Transportation in plants
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Phloem & translocation
- [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.