In this screencast, we're going to talk
about the structure of the inside of a
leaf in order to jump start our
discussion of photosynthesis. For the
purposes of this screencast, we're going
to imagine that we've taken a leaf and
sliced it in half crosswise, so that we can
look at this cross-section of the leaf. The
cross-section of the leaf is arranged in
layers from the top to the bottom. The
uppermost layer is called the waxy
cuticle; the cuticle is made up of
hydrocarbons and wax. This makes it
hydrophobic, meaning that it repels water.
Its hydrophobic nature is what causes
water droplets to bead on a leaf rather
than soaking in. Just underneath the waxy
cuticle is the second layer, which is
called the upper epidermis. The cells of
the upper epidermis are very closely
packed together. The upper epidermis has
mostly protective functions; it allows
light to come in, but also prevents water
loss in conjunction with the waxy
cuticle. The third layer moving down from
the top is called the palisade mesophyll.
If we were to look very closely at
the cells of the palisade mesophyll, we
would notice that each of them has a
nucleus and also has a very large
vacuole for water storage.
However, arguably the most important
thing about the palisade mesophyll
is the fact that these cells are full of
chloroplasts. This makes the third layer
the palisade mesophyll the main site of
photosynthesis in a leaf. Photons of
light will shine down through the waxy
cuticle and the upper epidermis, which
are both mostly clear, and allow
photosynthesis to take place in the
chloroplast of the palisade mesophyll.
The fourth layer from the top underneath
the palisade mesophyll is called the
spongy mesophyll. Now, before you start
drawing, please leave this space in the
box blank for a minute.
The cells of the spongy mesophyll are
a little bit more irregular in shape, but
if we look a little bit more closely, we
can see that they actually do contain
some chloroplasts. This means that they
can do some photosynthesis; not as much
as the palisade mesophyll. The cells of
the spongy mesophyll are also spaced
pretty far apart, and the area in between
them is called the intercellular space.
In this space is usually water and
oxygen. Now we're going to fill up this
empty space right here on the right and
we're going to fill it with a structure
called the vascular bundle. If we go back
to our original leaf cross-section, the
vascular bundle is going to be this area
right here, what we would originally
called the stem of the leaf, but we're not
going to call it a stem. The vascular
bundle is round and is responsible for
moving products around the leaf. Just
like you have a water pipe that brings
water into your house and a sewage pipe
that removes the wastewater, the vascular
bundle works similarly. The outside
coating of the vascular bundle is made up
of a large group of closely packed cells;
these cells are called bundle sheath
cells and they keep all of the
vasculature together. Inside the bundle
sheath cells is a tightly packed group
of tubules called xylem and phloem. The
purpose of the xylem is to bring the
water up from the roots of the plant in
order to nourish the leaves. Remember
that water is a key component in
photosynthesis. After photosynthesis, the
phloem will bring the glucose
molecules all around the plant to all the
parts that need it.
Even though these two types of tubules
are doing different things, they are
right next to each other which makes
for a very efficient
system.
Don't forget that even though there are
multiple types of structures within the
vascular bundles, the vascular bundle
only refers to the structure as a whole.
There are still three types of tissues;
phloem and xylem and
bundle sheath cells. Now we're going to move on
to drawing the lower layers of the leaf
and some of these will look pretty
familiar. Just underneath the vascular
bundle and the spongy mesophyll is
another layer of epidermis.
However this is called the lower
epidermis because it's on the lowest
part of the leaf. Just like the upper
epidermis, the lower epidermis has cells
that are packed very closely together in
order to prevent water loss. Finally,
underneath the lower epidermis we have
one more waxy cuticle, just like on the
top. It helps to prevent water loss. So
now we have a nice picture that shows
all of the different layers of the leaf,
but the problem is that this current
leaf as it stands is completely
watertight, it doesn't allow for any gas
exchange. Remember that in photosynthesis
photons of light shine onto leaves, we
add carbon dioxide and water, and then
after the light and dark reactions, we
produce 2 PGAL which become glucose and
also oxygen. But, our leaf as it currently
stands can't undergo any gas exchange, so
we're going to need to introduce a
couple of modifications to the bottom
layer of our leaf. We've now zoomed in on
the lower waxy cuticle and the lower
epidermis of the leaf, and we can also
see a little bit of the vascular bundle
up top. At the very bottom of the leaf,
we're going to add a small opening to
permit gas exchange. This structure here
is called a stomate, and if we were to
zoom out a little bit we would notice at
the bottom layer the leaf actually has a
fairly sizable number of them. The plural
of stomate is stomata. This opening
allows for carbon dioxide gas to flow in,
oxygen gas to flow out, and water vapor
to also flow out.
However, if we left it open all the time,
the leaf would rapidly dry out. These two
structures making up either side of the
stove mate are called guard cells; the
guard cells can pinch together in order
to open or close the stomate as
necessary.
Here you can see what the leaf actually
looks like if you took a cross-section
and examined it under the microscope
after giving it a little dye. The waxy
cuticle is very hard to see because it's
so thin, but if you look at the area
circled in red you can see a little hint
of it. This particular picture shows this
vascular bundle is being called the vein,
which is also correct, but for our
intents and purposes, we will call it a
vascular bundle. Finally here is an
unlabeled cross-section of a leaf, again
under a microscope. See if you can
identify all of the structures labeled
here, but please ignore this one. Here's
the answer key for this particular cross
section; even though all the layers were
originally the same color it's pretty
easy to tell them apart by cell shape.
The waxy cuticle is challenging to see,
but the epidermis cells tend to be
square shaped and packed very closely
together. In the palisade mesophyll, which
is outlined in green, the cells are what
we call columnar, they're very long and
skinny. In the spongy mesophyll, the cells
are easy to pick apart because they have
a large space in between them and
they're also more irregularly shaped. The
lower epidermis and the waxy cuticle on
the bottom look very similar to what's
on the top. Stovmata are easy to see
because they look a little bit different
than the lower epidermis cells; you only
find stomata on the lower surface of
the leaf and never on the top.
Finally, the vascular bundle is distinct
because the cells are very closely
packed together and they tend to be more
rounded shape. For our intents and
purposes you don't have to be able to
tell apart xylem and phloem; just know
that they're both there and what the
functions of both of them are. That's it!
See you all in class!