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.

Definition: Physiological Support

Plant physiological support is temporary as it depends on a high volume of water in cell vacuoles exerting turgor pressure on cell membranes to maintain cell shape for their protection.

Definition: Structural Support

Plant structural support is the permanent deposition of polymers such as lignin or cellulose in cell walls to maintain the shape of the plant and its cells.

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.

Key Term: Cell Wall

The cell wall is a rigid structural layer found outside the cell membrane of plant, fungal, and bacterial cells.

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.

Definition: Osmosis

Osmosis is the movement of water from an area of low solute concentration to an area of high solute concentration across a semipermeable membrane.

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.

Key Term: Turgidity

Turgidity is the point at which the cell’s membrane pushes against the cell wall, making the cell swollen and firm, usually because of being full of liquid.

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.

Wilted houseplant in need of water

Figure3

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.

Example 2: Describing the Function of Structural Support Materials

What is the primary function of structural support materials in plants?

  1. To maintain the shape of the plant and the plant cells
  2. To increase the rate at which materials pass through the plant
  3. To allow flexibility and movement for the plant
  4. To control which substances enter and leave plant cells

Answer

Structural support is permanent, and it depends on the deposition of hard substances in specific parts of the plant, specifically in cell walls. Different compounds are deposited into the walls of different cells, depending on their function. Many of these compounds can make a cell waterproof and impermeable to microorganisms. Some compounds can maintain the shape of the plant cells and therefore the plant itself.

The deposition of these compounds may decrease the rate at which certain materials, such as water, can pass through the plant surface, but they certainly do not increase the rate.

They may also, in some cases, decrease the flexibility of the plant. They do not primarily function to allow flexibility.

It is the role of the cell membrane, not the cell wall, that provides the majority of structural support, to control the movement of substances into and out of plant cells.

Therefore, the primary function of structural support materials in plants is to maintain the shape of the plant and the plant cells.

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 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.

Key Term: Cellulose

Cellulose is the main constituent of plant cell walls and is an insoluble polymer consisting of chains of glucose molecules.

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.

Key Term: Cutin

Cutin is a waxy water-repellent substance in the epidermis and cuticle of plants.

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.

Key Term: Suberin

Suberin is an impermeable waxy substance present in the cell walls of corky plant tissues.

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.

Key Term: Lignin

Lignin is a polymer that is found in some specialized cell walls to mainly provide mechanical support.

Key Term: Xylem

Xylem is a vascular tissue in plants that transports water and dissolved mineral ions from the roots to the other parts of the plant.

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.

Example 3: Describing the Role of Cutin and Suberin in Plants

What is the primary role of cutin and suberin in plants?

  1. To determine what enters and leaves the plant cells
  2. To provide structure and support to the vascular tissues
  3. To maintain the shape of cell walls
  4. To act as impermeable and waterproof barriers
  5. To maintain the shape of the reproductive organs

Answer

The cells in the cuticle of a leaf 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.

Plant cell membranes determine and control what enters and leaves the plant cells.

It is primarily the role of lignin to provide structure and support to plant vascular tissues, such as xylem.

Plant cell walls are made primarily of a carbohydrate called cellulose. Cellulose is very strong, and the mesh that it forms in the cell wall forms a physical barrier to support the cell and maintain cell shape.

Therefore, the primary role of cutin and suberin in plants is to act as impermeable and waterproof barriers.

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.

Example 4: Comparing Structural and Physiological Support in Plants

Which of the following tables correctly differentiates between examples of structural and physiological support in plants?

  1. TypeStructuralPhysiological
    ExamplesCellulose in cell walls
    Lignin in xylem walls
    The vacuole increasing/decreasing the turgidity of cells
  2. TypeStructuralPhysiological
    ExamplesThe vacuole increasing/decreasing the turgidity of cells Cellulose in cell walls
    Lignin in xylem walls

Answer

There are two main categories of plant support: 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.

An example of physiological support is the vacuole of a plant cell either filling with water or losing water depending on its availability. When there is lots of water available, it moves into the plant cells via osmosis, filling the vacuole. This increases the pressure exerted on the cytoplasm of the plant cell, which in turn increases the pressure on its cell membrane. The cell membrane, therefore, presses against the cell wall and the cell becomes turgid and rigid.

An example of structural support is the deposition of cellulose in plant cell walls, making cells stronger and helping them keep their shape. Lignin is another example, which is deposited in the walls of xylem vessels, making them waterproof and rigid.

Therefore, the table that correctly differentiates between examples of structural and physiological support in plants is this one:

TypeStructuralPhysiological
ExamplesCellulose in cell walls
Lignin in xylem walls
The vacuole increasing/decreasing the turgidity of cells

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

Key Points

  • Physiological support is temporary and refers to the ability of plant cells to increase or decrease their turgidity via osmosis.
  • Structural support is permanent and refers to deposition of strong compounds into cell walls in specific locations in the plant that require additional support or waterproofing.
  • Support mechanisms are advantageous to a plant to allow it to conserve water, obtain light, and protect it against herbivores, damage, and disease.

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