Lesson Explainer: Excretion in Plants Biology

In this explainer, 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.

All living organisms carry out several metabolic processes for their growth and survival. In plants, food is synthesized through photosynthesis and is then broken down to release energy through cellular respiration. The minerals absorbed from the soil are used to synthesize proteins, pigments, and other essential compounds.

Key Term: Metabolism

Metabolism describes all the chemical reactions that occur within living organisms to maintain life.

Each of these metabolic processes is essentially a chemical reaction, meaning the conversion of reactants into products that the plant needs for its growth and survival. These chemical reactions also generate by-products, which the plant does not need. These by-products, which are potentially toxic to the plant, are called metabolic waste and must be eliminated through excretion. Excretion is the removal of the waste products of metabolism from the body of an organism.

Key Term: Metabolic Waste

Metabolic waste is the products of metabolic reactions that are unnecessary or harmful to the organism and must, therefore, be eliminated through excretion.

Definition: Excretion

Excretion is the removal of the waste products of metabolism from the body of an organism.

Unlike animals, plants do not have a specialized organ system for excretion. Instead, they have several different mechanisms by which potentially harmful waste products are either recycled or excreted. Let’s learn about the different types of waste products that plants generate and how these products are eliminated.

As we know, plants are autotrophic, which means they synthesize their own food, in the form of glucose, through photosynthesis. The process of cellular respiration involves the breakdown of carbon-containing compounds, such as glucose, into carbon dioxide and water and the release of energy.

Both of these reactions release gaseous waste products. Photosynthesis liberates oxygen, whereas respiration produces carbon dioxide and water vapor. In plants, photosynthesis and aerobic respiration go hand in hand. The main products generated by each of these reactions are reused as the reactants of the other. To understand this clearly, let’s take a look at the word equations of the two processes.

Equation: Photosynthesis

Carbondioxidewaterlightenergyglucoseoxygen+(+)+

Equation: Respiration

Glucoseoxygencarbondioxidewaterenergy++(+)

Definition: Aerobic Respiration

Aerobic respiration is the process by which energy is released in the cells in the presence of oxygen.

You may notice that these two reactions are almost the exact opposite of each other. The products of the photosynthesis reaction are the same molecules as the reactants of the respiration reaction, and vice versa. This is represented in Figure 1.

Example 1: How Plants Reuse the Waste Products of Respiration

Plants, like most living organisms, respire. Water is a waste product of respiration. How can this water be reused by plants?

  1. As a source of energy for metabolic processes
  2. As a source of nitrogen for the synthesis of nitrogenous compounds
  3. As a reactant in photosynthesis
  4. As an insulating gas to regulate temperature

Answer

As we know, plants are autotrophic, which means that they synthesize their own food, in the form of glucose, through photosynthesis. The process of cellular respiration involves the breakdown of glucose into carbon dioxide and water and the release of energy. In plants, photosynthesis and respiration go hand in hand. The products generated by each of these reactions are reused as the reactants of the other. To understand this clearly, let’s take a look at the word equations of the two processes.

Respiration: Glucoseoxygencarbondioxidewaterenergy++(+)

Photosynthesis Carbondioxidewaterlightenergyglucoseoxygen+(+)+

You may notice that these two reactions are almost the exact opposite of each other. The products of the photosynthesis reaction are the same molecules as the reactants of the respiration reaction, and vice versa. This is represented in the diagram below.

Therefore, the water generated as a waste product during respiration can be reused as a reactant in photosynthesis.

Although these products can be reused in plants, in some cases, they may need to be removed through excretion. Plants eliminate excess quantities of these gaseous waste products by releasing them into the atmosphere, via a process known as gas exchange. The stem and leaves of a plant have specialized openings on their surfaces through which gaseous molecules, such as oxygen, carbon dioxide, and water vapor, may diffuse into the atmosphere.

Let’s have a look at each of these structures and understand their role in excretion. Leaves have openings called stomata, which are tiny pores found in their epidermis. Through these pores, gaseous metabolic waste products can diffuse into the atmosphere. Stomata open and close depending on the need for gas exchange, as represented in Figure 2.

Definition: Stomata

Stomata (singular: stoma) are pores in the epidermis of leaves through which gas exchange takes place with the atmosphere.

The stem of the plant also plays an important role in gas exchange. In some plants, the stems have pores on their surfaces, called lenticels, through which oxygen, carbon dioxide, and water vapor can be released into the atmosphere. Lenticels can be seen as raised circular, oval, or elongated openings on woody stems and trunks, as you can see in the photo below.

Lenticels on woody stems

Figure3

Definition: Lenticels

Lenticels are openings on the surfaces of stems and roots through which gas exchange takes place with the atmosphere.

Plants primarily release excess water into the atmosphere through transpiration, in the form of water vapor. Water molecules evaporate from the surfaces of the plant to the air. There are three different types of transpiration: stomatal, lenticular, and cuticular.

Definition: Transpiration

Transpiration is the loss of water through evaporation from the aerial parts of a plant to the atmosphere.

Stomatal transpiration is the evaporation of water molecules from the stomata. As we have learned, stomata are capable of opening or closing depending on the need for gas exchange. During the daytime, when the light intensity is high, plant cells carry out photosynthesis. The stomata therefore open to allow the diffusion of carbon dioxide from the atmosphere into the leaves. At the same time, since the stomata are open, transpiration takes place. In most plants, the leaves contain a type of tissue made up of mesophyll cells. This type of tissue can be spongy and, therefore, has large intercellular spaces. Water vapor usually accumulates in these intercellular spaces before being eliminated via transpiration through the stomata. Overall, stomatal transpiration accounts for about 90% of the water lost from a plant through transpiration.

Lenticular transpiration is the evaporation of water molecules from the lenticels on the stems of woody plants. Only a minimal volume of water, around 0.1% of the total water lost through transpiration, is lost in this way.

Cuticular transpiration is the evaporation of water molecules through the cuticle. The cuticle is a waxy layer that coats the epidermis of the aerial parts of a plant. Cuticular transpiration can happen when the stomata are closed. Overall, cuticular transpiration accounts for less than 10% 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 thick cuticles to prevent excess water loss through transpiration.

Definition: Cuticle

The cuticle is a waxy layer that coats the epidermis of the aerial parts of a plant.

Lenticular and cuticular transpiration occur throughout the day and night. However, at night, when the stomata are generally closed, since photosynthesis cannot take place, transpiration is accomplished primarily through the lenticels and cuticle.

Let’s quickly recap what we have learned about transpiration. In Table 1, you can see a comparison between the three different forms of transpiration.

Example 2: Loss of Water through Pores in the Stem

Water can also be lost from a woody plant through small pores in the stem. What are these pores called?

  1. Glands
  2. Hydathodes
  3. Lenticels
  4. Stomata

Answer

Plants primarily release excess water to the atmosphere through transpiration. Water molecules evaporate from the surfaces of the plant to the air. There are three different types of transpiration: stomatal, lenticular, and cuticular.

Plant stems play an important role in gas exchange, as well as in the absorption and diffusion of water. They have pores on their surfaces, called lenticels, through which oxygen, carbon dioxide, and water vapor can be released to the atmosphere. Lenticels can often be seen as raised circular, oval, or elongated openings on woody stems and trunks.

Lenticels on woody stems

Lenticular transpiration is the evaporation of water molecules from the lenticels on the stem of a plant. Only a minimal volume of water is lost in this form of transpiration.

Therefore, the pores in the stems of woody plants through which water is lost are called lenticels.

Aside from transpiration, water can also be eliminated from the bodies of some plants in the liquid form, through a process called guttation.

The water carrying dissolved minerals that travels upward from the roots through the xylem vessels is called the xylem sap. The absorption of water molecules from the soil into the roots creates an upward pressure through the xylem vessels, called the root pressure. Excess xylem sap is exuded in the form of water droplets through structures called the hydathodes, which are found in the margins of leaves, as you can see in the photo below.

Water droplets on margins of leaves

Figure4

This process is called guttation and is due to the upward root pressure. The water droplets exuded through guttation are rich in dissolved minerals. These water droplets should not be confused with dewdrops, which are formed by the condensation of water molecules from the atmosphere on the surfaces of plants.

While transpiration generally occurs during the daytime, guttation is more likely to happen at night or in the early morning, when the stomata are closed and the plant needs to eliminate large amounts of water.

Definition: Guttation (Droplet Exudation)

Guttation is the process of excreting xylem sap through the hydathodes at the margins of leaves.

Definition: Hydathodes

Hydathodes are pores in the epidermis of leaf margins through which xylem sap is excreted during guttation.

Example 3: The Primary Substance Excreted through Guttation in Plants

What is the primary substance excreted from the plant via guttation?

  1. Urea
  2. Nitrogenous waste
  3. Xylem sap
  4. Glucose
  5. Excess salts

Answer

Plants absorb water from the soil through their roots. This water, which contains dissolved minerals from the soil, must be transported upward to all the other organs of the plant. The vascular tissue in plants that transports water is called the xylem.

Plants can eliminate excess water through two different processes: transpiration and guttation. Transpiration is the evaporation of water from the surfaces of the plant to the atmosphere, whereas guttation involves the exudation of water in the liquid form.

During guttation, water is eliminated from the plant through structures called the hydathodes, which are found on the margins of the leaves. Let’s understand how this process happens. The absorption of water molecules from the soil into the roots creates an upward pressure through the xylem vessels of the plant, called the root pressure. The process of guttation occurs as a result of this upward pressure, which causes water from the xylem vessels to be excreted through the hydathodes in the form of water droplets.

The water carrying dissolved minerals that travels upward through the xylem vessels is called the xylem sap.

Therefore, the primary substance excreted from a plant via guttation is the xylem sap.

Another type of plant waste is nitrogenous waste. Just like animals, plants generate nitrogenous waste products like urea. These are the result of protein metabolism, in which proteins are broken down into smaller peptides and amino acids. Nitrogenous waste products can be excreted, but they can also be reused for protein synthesis. Nitrogen, in the form of ammonium and nitrates, is used to synthesize amino acids, which are the building blocks of proteins. Plants therefore convert their nitrogenous waste into reusable forms to synthesize the proteins they need for further growth and development.

Sometimes plants can generate waste products in the form of mineral salts or acids, which could have a toxic effect on the plant if allowed to accumulate. These compounds are converted into crystals and stored in the cytoplasm or vacuole of some plant cells. This prevents these potentially toxic compounds from spreading to different parts of the plant and causing harmful effects. These insoluble crystals accumulate in the leaves, bark, or fruits, which are eventually shed, leaving the plant free from toxic substances. For example, plants growing in soils containing excess calcium tend to accumulate insoluble crystals of calcium oxalate, called raphides, in their roots, leaves, or tubers.

Some plants can store certain waste products in resins or gums, which accumulate in old xylem vessels. Some plants may also produce these substances in response to injuries. Secretions like latex and oils can contain metabolic waste, and they accumulate in the bark, leaves, or stems. The photo below shows an example of resin secreted from the trunk of a tree.

Resin secreted from the trunk of a tree

Figure5

Plants are also capable of excreting certain compounds like organic acids, amino acids, and sugars into the soil through their roots. Some of these organic acids help the plant solubilize certain nutrients in the soil so that they can be reabsorbed by the roots.

Example 4: Storage of Waste Products in Plant Organs

Complete the statement to describe one form of waste removal in plants: Waste products can be stored in the of a plant, which can then drop off when no longer required.

  1. stem
  2. spores
  3. ovules
  4. leaves

Answer

Plants survive by performing metabolic reactions, in which a set of reactants are converted into products that the plant needs for its growth and survival. Aside from the products that the plant needs, certain other compounds are also generated through metabolic reactions, which are beyond the plant’s requirements. These are called metabolic wastes and must be eliminated from the plant’s body.

Sometimes plants can generate waste products in the form of mineral salts or acids, which could have a toxic effect on the plant if allowed to accumulate. These compounds are converted into insoluble crystals and stored in the cytoplasm or vacuole of some plant cells. Since these crystals cannot dissolve, this prevents these potentially toxic compounds from spreading to different parts of the plant and causing harmful effects. These insoluble crystals accumulate in the leaves, bark, or fruits, which are eventually shed, leaving the plant free from toxic substances. For example, plants growing in soils containing excess calcium tend to accumulate insoluble crystals of calcium oxalate, called raphides, in their roots, leaves, or tubers.

Let’s take a look at the options provided in the question. Of the listed options, leaves are the only organs in which waste products can be stored, which then drop off from the plant when no longer required.

Therefore, waste products can be stored in the leaves of the plant, which then drop off when no longer required.

Let’s summarize everything that we have learned about excretion in plants.

Key Points

  • Plants generate metabolic waste products that must be either excreted or reused.
  • The gaseous waste products generated by photosynthesis and respiration are used as substrates for the opposite reaction.
  • Gas exchange occurs with the atmosphere to eliminate excess gaseous waste.
  • Water can be eliminated through evaporation via three different types of transpiration: stomatal, lenticular, and cuticular.
  • Water can be eliminated as a liquid, in the form of xylem sap, through guttation, in which it is exuded through pores called the hydathodes.
  • The nitrogenous waste products generated through protein metabolism are reused in protein synthesis.
  • Waste products consisting of mineral salts, such as calcium oxalate, are stored as insoluble crystals called raphides in some plants.

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