- [Instructor] Plants and
trees are not just great
at creating their own food from sunlight,
but they're also excellent
in moving that food around.
When they have excess of food,
they move it to their storage organs.
For example, if this was a
carrot or a potato plant,
then they would move
that excess food down.
So they move the excess food
down and store it for future.
And now let's say maybe
during the winter season
there isn't much sunlight so
they cannot photosynthesize.
So they're not able to get a lot of food.
Now, they can move this food
up to wherever they want.
So they can move it back
from the storage organs
to the place where they need,
maybe the growing areas.
And this process of moving the food around
wherever they want, we give a name to it,
we call it translocation.
Translocation.
This is basically plants and
trees moving the food around
to the places they need.
But the big question
is, how do they do that?
I mean, for one, how
do they even understand
which part of the plant body needs food
because they don't have a brain?
And secondly, what mechanism do they use
to transport that food up or
down according to their needs?
Well, let's find out.
Now before we continue,
let's back up a little bit.
We've already seen in previous videos
that plants and trees consists
of pipe-like structures
to transport stuff around,
that starts from the roots
all the way to the leaves,
and we call these
structures vascular tissues.
And there are two kinds,
xylem that transports water and minerals
from the soil to the
different parts of the body,
and phloem, which mostly transports food.
Now if these terms look new to you
or you need a refresher on this,
then you can go back and
watch our previous videos
on intro to vascular tissues
and videos on xylem and transportation.
But if you feel comfortable,
then let's continue.
Now in this video, since we
wanna talk about translocation,
meaning transportation of food,
we are going to be
concentrating on phloem.
So translocation happens
inside the phloem.
Food gets transferred in them, but how?
Well, let's see.
Let me draw xylem and phloem.
So let's say this is the xylem
tissue which has water in it,
this is going to be
important for us as well,
and here is our phloem.
Phloem mostly contains sugar
molecules dissolved in water
forming a very thick sugary
sap, which is food for them.
And if you're wondering
what these things are
over here at the sides, that
is some leftover cytoplasm.
The cells don't lose
all of their cytoplasm.
And similarly, this is the cell walls.
They have developed gaps in between.
So they have not lost their
cell walls like in xylem,
some gap is developed so that
the food can, this whole sap
can move throughout the phloem.
And if you're wondering why
it is structured like this,
we'll talk a little bit
about that towards the end.
Anyways, imagine a couple
of cells next to phloem.
Let's assume these are far
apart, okay, not so close
and let's assume this cell
has a lot of sugar molecules in it.
And imagine this cell doesn't
have much sugar molecules,
it needs a lot.
Let's say it's one of the
growing regions of this plant.
So it needs a lot of sugars.
So how do we transport the
sugar from here to here
is the question?
Well, you can kind of guess the process.
We take some of the sugars,
put it into the phloem,
then we make sure it transports
through the phloem till here,
and then we remove the sugar
and put it into this cell.
That makes sense, right?
But a couple of questions could
be how does the phloem know
in which direction to transport this food?
Because that cell which
is in need of sugars
can be anywhere.
It can be below it or it
can be above this as well.
So how does it know whether
it has to go down or up?
And secondly, how do you even
move this thick sugary sap
through the phloem?
So let's put this back over here
and let's see how this works.
So the first step as you predicted
is to put the sugar into the phloem tube.
I'm just gonna call that loading.
And this process is an active process.
What does that mean? Active.
Well, think about it.
If the concentration over here of sugars
is less than over here,
then sugars will automatically diffuse
from higher to lower concentration.
But pretty soon the
concentration will equalize.
How do you move even more sugars there?
Well, that requires energy.
The cells have to spend some energy
to move the sugar from lower
to higher concentration.
So, by using energy,
the sugars need to be
transported into the phloem.
And because energy is
utilized for this process,
we say it's an active process.
Now see what happens.
Because of a lot of sugar,
the concentration of sugar
is much higher in this region
compared to any other regions.
And remember, nature always likes
to balance the concentration out.
So one way to reduce the concentration
is to put the sugar back.
But remember that's not
possible, we're not doing that.
So what else can nature do?
Well, there's xylem right
next with a lot of water.
So you know what's gonna happen?
Water will start flowing
from xylem into the phloem,
to this part of the
phloem because of osmosis.
So let me write that as step two.
Remember osmosis it's the
process in which solvent flows,
like water starts flowing from
a low concentration region
to a higher concentration region.
Basically water is flowing
to try and dilute this concentration.
Okay, what does that do?
Well, because a lot of
water is flowing in,
this region of the phloem
starts puffing up over there.
This increases the pressure over here.
And what I mean by that is
you can imagine these walls
are pressing on this solution,
squeezing that solution a lot.
Now, what do you think will happen
if I squeeze this solution a lot?
Well, it'll automatically
start moving from here
to a region where there is low pressure.
It's kind of like
squeezing a water balloon.
And where do you think
is the pressure lower?
The pressure is lower
where there is less sugar
concentration, right?
Because if there's less
sugar concentration,
there will be less water over here
and that's exactly where you need
to send your sugar molecules.
So if on the top there is a cell
which has very low sugar concentration,
automatically that part
will have low pressure,
automatically this side will move up.
If it turns out that
somewhere in the bottom
there is low concentration,
automatically the pressure
over there will be very low
and the sap will move down.
And this is how the pressure decides
in what direction the sap will move.
So in our example, the sap will
move down from here to here.
Let's call that as our step three,
and we can call this the bulk flow,
the bulk flow driven by pressure.
And the reason we are calling it bulk flow
is because this whole thing
is a solution, remember.
Even though I've put dots over here,
this is one single solution.
And so the whole solution
moves down, not just this part.
All right?
So let me show you what
that will look like.
So as the solution moves,
this pressure is relieved.
Let me get that back to normal.
And so due to the high pressure over here,
this whole solution will
start moving like this.
The whole thing will move from
high pressure to low pressure
until it reaches over here.
And then finally the sugar
reaches the low pressure region.
Because there is less
concentration of sugar,
it will move out of phloem
and it can now be taken up by this cell.
And so this we'll call it as
step four, unloading happens.
The sugars get unloaded
from the phloem tube.
So let's unload that sugar from the phloem
into the required cell.
And finally, remember that region
which had a lot of concentration
of sugar had a lot of water
due to the osmosis.
Well now, that concentration has lowered
so, that water will move back to xylem.
So in step five, again, osmosis happens
and the water moves back
and that's how translocation
happens inside plants.
Now before we summarize
and wind up this video,
one question we need to
address is why phloem structure
is a little different than that of xylem.
Why does it have cytoplasm,
little bit of cytoplasm left?
Why are the end walls not
completely gone like in the xylem?
Well, that's because
these cells are alive.
Phloem has life cells.
In contrast, xylem has dead cells.
And so xylem cells can afford
to lose all of their stuff
because they are dead, right?
But phloem cells need a
little bit of cytoplasm
and their cell walls to stay alive.
Now that could raise even more questions.
First of all, you may be
wondering, how can cells stay alive
without a nucleus or
without a mitochondria?
How can these cells be alive?
Well, in fact, you know
how they are staying alive?
They have a partner cells which
I have not shown over here,
but each cell is connected to a partner.
Let me show you what would that look like
if I were to draw those.
So these are the partner cells
and they have all the stuff needed
to keep these cells alive.
In fact, they are life partners, okay?
And that's why these cells
are called companion cells.
All right?
And so in reality, if you're wondering,
when you want to load the sugars,
you have to first load it
into the companion cell
and then it goes into the phloem.
Similarly, while unloading,
it first goes into the companion cell
and then it goes out of the phloem.
But that's a small detail.
We don't have to worry
too much about that,
so let me get rid of those.
So it's those companion
cells that keep them alive.
But another question
you might be wondering
is why do we have to go
through all that trouble
to keep these cells alive?
Why is it necessary that
these cells need to be alive?
Well, the answer is in this process.
You see, we just saw that
in order to load the sugar
into the phloem, that requires energy.
It's an active process
and the cell can only generate
energy if it is alive.
So if the cells were dead, like in xylem,
they wouldn't be able to generate energy,
they wouldn't be able to load sugar,
they wouldn't be able to
accept that sugar molecules.
It would just diffuse back
and then the phloem
transport wouldn't work.
And xylem cells don't need to be alive
because the mechanism of xylem
is only based on physical
forces like suction
or the pressure from the
bottom, from the roots, right?
You don't need the cells.
The cells don't need to use any energy
so they can afford to be dead,
but phloem needs to be alive.
All right, that's pretty much it.
So, let's quickly summarize.
What did we learn in this video?
We saw that plants and trees
can move the food up or down
based on their requirement and
we called this translocation,
and this happens inside
the phloem tissues.
And how do these sugars go
from one place to another?
Well, in the first step, we have loading
where the sugars get loaded
actively into the phloem tubes
and this requires energy,
that's why it's called
as an active process
and that's why these
cells need to stay alive.
And in doing so,
because the concentration
over here has increased,
water starts flowing from
xylem into the phloem
to try and decrease that
concentration due to osmosis.
And because of that, a
lot of water comes in
and the cell now kind
of all there puffs up.
I like to imagine that way
because now you can see
that there's a lot of pressure over there.
And then because there is high
pressure region over here,
it automatically wants to move
towards the low pressure region.
And the low pressure region is the region
where there is less
concentration of sugars.
And as a result, the
whole sap starts moving
towards the low concentration region.
And so this is that step
three, which is a bulk flow.
It starts moving, the whole sap moves
from higher to lower concentration,
and now that pressure is relieved.
And then in step four, the
sugars automatically diffuse out
because outside there
is less concentration
and that can now be taken up by the cell,
which we will call it as unloading.
And then finally,
the excess water drains
back into the xylem
because the concentration has decreased.
So it goes back
and that's how translocation
works in plants and trees.
