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Xylem & transpiration

Let's learn how transpiration helps water transport in xylem. Created by Mahesh Shenoy.

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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.