Lesson Explainer: Support in Plants Biology

In this explainer, we will learn how to describe examples of physiological and structural support in plants.

Have you ever noticed that if a pea rolls under the fridge and you find it, months later, it will have become wrinkled and dry? If you try putting this same pea into a pot of water, it will swell up to almost its regular size again, though eating it would not be a good idea at that point! This shrinking and swelling of the pea is due to it losing and gaining water. It is one example of a typical method of support in plant cells.

While humans, like many animals, have a skeleton to support them, plants do not. Plants, therefore, need other support mechanisms to maintain their cell and, overall, shape and to protect them.

There are two methods that a plant uses to maintain its shape and structure: physiological and structural. Physiological support is temporary and depends on the water content in a cell to keep its shape. Structural support is more permanent, and it depends on the deposition of hard substances in specific parts of the plant.

The pea shrinking and swelling displays an example of a physiological support mechanism, and we will look at this support method first.

You can see the cell wall, vacuole, and cell membrane of a typical plant cell in Figure 1. Plant cell walls are rigid structural layers surrounding their cell membrane. The vacuole in a plant cell is usually a large structure filled with a liquid called cell sap that consists of water and dissolved substances like sugars or enzymes. Lots of water can be stored in the vacuole, and as the volume of water stored in the vacuole increases, the cell swells. Let’s have a look at how this happens and why it is important.

Plant cells, like those in the pea, wrinkle and shrink when they have little water available in their surroundings. This causes the plant to wilt. As the availability of water outside the cells increases, however, water will move into the plant cells across the cell membrane by osmosis. Osmosis is the movement of water molecules from an area of low solute concentration to high solute concentration.

When the water concentration is low within the cells at this point, the solute concentration is high, so water moves into the plant cell’s vacuole from the cell’s surroundings. As it now contains a larger volume of water, the vacuole increases in size and so exerts more pressure upon the cell cytoplasm, pushing the cell membrane against the cell wall. This makes the cell wall appear swollen, and at this point, the turgidity of the cell is high. You can see the effect of high water availability outside cells on the turgid cell in Figure 2.

This process is temporary. If the plant becomes dehydrated from not receiving enough water, the cells will lose water. The loss of water from the vacuole makes the cell membrane pull away from the cell wall as less pressure is exerted upon it. This lowers the turgidity, making the cell eventually become shrunk and wrinkled as you can see in Figure 2. When several plant cells shrink in this way, this causes the whole plant, especially its leaves, to visibly wilt as you can see in the image below.

Example 1: Describing the Effects on a Plant When Cell Turgidity Is Not Maintained

What will visibly happen to a plant if cell turgidity is not maintained?

1. The stem will grow at a faster rate.
2. The surface area of the leaves will expand.
3. The leaves and the plant will wilt.
4. The leaves will turn yellow.
5. The flowers will drop off.

Answer

When the water concentration surrounding plant cells is higher than the water concentration inside them, water will move into the cells by osmosis. Osmosis is the movement of water molecules from an area of low solute concentration to high solute concentration.

As the water concentration is low within the cells, the solute concentration is high, so water moves into the plant cell’s vacuole. The vacuole increases in size and so exerts more pressure upon the cell cytoplasm, pushing the cell membrane against the cell wall. This makes the cell wall appear swollen, and at this point, the turgidity of the cell is high. You can see this on the right of the figure below.

This process is temporary. If the plant does not receive enough water, the cells will lose water. The loss of water from the vacuole makes the cell membrane pull away from the cell wall as less pressure is exerted upon it. This lowers the turgidity, making the cell eventually become wrinkled as you can see on the left of the figure above. When this happens to several plant cells, this causes the whole plant, especially its leaves, to visibly wilt.

Therefore, if cell turgidity is not maintained, the leaves and the plant will wilt.

Let’s look at the different forms of structural support in plant cells.

Structural support involves specific tough compounds being permanently incorporated into a plant cell wall. Different compounds are deposited into the walls of different cells, depending on their function. Most of these compounds function to provide a waterproof or impermeable barrier. The compounds can also help to maintain the shape of the plant cells, and therefore the plant itself, keeping the plant upright and strong.

Plant cell walls are made primarily of a carbohydrate called cellulose. Cellulose is an insoluble polymer made up of thousands of glucose molecules joined together into a chain.

You can see how cellulose makes up these cell walls in Figure 4, which magnifies an image of a leaf gradually to reveal the composition of its cells, the cell walls, and cellulose itself. Cellulose is very strong, and the mesh that it forms in the cell wall creates a physical barrier to support the cell.

You can see the location of the epidermis in Figure 5. Cellulose also helps to maintain cell turgidity in the physiological response we have just looked at. Cells without a cell wall, like animal cells, burst when they absorb too much water. In the same situation, plant cells will just become relatively stronger as the rigid cell wall prevents bursting! Especially in cases where cellulose is in the outer tissues of a plant, such as in the epidermis, cellulose cell walls form a barrier to prevent disease-causing microorganisms from entering the inner tissues.

The cells in the cuticle of a leaf, which you can see in Figure 5, have a substance called cutin deposited in their walls. Cutin is impermeable to water, so it prevents excess loss of water, thereby maintaining the plant’s shape. It also increases the effectiveness of this physical barrier by thickening the cell walls.

Some plants have a cork layer surrounding organs, such as the stem. Cork usually forms following an infection or leaf drop, and once made, it is a permanent feature. Cork is impermeable, formed by deposition of suberin into cell walls. As it is impermeable and waterproof, cork provides another layer of protection and support against both losing water and entry of disease-causing microorganisms.

Lignin is a compound that is deposited into certain plant cell walls, such as in the xylem. The xylem is part of the plant’s vascular transport system responsible for transporting water and minerals from the roots to the rest of the plant. You can see the basic structure of a xylem vessel in Figure 6.

When lignin is deposited into cell walls, it makes them waterproof. This is very helpful in the xylem, as it reduces the chances of water leaking out of the xylem vessels, increasing the efficiency of water transport. It also provides additional structural support to the xylem by making its vessels more rigid, helping them remain upright to form a continuous column of water.

Lignin and cellulose can also be incorporated into other parts of cell walls in different tissues in a plant.

There are three types of simple tissue in a plant: parenchyma, collenchyma, and sclerenchyma. Parenchyma cells have thin cellulose cell walls and usually contain many chloroplasts for photosynthesis.

Sclerenchyma cell walls are reinforced with lignin and extra cellulose. Sclerenchyma cells are generally found in nongrowing parts of the plant to add structural support to them. For example, sclerenchyma cells can be found supporting the vascular bundles in a stem, the plant tissues responsible for transport. Asparagus stems are full of sclerenchyma tissue, giving them a signature “snap” when broken!

Collenchyma cell walls are reinforced with extra cellulose and additional substances to provide extra support, typically in young stems as it is more flexible than sclerenchyma. A relatable example is in the flexible and stretchy strands in celery stems. Though the majority of the stem is sclerenchyma making it tough and crunchy, collenchyma forms the basic structure of the stretchy veins running along the stem.

Plants depend on these support methods for their survival. Conserving water is advantageous to plants as, among its other functions, it is a reactant in photosynthesis and so is vital to them being able to synthesize food. This food will be used to release energy through cellular respiration.

It is ideal for a plant to remain upright as it allows them to access more sunlight. This allows them to carry out more photosynthesis, which is the biological process by which they make their own nutrition. Plants compete for light to be able to produce their food. Being tall and strong can give one plant the edge over its competitor for light, helping it to outcompete other organisms for survival.

Additionally, a strong stem allows plants to withstand environmental pressures such as strong winds and to support heavy branches, fruits, or flowers in the upper parts of the plant. Keeping fruit and flowers off the ground is helpful as it protects them from some herbivorous insects, rot, decay, and damage.

Thick and impenetrable cell walls are useful in restricting the entry and movement of disease-causing microorganisms into the inner tissues of the plant.

Let’s recap some of the key points we have covered in this explainer.