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Class 10 Biology (India)
Course: Class 10 Biology (India) > Unit 1
Lesson 6: Transportation in plantsXylem & transpiration
Let's learn how transpiration helps water transport in xylem. Created by Mahesh Shenoy.
Want to join the conversation?
- why transpiration does not occur at night?(2 votes)
- There is no source of heat energy at night transpiration requires heat in order to carry out evaporation. Hope it helped!(16 votes)
- sir, what about capillarity as a force that aids the transportation of water(4 votes)
- atmahesh says that the absorption of water and minerals from the soil solution requires energy and hence is an Active process. But if we analyze the concentration difference, the concentration of solutes and water is greater outside (in the soil solution) that it is inside (the root hair cells). And hence the concentration gradient is from out to in. So this should be a passive process right? 10:15(3 votes)
- Roots grow upto a certain length in the soil. For example:
In a small patch of soil about 20-30 cm wide (rough estimates), the plant roots have been distributed evenly. So the water concentration gradient might be higher in the other portions of the soil but it might not be that high in the patch of soil we are talking about.
So as per the scenario given by Mahesh sir, the water concentration level is low in the patch of soil than it is in the root hair.
I hope this long (extra-long actually) answer might help you understand and clarify your doubt.(3 votes)
- What exactly is cuticular transpiration?(3 votes)
- Cuticular transpiration a process that occurs in the cuticle. Cuticle is a layer covering the epidermal layer. This occurs in plants which have less number of stomata and this transpiration depend upon the thickness of cuticle and the presence of wax . Cuticle is permeable to water. It is one of the 3 types of transpiration. Namely lenticular, stomatal and cuticular.(3 votes)
- how can water evapotate when it is in stomata without heat?(2 votes)
- Water is evaporated when the stomata is open and when there is excessive evaporation the stomata closes and stops evaporation. If water loss is greater than water uptake, air bubbles can form in the xylem. Plants reduce water loss by closing their stomata, developing thick cuticles, or by possessing leaf hairs to increase the boundary layer.(4 votes)
- How do the Xylem tissue lose their horizontal cell wall? Watch at2:39(2 votes)
- A possibility is that since the cells are dead gravity takes over and pulls the horizontal wall down and they break. Another possibility is erosion, maybe the wall eroded, but these are my possible answers(2 votes)
- plz tell me about ...Turgidity and Flaccidity(2 votes)
- Turgid is when water enters the cell, swelling up the cell, but flaccid is when water exits, making it rather loose. These all happen due to osmosis, which is the net movement of water molecules from an area of high water potential to low water potential.(2 votes)
- Xylem transports both water AND minerals right? So, how are minerals transported?(2 votes)
- Minerals don't get transported like chunks in the xylem. They are mixed with the water in the xylem making it a dilute solution. Therefore along with the water, the minerals mixed in the water also get transported to the leaves and other parts of the plants.(2 votes)
- why do only the horizontal cell walls decompose in the formation of xylem, why not the vertical cell walls.(2 votes)
- How does the water flow upwards using the root pressure atas just due to the concentration difference the water is coming inside but how is it flowing upwards? 11:46(2 votes)
Video transcript
- [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.