Biology: Phloem: The Movement of Sap

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So, plants suck water up out a leaf; they suck water up the stem to the leaf; they suck water out of the ground into the root, up the stem, out the leaf. Is it a circulation? Does it go in a circle like ours? No, it does not. Okay, and this is where plants are going to differ from you and I, in the sense that they don't have the so-called closed circulatory system, like you and I do. See, we've really covered the movement of water as a separate water fountain unto itself. But what about phloem? What about the idea that sap, that sugar has to move in, out of the leaf, into the stem, down the stem to the roots and feed the root? Let's see the answer to that one. Let's review a little bit, what we know about phloem.
Phloem is alive, and phloem has literally, two cells. The two, remember, it's derived from the proto cambium in the embryo, as it's growing. And what we're going to see here is that there is something called a `companion cell' and a `sieve tube.' And these are called sieve tubes because at the end, there's kind of a sieve-like arrangement in the cell, of the cell wall. Now you're saying, "well, isn't that what xylem had?" No, xylem had holes, remember xylem was a straw. This is a living cell, and in this living cell we're going to have connections, and that's what this sieve tube is all about.
So, how does a phloem work? Well, the phloem works; you know, xylem kind of works the same way. We use a term to use this called `bulk flow.' Bulk flow simply means there is some kind of a driving force behind it, and in the xylem the bulk flow was caused by transpiration. In phloem, it's going to be caused by, not transpirational pressure, but by a different kind of pressure. And in fact, we're going to refer to this as `pressure flow.' The flow is going to be caused by pressure, but by pressure of another type. It's actually going to be an osmotic pressure, if you will. So pressure flow.
Let's take a look at how this works and how we're going to accomplish something called `translocation.' How we are going to translocate sugar from the leaf, from the leaf, in the leaf, sugar, in the leaf, to the rest of the plant. How are we going to that? How are we going to translocate it? Well, you know every once in awhile, we can explain a living-all the time-we can explain a living system using an experimental system. And I want to use two words for you here. First, lets draw a little tub of water. And in this tub of water, I'm going to put a tube, like a glass pipe. And on that glass pipe, I'm going to put some selectively permeable tubing. And what I'm going to do (and now this is full of water), and what I'm going to do is, I'm going to put on this bag or on this side a bag with sugar solution in it. The bag is impermeable to sugar, but permeable to water. And, I am going to call this the source; that's one of the words I want to tell you about.
The other word I want to tell you about, is this word: sink. Engineering terms... we use them all the time. The thing with a lot of something, is the source, and the thing to which it's going to flow, is the sink. We can have a heat sink, for example. Anyway, this just has water in it. Watch what pressure is going to cause. What's going to cause it in this case? Osmotic pressure. Water is going to go shooting in there. Why? Because there's a high concentration of sugar and a low concentration of water and water's going to go in there. And if we take a look at what's going to happen to the sugar molecules, the sugar molecules (and remember, these bags are permeable to water), so water, if need be, can be pressured out of here.
And so what would possibly push water out of there would be maybe movement in that direction. Perhaps movement of sugar molecules. And as sugar goes in here and goes in that direction, you're going to eventually push sugar right through. That being said, let me ask you this. Will it ever stop? And the answer is, of course. It will stop when the sugar concentration here equals the sugar concentration here. And here come what makes a plant alive. What the plant is going to do is, it's going to take this sugar out of here, which I can't do in this bag, and we're going to unload this sugar.
So we have a couple of things we have to do. Number one: we have to load the sugar into this bag through the process of photosynthesis. Then we have to translocate to a sink through pressure flow, and then we have to unload it, through-I'm not going to tell you... Okay-act of transport. But don't tell anyone I told you.
Okay, let's take a look at how we're going to do this. Let's talk about the loading step, let's load. Well, pretty much, this is going to be a situation where it's going to be simplastic movement. In other words, we are going to literally-here's my mesophyll, and here's my photosynthetic cells that are making the sugar-and our job is to get it here into the xylem, okay, or excuse me, into the phloem. Because we want it to leave from the phloem and flow downward; the phloem: it flows downward into the parts of the plant. And you can see that what we've done here is created a symplastic pathway, which is absolutely makes sense. In fact, it probably enters the tonoplast, through the tonoplast and stores in water vacuoles also, as its moving through here-there's no question. But I want to show you one very interesting thing that we can add to this story.
And what we can add, is that some take the appleplastic pathway. And what's interesting about that is that, if you'll notice, there is a companion cell. Remember the companion cell for phloems? Well, it turns out that in the species of plants, that use the appleplastic pathway, the companion cell-which you recall is right next to the phloem tube- the companion cell has all sorts of involutions in it, for increased what? Increased surface area. And remember any cell that's all bent and twisted, like this one, and if that's its cell wall, and you're moving liquid along that pathway, you've just come up with a way to move more liquid through that cell. So, although I'm telling you that it can be a symplastic pathway, and it usually is, some species have gone one more step and added an appleplastic pathway with the companion cell the one right next to the sieve tube element, having increased surface area.
But that's not all. Some plants even use active transport, and that's very cool. Corn for example, in its phloem cell, concentrates-corm is a good one, corn produces a lot of sugar, you know that, corn syrup-corn uses a cold transporter mechanism to pump sugar into the phloem. And in fact, you'll find out, you can actually measure this, that in corn, the phloem has three times the amount of sugar as the mesophyll. Now why is this? Well, interestingly enough, it's co-transport. Let's see what happens.
Remember what co-transport is all about? First thing we do is we use ATP. And using ATP and a hydrogen pump, we take hydrogen ions from the inside of the cell and put them to the outside. And so we put our hydrogen ions out here. And then what's going to happen is, when the hydrogen ions are out here, they will have a natural tendency to flow back into the cell. So, here we go. We have... what cell are we talking about here? We're talking about a phloem cell. The phloem cell, and out here we're talking about a mesophyll cell. Here's what's going to happen: normally if you went the whole symplastic route and they're connected by plasmodesmata, you're not going to have a difference between the phloem cell and the mesophyll cell. Right? Cause it is just simple flow. Right? But, imagine you can do this: Imagine you can take this hydrogen and have a tendency to pass through, but it can't pass through unless it grabs a molecule of sugar; for example, sucrose. And if grabs sugar and pulls that through, you're going to end up with more in here, than out here. You're going to pump it, actively pumping it. Okay? So, you can do take with any sugars. You can do that with sucrose, you can do it with glucose, and that whole hydrogen transport system is going to concentrate the sugar in there. Oh, that's great stuff.
So, now that you have it down there, what are going to do next? Now you got it in the phloem. So now you've got to bring it down, and you've got to get it out. So, now you're down, so where are you going? You're going to the sink. Remember my diagram? We're going to the sink. So now what are we going to do. Well, now comes the final step: we want to drive the sap.
So, what we're going to do next is, as the source increases-so we have our source, and the source up in the leaf is going to increase it's amount of sugar molecules. And they're going to get higher and higher and higher. Let me ask you a question: as it gets higher and higher and higher, what's going to happen to the water pressure in there? Well, as you get a cell that has very high sugar, water from the surrounding cells is going to pour in there. Sound familiar. And, like my diagram with the two bags, as water pours in there, into the cell, it's going to push the sap along the phloem cells, to different places. Where? Well, generally down, but it might pivot it sideways, depending on how your plumbing is hooked up. And you're going to eventually get to cells that need sugar. So, now what we can do, is an amazing thing.
So, by the way, where do you think the phloem gets its water from? Could there be tubes right near by, that might have water in them? Could there be tubes that might have water and holes in them, so that water could flow where there is a high concentration of sugar-like say right here-and there's this cell, like a xylem cell perhaps, right near it. And if water is going to flow laterally into this-where there is a high concentration of sugar-and it's going to push it along; as the sugar passes by sinks (here's a sink, here's a sink), it's going to give up some of is sugar to the sink. And it's going to become less and less concentrated. So the water is going to be coming out of the xylem laterally into the phloem; pushing it along, which it's going to do some. And then what's going to happen is, eventually you're going to get to the point where the sugar is literally, completely used up, and we can have another flow of water in the other direction back to the xylem. So in essence it's going to become kind of a circulation.
So, in essence what we have is literally high sugar in the phloem: osmotic radiant; osmotic radiant: push the sugar solution; push the sugar solution: rip-out the sugar; you're back to water; start all over again. Where does it start? Up at the top. So it's a constant rebirth, if you will, of sugar. Of course, if photosynthesis is going to happen, structure follows function, even in plants.
Plant Systems and Homeostasis
Plant Transport
Phloem: The Movement of Sap Page [1 of 3]