45: How Plants Use Water Transcript
Hallie: Hello and welcome to One to Grow On. A show where we dig into questions about agriculture and try to understand how food production impacts us and our world. My name is Hallie Casey and I studied and currently work in agriculture.
Chris: I’m Chris Casey, Hallie’s dad. Each episode we pick an area of agriculture or food production to discuss and this week we are finally talking about how plants use water.
Hallie: You have been asking me this question for a little while.
Chris: It’s true and we did like what? 10 episodes on water or two or four or something. I don’t know.
Hallie: Only two.
Chris: Okay. We did not answer this question and now we can answer this question and I assume I’ve heard of xylem and phloem, which are the things in the trees and water goes up, water comes down, never a miscommunication.
Hallie: [Laughs]. Yeah, we’re going to be talking about mostly four different roles that water plays in plants and how they work. This is not completely everything that goes on in a plant that involves water because plants are mostly water.
There is a lot that goes on, but these are like the four begins.
Chris: Are they mostly water the same way that we are mostly water?
Hallie: The same way that most things are mostly water, yeah.
Chris: Most of the plant is mostly water. Okay.
Hallie: The first one we’re going to be talking about is structure. Dad, do you know what makes plants stand up?
Chris: Presumably, fiber that our doctors keep telling us to eat.
Hallie: Right. Humans have bones. Bones is what makes us be able to stand up and things like that, but plants do not have bones.
Chris: That would be super creepy if there were a plant with bones, super creepy.
Hallie: Yes, for some plants, it is cell walls. For some plants, it’s not cell walls. Do you know which plants it’s not cell walls for?
Chris: No, bananas would have to be cell walls. I don’t know. What do you even mean? What are you asking me? Like plants that stand up, but not because of their cell walls?
Hallie: We’ve talked about this a little bit in the past. There are different ways we categorize plants, right? One of the ways is between woody plants and herbaceous plants.
Chris: Got it. Are you going to tell me that the woody plants is not their cell walls, but I would argue that woody tissue is still some sort of cell wall. I don’t know.
Hallie: The main thing that keeps woody plants standing up is something called lignin.
Chris: Oh, lignin. I forgot about lignin.
Hallie: Your old friend lignin.
Chris: That’s right. For the listener who hasn’t listened to an episode where we have defined lignin. Lignin is?
Hallie: Oh me?
Chris: Yeah, you.
Hallie: I thought you are going to give the definition of lignin.
Chris: I wish.
Hallie: Lignin is basically like, if you look at branches, trunks, it’s what makes sticks sticky. It’s kind of those crunchier little cells and it gives those trees structure. It’s what builds out woody material. Whereas when you look at something like a banana or a sunflower is a really good example of this, they typically don’t have a lot of that woody tissue and so the thing that is keeping them stood up is water.
Chris: It’s like that old joke. What’s brown and sticky? A lignin based plant structure.
Hallie: A stick with lignin.
Hallie: We said cell walls, right? I just mentioned water. What do you know about plant cells?
Chris: I know they are cells.
Hallie: Yeah, true.
Chris: I guess for the ones on the outside it’s called cellulose. I don’t know why it’s called cellulose. I don’t know what cellulose is. I don’t know. I got nothing else.
Hallie: Do you know what the largest part of a plant cell is?
Hallie: The largest part of a plant cell is what’s called the vacuole.
Chris: Okay. That’s a word that was probably on some worksheet in ninth grade biology.
Hallie: [Laughs]. The vacuole is basically the goo of it all. Within a plant cell, you’ve got a mitochondria or two. You have some chloroplasts. You have lots of different things in there. All of that is sitting within the goo of it all, which is the vacuole. We can kind of think about plant cells like a balloon or I guess more accurately like a water balloon, but it’s easier for us to think about it as an airfield balloon because those are the ones that we really see stretched out. A water field plant cell is usually pretty tight. If it’s filled all the way up, those walls are really sturdy. It has a lot of structure to it. It’s not caving in. If you put pressure on it, it’s able to kind of keep its shape. That’s similar to how a cell wall works in a plant if you have a full vacuole. That’s what’s called turgor pressure. It’s that pressure within a cell that’s blowing up those cells. It pushes those cell walls out and it lets the plant stand up, which is why, if a plant isn’t watered enough, it loses that turgor pressure and those cell walls start to collapse in on each other and then that’s when we start to see wilting. It’s because those cells aren’t able to fill up the balloons with all that water.
Chris: I almost feel like saying the vacuole is the largest part of the cell. It’s like saying the biggest part is the part that’s not there.
Hallie: Yeah, I know.
Chris: Except that it’s goo that requires water to be present.
Hallie: Right. One of the really nice things about this for plants is that water is not a solid right. It lets plants be a lot more flexible. Of course, trees are not as flexible as little bitty flowers on the ground or grasses or things like that.
Hallie: But having their structure being made up of water lets them be a lot more flexible for the wind and stuff. If wind comes along, it can blow about in the wind and it’s not going to be breaking because it has a lot more flexibility, which is great.
Hallie: That’s the number one thing that plants use water for. That’s like the thing that a lot of the water is. Most of the water in plants is within the vacuole. Another thing that plants use water for is thermal regulation.
Chris: Staying not too hot, not too cold, but just right.
Hallie: Mostly not too hot. Water is not going to keep you super warm, generally speaking.
Chris: Fair enough. Staying cool in the summer.
Hallie: Yeah, staying cool in the summer. This is very important for plants. The plant type term is actually called transpiration. Transpiration is the thing that keeps plants cool. It’s them losing that water off of their leaves. It’s like when we sweat. When you lose that water, the energy it takes to have that water evaporate pulls heat energy off of the plant.
Chris: Right. Heat of evaporation, good stuff.
Hallie: Exactly. This happens through the xylem, which you mentioned earlier.
Chris: Okay. What is that?
Hallie: How much do you know about the xylem?
Chris: I know it’s not the phloem.
Hallie: Is that it?
Chris: That’s it. I think one goes up. One goes down.
Hallie: That is very helpful. The xylem goes up. The xylem is different from the phloem in that the xylem is made up of dead cells, so there’s no actual energy. There’s no living cells that hold the water in the xylem. There’s no energy to move the water. It’s just dead cells and it’s what’s called passive transport.
Chris: Kind of like hair for the plant.
Hallie: Like hair?
Chris: Like hair. Hair’s dead cells, fingernails are dead cells. Now plants have like little fingernails type of stuff.
Hallie: Like how we lose our water through our hair.
Chris: I mean, maybe it doesn’t work quite the same way in animals.
Hallie: Like how our hair is little tubes of water.
Hallie: Sorry, I keep thinking about tubes of hair water.
Chris: Okay. Just to be clear, plants don’t have hair, but the thing that they do have called xylem is dead cells that transports water from the inside of the plant to the outside of the plant.
Hallie: You did just say plants don’t have hair, but in this section we are actually talking about two different types of hair that plants have.
Chris: Oh, great.
Hallie: Let’s look into that. In the soil, you have roots and on the main parts of the roots you have little root hairs. Root hairs are really, really small oftentimes they’re microscopic but they have a lot more surface area than the root itself, so most of the intake and output from the root system is actually happening on those root hairs.
Chris: Got it.
Hallie: The way that plants take up water, because again, there’s no energy expended to take up water is just a concentration gradient, which means that the water potential is lower inside of the root hairs than outside in the saturated soil. If there’s a puddle of water on the countertop and you have a dry sponge and you put that dry sponge in the teensiest bit in contact with that, that water is going to slowly move into the sponge, right?
Chris: Yeah, okay.
Hallie: That’s because of a concentration gradient. The water wants to be where water is not.
Chris: It’s exactly the same way in the root of the plant you’re saying.
Hallie: Well, I’m saying inside of the root hair there is going to be a lower water potential than outside. If you’ve just watered, the soil is fully saturated. There’s a lot of water out in the soil itself. That water is going to move to where water is not, which is inside of the root hair.
Chris: Right. The root hair functions the same way a sponge functions basically.
Hallie: In this case, yes. Water molecules H2O are cohesive, right?
Chris: Yes, they adhere to each other.
Hallie: They adhere to each other because they have those little hydrogen bonds. That’s why if you fill a cup up too much, you have a little bubble above the rim of the cup or it’s the same reason why you get droplets of water. It’s because those little molecules of water want to stick to each other. They have this cohesion property.
Chris: They’re like tiny magnets for each other.
Hallie: They’re like tiny little magnets. They love to stick together.
Chris: That’s why little bugs can skate across the ponds.
Hallie: Exactly. It’s the exact same reason. This is kind of wild, so prepare yourself. The way that transpiration works in a plant is you have the xylem. It’s these dead cells. It’s basically like a straw inside the plant. It goes all the way up to the leaves. The sun is shining or the wind is blowing and it pulls off a water molecule from the leaf. Every single water molecule within the plant is stuck together. They have those cohesive properties. As one water molecule gets pulled off of the leaf, it’s pulling one more up through the roots and into the stem. It’s like one big chain of links and it’s just slowly moving up through the plant.
Chris: That makes sense. This sounds similar. Not exactly the same, but similar to when you have a cup of water and you’re in a restaurant and you really want to know your parents, so you put the straw in the water and then you blow across the top of the straw and the water comes up the straw and out where you’re blowing. It sounds like a similar principle only you’re not using wind to do it. It’s just the pressure gradient is created by the fact that the water is exiting from the leaf and pulling out more.
Hallie: This does happen because of wind sometimes. The reasons why plants lose water are different reasons. It could just be there’s a really low humidity outside and so it’s being pulled out. There could be a high temperature. Sunshine could be evaporating the water or wind is something that can pull water out of a leaf.
There are different properties to a leaf to make it more or less likely that water will come out of the leaf. If they have a waxy cuticle, if they have more or fewer stemmata, which are like little pores, which is what the water actually comes out of or how open those pores are, they can be more or less open.
Chris: I’m sorry, you said waxy cuticle and now all I can think of is a plant like sitting in a chair getting a pedicure, getting the files down, talking to gossip and all those kind of stuff.
Hallie: That’s exactly what I mean.
Chris: What’s the cuticle on a plant?
Hallie: It’s something like if you think of the difference between a basil leaf versus like a holly leaf. A holly leaf has a really waxy cuticle because they kind of have that waxiness to them. They’re really thick. Basically, it just means that on top of the leaf skin, there is a really like thick layer of wax that’s protecting those pores from having water pulled out of them.
Chris: Got it.
Hallie: Another good example is like succulents. If you have like a jade plant or something in your house, then those have pretty waxy cuticles usually. But another difference could be the leaf size or the leaf shape, whether or not the leaf is folded inward.
If you look at desert plants, then usually they have really, really small leaves because they’re less likely to be losing water through those and then another factor is actually the pubescent on the leaf. How much pubescent there is, which is like little bitty leaf hairs.
Chris: This is something that makes middle schoolers laugh hysterically I’m sure.
Hallie: I’m sure it is.
Chris: All right.
Hallie: Maybe that’s why we don’t teach plant anatomy to middle schoolers.
Chris: Maybe so.
Hallie: But yeah, we were talking about hairs earlier. This is another way that have hairs. They have root hairs and they have leaf hairs. The leaf hairs are called pubescent.
Chris: I had no idea and these are ways for plants to release water.
Hallie: These are basically ways for plants to not release water.
If you’re an understory plant and you might not be getting a lot of water, maybe you’re a small little bush and you’re sitting next to a big tree, you might develop a lot of pubescent so that you can hold onto as much water as possible or if you’re out in the middle of a prairie and it’s really sunny and there’s not a lot of water to go around because you’re surrounded by all these hungry grasses, then maybe you develop really small leaves so that you’re not losing water whenever willy nilly so that you’re able to survive or you know we have live oaks here in Texas and we have really unpredictable rainstorms. We have a drier season and in the wetter season, usually our live oaks have these pretty waxy leaves, so they’re not losing water throughout that dry season.
Chris: A plant on a prairie is wetter when it’s hairy.
Hallie: [Laughs]. Great work, dad. Absolutely great work.
Chris: Thank you. I mean, it’s basically like an animal in the cold is hairier and they don’t lose as much heat. A plant in the shade has these pubescent and doesn’t lose as much water.
Hallie: Right. We have been talking about this from the beginning as a way to regulate temperature as well.
Chris: Got it.
Hallie: All accurate.
Chris: All right. Well, I think about these poor little plants when they do lose too much water, part of what they need other than more water is a break.
Hallie: Here we go.
Chris: Welcome to the break.
Hallie: Hello. I would like to talk to you about the census.
Chris: Why do you want to talk to people about the census? I mean, really.
Hallie: Well, if you live in the US once every 10 years, we do this big count of everyone living in the US and it’s extremely important for things like representative apportionment, which is how we decide what your political power as an area or region looks like or things like funding your schools and hospitals. The deadline to fill out the census is September 30th, 2020. It takes like two and a half minutes. It’s not a very long process and it’s extremely, extremely important. You don’t have to be a US citizen. All you have to be is living in the US. You don’t have to be a voting age. You don’t have to be anything like that. You should just be filling it out for your household. You can go to census2020.gov to fill out the census. It will not take you very long.
Chris: You know who I bet has filled out the census?
Hallie: Who’s that?
Chris: Our patrons, especially our starfruit patrons, Vikram, Lindsay, Mama Casey, Patrick, and Shianne. Thank you so much patrons. We can’t do this without you.
Hallie: All of our patrons, including and especially our starfruit patrons are absolutely amazing and we are so, so grateful for you all.
Chris: You’re all counted.
Hallie: In the census of our heart.
Chris: That’s right. [Laughs]. Back to the episode.
Hallie: We talked about thermal regulation. We talked about structure. Now, let’s talk about the use of water as a solvent. Plants use water as a solvent inside of the phloem.
Chris: All right and the phloem is down.
Hallie: Phloem is not the xylem. It is in fact both up and down.
Chris: Oh, all right. Xylem goes from the root to the leaf.
Hallie: There’s not another way it can go because it’s passive transport. Phloem is active cells, so there is actual energy that is expended because it is moving against a concentration gradient. The phloem moves things like glucose, amino acids, some nutrients. The xylem also moves some nutrients, but it does not move all of the nutrients. The main thing that the phloem is moving is these sugars and proteins.
Chris: Wouldn’t have known that plants need proteins, but, okay.
Hallie: Where did you think the proteins in the plants come from? We eat plants and they have protein.
Chris: That’s true. I never really thought about it. I thought maybe they could just build proteins. But to me, if you need proteins, it’s because you have muscles you got to build because you’re working out. You’re getting jacked. I’ve never seen a rose with a six pack.
Hallie: Plants do build proteins. They don’t eat other plants that have proteins in them. They just build them themselves. But usually they’re building them and then they have to go around to all the different cells because one of the main things that we use proteins for is things like DNA synthesizing.
Chris: Oh, that’s important.
Hallie: There are lots of important amino acids out there. Lots of important sugars as well, but every single cell has to have sugars and amino acids. You have these little cells they’re alive. They have to expend little ATPs, little energies as they’re moving these proteins and sugars around the plant because oftentimes they’re going against a concentration gradient. When we’re thinking about like a straw for the xylem, the phloem is more kind of a well, so like pulling something up with the well. You’re moving energy to get a resource out of somewhere and into somewhere else.
Chris: Xylem is like a straw. Phloem is like a well or like something with an active pump.
Chris: Got it.
Hallie: That’s the third one. The fourth one is bio chemical reactions.
Chris: Chemical reactions are great.
Hallie: You’re a fan?
Chris: I am a fan. Good old chemistry.
Hallie: Cool. Do you know the reaction for photosynthesis? Don’t look at the notes.
Chris: I don’t, but I think it’s like glucose in light makes carbon dioxide in water or something like that.
Hallie: Well close, but no. If we think about how plants function, it’s actually carbon dioxide and water plus light makes glucose and oxygen. When we think about what plants are creating and giving off, the end product is that glucose they’re able to store and utilize and then the oxygen is coming off of the plant. We’re starting with the carbon dioxide. That’s what the plants are taking in and they’re also of course taking the water in from the roots. Then once you have energy from light, then they’re able to convert that into glucose and oxygen.
Chris: It’s weird to me that it’s synthesizing sugar from carbon dioxide and water because I mean, I never studied biochemical reactions I guess, but I don’t think of carbon dioxide and water as reactive, but I guess this is how we talk about plants taking carbon dioxide out of the air. I guess this is reaction where they do it and they make sugar for us to eat and for them to use and oxygen for us to breathe, which is very nice of them. Thank you, plants.
Hallie: Exactly. That’s how it works. Of course, that’s not just how it works and I don’t have time to go all the way into all the intricacies of the photochemical reactions that are happening throughout photosynthesis.
Hallie: But I did want to talk about one example of where water comes into play because like you said, you don’t just like take a carbon dioxide molecule and a water molecule and you stick them together and you make sugar. There are a lot more biochemical reactions that have to go into it. I will link in the show notes an incredible comic that is made by a guy called Jay Hosler who’s a great science communicator. I was actually shown this comic when I was first learning about photosynthesis when I was in grad school and really trying to understand every single step of photosynthesis. This comic is like a little ant and a little bee and they jump inside of the plant and they walk you through photosynthesis in the most engaging way.
Chris: That sounds awesome.
Hallie: It’s great. I am going to link it in the show notes. I very strongly recommend anyone who’s interested to go check it out.
Chris: Why haven’t I rewritten something like this? Gosh.
Hallie: [Laughs]. In order to create the sugar and the oxygen you have to have water to be present. The xylem and the phloem are moving that water up and down the plant. How is the water actually used in the reaction of photosynthesis? Here’s one example. Inside of the plant, there are plant cells. Inside of a plant cell there are these things called chloroplasts.
Inside of chloroplasts, there are these little things that look like a stack of pancakes and they’re called thylakoids.
Chris: I like pancakes. That’s a word I’ve never heard before.
Hallie: The thylakoids is actually the thing that absorbs light during the photosynthetic reaction.
Chris: It’s like a little plant solar cell.
Hallie: Kind of like that. The part that actually grabs the light energy are what are called photosystems, which are inside of a single thylakoid. They’re basically clusters of chlorophyll molecules along the thylakoid membrane. There are two photosystems. There’s photosystem one and photosystem two. I’m going to talk just about photosystem two.
Chris: Oh, you’re going to make photosystem one pill all left.
Hallie: Listen, there’s a lot of steps to photosynthesis. It’s extremely complex. I’m amazed that plants do it every single second of the sunshiny day. It’s an amazing, amazing process. I cannot possibly walk through every single part of it because I do not understand it. It is so complex. It is so beautiful. It is so amazing.
Chris: Yet we eat them.
Hallie: I know. We eat them after they’ve made these beautiful sugars with this amazing miraculous process.
Chris: Thank you, plants.
Hallie: Photosystem two. What photosystem two does is it chops a water molecule into two hydrogens and an oxygen and an electron. The oxygen just jumps off and it gets given off as a waste product. It left the picture. Inside of the thylakoid pancake, one of the thylakoids is called a lumen. Inside of that lumen, it’s like the vacuole. It’s the inside part. Inside of that lumen, it’s chock full of hydrogens, which creates what’s called a proton gradient.
Chris: Ah, that’s a word I’ve heard before.
Hallie: What’s that?
Chris: It’s a proton gradient.
Hallie: Correct. Yes.
Chris: I did take a semester of biochemistry. It’s something I remember us talking about and areas of differing charge, basically.
Hallie: Exactly. That’s exactly what it is.
There are several different ways that plants use proton gradients. Here is an example of a proton gradient, but one of the things that they do with all these hydrogens, they have a bunch of them chock-a-block in the lumen. One of the things that they do is once you have 14 of these hydrogens to rub together, there’s an ATP synthase guy who’s able to go in there gets 14 hydrogens together, builds an ATP.
Hallie: An ATP is adenosine triphosphate and it’s kind of known as what a lot of people refer to as molecular unit of currency. It’s basically what plants use whenever they need to expend energy. ATP is what’s used in the phloem when they’re trying to move nutrients, glucose, amino acids, all up and down. They have to use those ATPs to get that movement.
Chris: These little energy cell guys build the ATPs from the photosynthesis.
Hallie: Here’s one example. You need ATP in order to eventually build a glucose. Photosystem two has a water molecule chops it into half, gives off the oxygen as just a waste product, goes off, gets hooked up with another two, goes out into the atmosphere, we breathe it in. That’s great. But eventually you get 14 of those hydrogens, right? You’ve got one oxygen, two hydrogens. You go through that a couple of seven times or whatever. You get 14 hydrogens and then you get an ATP. That’s one example of one of the many steps in photosynthesis. It’s not comprehensive. Water is used a lot of other places in photosynthesis, but there’s one example.
Chris: Nice. It’s the miracle of life.
Hallie: The miracle of photosynthetic life. The miracle of it. It’s amazing. I love plants.
Chris: It sounds pretty amazing. I think we’ll need to talk about photosynthesis more in another episode.
Hallie: We can do that. Do you want to do a little summary?
Chris: No. I mean, that was a lot. Plants need water to stay cool and to move nutrients and to make energy and oxygen for us to breathe. Everyone, make sure the plants get water.
Hallie: Water your plants, friends.
Chris: Water your plants, friends and water your plant friends. Plants are friends and food.
Chris: Thanks for listening to this episode of One to Grow On.
Hallie: This show is made by me, Hallie Casey and Chris Casey. Our music is Something Elated by Broke for Free.
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Chris: Be sure to see what sprouting in two weeks.
Hallie: But until then keep on growing.