Why Different Organisms Use Different Excretion Methods
Excretion is the removal of waste products from the body of an organism. Based on the type of product, there are three types of animals. The kidney is the primary excretory organ in humans. Lung, skin, liver and intestine are accessory excretory organs.
Physiology of Excretion
The process of excretion has two parts
1. Formation of urea in the liver by urea cycle or ornithine cycle.
2. Formation of urine.
Different Modes of Excretion
Depending upon excretory products, all total there are five modes of excretion in animals. They are:
Ammonotelism: Excretory product is ammonia.
Ureotelism: Excretory product is uric acid.
Aminotelism: Excretory product is amino acids.
Uricotelism: Excretory product is uric acid.
Guanotelism: Excretory product is guanine.
Ammonotelism
The process in which ammonia is eliminated from the body is called ammonotelism. And the organism which exhibits this as an excreted material known as ammonotelic. Examples of organisms are fish, protozoans, echinoderms, poriferans and crustaceans. Such excretion is mainly shown in aquatic animals, as they excrete ammonia directly into the environment, where dilution of compounds occurs very quickly as this compound is very toxic to tissues.
Ureotelism
In organisms like mammals and amphibians, waste material is in the form of urea and organisms which excrete urea are known as ureotelic. As ammonia is converted into urea in the liver of animals and released back into the blood. Further kidney filters the urea and excretes it outside the body. Some of the urea remain inside the matrix of the kidney so that they can maintain the required osmoregulation in the organism. Humans always excrete urea and because of this, they are ureotelic in nature. However, in comparison to ammonia, urea is less toxic.
Uricotelism
Organisms that are uricotelic in nature remove their nitrogenous wastes as uric acid in the form of pellets or paste. In these organisms loss of water is very less and is least toxic also. As uric acid is insoluble in water because of this reason, the excreta is in the paste form. Examples of uricotelic organisms are birds and insects.
Aminotelism
In this type of excretion, mode excreta is in the form of excess amino acids. This process is known as aminotelism. Example: Mollusca.
Guanotelism
In this mode of excretion, organisms convert ammonia into guanine before excretion. Some which show such type of excretion are reptiles, birds, earthworms. As guanine is not soluble in water so no water is required.
FAQs on Understanding the Various Modes of Excretion
1. What are the major types of nitrogenous wastes excreted by animals?
Animals primarily excrete three major types of nitrogenous wastes based on their habitat and water availability. The classification of animals based on their primary excretory product is a key concept in understanding excretion. These types are:
- Ammonia: Excreted by ammonotelic animals, such as most aquatic invertebrates and bony fishes. Ammonia is highly toxic and requires a large amount of water for its elimination.
- Urea: Excreted by ureotelic animals, including mammals, terrestrial amphibians, and marine fishes. Ammonia is converted to less toxic urea in the liver, which helps conserve water.
- Uric Acid: Excreted by uricotelic animals like reptiles, birds, and insects. Uric acid is the least toxic and is excreted as a paste or pellet with minimal water loss, a significant adaptation for dry environments.
2. What are the three main steps involved in urine formation in the nephron?
Urine formation is a complex process that occurs in the nephrons of the kidneys and involves three main steps. Understanding these steps is fundamental to explaining how the body filters waste.
- Glomerular Filtration: This is the first step where blood is filtered under high pressure as it passes through the glomerulus. Water, glucose, salts, and urea are forced out of the blood into Bowman's capsule, forming the glomerular filtrate. Larger molecules like proteins and blood cells are retained in the blood.
- Tubular Reabsorption: As the filtrate passes through the renal tubule, the body reabsorbs essential substances. About 99% of the filtrate is reabsorbed, including most of the water, glucose, and essential ions. This process is crucial for preventing the loss of vital nutrients.
- Tubular Secretion: In the final step, waste products like hydrogen ions, potassium ions, and certain drugs are actively secreted from the blood into the filtrate in the tubule. This process helps maintain the pH balance of the blood and eliminates additional wastes.
3. How is the process of excretion different from egestion?
Excretion and egestion are two distinct biological processes that are often confused. The key difference lies in the type of waste and the metabolic pathway involved. Excretion is the removal of metabolic waste products from the body, such as urea, carbon dioxide, and excess salts, which are produced by the body's cells. In contrast, egestion is the process of discharging undigested or unabsorbed food from the digestive tract in the form of faeces. Excretion involves organs like the kidneys, lungs, and skin, while egestion is the final step of digestion.
4. How does ureotelism offer a significant advantage over ammonotelism for terrestrial animals?
Ureotelism provides a crucial survival advantage for terrestrial animals primarily related to water conservation and toxicity management. Ammonia, the waste product in ammonotelism, is extremely toxic and requires a large volume of water to be diluted and safely excreted. This is feasible for aquatic organisms with constant access to water. However, terrestrial animals must conserve water. They convert highly toxic ammonia into urea, which is about 100,000 times less toxic. This allows the body to transport and store the waste in the bladder temporarily and excrete it in a concentrated form (urine), leading to significantly less water loss compared to excreting ammonia directly.
5. Are the lungs and skin also considered excretory organs? Explain their role.
Yes, besides the kidneys, other organs play a supporting role in excretion. The lungs and skin are important accessory excretory organs. Here's how they contribute:
- Lungs: The primary function of the lungs is respiration, but in this process, they excrete a significant metabolic waste product: carbon dioxide. A large amount of CO₂ and a considerable quantity of water vapour are removed from the body during exhalation.
- Skin: The skin contains sweat glands that produce sweat, which is mostly water and salts (like NaCl). While the primary purpose of sweat is thermoregulation (cooling the body), it also helps in the removal of small amounts of urea and lactic acid, thus contributing to excretion.
6. Why is the glomerular filtrate described as an 'ultrafiltrate' of plasma?
The term 'ultrafiltrate' is used to describe the glomerular filtrate because the filtration process in the glomerulus is extremely fine and selective based on molecular size. The membrane between the glomerular capillaries and Bowman's capsule acts as a barrier that allows small molecules like water, glucose, amino acids, salts, and urea to pass through freely. However, it prevents the passage of large molecules, specifically plasma proteins (like albumin) and blood cells (RBCs, WBCs). Therefore, the resulting filtrate is essentially blood plasma minus these large protein and cellular components, making it an 'ultrafiltrate'.
7. How do plants excrete their waste products without a specialised excretory system?
Plants lack a specialised excretory system like animals but have developed various mechanisms to manage their waste products. Key methods include:
- Gaseous Waste Removal: Plants excrete gaseous wastes like carbon dioxide (from respiration) and oxygen (a by-product of photosynthesis) through stomata on leaves and lenticels on stems.
- Storing Waste: Many waste products are stored in cellular vacuoles or in tissues with dead cells, such as old xylem (heartwood). They also store waste in leaves, bark, or fruits that are eventually shed from the plant.
- Secretion: Plants secrete some waste products into the soil around them through their roots. They also excrete wastes in the form of gums, resins, and latex, which are often stored in special ducts or canals.
