Roots and Shoots - Online Tutor, Practice Problems & Exam Prep

Plants consist of two major structures: roots and shoots. Roots primarily absorb water and nutrients, while shoots facilitate photosynthesis by capturing sunlight and carbon dioxide. The stem, composed of nodes and internodes, supports vertical growth. Leaves, with their petiole and blade, are crucial for gas exchange and photosynthesis. Various adaptations, such as adventitious roots and modified stems, enhance plant survival in diverse environments. Understanding these structures and their functions is essential for grasping plant biology and ecology.

Plant Cells Review

Hi. In this video, we'll be talking about the two major structures of the plant body: the roots and the shoots. Now, before we get there, let's actually start by reviewing some of the plant anatomy we've already discussed. Hopefully, you remember that plants are eukaryotic organisms that synthesize sugars by using sunlight energy to generate ATP, and then using that ATP to carry out carbon fixation in the Calvin cycle. This process, of course, is photosynthesis that I'm talking about, and if you don't remember this or you feel like you need to review it maybe, I highly recommend you go back and check out the videos on photosynthesis and the Calvin cycle. There are also some videos on general plant biology, let's just say, that can cover some topics to maybe give you a little more background before this.

Now, moving on, the chloroplast is the organelle in the plant cell that carries out photosynthesis. Usually, there are many chloroplasts in there, not just one carrying out photosynthesis. Chloroplasts get their name from chlorophyll, which is the green photopigment that plays a big role in absorbing sunlight energy. Now chloroplasts, like the one we see here, only take up a little space in the cell. They're, you know, many need to fit in a cell, so they don't take up tons of room. However, plant cells have this large structure called the central vacuole. Oops. And the Central Vacuole has a variety of functions in the plant cell. For one thing, it's filled with water, sugar, amino acids, and other stuff. Sometimes plant cells will sequester toxins in the central vacuole to protect the cell from them, and we call this solution in the central vacuole, the cell sap. And in addition to just holding this material, the central vacuole also is responsible for maintaining a plant cell's turgidity, right, that rigidness that the cell gets. Now the vacuole is only one piece of maintaining that rigidness. The other piece is the cell wall, which provides structure and protection for the plant cell.

Now, you might recall there are actually two types of cell walls: the primary cell wall and the secondary cell wall. The primary cell wall is made of cellulose, and cellulose, you'll remember, is that polysaccharide, and it surrounds the plasma membrane while the cell is growing. Now, the secondary cell wall is only found in some plant cells. The secondary cell wall is a thicker structure, and not only is it made of cellulose, it's made of lignin. Lignin is a polymer, you might recall, that's found in vascular plants and it helps the cells in those plants maintain rigidity. It's also a very important component of wood, so that might give you a sense of the sort of hard material that results from lignification. Now the secondary cell wall will actually form after the cell has grown. So, the primary cell wall is going to form during cell growth. After cell growth, some plant cells will form that secondary cell wall. You might recall that plant cells actually have channels between them called plasmodesmata. These channels allow for transport of materials between cells and cell signaling. So you can see, basically the image we have here is a blown-up version of one of these junctions between plant cells. You can see the plasmodesmata create these channels between cells. However, these plant cells still have their cell walls and plasma membranes. And of course, one last thing that's worth noting is, this substance, the middle lamella, mostly made of pectin. It's kind of like a sticky goo that's going to help glue these plant cells together, and you will find it on the outside of the primary cell wall.

Now, let's flip the page.

Root System

Hopefully, you remember from our discussion on photosynthesis that plants need water, carbon dioxide, and sunlight to carry out that process. Water provides the source of electrons, carbon dioxide is the source of carbon, and sunlight is going to provide the energy needed for this process. In addition to these things, plants also need nutrients. They need nutrients like nitrogen, phosphorus, potassium, and magnesium. In particular, they need these things to build molecules and to maintain their cells, basically to live. They'll die without these things. Hopefully, you already are, you know, your brain gears are already turning, and you're thinking, ah, nitrogen, phosphorus. What could those be used for? Proteins, right? Proteins, nucleic acids too. I mean, the backbone of nucleic acids has all those phosphate groups, proteins and nucleic acids filled with nitrogen. So, you know, these nutrients are essential to the basic function of a plant's cells. Now, usually, these nutrients are obtained from the soil where they're found as ions, right? So, we're not just going to see plain old potassium sitting around. It's going to be like potassium with a charge on it. Of course, potassium is a cation, so it's going to have a positive charge. Now, plants absorb, you know, these nutrients and water through their roots. And, of course, they absorb sunlight energy through their leaves. And this absorption is going, or this need for absorption is going to play into the morphology or the appearance of those structures. So leaves have to absorb that sunlight energy, right? They take the appearance of sheets. This is actually giving them a really high surface area to volume ratio. You might recall that that surface area to volume ratio has a huge effect on absorption. The higher that ratio, the more absorption is possible. So leaves being these broad, very thin structures have very low volume for a huge amount of surface area. Roots have a slightly lower ratio. They're tube-like structures, right? So they're going to have more volume to, more volume compared to their surface area, but they form a really diffuse network. So they are able to still generate a large amount of surface area for absorption. Now, the reason they need a little more volume is that roots are also going to be super important for transporting materials, right? So you need a little space in there, in order to move stuff around effectively. Now storage structures which, you know, can be found, you know, in roots, for example, like tubers, where they, you know, the root will swell up with sugars or something like a carrot, for example. Those are structures that are not involved in absorption, and that's why they will actually have the lowest surface area volume ratio of these parts of the plants. Now, you know, you can see the two major parts of a plant right here. We have the shoots, and down here, the roots. And of course, you can also see that in this, picture of a real tree over here, where the earth's been cut away. So you can actually see all these roots below the ground here. And, of course, all these, branches and the actual trunk of the tree itself are the shoots, the proverbial shoots. So the root system has, a couple of jobs. It's not just there to absorb water and ions from the soil. It also anchors the plant into the ground, which is quite important. I mean, you know, you don't want to just blow away if a gust of wind comes by. Roots can also store materials produced in the shoots for later use, you know, think of a carrot, for example. The carrot, those are the shoots. We actually eat the root of this plant. We don't really eat the shoots of this plant. And that's because these roots are full of sugar. So they're delicious, right? And that's because they're actually being used for storage purposes, right? It's these green tops that are actually producing the sugars. And then, they're being stored in, the carrot root. Now, when talking about roots, it's important to get a little terminology straight. The main dominant root is called the taproot. Right here, you can see the taproot. I'm drawing a line through it. That is our taproot. And it's the dominant root from which all the other roots of the plant will project. Those other roots of the plant are things we call, oops, lateral roots and here I'm going to draw a lateral root, or a few lateral roots in red, so those are lateral roots. Now it's important to note that not all roots are actually going to arise from, arise from, like, some taproots. Some plants are what we call adventitious. And basically, these have roots that develop from the shoot system rather than the root system. So here you can see the shoot of a plant. So that is the shoot. Right? And, here are some roots sprouting from it. These structures right here. Let me jump out of the image. Sorry you guys can't see what I'm pointing at. Those are the roots. They're sprouting off the shoot, making this an adventitious plant. There are a variety of plants that show this sort of, morphology. A lot of vines are adventitious. More famously, fig trees send down these, long, you know, tendrils from their shoots down to form roots into the ground. Very cool-looking if you've ever seen one in person. It's almost like a canopy of these aerial roots. Now, let's turn the page.

Shoots and Leaves

The root's main job is absorbing water and nutrients. The shoot's main job is to absorb CO₂ and light, and to then carry out photosynthesis. The stem of the plant is the vertical growth structure. This is how the plant is going to reach its leaves up to get access to light. And when we talk about the stem of a plant, it's important to get some terminology straight. There are these points on the stem we call the nodes. This is basically the part of the stem where leaves and buds will grow out. And the space between these nodes is called an internode. So, basically, the stem of a plant is made up of a series of internodes and nodes.

Now, at these nodes, we have what is called a lateral, or sometimes an auxiliary bud, and you can see a picture of that here. We have these little buds represented in yellow, appearing at the nodes of the stem. Now, at the top of the plant, we'll have what's called the apical bud. You can see that up here. This image is labeled 'terminal', but this is also called the apical bud. Now, this apical bud is the primary growth point of the plant. This is where the plant is going to basically grow upward from. Those lateral buds are where leaves, branches, or flowers, you know, various lateral structures will develop. So apical bud is for vertical growth, lateral bud is for lateral growth.

Now, very quickly, branches are a special structure. That is a woody projection from the plant. Not all plants are woody plants, so not all plants are going to have those woody projections. Some stems are non-woody. So turning our attention to what grows on the stem, let's look at leaves. These are the organs of vascular plants that are the sites of photosynthesis and gas exchange, and they actually have 2 parts. The petiole, which is the stalk or this portion that connects the leaf to the stem. So this is the petiole, right here. And then the other portion is called the blade, like a blade of grass. That's really the portion we think of as the leaf, so to speak. Now, the interior tissue of the leaf is where the magic of photosynthesis, so to speak, happens. We call that area the mesophyll. So here, this is all mesophyll. And you can actually see, this layer on the outside, that's the cuticle, a structure we've talked about before that prevents water loss, from land plants.

And also, you might notice here in the bottom, the guard cells, these cells around the opening in the leaf known as the stoma, or stomata as the plural form you see here. So, those stomata are the pores in the leaf that control gas exchange, and help regulate water loss. So, they open and close based on the swelling or shriveling of the guard cells, which are these special cells that fluctuate their turgidity to open and close that pore, that stoma. And you can see that going on in these images here. And actually the way these plants swell or shrivel is by moving Potassium ions, and water follows those potassium ions. So either they're drawn into the vacuoles. Right? The central vacuoles, water follows, they get turgid. So here they're turgid, they swell up, and that pops open the pore, or, the potassium ions leave the vacuole, go to the extracellular environment, drawing water out of the vacuole, making those guard cells shrink and lose turgidity, and that causes the stoma to close. Alright. That's all I have for this page. Let's flip.

Phenotypic Plasticity

Plants have an amazing range of phenotypic plasticity. Meaning that they're able to take these basic structures, the roots, stems, and leaves, and modify them to gain a significant variety of functions. Now, I don't really want you to memorize any of this; I more just want you to get a qualitative sense of the breadth of morphological variety that plants can take on. So, let's start with roots. Now, we talked about adventitious roots before, which are those roots that are going to come from a stem. And there is a special type called a prop root, which is basically an adventitious root that props up the plant. It gives it aerial support. So these are modified roots that give the plant a little boost above the ground. Right here we have a picture of a corn plant, which has these prop roots, but these prop roots are also seen in, for example, mangrove trees, which live in brackish water and use the prop roots to kind of prop themselves out of the water. Go figure. Now, pneumatophores are generally advantageous that they don't have to be, and these are special modified roots that allow for gas exchange. You can see them in this image, all these little, they look like sticks popping up from the ground. Those are actually roots, technically they're pneumatophores, so those are roots that are doing gas exchange. Plants, again, like mangroves, have these because, for example, with the mangroves, they live in that brackish water, they still need to extract gas, so these pneumatophores allow them to pull gas out of the air by sticking up above the water.

Now, let's move on and take a look at some modified stems. So, for example, we have cacti. Right? A cactus is basically a stem, but it's been modified for water storage. Right? You know, it's the old myth, if you're stuck in the desert, find a cactus and drink it. I don't recommend doing that. Drinking from a lot of cacti will actually make you very sick, or maybe even a little more than that, see some crazy things, let's put it that way. Now, stolons are a type of modified stem that allows a plant to produce a new individual above ground. You can see an example of a stolon right here, this is the stolon, it's being kinda like shot out of the plant, and it where it lands, it produces a new individual. Sort of like the opposite of a stolon is a rhizome, and this is basically the same type of thing, a stem projection that comes out, this time it goes underground instead of going out above ground, and where it lands, it produces a new individual. Now, stems can also be modified into what are called stem tubers. Most notably, things like potatoes. Yeah. Potatoes are actually just modified stems, modified to store carbohydrates. And another structure you probably didn't realize was actually a modified stem, is a thorn. Thorns are just stems that have been modified into these defensive structures.

Let's lastly take a look at some modified leaves. Bet you didn't realize that an onion is actually a modified leaf. Yes. The layers of an onion are all modified leaves. It's called a bulb, and it's basically a leaf that's been modified for food storage. So, just like a stem can be modified into tubers, leaves can also be modified for food storage. And don't forget, earlier we looked at the example of a carrot, which is a root that has been, in some cases, in some sense, modified for food storage. Now, succulents are not only drought-friendly plants, and hopefully, if you live in a drought-stricken area, you'll replace your lawn with some nice succulents. But succulents have modified leaves that improve water storage. They're very often compared to cacti because of those similarities. You know, they have the same kind of, almost like feel to them. A nice example of a succulent is the Agave plant, which is what tequila comes from. Also, mezcal. Don't discount mezcal. Very underrated. Now, tendrils are another type of modified leaf. These are how climbing plants, like vines, are able to grip onto surfaces. So, they have these modified leaves that almost act like grappling hooks. Right? They wrap onto a surface and grip it, allowing the plant to climb vertically. There's also leaves that act as floral mimics, basically like pretending to be a flower. A perfect example of this is the poinsettia, which has these leaves that turn red to mimic the appearance of a flower, even though it's actually just a leaf, and this is done to help with insect pollination. There are also traps like the Venus flytrap. Right? That eat animals, those are also modified leaves. And lastly, cactus spines are modified leaves. So, just want to make a quick note here, remember thorns are modified stems, these cactus spines are modified leaves. So they may seem like more or less, the same structure, but they're coming from different sources. Now that's all I have for this lesson. See you guys next time.

Here’s what students ask on this topic:

What are the main functions of roots in plants?

Roots primarily serve three main functions in plants: absorption, anchorage, and storage. Firstly, roots absorb water and essential nutrients from the soil, which are vital for the plant's growth and metabolic processes. Secondly, roots anchor the plant securely in the ground, providing stability and preventing it from being uprooted by wind or other forces. Lastly, roots can store food and nutrients produced by the shoots, such as in the case of carrots and tubers, which store sugars and other carbohydrates for later use. These functions are crucial for the overall health and survival of the plant.

How do leaves facilitate photosynthesis and gas exchange?

Leaves are the primary sites for photosynthesis and gas exchange in plants. They contain chloroplasts, which house chlorophyll, the pigment that captures sunlight energy. This energy is used to convert carbon dioxide (CO₂) and water (H₂O) into glucose and oxygen (O₂) through the process of photosynthesis. Leaves also have structures called stomata, which are small pores that open and close to regulate gas exchange. Guard cells control the opening and closing of these stomata, allowing CO₂ to enter for photosynthesis and O₂ to exit as a byproduct. This process also helps in regulating water loss through transpiration.

What is the difference between primary and secondary cell walls in plant cells?

The primary cell wall is the first layer formed during cell growth and is mainly composed of cellulose, a polysaccharide. It is flexible and allows for cell expansion. In contrast, the secondary cell wall forms after the cell has stopped growing and is found in some plant cells. It is thicker and contains both cellulose and lignin, a polymer that provides additional rigidity and strength. The secondary cell wall is crucial for supporting the plant's structure, especially in woody plants. These differences in composition and function are essential for the plant's growth and structural integrity.

What are adventitious roots and their functions?

Adventitious roots are roots that develop from non-root tissues, such as stems or leaves, rather than from the primary root system. These roots can serve various functions depending on the plant species. For example, in some plants like mangroves, adventitious roots called pneumatophores facilitate gas exchange in waterlogged soils. In other plants, such as corn, adventitious roots known as prop roots provide additional support and stability. These roots can also help in vegetative propagation, allowing plants to spread and colonize new areas. Adventitious roots are a versatile adaptation that enhances a plant's ability to survive in diverse environments.

How do plants adapt their structures for different environmental conditions?

Plants exhibit phenotypic plasticity, allowing them to modify their structures to adapt to various environmental conditions. For instance, roots can develop into prop roots for additional support or pneumatophores for gas exchange in waterlogged soils. Stems can be modified into structures like stolons for above-ground propagation or rhizomes for underground growth. Leaves can also adapt; for example, succulents have thick leaves for water storage, while tendrils in climbing plants help them grip surfaces. These adaptations enable plants to optimize resource acquisition, support, and survival in diverse habitats, demonstrating their remarkable ability to adjust to changing environments.