Paul Andersen explains how nutrients and water are transported in plants. He begins with a brief discussion of what nutrients are required by plants and where …

Hi. It's Mr. Andersen and in
this podcast I'm going to talk about plant
nutrition and transport. I'm going to use
a number of terms that you should be familiar
with. Like monocot, dicot, or ground tissue,
vascular tissue. And if you're not sure what
I'm talking about, make sure you go back and
watch the previous podcast on plant structure.
But before we get into that we should talk
about what plants need. In other words, what
is the nutrition that plants count on? Well
it's the same thing as in you. They need CHNOPS.
So basically what they need is carbon, hydrogen,
nitrogen, oxygen, phosphorus and sulfur. And
so where do they get the carbon? Well, unlike
us, they're going to take carbon in in the
form of carbon dioxide. So that's going to
come in through their leaves. They're going
to absorb their water, so they're going to
get their hydrogen and oxygen through their
roots as well. But the other things like nitrogen,
phosphorus and sulfur, they're all going to
take in through their roots. Now how does
that differ from animals? Well we simply eat
things, like plants. Or we eat things that
eat plants. And so we can get all of those
nutrients. And so basically we are kind of
talking about the digestive, circulatory and
respiratory system all in one, in relation
to plants. And so basically plants live in
two worlds. They live in the world above ground
and that's the world of the shoots. And then
they live in a world below ground. And that's
in the world of the roots. And so basically
the one thing that they're taking in through
their leaves is carbon dioxide. And they are
using that carbon dioxide to make sugars.
And those sugars are going to make up the
bulk of a plant. But they are also going to
be used for energy within the plant. Now,
as far as roots go, they need water. And so
that water is going to flow in through their
roots. It's going to move up through the xylem
and it's eventually going to evaporate through
their leaves. And so they're taking that water
in. And the one thing that may puzzle you
a little bit on this diagram is that they
are also taking in oxygen. They are taking
in oxygen through their roots. And you might
think that seems a little odd. What do they
need oxygen for? Well remember they do photosynthesis.
And they're using photosynthesis to take in
the energy of the sun and convert that carbon
dioxide into sugars. But they are going to
use those sugars as well. So they do cellular
respiration. What do you require for cellular
respiration? You need sugars. And they are
going to produce those through photosynthesis.
But they also need oxygen. And so any part
of a plant that's growing or requires energy
is also going to require oxygen. Okay. Let's
start at the roots and then kind of work our
way up through the shoots. I'm going to have
examples here. These are the dicots and the
monocots. Remember a quintessential example
of a dicot would be like a sunflower. And
a monocot is going to be corn. Or a dicot
could be a dandelion. And a monocot could
be grass. But basically what were going to
do is we're going to work our way up from
the roots to the stems and then finally to
the leaves. And you'll find that there are
similarities between the two, but subtle differences.
And so if we start in the roots what we are
looking at here is a cross -section. So imagine
that a root looks kind of like this. And there's
going to be root hairs coming out of it. Basically
what we'll do is we'll just take a cross-section.
So we're cutting right across like that. And
that's why we have this kind of a circle.
And so basically what do we have here? Well
there are three types of tissues. Remember
there's going to be an epidermis on the outside.
So you can see the epidermis here. There's
going to be ground tissue here in the middle.
So on the monocots same thing. Epidermis on
the outside and then it's going to be ground
tissue right here. But you're going to have
vascular tissue on the inside. So that vascular
tissue is going to be made up of xylem and
phloem. And it's kind of hard to see with
that color. Likewise here we're going to have
xylem and we're going to have phloem. The
phloem is going to be right outside of that.
And so basically if you think about it, in
the roots the vascular tissue, that's going
to be the movement of water and the movement
of nutrients, is going to be centered in the
middle of the root. Now if you think about
it, that doesn't make sense. How is the water
going to get from the outside to the center?
Well every time we build new roots you can
see, this is roots actually in a potato, you'll
see that they're actually starting at the
bottom and then they're growing up from the
center. And so it's kind of like a plumbing
network where it's going to start on the outside,
it's going to move to the inside. And then
three dimensionally it's going to go up in
the plant. Now if you look at the edge, some
of these cells are modified into what are
called root hairs. And the function of that
is to increase surface area. And a lot of
these have fungus that are living in and around
them. Symbiotic fungi. It allows them to absorb
even more nutrients. Now we're going to go
above ground into the shoot system. And we're
going to look at the stems. And so what you'll
find is that we're still going to have the
epidermis on the outside. You could see that.
So here's the epidermis on the outside. It's
going to have that cuticle on the outside
of that. But what we start to see in the dicot
is that those vascular tissues moving from
the center of the root towards the edge of
the stem. And the reason why is that it eventually
has to flow out into the leaves. And so right
here would be the xylem, closer to the edge.
We're going to have the phloem here. So this
would be the xylem here, the phloem here in
a dicot. Likewise here we're going to have
the phloem here, excuse me xylem and then
the phloem right here. I've always thought
these looked kind of like little monkey heads
in a monocot. And then we're going to have
the epidermis out here. What are all the other
cells then? The other cells are going to be
ground tissue. So that's going to be ground
tissue here as well. Alright. Let's keep going,
because remember those stems are eventually
going to bud off and those are going to reach
to the leaves. And so if we look at leaves,
remember what makes a dicot a dicot is it's
going to have this net-like venations. So
you can see these nets that are branching
off. Those nets, those veins are actually
the vascular tissue. And so if we look at
a cross section right here of a dicot you
could see that vascular tissue moving right
through the middle. Likewise, if we go to
a monocot, what makes them a monocot? Well
one of the characteristics is you're going
to have one of these parallel veins that are
going through the leaf. And so you can see
that those veins are vascular tissue that's
moving right through the leaf, if I kind of
try to draw this three dimensionally. But
both of them are going to have essentially
the same characteristics when you get to the
level of the leaves. And now we're taking
a cross section of the leaf. We've got our
epidermis on the top. You got the waxy cuticle
above that. We're going to have epidermis
on the bottom. You can see right here that
there's a stomata there. What the stomata
for? That's to take in carbon dioxide and
also to lose water. Some of the other levels
that we have in here is this. We're going
to call this the palisade mesophyll. And then
we're going to have this down here which is
going to be the spongy mesophyll. And so basically
what is that? And it's organized different
in dicots and monocots. But basically this
palisade mesophyll is like palisades, like
a waterfall, is going to be an area where
we have photosynthesis going on. So we're
going to have light coming in and that light
is hitting chloroplasts. And that's where
the photosynthesis is taking place. But what
else do we need for photosynthesis. Well,
we need that water. That's going to come through
the vascular tissue. And then we need carbon
dioxide. And so the bottom part here, all
of that space down here is for the carbon
dioxide gas to diffuse in and then can be
used in photosynthesis. One quick thing I
should mention going way back down to the
roots again, our immune system kind of protects
us from it's environment. And also our skin
is going to protect us and is going to allow
things in that we want and and things out
that we don't. And so basically the way it
works in the roots of a plant is a little
bit different. And so imagine now we're down
right in here. We're down in the root hairs
like this. So how is water getting in? Water
is flowing in through osmosis. So it's flowing
in here to the middle. And remember it's going
to go to the center of the root and then it's
going to work it's way up. But we don't want
to let just anything come in. And the way
water flows is kind of odd. Some of the water
is going to flow, so this is one cell and
this is another cell, so some of the water
is going to flow within the cell. So it will
go across a membrane. And it will flow through
these little pores between the cells called
plasmodesmata. But some of it will actually
move through the cell wall. So you can see
this water right here would be moving through
the cell wall and it's never having to go
across a cell membrane. And so basically what
plants have, now let me do a quick cross-section.
So if this is the epidermis and this is the
vascular tissue on the inside of a root, so
the water is going to flow in like this, basically
we don't want it to flow up in a plant unless
we have had some say over what has coming
in. And remember what protects our cells,
what allows certain things in and certain
things out, that's going to be the cell membrane.
And if you look at this water, since it's
going all the way across here, we call that
apoplectically. It's moving through the cell
wall. It's never had to go across this cell
membrane. And so basically what happens is
you're going to have an endodermis here. This
has a lining around it. And there's going
to be a waxy cuticle called the casparian
strip. You can kind of see it right here.
This is that casparian strip that's connecting
all of the cells. And basically what it does
is it acts almost as, I don't know if you've
seen the Lord of the Rings, but there's a
scene where Gandolf, you know, puts his staff
down and says, "You shall not pass". Basically
what the casparian strip is doing is it's
this strip that's goes all the way across
here and it's basically forcing that water
that's made it all the way to the inside of
the vascular tissue, it can't keep sneaking
through this cell wall. It will have to force
its way into the cell. And it has to go symplastically
at that point. So the function of the casparian
strip is to force the water eventually to
go across the membrane so we have control.
And so we just have two things left to talk
about. And that's basically how water moves
up a plant and then how sugar moves in a plant.
And we've talked about transpiration before.
But I want you to think about it as a pull.
In other words there's going to be xylem,
which are these vascular tissues that go all
the way from the roots, all the way up in
the tree. It's going to go all the way up
the inside of the bark. All the way up to
the leaves. And eventually evaporate. And
so there's a connection all the way up from
the roots to the leaves. But there's also
something important you should know about
water. Water is slightly charged. And that
means that this right here, the oxygen is
going to have a slightly negative charge.
And the hydrogen is going to have a slightly
positive charge. And so basically water will
line up so the negative oxygen is attached
to the positive hydrogen. And we call this
bond right here a hydrogen bond. And so basically
water is all connected through these hydrogen
bonds. And so once one bit of water starts
flowing up the tree, the ones are going to
follow. And we call that process, or we call
that phenomena cohesion. There would also
be some adhesion to the inside of the xylem.
So two quick things you should know about
xylem. It's dead at maturity. So these are
cells that are now empty. They're not alive
anymore. It's simple just a container or a
vessel for the water to flow through. But
basically if we go all the up here to the
leaf, water is evaporating out. So the H2
is evaporating out. That water is connected
all the way back through the vascular tissue
through the leaves, through the veins. All
the way back down through down here to the
root hairs. And so we have a continuous pathway
all the way up of water. And so every time
one water molecule evaporates here in the
leaf, that's going to pull this whole chain
up. And so really xylem and the water is moved
through a pulling process. What's generating
all of that energy to pull it up? It's going
to be the sun is pulling it all the up like
that. Okay. Now last thing then is how does
phloem move? Well phloem is different. Phloem
are cells. So here is one cell. Here's another
cell. Here's another cell. They're empty.
And so how do you be alive as a cell and be
empty? Well basically they'll have these companion
cells that are going to have a nuclei. And
they're connected to the phloem cells. And
they're going to provide the metabolic support
for phloem. But phloem is a living cell. And
xylem we said is dead. They're going to be
little connecting holes all the way through
here. And same thing over here. It will be
connectors like that. And so basically I like
to think of it as being pushed. And so phloem
and sap and sugar is actually pushed around
in a tree. And you might be thinking how does
that work? Well let's look at this carrot
here. So we've got a carrot. It's planted
underground. And let's say that it's the second
year. And this carrot happens to be coming
back again. Well if you think about it, where
is the sugar going to be if this plant is
just starting to grow? Well more of the sugar
is going to be in the carrot. And so if more
of the sugar is going to be in the carrot,
so we have sugar here, that sugar is going
to flow out from an area of high sugar concentration
down in the carrot, into low sugar concentration
in here. We're also going to get flow of water
in this direction. But basically the sugar
is going to be pushed in this direction. Since
there is more sugar down here we call that
the sugar source. And so phloem is actually
going to be pushed from the source to the
sink. Okay. So there's more sugar down here.
Where's it going to go? Well there's sugar
all the way down here, so that sugar is going
to start to flow up to here. The leaves, it's
early in the spring we'll say, and they're
starting to grow. So they're not making any
sugar yet. That means that there's no sugar
here. And so we call this, a place where there's
no sugar, the sink. It's just like a sink.
It's just like a sink. So the water flows
from the source to the sink. Sugar does the
same thing. So the sugar is going to flow
up here and then it's going to move into these
cells. And so basically we get this flow of
sugar like this. This is going to be early
in the spring. But let's say the carrot starts
to grow. And now we've got photosynthesis
going on. And not we've got these leaves producing
a bunch of sugar. Well now this is the sugar
source. So we're making a bunch of sugar here.
That's going to push the sugar down here from
an area of high sugar to low. That's going
to be stored down here in the carrot. And
so that's going to become the sink when it's
active. And so basically what we get is this
flow of sugar, always from the source to the
sink. But we have this flow of water all the
way up the tree continuously as long as the
sun in shining. And so that's basically how
material is moved around in a plant. It tells
you a little bit about what they need. How
they get it. And I hope that's helpful.}

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