Lesson Video: Excretion in Plants Biology

In this video, we will learn how to describe the processes by which plants excrete substances and outline how some waste products can be reused by the plant.


Video Transcript

In this video, we will learn how plants can produce metabolic waste products such as carbon dioxide, water, and nitrogenous waste. We will then explore how some plants are adapted to excrete some of these waste products through various mechanisms and structures.

All living organisms, including plants, carry out several metabolic processes for their growth and survival. This diagram shows a highly magnified view of a typical plant cell. In the chloroplasts that are found in many plant cells, food in the form of glucose is synthesized through photosynthesis. Many plant cells also contain mitochondria. These mitochondria are the site of cellular respiration, where the glucose that was synthesized in photosynthesis is broken down to release energy. Plants also need to absorb mineral ions into their roots from the soil to synthesize proteins, pigments, and other essential compounds. These compounds are involved in various different reactions, the sum of which make up the plants’ metabolism.

Metabolism describes all of the chemical reactions that occur within living organisms to maintain life, so plants must carry out these metabolic reactions in order to survive. These chemical reactions all generate products, which the plant does not always need and may even potentially be toxic. These byproducts are called metabolic waste and must often be eliminated via excretion if they cannot be reused or recycled by the organism in another metabolic process. Excretion is the removal of metabolic waste products from an organism’s body. Unlike animals, plants do not have a specialized organ system for excretion. Instead, they have several different mechanisms by which potentially dangerous waste products can either be recycled or excreted.

Let’s learn about the different types of metabolic waste products that plants generate and how these waste products are eliminated. Plants are autotrophs, which means that they synthesize their own food or nutrition. In this case, through photosynthesis, they synthesize glucose. We’ve also learned that cellular respiration releases the energy from carbon-containing compounds like glucose. Let’s look at this in more detail. The primary form of cellular respiration involves breaking down glucose by reacting it with oxygen to produce carbon dioxide and water. This process also releases energy that the cell can use. The reactants of cellular respiration are shown in orange, while the products are shown in pink.

Photosynthesis, on the other hand, reacts carbon dioxide with water in the presence of light energy to produce glucose and oxygen. In this case, we’re shown the reactants in pink and the products in orange. This is because these two reactions are almost the exact opposite of each other. Both of these reactions liberate gaseous waste products. While photosynthesis liberates oxygen, cellular respiration liberates carbon dioxide and water vapor. In plants, photosynthesis and cellular respiration go hand in hand.

While the products of photosynthesis form the reactants of cellular respiration, the products of cellular respiration form the reactants of photosynthesis. This shows an example of how some products of metabolic reactions can be reused in plants. However, in some cases, these gaseous waste products need to be excreted. Plants can eliminate excess quantities of these gaseous waste products by releasing them into the atmosphere by a process known as gas exchange. This is also the way in which the plants will absorb gases like carbon dioxide. However, water, like mineral ions, will need to enter the plant through its roots. The stems and leaves of a plant have specialized openings on their surfaces through which gaseous molecules like oxygen, carbon dioxide, and water vapor may diffuse into the atmosphere.

Let’s take a look at each of these structures to understand how excretion of gases in plants can occur. Leaves contain openings called stomata or a singular stoma. These are tiny pores found in amongst the epidermis cells mostly on the underside of leaves. The stomata are the site at which gas exchange between the leaves and the external atmosphere takes place. Through these pores, gaseous metabolic waste products, like the oxygen that’s produced in photosynthesis and not used in cellular respiration, can diffuse out of the leaf and into the atmosphere. Other gases can also diffuse between the leaf and the external environment through the stoma, and we’ll come back to these in more detail in just a little while.

The stem of some plants can also play a key role in gas exchange. For example, the stem that makes up the woody trunk of this tree contains many pores on its surface called lenticels. Through these lenticels, oxygen, carbon dioxide, and water vapor can be exchanged with the atmosphere. Lenticels are raised, oval, circular, or in this case elongated openings on woody stems in trunks and even on some roots. Plants primarily release excess water into the atmosphere as the gas water vapor through a process called transpiration. Transpiration is the loss of water through evaporation from the aerial, or upper, parts of a plant into the atmosphere. So water won’t only be lost in the form of water vapor through transpiration from the lenticels, but also from the stomata, which you’ll recall are pores on the underside of many leaves.

If we take a look at a cross section of some of the main cells in a leaf, we can see how transpiration occurs more clearly. We can still see the stoma on the underside of the leaf. We can also still see the surrounding lower epidermis cells at the bottom of the leaf and the upper epidermis cells that are found at the top of the leaf. Sometimes the cells of the epidermis produce a waxy cuticle to coat them. There are three main types of transpiration: stomatal transpiration, lenticular transpiration, and cuticular transpiration. Let’s look at stomatal transpiration in a little more detail first.

We already know that this includes the evaporation of water molecules from the stomata. But let’s have a look at this in a side view of the leaf to see how it happens more clearly. During the daytime when light intensity is high, many plant cells will be carrying out photosynthesis. As we know, oxygen is produced in photosynthesis. And the oxygen molecules that can’t be used in respiration will diffuse through the air spaces in between the cells and the leaf and out of the stomata. And carbon dioxide, which is needed for photosynthesis, will diffuse from the atmosphere through the stomata between the air spaces and into the cells that require it for photosynthesis.

At the same time, however, stomatal transpiration will take place. Liquid water that’s produced in cellular respiration in cells will accumulate in the intercellular spaces. There, the water molecules can evaporate into water vapor and the water vapor exits the leaf through the stomata into the atmosphere. Overall, stomatal transpiration accounts for about 90 percent of the water lost from a plant through transpiration.

But you might remember that water is a key reactant in photosynthesis. So in order to make its own food and survive, the plant can’t afford to lose large volumes of water. Therefore, at night, when the light intensity is too low for photosynthesis to occur, the stomata close as there’s no reason to absorb carbon dioxide into the leaf if photosynthesis cannot occur. This prevents the excess loss of water through stomatal transpiration when light intensity is low.

Lenticular transpiration is the loss of water through the lenticels as water vapor. Only a minimal volume of water, about 0.1 percent of the total water lost through transpiration, is lost through the lenticels. Cuticular transpiration is the evaporation of water from the cuticle. The cuticle is a waxy layer, and this waxiness makes the surface of the plant slightly less prone to water loss. But it’s still possible and cuticular transpiration can occur even when the stomata are closed. Overall, cuticular transpiration accounts for less than 10 percent of the total water lost through transpiration.

The rate of cuticular transpiration depends on the thickness of the waxy cuticle. Plants growing under extremely hot and dry conditions can develop extra thick cuticles to prevent extra water loss through transpiration. Lenticular and cuticular transpiration can occur throughout the day or night, while stomatal transpiration can only occur in the daytime.

Let’s summarize the information we’ve learned about stomatal, lenticular, and cuticular transpiration in a table. Stomatal transpiration occurs through pores on the leaf surface called stomata, and it accounts for about 90 percent of the water that’s lost through transpiration. But it can only occur during the daytime as stomata only open when light is present for photosynthesis to occur. Lenticular transpiration occurs through lenticels, which are raised openings of various shapes and sizes on the stems and sometimes the roots of woody plants. It only accounts for about 0.1 percent of the total water lost through transpiration. But lenticels don’t close, so lenticular transpiration can occur throughout the day and the night.

Cuticular transpiration occurs from the cuticle, which are waxy layers that coat the epidermis generally of the leaves. Cuticle transpiration accounts for less than 10 percent of the total water lost through transpiration, and it can occur throughout the day and the night.

Aside from transpiration, water can also be eliminated from the bodies of some plants in liquid form through a process called guttation. Water is first absorbed in plants from the soil into their root hair cells. Mineral ions, which have been shown here in green, are also absorbed from the soil in a similar way. Water is then carried along with dissolved mineral ions up through the plant to its aerial organs like the leaves through long tubelike structures called xylem vessels. When in the xylem vessels, the water and dissolved minerals are known as xylem sap. The absorption of water molecules from the soil and into the root cells creates an upward pressure through the xylem vessels. This is aptly named the root pressure.

Excess xylem sap is exuded in the form of water droplets through structures called hydathodes, which are found in the margins of leaves. This process is called guttation, or sometimes droplet exudation, where the xylem sap rich in dissolved minerals is exuded through the hydathodes. And it’s a result of the root pressure that’s exerted by water moving into root hair cells. The water droplets produced by guttation should not be confused with the dew drops that are often found on grass in the early morning. Instead, dew drops are formed through the condensation of atmospheric water onto the surface of plants. While transpiration occurs mainly in the daytime, guttation is more likely to occur at night or in the early morning when the stomata are closed but the plant needs to eliminate excess volumes of water and mineral ions in large quantities.

Another type of plant waste is nitrogenous waste. Just like some animals, plants can generate nitrogenous waste like urea, nitrates, and ammonium. These are formed as a result of protein metabolism in which proteins are broken down into smaller peptides, which can subsequently be broken down into smaller amino acids. Amino acids can then be converted into other substances or used in various metabolic reactions. These metabolic reactions will produce metabolic waste that needs to be excreted. Alternatively, these amino acids might be recycled in protein synthesis to make new proteins for growth and repair.

Nitrogen in the form of ammonium and nitrates can actually be used to synthesize amino acids again too. As amino acids are the building blocks of proteins, these waste products can theoretically be recycled through protein synthesis to form proteins that are needed for growth and development.

Let’s take a look at another type of plant waste. Sometimes these waste products are in the form of mineral salts or acids. These compounds might have a toxic effect on a plant if they’re allowed to accumulate. Instead, these compounds can be converted into crystals. In this crystal form, which is represented in this diagram as pink dots, they can be stored in the vacuole or cytoplasm of certain cells. This can prevent these potentially toxic compounds from spreading to different parts of the plant and causing harmful effects. These crystals can accumulate in structures that are fairly disposable like leaves, bark, and fruits. These structures can eventually be shed, leaving the plant free of toxic substances, so it can regrow these new structures from scratch.

For example, plants like this potato plant grown in soil with excess calcium tend to accumulate insoluble crystals of calcium oxalate inside their leaves, roots, and tubers. These are sometimes called raphides. Sometimes organic acids can be helpful to the plant to make nutrients in the soil more soluble so they can be reabsorbed by the roots.

The final method of plant excretion that we’ll look at in this video is how they can accumulate certain substances that can then be removed via secretions. For example, some plants can store certain waste products in resins and gums, which accumulate in old xylem vessels. These substances can then be excreted out of a plant, for example, in response to damage. In fact, some plants can even produce these substances in response to injury to block up a site of damage to prevent the entry of pathogens. Secretions like latex and oils can also contain metabolic waste products that accumulate in bark, stems, and leaves. Let’s see how much we can remember about excretion in plants by applying our knowledge to a practice question.

Water can also be lost from a woody plant through small pores in the stem. What are these pores called? (A) Glands, (B) hydathodes, (C) lenticels, or (D) stomata.

The stem of a plant plays an important role in gas exchange, as well as in the absorption and diffusion of water. The surfaces of the stems of some woody plants like those mentioned in the question contain raised openings called lenticels. Lenticels are a site of gas exchange between the stem and the atmosphere surrounding the stem. More precisely, they are the site of lenticular transpiration. This describes how water vapor, which is shown in our diagram as blue dots, can move from the stem through the lenticels and into the external environment. Though only a minimal volume of water is lost from the plant through lenticular transpiration, it does describe one of the ways that water can be lost from a woody plant stem.

Water can also be lost from a plant through stomatal transpiration. But as stomata are only found on leaves and the question is asking us about pores found on the stem through which water can be lost, stomata cannot be our correct answer. Water can also be lost from plants through a process called guttation through structures called hydathodes. But as hydathodes, like stomata, are found on plant leaves and not on the stem, this cannot be our correct answer.

Glands are structures typically found in animal bodies, which produce hormones. Although plants do produce hormones, they don’t have glands. Also, glands are not generally associated with water loss, so this is not our correct answer. So we’ve worked out that the pores in a woody plant stem through which water can be lost are called lenticels.

Let’s review some of the key points that we’ve addressed in this video about excretion in plants. Plants generate metabolic waste products that need to be either excreted or reused. The products generated in photosynthesis are used as reactants in respiration, and the products of respiration can be used as reactants in photosynthesis if they’re not released into the atmosphere. Water can be eliminated through evaporation via stomatal, lenticular, and cuticular transpiration. Water can be eliminated in the form of xylem sap by guttation through hydathodes. Nitrogenous waste products can often be reused in protein synthesis.

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