A Study on Various Modes of Excretion
Excretion is vital for removing nitrogenous wastes and maintaining balance in organisms. Different species follow modes like ammonotelism, ureotelism, uricotelism, aminotelism, and guanotelism depending on habitat and adaptation. Learn their examples, differences, and NEET MCQs here.
This Story also Contains
- What is Excretion?
- Modes of Excretion in Various Organisms
- Comparison of Different Modes of Excretion
- Adaptations in Excretion for Water Conservations
- Modes of Excretion NEET MCQs
- FAQs on Modes of Excretion
- Recommended Video on Modes of Excretion and their Functions
As body cells carry out metabolic activities, they consume oxygen and nutrients and produce waste products such as carbon dioxide, urea, and uric acid. Wastes must be eliminated from the body because they can be toxic to cells if they accumulate. While the respiratory system rids the body of carbon dioxide, the excretory system disposes of most other wastes. The excretory system also helps regulate blood composition, pH, volume, and pressure; maintains blood osmolarity; and produces hormones.
Modes of excretion refer to the various ways organisms eliminate metabolic waste products from their bodies. Depending on the physiology and habitat, organisms have different excretory products and their elimination. Organism may excrete ammonia (ammonotelism), urea (ureotelism), and uric acid (uricotelism).
What is Excretion?
Human excretion is a natural process using which living organisms discharge unwanted matter and substances from their bodies for the regulation of internal balance. This process involves the excretion of substances that are waste products in the given body like urea, carbon dioxide and salts which in case of build-up, the body would be harmful to itself. Excretion is very important in control of the internal milieu, concerning water, electrolyte, and acid-base composition. The excretory systems are not passive structures, they are tightly related to the type of life forms and serve as a proof of evolution which has adapted itself to different ways to make a proper disposal of wastes.
Modes of Excretion in Various Organisms
The various types of excretion are:
Ammonotelism
Ammonotelism is the process in which animals release highly toxic ammonia. This mode is most often observed in Fishes and Amphibians since they can excrete ammonia more easily straight to the water they live in. This is because ammonia is highly soluble in water and thus it disperses quickly within the water and water is diluted very fast thus eliminating toxicity.
Ureotelism
Ureotelism is the process of eliminating urea, a less evil nitrogenous waste apart from ammonia. This mode is seen mostly in mammals, amphibians, and cartilaginous fish. Urea is synthesised in the liver via the urea cycle and is expelled through the kidneys; thus, these animals can effectively rid their body of nitrogenous waste products without using much water.
Uricotelism
Uricotelism includes the excretion of uric acid, which has less toxicity than ammonia and is also insoluble in water, turning into a paste. This mode is common in birds, reptiles, and terrestrial insects. In these animals, uric acid clearance does not need as much water as urea or creatinine; hence it helps in further water conservation in regions of drought.
Aminotelism
Aminotelism is another type of regulation and it involves the direct excretion of those amino acids. This mode is quite uncommon, and it is only recorded in some organisms belonging to the invertebrates category. Trimolecular organisms discharge amino acids without the need to metabolize into such compounds and spread them in water channels where the effects of toxicity are eased off.
Guanotelism
Guanotelism is the condition that results in the production of guanine or nitrogenous waste material. This mode is observed in some arthropods such as spiders and some of the insects. Since guanine is fairly non-soluble in water, these organisms can discharge waste in a solid state with no loss of water; more so, in desert-type conditions.
Comparison of Different Modes of Excretion
Mode of Excretion | Primary Waste Product | Toxicity | Water Requirement | Energy Requirement |
Ammonotelism | Ammonia | Very Toxic | Very high (needs continuous dilution) | Very low (no conversion needed) |
Ureotelism | Urea | Moderate | Moderate (less than ammonia) | Moderate |
Uricotelism | Uric acid | Low | Very low (excreted as paste/solid) | High (energy - intensive synthesis) |
Aminotelism | Amino acids | Moderate | High (diluted in water) | Very low (no conversion) |
Guanotelism | Guanine | Very low | Negligible (excreted in solid form) | High |
Adaptations in Excretion for Water Conservations
Despite the harsh conditions of deserts like intense heat during the day, cold at night, and scarcity of water, vegetation, many kinds of animals have adapted to live there successfully. The smaller desert mammals are mostly fossorial (living mainly in the ground) or nocturnal (active at night). The lower temperature and higher humidity of burrows help to reduce water loss by evaporation. Desert animals such as the kangaroo rat can, if necessary, derive the water they need from metabolism of their dry food, drinking no water at all. Such animals produce a highly concentrated urine and form almost completely dry feces.
Modes of Excretion NEET MCQs
Q1. Choose the incorrect statement
The process of excreting ammonia is Ammonotelism
Kidneys play any significant role in the removal of ammonia
Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions
Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature
Correct answer: 2) Kidneys play any significant role in the removal of ammonia
Explanation:
Types of Animals Based on Excretory Wastes -
Ammonotelism:
The process of excreting ammonia is Ammonotelism.
Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature.
Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions.
Kidneys do not play any significant role in its removal.
Hence, the correct option is Kidneys play significant role in the removal of ammonia
Q2. Uric acid is the chief nitrogenous component of the excretory products of
Earthworm
Cockroach
Frog
Man
Correct answer: 2) Cockroach
Explanation:
Uricotelic Animals -
Reptiles, birds, land snails, and insects excrete nitrogenous wastes as uric acid in the form of pellets or paste with a minimum loss of water and are called uricotelic animals. The cockroach is an insect which excretes uric acid.
Hence, the correct answer is option 2) cockroach
Q3. Assertion: Sharks are categorized as ammonotelic animals.
Reason: Sharks have the ability to retain significant amounts of ammonia in their blood.
Both the assertion and reason are true, and the reason is a correct explanation of the assertion.
Both the assertion and reason are true, but the reason is not a correct explanation of the assertion.
The assertion is true, but the reason is false.
Both the assertion and reason are false.
Correct answer: 4) Both the assertion and reason are false.
Explanation:
In this case, the assertion claims that sharks are categorized as ammonotelic animals. However, this assertion is false. Sharks are ureotelic animals, meaning they excrete urea as their primary nitrogenous waste product, not ammonia.
The reason given is that sharks have the ability to retain significant amounts of ammonia in their blood. This reason is also false. Sharks do not retain ammonia in their blood; instead, they convert ammonia into urea through their liver, which is then excreted.
Hence, the correct answer is Option (4) Both the assertion and reason are false.
Also Read:
FAQs on Modes of Excretion
What are the different modes of excretion?
Excretion refers to the process of eliminating nitrogenous waste products formed during metabolism. Different animals have evolved distinct modes of excretion depending on habitat and water availability. These mechanisms help maintain osmotic balance and nitrogen metabolism in organisms. The major types are:
Ammonotelism (ammonia excretion)
Ureotelism (urea excretion)
Uricotelism (uric acid excretion)
Aminotelism (amino acid excretion)
Guanotelism (guanine excretion).
Which animals are ammonotelic?
Ammonotelic animals are those that excrete ammonia as their main nitrogenous waste. Ammonia is highly toxic and soluble in water, so it must be eliminated quickly and requires large amounts of water for dilution. Hence, freshwater fishes, aquatic amphibians, and invertebrates are ammonotelic. This mode of excretion is economical in terms of energy but is only possible for animals living in aquatic environments where water is abundant.
What is ureotelism?
Ureotelism is the mode of excretion in which organisms excrete urea as the chief nitrogenous waste product. Urea is less toxic than ammonia and can be stored in the body for some time before elimination. This adaptation helps animals conserve water while maintaining nitrogen balance. Mammals (including humans), cartilaginous fishes (like sharks), and amphibians follow ureotelism. The liver converts ammonia into urea through the ornithine cycle, which is then excreted by kidneys.
Which mode of excretion is seen in birds?
Birds excrete nitrogenous waste primarily in the form of uric acid, a white paste-like substance with very little water. This process is called uricotelism. Uric acid is almost insoluble in water, making it an adaptation for water conservation. This is essential for flight, as carrying less water reduces body weight. Apart from birds, reptiles and insects also show uricotelism. It represents the most water-efficient excretory strategy among animals.
Recommended Video on Modes of Excretion and their Functions
Frequently Asked Questions (FAQs)
Bony fish in saltwater environments face the challenge of constant water loss due to osmosis. They maintain water balance by drinking seawater, absorbing both water and salt through their intestines, and then excreting excess salt through specialized chloride cells in their gills. Their kidneys produce small amounts of concentrated urine to conserve water.
Yellow bodies, also known as brown bodies, in earthworms are accumulations of waste materials and worn-out tissues. They form when the earthworm's immune system encapsulates foreign particles or dead cells that cannot be excreted through normal means. These bodies are stored in the coelomic cavity and may be eventually eliminated through the nephridia or stored indefinitely in the body.
Terrestrial gastropods have adapted their excretory system to conserve water in a land environment. They have a single kidney that produces uric acid as the main nitrogenous waste product, which requires little water for excretion. Some species can reabsorb water from their excretory products in a specialized ureter. Additionally, they may excrete some waste products through their skin or incorporate them into their shells.
Echinoderms have a unique water vascular system that serves multiple functions, including waste removal. They use ciliated cells called podocytes, similar to those found in vertebrate kidneys, to filter their coelomic fluid. Waste products are then expelled through their tube feet or through small aboral pores. Some echinoderms also have specialized excretory structures called axial glands.
Sharks have a unique excretory system adapted to their marine environment. Unlike bony fish, sharks retain urea in their blood at high concentrations. This helps them maintain osmotic balance with seawater. They have a rectal gland that excretes excess salt, similar to the function of chloride cells in bony fish gills. Sharks produce small amounts of dilute urine, in contrast to the concentrated urine of bony fish.
Aquaporins are specialized protein channels in cell membranes that facilitate the rapid movement of water molecules. In the excretory system, particularly in the kidneys, aquaporins play a crucial role in water reabsorption. They allow for the fine-tuning of water balance in the body by controlling the amount of water that is reabsorbed from the filtrate back into the bloodstream.
Cnidarians, like jellyfish and coral, lack specialized excretory organs. Instead, they rely on diffusion across their body surface to remove metabolic wastes. Their simple body plan, often just two cell layers thick, allows for efficient diffusion of waste products directly into the surrounding water. Some waste products may also be expelled through the mouth opening.
The vasa recta are specialized blood vessels that run parallel to the loop of Henle in the kidney. They play a crucial role in maintaining the concentration gradient in the medulla of the kidney. The vasa recta's countercurrent exchange system helps preserve this gradient, which is essential for the kidney's ability to concentrate urine and conserve water.
Tapeworms, being endoparasites, have a simplified excretory system compared to free-living planaria. Tapeworms use a system of branching tubules called protonephridia, which end in flame cells. These cells help in osmoregulation and removal of some metabolic wastes. Planaria, on the other hand, have a more developed network of flame cells and tubules, allowing for more efficient waste removal in their free-living aquatic environment.
Aldosterone is a hormone that plays a crucial role in regulating electrolyte balance and blood pressure. In the kidneys, it acts on the distal tubules and collecting ducts, promoting the reabsorption of sodium and the excretion of potassium. This helps maintain proper sodium levels in the blood and influences water retention and blood pressure.