Why Is Excretion Essential for Living Organisms?
Excretion Meaning
Excretion definition in biology is a process through which living organisms expel waste or toxic substances from their bodies. It is like eliminating unwanted substances from a living body. The product that comes out of the excretion process is called excretory products. Excretory products are generated through metabolic activities or non-metabolic activities. Metabolism helps in stimulating the excretory process. In unicellular organisms, excretion takes place through cells whereas in multicellular organisms, like animals and human beings, excretion takes place through the body.
Types Of Excretory Waste
Metabolic wastes are excreted in the form of solid, liquid, and gas like oxygen in plants, sweat in humans. Non-metabolic wastes are generally substances in a living organism’s body that are no longer required or are no more useful to the body like urine and excreta.
Excretion In Humans
The human excretory system is also the urinary system of the human body. This system consists of the kidneys, the ureters, the bladder, and the urethra. Kidneys are the main functional unit of this system. Nephron, a part of the kidney, helps in filtering the blood and collecting waste from it and storing it into the kidney. The waste from the blood is stored in the form of urine in both the kidneys. Urine leaves the kidneys through the ureters and then passes into the bladder. The bladder stores the urine, but it cannot store for longer. The bladder pressurizes the urine to move to the urethra and straight out of our body in the form of urine.
Functions of Kidney
Excretion of toxic substances through metabolic activity such as urea, uric acid, and ammonia in our sweats or urine.
Maintains the homeostatic conditions of our body, including our body temperature, pH balance of our extracellular fluid.
It also manages the volume of extracellular fluid and also helps in maintaining its ionic balance.
Excretion in Animals
In animals, bodies with a single layer of cells, excretion generally occurs through the process of diffusion. For example, sponges that are multicellular animals but with a single layer of cells excrete through specialized cells. It diffuses gaseous wastes into the water. It takes place at the site of the elimination of the outer environment of the organism. In more complex animals, the process of excretion takes place through an overly complicated excretory system. For example, in vertebrates, the excretion of wastes takes place through an excretory organ like the kidney and urinary ducts associated with it.
For example, an elephant eats his food and swallows it down his stomach. This process is called ingestion. In the stomach, the food is broken down into simpler and soluble products. This is digestion. The soluble parts are absorbed in the body by the process of assimilation. After absorption, enzymes act on the product, and through metabolism, it undergoes a chemical reaction. This leads to the production of carbon dioxide which is exhaled to the environment with the help of the lungs in the respiratory system, oxygen that is produced is used, and other nitrogenous wastes like uric acid, ammonia, etc. are excreted out of the body through the excretory system and its organs-kidneys and various urinary ducts. This system is more or less similar to the excretory system of human beings.
Excretion in Plants
We should not forget that plants are living things and, therefore, can excrete too. Small plants can simply go through their excretory process with the help of their cells. These plants excrete wastes on the surface of their cells. Large plants cannot use these cells as they do not have much access to the outside environment. Therefore, like animals, they use their cells to secrete wastes from the extracellular spaces, especially in the leaves. The main product of the excretion in plants is oxygen or O2, which is our life support. Plants exhale oxygen through large openings in the leaves called stomata. This very much sums up the process of photosynthesis in the plants as well. These stomata inhale the carbon dioxide that we give out and exhale the oxygen that we breathe. The major difference between the excretion in plants and animals through extracellular spaces is that the main byproduct of the excretory system of the plant is in a gaseous form, unlike animals.
FAQs on Excretion in Biology: Meaning, Types & Importance
1. What is the fundamental definition of excretion in biology?
Excretion is the biological process of removing harmful metabolic wastes and other non-useful substances from the cells and tissues of an organism. This process is crucial for maintaining a stable internal environment, a state known as homeostasis, by preventing the toxic buildup of substances like urea, ammonia, and uric acid.
2. What are the major types of excretion, classified by the primary nitrogenous waste product?
Organisms exhibit different modes of excretion based on the main nitrogenous waste they produce. The primary types are:
- Ammonotelism: The excretion of ammonia. This is common in aquatic animals like bony fish and amphibians that have access to plenty of water to dilute the highly toxic ammonia.
- Ureotelism: The excretion of urea. Mammals, including humans, and some terrestrial amphibians convert ammonia into less toxic urea, which requires less water for removal.
- Uricotelism: The excretion of uric acid. Birds, reptiles, and insects excrete uric acid as a semi-solid paste, which is an excellent adaptation for conserving water.
3. What is the key difference between excretion and secretion?
The key difference lies in the nature and purpose of the substance being released. Excretion is the removal of unwanted and toxic metabolic waste products like urine and carbon dioxide. In contrast, secretion is the process where useful substances, such as hormones, enzymes, or saliva, are produced and released by cells or glands to perform a specific function within the body.
4. How does the human excretory system work to filter waste from the blood?
The human excretory system primarily involves the kidneys. Blood enters the kidneys, where millions of tiny filtering units called nephrons perform their function. The process involves three main steps: glomerular filtration (filtering blood), selective reabsorption (reclaiming useful substances like glucose and water), and tubular secretion (adding more waste to the filtrate). The final waste fluid, now called urine, travels from the kidneys through the ureters, is stored in the urinary bladder, and is finally expelled from the body through the urethra.
5. What are the consequences of kidney failure, and how is it medically managed?
If the kidneys fail to function, toxic metabolic wastes like urea accumulate in the blood, a condition called uraemia. This can lead to serious health issues, including fluid retention, electrolyte imbalance, and organ damage. The most common medical intervention for kidney failure is dialysis, an artificial process that filters the blood outside the body. In severe cases, a kidney transplant may be required.
6. How is excretion in plants different from excretion in animals?
Plants and animals have fundamentally different strategies for excretion. Animals possess complex, specialised excretory organs (like kidneys, lungs, and skin) to actively remove waste. In contrast, plants lack such organs. They manage waste through simpler methods such as: storing waste in vacuoles or aging leaves that are later shed; releasing gaseous wastes (like oxygen during photosynthesis) through stomata; and secreting waste products like resins and gums.
7. Why is the removal of metabolic waste so critical for the survival of an organism?
The removal of metabolic waste is critical because these substances are often toxic and can disrupt vital bodily functions if allowed to accumulate. For instance, high levels of ammonia or urea can alter the pH of blood and tissues, interfere with enzyme activity, and damage cell membranes. By eliminating these toxins, excretion plays a direct role in maintaining homeostasis, ensuring that the internal chemical and physical conditions remain stable and conducive to life.
8. What is the difference between excretion and egestion, and is defecation a form of excretion?
This is a common point of confusion. Excretion is the removal of metabolic waste produced by the body's cells. Egestion, on the other hand, is the removal of undigested, unabsorbed food materials from the digestive tract. Therefore, defecation—the act of passing faeces—is an example of egestion, not excretion, because faeces primarily consist of undigested food and bacteria, not metabolic by-products from cellular activity.
