Lesson Video: Plant Tissues Biology

Video Transcript

In this video, we will learn how to describe the structure and function of different tissues found in plants. We will discover how to distinguish between simple and compound plant tissues, specifically looking into the structure and function of all three simple plant tissues parenchyma, collenchyma, and sclerenchyma. We will also learn about the structure and function of the compound tissues that make up the plant’s vascular system, the xylem and the phloem.

Have you ever wondered why celery is so crunchy or why pears are a little gritty in their texture? Different plant tissues contain various types of cells with diverse chemical and structural components that give them their unique properties, the specifics of which we will discover in this video.

A tissue is a group of specialized cells that carry out a particular function. There are two main types of tissues in plants, simple tissues and compound tissues, which are otherwise known as complex tissues. Let’s look at simple tissues first. Simple tissues are generally composed of one type of cell, or at least the majority of their cells have very similar structures and functions. For example, this micrograph shows some cells in one type of simple tissue in a plant leaf. And as you can see, all the cells have a similar large rectangular shape and contain small green circles, which are their chloroplast organelles.

Compound issues, otherwise known as complex tissues, consist of multiple different types of cell, which vary not only in their structure but also in their function. Compound tissues are mainly found in the vascular regions of the plant, called the vascular bundles. The vascular bundles are responsible for transporting substances such as water or the products of photosynthesis to the parts of the plant that require them. You can see several vascular bundles in this micrograph of the cross section of a sunflower stem, one of which has been circled in pink. Each vascular bundle consists primarily of just two compound tissues, the xylem and the phloem.

Each of these compound tissues are made up of different cells with very different structures. For comparison with these compound tissues, we can also see the three simple tissues found in plant parenchyma, collenchyma, and sclerenchyma, which sits on the outside of each vascular bundle. Each of these simple tissues contain cells that are very similar in structure and function, though their size does vary slightly. While parenchyma tissues make up the bulk of the central region of plant stems, leaves, and roots, collenchyma tissues mainly provide structural support and some flexibility and are usually found just beneath the outer layer of plant cells, called the epidermis. Sclerenchyma tissues also provide strong mechanical support, mainly to the vascular bundles.

Let’s have a look at the structure and functions of these different tissues in a little more detail, starting with simple parenchyma tissues. Parenchyma cells make up the soft, fleshy tissues inside various parts of nonwoody plants, such as the leaves, stem, and roots. This diagram represents some parenchyma cells that have been magnified within a tuber of a potato plant. Tubers such as these make up part of the root system of the potato plant, responsible for storing sugars. And they are the part that we, humans, tend to consume. These sugars are stored as starch within these parenchyma cells, providing us with the first function of parenchyma tissue, nutrient storage.

Let’s keep a list of the distinguishing structural and functional characteristics of parenchyma cells as we observe them. You can see that parenchyma cells are all round or oval in their shape. Most parenchyma cells have not yet specialized. So they have the potential to differentiate into most types of plant cell. But they all share similar basic characteristics. For example, most parenchyma cells have a large number of chloroplasts and thin cell walls made of cellulose. Parenchyma cells are living and have many functions in addition to nutrient storage. They can store water in their large permanent vacuoles. And many parenchyma cells secrete sap that is stored in these vacuoles. Parenchyma cells can use the stored water and carbon dioxide from the air to carry out photosynthesis to produce their own food. Parenchyma cells also tend to have other varied roles, such as gas exchange, to obtain this carbon dioxide that they require for photosynthesis.

Let’s look at another simple tissue, collenchyma, next. Collenchyma tissues consist of long cells with thick cell walls that provide structure, support, and some elasticity to a plant, particularly to young stems, and are therefore essential in the growing regions of the plant. This diagram represents a cross section of collenchyma cells that would be found just below the epidermis of a celery stem. Celery is crunchy as the main role of collenchyma is to provide mechanical support to these growing regions. They also provide some flexibility. Collenchyma tissues, like parenchyma, usually consists of living cells. As you can see, collenchyma cells are typically rectangular in shape, and they tend to be longer than parenchyma cells. And they have thick cell walls where the cellulose is reinforced with another substance called pectin, which adds this mechanical support to the stem.

Let’s look at our final simple plant tissue next, sclerenchyma. Sclerenchyma cells are the toughest of the three simple tissue types. Their mechanical strength is provided by their thick cell walls that provide support in stems and leaves, for example, around the vascular bundles. This diagram represents the structure of sclerenchyma cells found in the fruit of a pear plant. Sclerenchyma cells vary more in their size and shape than the other two simple tissue types. But they are typically recognizable by their thicker cell walls. Plants do not have a skeleton as we humans do, but these strong sclerenchyma cells allow them to stand upright and remain rigid. The cell walls and sclerenchyma tissues are made of cellulose, hemicelluloses, and a chemical called lignin. Lignin kills cells as it waterproofs them. So lignified sclerenchyma tissues are technically dead. It is also this lignin which adds the gritty texture to the pear fruit.

Let’s look at the compound plant tissues next, starting with xylem. As we mentioned earlier, xylem is an example of a plant vascular tissue, which means it is involved in transport. The xylem is responsible for transporting water and dissolved mineral ions, which are absorbed from the soil into the roots of a plant, where they are then transported through the xylem to all the parts of the plant that might require them. For example, the parenchyma cells in the leaves will need water to photosynthesize. Xylem tissue consists of two main types of cell: xylem vessels, otherwise known as tracheids, and xylem fibers, both of which can be seen in this diagram.

The walls of xylem vessels are made of sclerenchyma cells, which are lignified and therefore dead. The lignin in their cell walls waterproofs them and provides extra structural support to the plant. The cells in a xylem vessel are stacked end to end with their end walls broken down to form a hollow tube. And this allows water and dissolved minerals to be pulled up through them as if through a straw. Xylem fibers are also lignified and nonliving, and their main role is to provide mechanical support.

Let’s look at another type of compound tissue now, phloem tissue. Phloem is another type of vascular plant tissue that transports primarily the products of photosynthesis to the cells of a plant. These products of photosynthesis, sometimes called assimilates, are organic solutes in the form of dissolved sugars and amino acids, which are needed in various regions of the plant. These substances are moved primarily from the leaves as this is where most photosynthesis will occur, both up and down the stem to all tissues in the plant, as sugars and amino acids are needed in almost all of the plant cells.

Phloem tissue consists of four main types of cell that you can see in this diagram: sieve tube members, otherwise known as sieve tube elements, companion cells, and fibers and sclereids, which are represented here as just a singular black cell. Sieve tube members are long, hollow columns of cells fused end to end. But unlike xylem vessels, their end walls are not completely broken down. Instead, there are sieve tube plates between each adjacent sieve tube member, which, much like a sieve, have holes to allow solutes to pass through them. In order to make the sieve tube members hollow, the majority of their organelles break down and mature cells have no nucleus.

Companion cells are linked to sieve tube members by channels in their cell walls called plasmodesmata, which link together the cytoplasm of the two cells. Companion cells are examples of specialized parenchyma cells, so these cells are living. Fibers and sclereids are examples of sclerenchyma cells, and they have thick cell walls to provide structural support to the phloem vessels. Now that we know some more information about plant tissues, let’s review what we’ve learned by having a go at a practice question.

Which type of simple tissue in plants is being outlined in the following description? The tissue has cells that are oval or round in shape, surrounded by thin cellulose cell walls, and may contain chloroplasts.

We are asked to determine which type of simple tissue has these structural features. So to figure this out, let’s compare the three simple tissues in plants using a table and define some key terms along the way. Simple tissues are generally composed of one type of cell. Or at least the majority of cells have very similar structures and functions. There are three different types of simple tissues in plants: parenchyma, collenchyma, and sclerenchyma. The drawings below the table correspond to the typical shape and structure of cells in each type of simple tissue.

Parenchyma cells are typically oval or round in shape, while collenchyma cells usually look like elongated rectangles. And as you can see, sclerenchyma cells vary widely in their shape and size. You might notice that parenchyma cells have the thinnest cell walls of all three tissue types. And these walls are made of cellulose. Collenchyma cells have thicker cell walls to provide mechanical support and are reinforced with a substance called pectin. Sclerenchyma cells have the thickest cell walls of all three types to structurally support and provide strength to the plant’s transport vessels, for example. These cell walls are reinforced with strong substances, such as lignin, to provide them with this strength.

Parenchyma cells also contain chloroplasts, as one of their many functions is to carry out photosynthesis. Collenchyma cells might also contain chloroplasts, but they are unlikely to be so many as in a parenchyma cell. Sclerenchyma cells are technically nonliving, partly due to the waterproofing lignin in their walls. Therefore, there is no point in sclerenchyma cells containing chloroplasts as they would not be able to obtain the water needed for photosynthesis across their walls. Let’s look back to the information in the question to discover which simple tissue type it is describing.

The question describes the cells’ overall shape as oval or round. This suggests that the cells might be found in parenchyma tissues rather than the long rectangular cells of collenchyma. As collenchyma cells vary in their shape, the cells in the question being round does not necessarily rule out sclerenchyma. So let’s continue to the other features. The question also states that the cell walls of these cells are thin and made of cellulose. This provides further evidence that they may be found in parenchyma tissues as both collenchyma and sclerenchyma cells have thicker cell walls that are reinforced with other substances, such as pectin and lignin.

The question tells us that the cells in this tissue may contain chloroplasts. We know that these organelles may be found either in parenchyma or in collenchyma cells, but not in sclerenchyma cells. Based on this evidence, we can deduce that these cells are unlikely to be found in collenchyma or in sclerenchyma tissues, which means that as they meet all the necessary criteria, these cells are most likely to be found in simple parenchyma tissues.

Let’s review some of the key points that we’ve covered in this video. Simple tissues consist of cells that are very similar in their structure and function, for example, in plants, the three simple tissues of parenchyma, collenchyma, and sclerenchyma. While parenchyma tissues have varied functions that include photosynthesis and water and nutrient storage, collenchyma and sclerenchyma both function to provide structural support to different regions of the plant, with collenchyma also providing some elasticity to the growing regions.

Compound issues are made up of different cells with varied structures and functions, for example, the xylem and phloem tissues that make up the vascular bundle in plants. Xylem tissues function to transport water and dissolve minerals from the roots to the other parts of the plant. And phloem tissues function to transport sugars and amino acids from the site of photosynthesis, usually in the leaves, to the parts of the plant that require them.