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.

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.

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.

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:

Photosynthesis

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.

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.

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.

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 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 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 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.

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.

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.

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.

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.

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.

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