Q.State role of liver in urea production.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Modes Of Nitrogenous Excretion
Why do we excrete nitrogen at all?
Every living cell runs on proteins and nucleic acids. When these molecules are broken down — either from old, damaged proteins or from excess dietary intake — the amino groups (−NH2) are stripped off. That nitrogen atom, now free, is toxic. Ammonia (NH3) is the simplest form, but it is highly soluble and poisonous even in small concentrations. The body must get rid of it, or convert it into something safer, before it accumulates.
So the fundamental problem is: how do you safely eliminate nitrogen waste, given the environment you live in?
The answer depends on how much water you can afford to lose, and how much energy you can spend.
The three strategies
1. Ammonotelism — dump it as ammonia
If you live in water, you can simply let ammonia diffuse out through your skin or gills. Ammonia is tiny, highly soluble, and requires almost no energy to produce (no conversion cost). The catch: you need a lot of water to keep the concentration low enough that it doesn't poison you.
Ammonotelism is the excretion of nitrogenous waste primarily as ammonia (NH3). It is common in aquatic animals — most bony fishes, aquatic amphibians, and aquatic invertebrates.
The advantage is energy efficiency. The disadvantage is water loss — you must constantly flush your body with water. That is fine in a river or ocean. On land, it is impossible.
2. Ureotelism — convert to urea
On land, water is scarce. You cannot afford to pour litres of water through your body just to wash out ammonia. So the liver converts two molecules of ammonia (plus one CO2) into urea — a much less toxic, neutral molecule.
2NH3+CO2ornithine cycleCO(NH2)2+H2O
Urea is still water-soluble, but you can hold it at a much higher concentration in urine before it becomes dangerous. That means you need far less water to excrete the same amount of nitrogen.
Ureotelism is the excretion of nitrogenous waste primarily as urea. It is found in mammals, most adult amphibians, cartilaginous fishes (sharks, rays), and some terrestrial turtles.
The trade-off: urea synthesis costs energy (about 4 ATP per urea molecule). But it saves water — a critical advantage on land.
3. Uricotelism — go solid
Some animals face extreme water shortage — birds, reptiles, insects, land snails. They cannot afford to lose any water at all through nitrogen excretion. Their solution: convert ammonia into uric acid, a purine derivative that is almost insoluble in water.
Uric acid is excreted as a semi-solid paste (the white part of bird droppings). It costs even more energy to make (about 7 ATP per molecule of uric acid), but it saves almost all the water.
Uricotelism is the excretion of nitrogenous waste primarily as uric acid. It is characteristic of birds, reptiles (including snakes and lizards), insects, and some land snails.
The trade-off at a glance
| Feature | Ammonotelism | Ureotelism | Uricotelism |
|---|---|---|---|
| Waste form | Ammonia (NH3) | Urea (CO(NH2)2) | Uric acid (C5H4N4O3) |
| Toxicity | Very high | Low | Very low |
| Water needed | Very high | Moderate | Very low |
| Energy cost | Very low | Moderate | High |
| Habitat | Aquatic | Terrestrial (with water access) | Arid / dry land |
The precise statement
Modes of nitrogenous excretion refer to the form in which an animal eliminates the nitrogen derived from protein and nucleic acid metabolism. The three major modes are:
- Ammonotelism: excretion of ammonia (requires abundant water, low energy)
- Ureotelism: excretion of urea (moderate water loss, moderate energy) …
The liver converts toxic ammonia into less-toxic urea through the ornithine (urea) cycle. …
Excess amino acids are deaminated, releasing ammonia, which is too toxic to be carried around the body or excreted directly at typical mammalian water-intake levels. The liver takes on the job of detoxifying this ammonia: through the ornithine cycle (also called the urea cycle, discovered by Krebs and Henseleit in 1932), liver cells convert ammonia into urea, at a cost of 3 ATP molecules per urea molecule produced. Urea is far less toxic and far less water-soluble than ammonia, so once released into the blood it can be safely transported to the kidneys and excreted using a much sma …
Recall the specific organ (liver) and specific cyclic pathway (ornithine/urea cycle) resp …
- Saying the kidney produces urea — urea is manufactured in the liver and only excreted by the kidney. …
- CBSE 2026Set ANNUAL1 markMCQQ.The excretory product in frog is(a) uric acid(b) uric acid and ammonia(c) urea(d) ammonia
›Reveal solutionSolution
The adult frog excretes nitrogenous waste mainly as urea (ureotelic excretion).
Animals are classified by the chief nitrogenous waste they excrete: ammonotelic (ammonia — most bony/aquatic animals such as tadpoles, many aquatic invertebrates), ureotelic (urea — most terrestrial/semi-terrestrial amphibians and mammals), or uricotelic (uric acid — reptiles, birds, insects, adapted to conserve water). The adult frog, being amphibious but spending significant time on land where water conservation matters more than in its aquatic larval stage, converts toxic ammonia to the less toxic, less water-demanding urea and is therefore classified as ureotelic. (Its tadpole, being fully aquatic, is ammonotelic.)
…
- CBSE 2026Set ANNUAL1 markMCQQ.Adult Frogs are similar to Human in the type of Excretory waste they produce. What is Primary Excretory waste in both?(a) Ammonia(b) Urea(c) Uric acid(d) Creatinine
›Reveal solutionSolution
Adult frogs, like humans, are ureotelic animals that excrete nitrogenous waste chiefly as urea.
Animals excrete nitrogenous waste in one of three main forms depending on their habitat and need to conserve water: ammonia (ammonotelism, seen in aquatic animals like bony fish and tadpoles, where ammonia can be diluted and washed away quickly since it is highly toxic and needs plenty of water), urea (ureotelism, seen in terrestrial and semi-terrestrial animals like adult amphibians, mammals, and humans, where the more toxic ammonia is converted in the liver to less toxic urea, which is more water-soluble and requires far less water to be safely excreted), and uric acid (uricotelism, seen in birds, reptiles and insects, an almost non-toxic, poorly soluble semi-solid form that con …
- CBSE 2026Set ANNUAL1 markMCQQ.Uricotelic animals excrete(a) Ammonia(b) Urea(c) Uric acid(d) Creatinine
›Reveal solutionSolution
Uricotelic animals excrete uric acid, so the answer is (C).
Animals are grouped by their main nitrogenous waste:
- Ammonotelic — excrete ammonia (e.g. many aquatic animals, bony fishes); needs a lot of water.
- Ureotelic — excrete urea (e.g. mammals, adult amphibians). …
- CBSE 2025Set zoology-hz21 markMCQQ.Frog is :(a) Uricotelic(b) Ureotelic(c) Ammonotelic(d) None of these
›Reveal solutionSolution
The adult frog is ureotelic — it excretes nitrogenous waste mainly as urea.
Animals are classified by the form in which they excrete nitrogenous waste:
- Ammonotelic: excrete ammonia directly (needs large amounts of water to dilute the toxic ammonia) — e.g., many bony fishes, aquatic amphibian larvae (tadpoles).
- Ureotelic: convert toxic ammonia to less toxic urea in the liver (ornithine cycle) before excretion — e.g., adult amphibians (including the adult frog), mammals.
- Uricotelic: convert ammonia to insoluble uric acid — e.g., reptiles, birds, insects (water-conserving). …
- CBSE 2020Set ANNUAL1 markMCQQ.Which is uricotelic?(a) Earthworm(b) Cockroach(c) Frog(d) Human being
›Reveal solutionSolution
The cockroach is uricotelic — it excretes nitrogenous waste chiefly as uric acid.
Animals are classified by the form in which they excrete nitrogenous waste: ammonotelic (ammonia — most bony fish, aquatic amphibians, aquatic insects, where earthworm and frog fall depending on habitat), ureotelic (urea — mammals, many terrestrial amphibians like frog on land, marine fish), and uricotelic (uric acid — insects, reptiles, birds, land snails). Uric acid is largely insoluble in water and can be excreted as a semi-solid paste with very little water loss, which is a major water-conservation adaptation for …
- CBSE 2016Set ANNUAL1 markQ.Why aquatic animals can afford to be ammonotelic?
›Reveal solutionSolution
Aquatic animals live surrounded by abundant water, so highly toxic ammonia can be rapidly diluted and diffused away through their body surface/gills as fast as it is formed, without needing to spend extra energy conve...
Unlimited surrounding water lets aquatic animals safely excrete toxic ammonia directly, without converting it to a less toxic form.
Ammonia is the most toxic of the nitrogenous excretory products, but it is also extremely soluble in water. Aquatic animals (most bony fishes, aquatic amphibians, many invertebrates) are constantly surrounded by a large volume of water, which allows the ammonia they excrete (mainly by diffusion across gills or the general body surface) to be immediately and continuously diluted to non-toxic levels and carried away. Because this dilution happens so efficiently, aquatic animals do not need to expend additional metabolic energy converting ammonia into the less toxic but energetically 'costlier' urea (as in ureotelic terrestrial a …
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