- [Instructor] Look at
all these giant trees
standing tall and mighty.
When I used to look at them,
I used to wonder about one question.
How do these trees take
up water from the ground
and push it all the way to the top
against the force of gravity?
I mean think about it.
Some of the trees can grow
hundreds of meters tall.
So, how do they get that
water all the way to the top?
The answer is evaporation, that's right.
It turns out and this blew my mind away
when I learned about this,
it turns out more than 95%
of the water that they absorb
just evaporates into the atmosphere.
But doesn't that sound crazy to you?
I mean how can evaporation
make water go up
and what's the whole point?
I mean if they are evaporating
most of that water,
why do they even need water?
What's going on?
Well, let's find out.
Now, before we dive into this,
let's back up a little bit.
We have seen before that plants
and trees can grow so tall
because they have evolved pipe
like structures within them
which are called vascular tissues.
They help in transporting
the food and water
from one part of the body to another.
And since we have to transport
two things, food and water,
there are two separate kind of pipes.
One is called xylem which
transports mostly water
and some dissolved minerals
and the other one is called phloem
which mostly transports food,
f for food, ph for phloem.
Now, if you seem to be pretty unfamiliar
with these fancy names then
we've talked a lot about them
in a previous video called
intro to vascular tissues.
Be great idea to go back and
watch that as a refresher.
If you feel you're confident,
let's go ahead then.
So, in this video, since
you wanna talk about water
we're going to be focusing only on xylem
and see how it works.
Now, when I was studying about this
even before thinking about water,
my main question was, where do
these pipes come from, right?
I mean how did they get there?
Turns out that they come
from modified cells.
For example let's think
about how xylem is formed.
Well, here's how we
like to think about it.
So, before xylem was formed
we had normal plant cells
but as they mature they start losing
all of this stuff from inside.
They start losing their
nucleus, their mitochondria,
their cytoplasm, all of that is gone
and eventually they lose even these
horizontal cell walls as well
and all the vertical cell
walls join together to form
a tall empty pipe, that's our xylem.
And since it's empty we can
now fill it up with water.
And so this might make you wonder,
if these cells have lost
all of their organelles
how are they staying alive.
The answer is they aren't.
They're all dead.
And so important to remember that xylem
is made of dead cells,
they're no longer alive
and just to contrast it
turns out that phloem
is made of living cells.
We'll get to that in another
video as to why and how
but xylem is made of dead cells.
And so now to the big question,
how does the water climb
up these xylem vessels?
Well, like I said before
it's due to evaporation.
When the water eventually reaches
all the cells of the leaves,
most of them go out of the
tiny pores which are present
in these leaves.
You might know the names of these pores,
they're called stomata.
They're useful in exchanging gases.
When the stomata opens up,
the water gets evaporated
in to the atmosphere.
But how does this evaporation
help in pushing the water up?
Well, to figure that out,
let's zoom into one of these leaves.
So, let's say that this
is that same xylem tissue
that we just talked
about made of dead cells
and let's say this is one of the leaves,
highly magnified view you imagine
and this is one of those pores the stomata
through which the water can go out.
Now, if we draw some
of the water molecules,
imagine these are individual
molecules of water.
I know they seem too big.
Molecules are not so
big but imagine, okay.
And so these are the water molecules
that are moving out of the leaves.
It turns out that these
molecules pretty strongly attract
other molecules as well
and as a result when
these molecules move out
they automatically start pulling
on the molecules behind them.
You'll learn in chemistry that this force
is called the hydrogen bonding, okay
but don't worry about the names as of now.
So, because of this attraction
they pull on the molecules behind
and as a result these start moving away
and then they start pulling
on the ones behind them
and they start moving
on the ones behind them.
And so can you see that
because of this the force,
this pulling force starts from the leaves
and goes all the way down to the roots
because this is a single column of water,
a giant very tall column of water
and that's how the force gets transmitted.
This is kind of like how
she's drinking that juice
from the straw.
She's sucking on that straw
and the juice comes up.
Similarly, you can imagine
the xylem to be a giant straw.
The evaporation causes that suction
because of which the water gets pulled up.
And so because there's
a lot of suction force
within the xylem,
the walls of the xylem tissue
needs to be super thick,
thick enough to withstand that force.
Now, what do you mean by that?
For example, just look
at this water bottle.
When I start sucking on
it look at how easily
the walls collapse.
There is a similar suction
over here but way stronger
and so the walls need to be
strong enough to not collapse.
And so xylem has pretty
strong walls because of which
it also acts like a backbone
to the plants and the trees.
And if you're wondering,
wood is made of xylem.
In fact, the word xylem
comes from the Greek xylon
which means wood.
So, the wood that is
used in the furnitures
and tables and everything
were once a part of this
amazing water conducting system.
Pretty awesome, right?
But you might be thinking,
why do plants have to go
through all this trouble
to suck that water and then
just make it evaporate?
Why are they doing it in the first place?
Well, for many reasons.
First of all, they need
some of that water.
They don't throw all of it away.
Some of that water is
useful in photosynthesis.
But more importantly this
running stream of water
helps in transporting the
minerals, the important minerals
from the soil to all
the cells of the plant.
So, this water acts like a train
where the minerals hop in at the soil
and then when they reach their
destination at the leaves
they get absorbed, the
water gets thrown away
and the process repeats.
So, it's because of this the the trees
get the important minerals
because of which they can grow.
And if that's not enough
evaporation is also useful
in cooling them down
just like how when you go out in a hot sun
you start sweating because
when that sweat evaporates
your body gets cooled down.
In a similar manner,
these trees and plants
are always in the hot
sun during the daytime.
So, they need to cool off as well
and so instead of sweat they
just evaporate these water
and that helps them cool down.
And so you can see
evaporation is a big deal
for these plants and trees.
It's such a big deal
that scientists thought,
let's give it a different name altogether.
So, the evaporation that
happens in the trees
from their plants, from the stomata,
it's called transpiration.
Okay, transpiration.
And why is this transpiration useful?
Because it causes suction force.
So, it causes suction
because of which the water gets pulled up
and all the minerals get transported
and also it causes cooling,
this is called evaporative cooling.
Both of these are essential
and that's how xylem pretty much works.
But if you're really curious,
you might ask one final question.
What happens during the night time?
Because transpiration requires heat
which you can get during the daytime
but during night time there's
hardly any transpiration.
So, what keeps that column
of water up and running?
Why doesn't it just fall down?
That's because there is a second force
that is causing this water to go up.
This force originates
from the roots itself,
it's called the root pressure
because it comes from the roots
and it's this force that makes
sure during the night time
the column just doesn't fall down.
So, there are both forces acting.
There's a suction force
due to transpiration
and there is this root pressure.
During the daytime, the
transpiration dominates
but during the nighttime because
there is no transpiration,
root pressure is the only force.
But how does that work?
Well, to quickly give you an overview,
here we have zoomed in to the tip
of one of the tiny root hairs.
And what you're seeing
over here is the water
and these dots represent minerals.
So, we have water and
dissolved minerals in the soil
and we also have water
and dissolved minerals
in these root hairs.
And the concentration is pretty much even
because diffusion makes sure of that.
But now the cells of these root hairs
start messing with the
concentration balance.
They start absorbing the
minerals from their surrounding.
So, look at the minerals carefully.
They start absorbing it from the soil
and this cannot happen all by itself,
the cells need to spend energy
to bring those minerals in.
And because of that it's
called an active process
or an active transport.
Active means they're using
energy to transport the minerals
inside their cells.
And now look at what has happened.
Because the soil lost the minerals,
this solution has become dilute
because there's a lot of water
but there's less minerals
and the solution inside the
root has become concentrated,
more concentrated than outside.
And nature does not like
that, nature needs balance.
So, either the minerals
have to flow back out
but the cells don't allow that,
they use energy to make
sure that doesn't happen.
But if that can't happen,
you know what nature does?
It makes sure that the
water starts going in,
the water starts rushing in.
Think about it.
Because the outside region is more dilute,
the water moves out from that
and enters the more concentrated region.
It starts diluting this as well.
And in the process notice
the water starts rushing in.
You may have heard of this process.
This process is called osmosis.
It's a special kind of diffusion in which
particles don't move
from higher concentration
to lower concentration but
instead the water starts moving
from lower concentration
to higher concentration.
but the idea is the same
to balance the concentration.
And as a result of this osmosis,
now can you see that
because water is rushing in
it starts pushing on
the column of the water
that's already present over here
and that's what causes the
pressure from the bottom.
This is called the root pressure.
Let me just write that down.
This pressure is called root pressure.
And in more general because
it is happening due to osmosis
you can also call this
as the osmotic pressure.
And so this root pressure forces
the water to go up as well.
It's not as strong as
their suction produced
due to transpiration but
during the night time
this is the only available force.
So, this makes sure that the
column doesn't fall down.
And that's pretty much it.
So, to quickly summarize, what
did we learn in this video?
We saw that xylem tissues
are made of dead cells
which help in transporting water up
and it happens due to two forces.
One is the transpiration in
which the water evaporates
and causes suction
which literally sucks
the water from the roots.
And we saw that transpiration also helps
in cooling the plants off.
And the second pressure
is the root pressure.
This is where the roots are
actively absorbing the minerals,
increasing the concentration
and thereby causing osmosis.
And so this continuous
upstream of water makes sure
that every single cell
gets the required minerals
from the soil and helps recycle
the water from the ground
into the atmosphere.
