In this explainer, we will learn how to describe the structure and function of specialized plant structures.
Many species of plants are complex, multicellular organisms. With over 390 000 species of plants worldwide, they are incredibly diverse and interesting! All plants are made up of specialized cells that carry out particular roles and work together to keep the plant alive and functioning. This is crucial for us—without plants and their ability to produce oxygen, we would not be able to survive!
The plant kingdom includes a wide range of organisms, including flowering plants like the tomato plant pictured in Figure 1, trees like giant coast redwoods, and even small nonflowering plants like moss! Like animals, plants also contain organs. Organs are defined as structures that are made up of multiple specialized tissues. The main organs of a flowering plant—like a tomato plant—are the roots, the stem, and the leaves, as shown in Figure 1.
Definition: Specialized Cell
A specialized cell is one that has differentiated to have a particular structure to serve its specific function.
As well as the specialized cells that are present in plants, plants also contain regions of unspecialized stem cells called meristem cells. Plants are especially interesting organisms, as they tend to retain these unspecialized cells for their whole life, which allows them to continue to grow as long as they are in appropriate conditions! Meristem cells can divide rapidly, which causes this growth, and they are able to specialize into any type of plant cell. Meristem cells are located in the tips of roots and shoots, as these are the regions where the majority of plant growth occurs.
Let’s have a look at some of the specialized cells found within plants, starting at the bottom.
The roots of a plant are crucial for providing the plant with key substances, like essential minerals and water. Roots themselves are long, branching organs of the plant that spread throughout the surrounding soil. Roots possess root hair cells, which are highly specialized cells that function to absorb water and minerals. We can see the general structure of a branching root and a root hair cell in Figure 2.
Let’s take a look at the adaptations of the root hair cells in a bit more detail.
Root hair cells, as we can see in Figure 2, have long extensions that reach through the soil. This is an adaptation that helps increase the surface area of the cell, which in turn increases the amount of water and minerals the cell can absorb from the soil. Root hair cells also help to anchor the plant in soil, fixing the plant into the ground and allowing greater penetration of the roots.
Some of the minerals that the plant requires will need to be actively transported into the cell. So, the root hair cell contains many mitochondria, which provide the energy needed to do this. Interestingly, root hair cells are continually replaced by the plant and will not survive for more than a few days or weeks!
Relative to other plant cells, root hair cells have a thin cell wall. This means it is easier for the water to move from the soil into the cell, as indicated in Figure 2. One of the main components of these cell walls, cellulose, will allow the movement of water and minerals. Other polymers that make up cell walls, like lignin or cutin, will not allow this. Water is absorbed by the root hair cell by a process known as imbibition. Imbibition occurs when the solid particles of the root hair cell wall absorb water, swelling up and increasing their volume. The root hair cells are covered with a thin layer that possesses a strong attraction to water molecules, and this encourages the water to enter the cell walls of the root hair cells by imbibition. The water molecules are able to pass through the gaps in the semipermeable membrane by osmosis.
The root hair cells have a large vacuole; this is a fluid-filled “sac” that helps maintain shape and structure. The vacuole also has a large concentration of solutes, which encourages the movement of water from the cell walls into the vacuole by osmosis. Due to the presence of a large concentration of dissolved solutes, the vacuole is said to have a low water potential. The movement of water from the soil and into the root hair cell is shown in Figure 3.
Figure 3 shows how water moves from the soil and into the vacuole of a root hair cell by osmosis. Osmosis is the movement of water from a low concentration of solutes (where there is a high concentration of water) to a high concentration of solutes (where there is a low concentration of water). Osmosis is a passive process, which means it does not require energy. As the concentration of solutes increases, the “osmotic pressure” that causes the movement of water also increases.
Generally smaller molecules, like water and salts, can pass through membranes, but large substances, like sugars and proteins, cannot. Due to allowing some molecules to pass through and preventing others from doing so, cell membranes are known as semipermeable or selectively permeable.
Osmosis is the movement of water from an area of low solute concentration to an area of high solute concentration across a semipermeable membrane.
Key Term: Semipermeable
A structure is semipermeable if it allows some molecules or substances to pass through but not others.
Definition: Osmotic Pressure
Osmotic pressure is the pressure that moves water through a semipermeable membrane.
Example 1: Recalling the Adaptations of a Root Hair Cell
Which of the following is not an adaptation that root hair cells have for their function?
- They have thin walls to allow water and minerals to move into the cell.
- They are numerous in number to increase water and mineral uptake.
- They contain many chloroplasts that photosynthesize and provide glucose for the root.
- They have a low water potential inside the root hair vacuole to encourage water to move into them.
Root hair cells are highly specialized cells found in the root of a plant. They have multiple adaptations to help them carry out their primary function, which is to absorb water and minerals from the surrounding soil to keep the plant alive and healthy.
Compared to other plant cells, the cell walls of a root hair cell are relatively thin. This allows water and minerals to move easily through the cell wall and then the semipermeable membrane. In the image, we have magnified a single root hair cell. The roots will contain many, many repeats of the root hair cells to allow them to take up the maximum amount of water possible.
The vacuole within the root hair cell is a fluid-filled sac. This fluid has a high concentration of solutes, like salts and sugars, dissolved into it. As it has a high solute concentration compared to its water concentration, we say that it has a low water potential. Put simply, this means that water will be encouraged to move into the vacuole.
Root hair cells contain many mitochondria, which are the site of cellular respiration. Cellular respiration breaks down carbon-containing compounds such as glucose to release energy that the root hair cells can use (e.g., to transport ions and molecules from the soil and into the roots).
However, unlike many other plant cells, root hair cells will not contain chloroplasts. Chloroplasts are the site of photosynthesis, which is the process that uses light energy to produce glucose for the plant. As they are underground, no light reaches the root hair cells, so the chloroplasts would not be able to carry out their primary function of photosynthesis.
Therefore, the only answer that is not an adaptation of a root hair cell is that they contain many chloroplasts that photosynthesize and provide glucose for the root.
The root hair cells absorb the water and minerals from the soil, but what happens to the water and minerals after that? To answer this question, we need to move up the plant and look at the specialized plant structures within the stem.
The majority of plants you may be familiar with (e.g., roses, daffodils, and all species of tree) are vascular plants. This means they contain specialized vascular tissue, which is tissue used primarily for transporting substances. The two vascular tissues we are going to look at here are xylem and phloem.
Xylem tissues in plants are made of primarily dead cells. There are two types of cells that make up the xylem vessels, called tracheids and vessel elements/members. These cells will form thick-walled, continuous tubes that run through the stem of a plant. The role of the xylem tissue is to transport water and minerals through the plant. This occurs in one direction only, upward, from the roots to the leaves.
Phloem tissues in plants are made of primarily living cells, including sieve tubes that join end to end with pores in between to allow the movement of substances. These pores in the end walls of each sieve tube element are called sieve plates. Sieve tube elements are joined to companion cells. Companion cells are highly specialized to provide the sieve tubes with energy to carry out transport, and they contain many mitochondria to do this.
The role of the phloem tissue is to transport the products of photosynthesis (e.g., dissolved sugars like sucrose, amino acids) around the plant. Generally, the phloem transports these dissolved substances to all areas of the plant from areas that photosynthesize, like the leaves. It is especially important to transport sugars to nonphotosynthetic parts of the plant, like the roots, as they will not be able to make their own sugars but will still require them. Therefore, unlike transport in the xylem, this transport is bidirectional; this means it happens in two directions, from the leaves to the rest of the plant and from the rest of the plant to the leaves.
The structure of the xylem and phloem vessels are compared in Figure 4.
Example 2: Describing the Function of Cells in the Phloem Tissue
What is the primary function of the companion cells in the phloem?
- Providing chloroplasts for a maximum rate of photosynthesis to take place
- Maximizing the available surface area for the diffusion of gases
- Connecting the xylem and the phloem for the exchange of substances
- Providing energy for the transport of substances in the phloem
The phloem is a specialized plant tissue, and the main function of the phloem is to transport sugars and amino acids around the plant. The transport of sugars by the phloem is “bidirectional,” which means it goes both directions: to and from the leaves and the rest of the plant. The phloem is largely composed of sieve tube elements, which are elongated, living cells. These cells have pores or perforations in the end cell walls, which allow substances to pass through them once the cells are joined end to end. They are also joined to a companion cell.
Transport of sugars in the phloem requires energy. In plant and animal cells, energy is provided via a process called cellular respiration. Cellular respiration takes place in organelles called mitochondria, which break down carbon-containing compounds such as glucose in a series of chemical reactions to release energy. Sieve tube elements do not contain mitochondria, but companion cells do. The companion cells therefore carry out cellular respiration and provide the energy needed for the phloem to transport sugars around the plant to where they are needed.
Therefore, the primary function of the companion cells in the phloem is providing energy for the transport of substances in the phloem.
Both the xylem and phloem tissues extend into the leaf, where we will find many more specialized plant cells! Let’s take a look at a cross section of a leaf, shown in Figure 5, to better understand the structure.
Plants are well adapted to carry out photosynthesis, as they contain many chloroplasts. Chloroplasts are organelles that contain the green pigment chlorophyll and are found in the majority of plant leaves; the chlorophyll pigment is responsible for giving these leaves their green color! The chlorophyll within each chloroplast is specifically adapted to capture and absorb sunlight. This is so the chloroplast can carry out its main function, which is photosynthesis.
In photosynthesis, plants take in carbon dioxide from the environment and water from the soil and, through a series of metabolic reactions, convert these compounds into sugars, such as glucose, and oxygen. Glucose is a carbohydrate that can be broken down in cellular respiration to provide the plant with energy and therefore acts as a major component of plant nutrition. The oxygen can also be used in cellular respiration by the plant or released into the atmosphere.
Photosynthesis is the process that converts carbon dioxide and water into sugars, such as glucose, and oxygen in the presence of sunlight.
The leaf is covered with a waterproof layer called the waxy cuticle. This minimizes water loss from the plant by evaporation through the leaf. It is called the cuticle as it contains a water-repellent substance called cutin.
This cutin that forms the waxy cuticle is produced and secreted by cells located just below the cuticle in a layer called the epidermis. You may have heard the term epidermis before, as the surface layer of our skin is called the epidermis! Much like our skin, the plant epidermis is a layer of cells that acts as a protective barrier for the plant. Plant leaves have an upper epidermis and a lower epidermis. The cells in the epidermis lack chloroplasts and therefore contain no chlorophyll. This means that the epidermis is transparent. This transparency is especially important for the upper epidermis as it allows the cells below it to access more light for photosynthesis.
Within the leaf, there are two mesophyll layers: the palisade mesophyll layer and the spongy mesophyll layer. These layers are a collection of specialized parenchyma cells and tissues that lie in between the upper and lower layers of the epidermis.
The palisade layer is made up of long, cylinder-shaped cells. The main function of these cells is to carry out photosynthesis for the plant, which is how plants make their own nutrition. Many cells that contain a large number of chloroplasts are located near the top of the leaf, which means they can capture the maximum amount of sunlight.
While the spongy mesophyll cells are still likely to contain chloroplasts, there will be fewer than in the palisade mesophyll cells as the spongy mesophyll layer is located lower in the leaf and is therefore less exposed to light. The spongy mesophyll layer is also adapted to help the leaf photosynthesize, but in a different way. The cells of the spongy mesophyll are more irregularly shaped and are largely spaced out. This adaptation allows the efficient transport and exchange of gases (e.g., oxygen for respiration and carbon dioxide for photosynthesis).
Example 3: Explaining the Location of Palisade Cells in the Leaf
Palisade cells near the top of the leaf contain many chloroplasts. Which of the following best explains why?
- Palisade cells have many chloroplasts to allow maximum diffusion of gases in and out of the leaf.
- Palisade cells have many chloroplasts to capture the sunlight needed for photosynthesis.
- Palisade cells have many chloroplasts to expand the cell and increase the surface area available for water uptake.
- Palisade cells have many chloroplasts to capture the sunlight needed for respiration.
The leaf of a plant contains many different specialized cells. One example is the palisade cells, which form a layer near the top of the leaf. These cells are elongated, cylindrical in shape, and contain many chloroplasts. To answer the question, we need to recall what the primary function of chloroplasts is.
Chloroplasts are organelles that contain the green pigment chlorophyll and are found in the majority of plant leaves; the chlorophyll pigment is responsible for giving these leaves their green color! The chlorophyll within each chloroplast is specifically adapted to capture and absorb sunlight. This is so the chloroplast can carry out its main function, which is photosynthesis. Photosynthesis is the biological process plants use that converts light energy into chemical energy in the form of sugars such as glucose, and this glucose is used by the plant as their food. The equation for photosynthesis is
We can also eliminate the other options. The spongy mesophyll layer contains the cells that have many air spaces between them to allow the maximum diffusion of gases. There is no need for the palisade cells to “expand” to increase the surface area, and they are not specialized for water uptake—this is the job of the root hair cells. As discussed, the reason the chloroplasts capture sunlight is for photosynthesis and not respiration.
Therefore, the best explanation for why palisade cells contain many chloroplasts is to capture the sunlight needed for photosynthesis.
The small openings shown on the underside of the leaf are stomata (singular: stoma). Figure 6 provides a diagram of stomata in the underside of the leaf when they are open and when they are closed.
The opening and closing of stomata is controlled by surrounding guard cells. When guard cells are "flaccid"—which means they are not full of water, so the cell contents shrink—the stomata will open to allow the exchange of gases with the environment. This allows the plant to take in carbon dioxide for photosynthesis and release the oxygen it produces. However, when the stomata are open, they also allow water vapor to diffuse from the leaf. To prevent too much water being lost, the stomata can be closed by the surrounding guard cells. To do this, the guard cells will absorb water and “swell up,” becoming turgid and reducing the gap between them. This response is particularly useful in times of water shortages, or during the night when photosynthesis cannot take place.
Let’s summarize what we have learned in this explainer.
- Many plants are complex, multicellular organisms that contain many specialized cells and tissues.
- Root hair cells are specialized cells, adapted to absorb minerals and water from the soil.
- Xylem tissue is specialized to transport water and minerals from the root to the leaves of the plant.
- Phloem tissue is specialized to transport sugars and amino acids to and from the leaves and the rest of the plant.
- Leaves contain many specialized cells and structures, including the waxy cuticle, epidermis, palisade and spongy mesophyll layers, stomata, and guard cells.
- The primary function of the leaves is to carry out photosynthesis, and the cells within the leaf are adapted to carry out different functions for this purpose.