct of metabolism.
Vacuoles are storage
containers in plants, mostly.
They do recycle some materials,
but they mainly store things
in those large storage containers.
Okay, so in the last few
minutes we have here,
we will go over two more organelles
and then we'll call it a day.
Chloroplasts and mitochondria,
these two organelles are
gonna get their own lecture
because they're key for understanding
how living things use energy.
They're key in metabolism.
These are the two, what we
call, energy organelles.
NO other organelles do
what these organelles do.
That's why they're so unique.
Now, what's fascinating
about these two organelles,
chloroplasts and mitochondria,
is they're the only
organelles besides the nucleus
that actually have DNA.
Chloroplasts and mitochondria actually
have these tiny little circles of DNA.
And it does influence how these function
because proteins can be
made and are made from this
to aid in the role of this organelle,
the same thing for mitochondria,
but they're the only ones.
No other organelle, lysosomes
don't, peroxisomes don't,
rough and smooth ER don't.
No other organelle besides the nucleus,
which contains the bulk of your DNA,
and chloroplasts, which is in plants
and other photosynthetic organisms,
and then mitochondria,
which is pretty much
in most organisms, these
have their own DNA.
Now, what is the job of chloroplasts?
Chloroplasts, by nature, are green,
and when we study photosynthesis
in a couple weeks,
you'll see why ultimately
comes down to the proteins
that are found in this organelle,
that they reflect green light,
and therefore, they look green,
but it's not about the
reflection of light.
It's actually about the
absorption of light.
Chloroplasts are the organelles '
that actually absorb
sunlight and use that energy
to make sugars and fats and proteins.
These are the organelles that make
the initial organic molecules
that are necessary for all life.
They're the ones that make the sugars
that you and I eventually
get from their fruit
or from an organism that ate their leaves
and whatever the case.
This is how we get energy
transferred through ecosystems.
So, more on that, but if
I were to talk about, say,
an autotroph, we learned
that back in Lecture One.
Autotrophs are organisms
that make their own food.
They have chloroplasts.
That's the way they make their
own food is this organelle.
Now, not all cells in an
organism have chloroplasts.
Trees only have chloroplasts
in their leaves.
They don't have them in the
other cells in their body.
So how do the other
cells, even in a plant,
make energy or get energy?
Well, even plants have this
organelle, mitochondria.
So you and I, we don't have chloroplasts.
Really, animals aren't
photosynthetic organisms.
Neither are fungi,
but in order to process
the sugars and the fats and the proteins,
we need this organelle
called mitochondria.
Mitochondria, as I mentioned,
has its own DNA as well
in addition to chloroplasts.
Well, there's one main job.
Pretty much, mitochondria has one job,
and that is make ATP.
Remember we talked about
ATP as an organic molecule.
It's a short-lived battery
that fuels metabolism.
When sugars come into a cell,
they get processed into ATP.
When fats come into the cell,
they get processed into ATP.
When proteins come into the cell,
they get processed into ATP.
So that's the job of mitochondria.
It takes any organic molecule
and turns it into the same energy, ATP.
So the cells in your body that need
the most amount of ATP tend to have
lots and lots of mitochondria.
What tissue in your body
do you think might have
a lot of mitochondria to it?
What requires a lot of energy?
- [Student] Muscles.
- [Professor] Muscles, good.
Muscles tend to have a
lot of energy consumption,
and therefore have a lot of mitochondria.
Other tissues in your body
might have less mitochondria
because they don't make a lot of,
they don't need a lot of energy.
They don't work like your muscles do
Neurons, neurons use more
energy, pound for pound,
about three times the amount of energy
than muscles do.
That's why if you sit down
and study for several hours,
and at the end you're
like, "Man, I'm tired."
And somebody's like, "Oh shut up."
You can say, "Hey, I burned three times
"the number of calories just by thinking,
"pound for pound, than I did working out."
So I get my workout
every day, mainly here,
not in the muscles.
So, your neurons use a
substantial amount of ATP,
and therefore have a lot of it.
So, one of the fascinating
things about mitochondria
is you can, like your other genes,
have defects in the DNA here,
and those can be hereditary.
And if you get defects in this DNA
and the proteins aren't made properly,
this leads to neuro-genitive
disorders and dystrophies.
Not all muscular dystrophies
are caused by this,
but a lot of them are.
So, in some cases, because your muscles
can't produce enough energy,
you get a muscular dystrophy.
In other cases, you might
get a neuro-genitive disorder
because the cells are dying
because they're not able
to make enough energy
due to a defect in the DNA in here.
And this is actually
inherited through the mother.
One of the fascinating
things about inheritance is
your mitochondria, all the mitochondria
you have in your body
came from your mother,
not from your father.
Your father is just a sperm donor.
So, essentially, that
nucleus from the sperm
is what fuses with the egg,
but the actual mitochondria
that your cells make from that zygote
are your mother's mitochondria.
So all of your DNA, whether
you're male or female,
is your mother's mitochondrial DNA.
Now, this last organelle,
is unlike most others
in that it's not what we call
a membrane-bound organelle.
And the reason for it
is because it's not made
of phospholipids like the other organelles
we've talked about, like
peroxisomes and vacuoles
and the rough and smooth ER and the like.
So, what is the cytoskeleton?
Well, it's mostly a series
of complex configurations
of proteins that play a
role inside of the cell.
So just like we have an internal skeleton
made of calcium, our individual cells
have an internal skeleton,
but it's not made of calcium.
It's made of protein.
So the cytoplasmic skeleton
or the cytoskeleton
has a number of different jobs.
We're gonna go through
each one of those jobs.
Now you're not gonna be tested
on the individual aspects,
but it will be collective.
So you gotta know all of these factors
that go into what role
the cytoskeleton plays
in the homeostasis of the cell.
So some of the simplest of them,
actin, actin can be
found not only in cells,
but also in a lot of your major organs
like your muscles, where
it plays a key role
in your muscles being able to contract.
So, some cells are gonna have a lot more
of this protein than others,
especially your muscle cells.
But, in all cells, actin primarily serves
as a matrix where the
cell maintains its shape
due to these filaments.
So we sometimes call them microfilaments
because they're the smallest
of the protein structures
and they just work on
the internal scaffolding.
Now the question becomes,
why and how do cells
and the animals cells, since
we don't have a cell wall,
how do we keep our cell shape?
How do we make them flat?
How do we project portions of them out
that are these microvilli
that we've talked about before
that increase the surface
area of the cells?
That's the cytoskeleton.
So the actin filaments are the ones
that are just beneath the surface,
pushing that membrane out and holding it
in that particular position
or they're structured in
such a way under the surface
that holds the cell in
that particular shape.
So this is how animals
maintain their cells' shape.
Now it's not just for shape.
If we go back to plants,
'cause plants use a cell
wall to maintain their shape,
and you say, "Well, why do they
"have these microfilaments, then?
"Why do they have these things
"if the cell wall helps to
maintain the plant's shape?"
Well, it's not just for shape of the cell,
but one of the things
is it anchors proteins
in the membrane, the cell membrane
so that they don't all
start floating around,
because this membrane is so fluid,
and if you watch the video
that I showed last time,
you saw that sometimes
things can just float
within the membrane.
Well, some proteins need to
be in very specific areas
on the surface of the cell.
They can't just keep moving
around wherever they want,
and so, another aspect
of this cytoskeleton
is to anchor these surface
proteins in the cell membrane
to make sure that they don't move
from where they're supposed to be.
So that's why, even plants
have an internal cytoskeleton,
even though this isn't necessarily
for maintaining the cell membrane's shape
as the cell wall typically is.
But here, in animal cells,
this is the primary way
in which our cells maintain their shape
and it also anchors our surface
proteins to the membrane.
So that's one aspect of the cytoskeleton.
Let's look at others.
Another question that
some scientists ask is,
how do these organelles, like
with the endomembrane system,
how do they stay where
they're supposed to be?
Why doesn't this just float
off over here and whatnot?
Well, that's the second
part of the cytoskeleton,
what we call the intermediate filaments.
These are a little bit bigger,
hence we call them intermediate Filaments.
They hold the major organelles in place.
They hold the nucleus in place.
They hold the rough and the smooth ER
and the Golgi apparatus in place.
They make sure that they
stay where they should,
so that they can work properly.
Now the last one is by far
one of the more intriguing ones.
The microtubules, which are the largest
of the portions of the cytoskeleton,
these are used primarily
for trafficking in the cell.
Now if you'll remember from that video,
you saw that vesicle that
was being pulled along
by those two little feet?
It was walking on microtubules.
So, the microtubule are
kind of like train tracks
where it tells it where to go.
It's not just free-floating
in the cytoplasm.
When the vesicle goes from
the rough to the smooth ER,
I'm sorry, from the smooth
ER to the Golgi apparatus,
and to the surface, it knows
exactly where it's going
because it's following these microtubules.
Another aspect we'll get into later on
is microtubules are also involved
in the cell division process
when the cell starts pulling
components of it in half
and the cell divides in half.
So you'll see microtubules
a little bit later on this semester
when we get into what's called mitosis.
Cilia are not the same as microvilli.
Let me explain the difference.
Microvilli, these are these projections
that I told you increase the surface area,
like in your intestines and whatnot.
And that's what you're seeing right here
is these little protrusions right here,
those are cilia, I'm sorry,
those are microvilli.
So what are those bit long things for?
I mean, these huge long things, the cilia.
Well, they're similar to flagellum.
They have mobility to them.
They whip around, but they're
not for moving the cell.
So where would you find these
and why is it important?
Well, you'll find them in
areas like your intestines
and like your trachea
and esophagus and whatnot
where, because of the nature
of what they're exposed to,
like food, bacteria getting
caught in the air you breathe,
and whatnot in that mucus membrane,
the cilia pretty much clean
the cell's surface off.
And they sweep things by it and get rid of
and kind of wipe away the surface.
That's why in your esophagus,
and that's why in your trachea and others,
you have this mucus membrane,
and when things get caught,
they just pretty much sweep 'em by
and get rid of them so that
the body can take care of them.
This is where you also deal with an issue
with individuals which
have cystic fibrosis
because the mucus
membrane becomes so thick
in this disorder, genetic disorder,
that the cilia can't really move the mucus
and the bacteria get caught
and people with cystic fibrosis have
a lot of digestive problems
as well as respiratory problems
and bacterial infections and the like,
primarily due to the
thickness of their mucus
in those particular areas.
