Cell wall Video Lecture - Class 9

we've talked a lot about cells in
general but what I thought I would do in
this video is focus on plant cells and
in particular focus on the cell walls of
plant cells so this right over here this
is a drawing of a plant cell and the
thing that might jump out at you
immediately is instead of drawing it as
just kind of a roundish shape like that
the way i've drawn a lot of other cells
i've drawn this it's kind of a cubic
structure or rectangular prism and
that's because plant cells can have a
structure like that and so the next
question is well what gives them that
shape what's allows them to to form that
kind of cubic rectangular prism shape
and the answer is it's the cell wall so
let's say cell cell wall so let's make
sure we can orient ourselves properly in
this picture so clearly if I was if I
didn't have this cutout all I would be
seeing is the outside all I would be
seeing is the cell wall but we've cut it
out and we can see the different layers
we have the cell wall on the outside
right below that right below that we
have the cellular membrane or the plasma
membrane so this is a cell cellular
cellular membrane cellular membrane
right under that and then under that the
cellular membrane is control is is
containing the cytoplasm and inside of
the cytoplasm we have all sorts of
things this big thing that is taking up
a lot of the volume inside of this plant
cell that's a vacuole which we have
described another in other in other
videos vacuole as a combination of this
internal pressure things like the
vacuole and just frankly the pressure
from from all of the fluid inside the
cell pushing outwards plus the cell wall
kind of holding it all in that's what
gives plants their structure that's why
a plant is able to grow and be a plant
is able to grow and be upright so that's
my drawing of a plant actually have a
plant in my room that I'm looking at
right now and it's able to grow and be
upright and so you have the cell wall
you have the cellular membrane you have
the other organelles I have some
chloroplasts here key for photosynthesis
have my our good friends mitochondria we
have our nuclear membrane or I should
say this yellow thing is the inner
nuclear membrane has the DNA inside
then you have the endoplasmic reticulum
kind of containing that the rough ER
containing the ribose or having the
ribosomes on the membrane the smooth ER
not having the ribosomes Golgi apparatus
so that's a little bit of a review but
our focus here is on the cell wall so
let's go back to that so if we zoom in
on this if we zoom in on the cell wall
right over here we can look at we can
look at this diagram and over here it
might be a little bit surprising to you
because what I've always imagined a wall
a cell wall I imagine something like a
brick wall something that's impenetrable
but this drawing shows us something
different and just to be clear what's
going on here
so this is our cellular membrane so I
already wrote cellular membrane so if
right over here I have my lipid bilayer
and then right on top of that I have the
cell wall but you see it isn't just a
thick like a brick wall something that's
impenetrable you see you have all of
these polysaccharide fibers running
across it so you have things like
cellulose which we saw as a polymer of
glucose arranged in a certain way Hemi
cellulose cellulose which has different
types of monomers associated with it we
have pectin which is another
polysaccharide and all of these things
you've actually probably eaten if not
today probably in the last week when we
talk about fiber in your diet you're
talking about things like the cellulose
and the pectin things that your body
can't digest when you eat a plant you're
getting it because you're eating their
cell walls and it does cool things like
slows the absorption of glucose in your
intestines it absorbs water so I guess
you could say things pass a little bit
easier but just to see but the the key
thing here is this isn't a wall this
actually allows or it is a wall it's
officially the cell wall but it's not a
thick it's not an impenetrable wall like
you might associate the wall of the room
that you're in you can see that it has
space for small molecules to flow and
it's really more like a mesh or like a
fabric and so the cellular membrane
actually has access to this this to the
fluid and to the molecules that are
between the cells and so just to be
clear what we're looking at this layer
editor that's a cellular membrane that's
the lipid bilayer this right over here
this is the cell wall than a different
color that is the that is the cell wall
and then right above the cell wall
that's the space between the cells which
we call the middle lamella so the space
between the cells we call the middle
lamella so this also is this right over
here is also the middle lamella so all
of that is interesting but you might say
okay well how hard is the cell I get
that it's a mesh but you know clearly
the cells are able to or the plants are
able to stand upright is that because
the cell wall provides all of that
rigidity and the answer is kind of the
cell wall is like this mesh it helps
these cells have their shape but if you
stop watering a plant you're going to
see it you're going to see it kind of
wilt over and that's because part of
that that it's ability to stand up is
from the internal pressure of the cells
but also part of its shape is the actual
cell wall now some of you might say well
I've seen plants that are much much more
rigid than this plant you've just drawn
what about things like trees what about
wood wood seems very rigid in fact so
rigid that we can build actual walls out
of wood and the answer there is these
more mature plants actually once the the
cell has stopped growing and you have
your cell wall more layers of cellulose
and other molecules can be built to form
what's called a secondary cell wall so
this could be viewed as a primary cell
well then a thicker secondary cell wall
could be built which gives much much
much more rigidity and so when you look
at wood what gives wood its structure
even if you were to take out all of the
water I mean if you were to dehydrate
the wood it's still going to have its
rigidity because the cellulose layers
and the other molecules are so thick
that's able to have its rigid form now
the last thing I want to talk about
we've already seen that the cellular
membrane has access to the the molecules
floating around between between cells
but there's actually also direct tunnels
between adjacent plant cells and those
direct tunnels I've drawn here on this
outside of the cell wall as these little
yellow circles these are plasmodesmata
plasma these are plasmodesmata and to
forget about understanding what they're
like imagine this is one cell so I'll
write mirror cell one and let's say this
is cell to cell two right over your
never crow
section you see the plasma you see the
plasmodesmata are these tunnels that
form between not just the membrane and
the cell wall and the plasma deck it
forms between the two cells and so you
can actually have a flow of cytosol and
small molecules directly directly
between these two cells if you want to
get a little bit more involved in the
structure you have this kind of smooth
endoplasmic reticulum type going through
it but I want to make it very clear
because a lot of times when you study
biology it's all explained it seems all
neat and clean and textbook but people
are still studying exactly why do we
have these things why are they why why
are they necessary what gets transported
across these things and how are they
able to transport it under what
conditions are they so all of these
areas when you were to kind of dig one
layer deeper than frankly I'm talking
about you're getting into an area of
active research so anyway hopefully this
whole thing gives you a little bit more
appreciation for the wood around you the
plants the house plants around you and
even the salad that you might have for
lunch