Biology · Excretory Products and Their Elimination · NEET
Ammonia is the MOST toxic of the three; uric acid is the LEAST toxic. Urea sits in the middle. That is why animals living in water (with plenty of water to dilute the poison) can afford ammonia, while land animals that must save water switch to the safer urea or uric acid. Toxicity order: ammonia > urea > uric acid.
It is about water. Ammonia is very soluble and needs a large amount of water to be flushed out safely, so bony fish just let it diffuse out through the gills into the surrounding water. Birds must fly and save water, so they turn nitrogen into uric acid, which is almost insoluble and leaves as a semi-solid pellet or paste with very little water loss.
An adult frog on land is UREOTELIC (excretes urea). NCERT clearly states 'the frog excretes urea and thus is a ureotelic animal.' Do not confuse it with an aquatic tadpole or many aquatic amphibians, which are ammonotelic. The switch from ammonia to urea happens as the animal shifts toward land life.
Uric acid needs LESS water than urea. The full order of water needed is: ammonia needs the most, urea needs a moderate amount, and uric acid needs the least (it comes out almost dry as a pellet or paste). Less toxic waste = less water needed to remove it safely.
The cockroach is URICOTELIC ONLY. Being a land insect, it saves water by excreting uric acid through its Malpighian tubules. This was directly asked in NEET 2023. Insects, reptiles, birds and land snails are all uricotelic.
Humans (and other mammals) are UREOTELIC. The liver converts toxic ammonia into urea through the ornithine (urea) cycle, and the kidneys remove this urea in urine. We are not uricotelic; the small uric acid in human urine is a minor by-product, not the main nitrogen waste.
Nitrogenous waste is excreted in the form of pellet or paste by:
The Cockroach is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
They are three modes of nitrogen excretion named by the waste product. Ammonotelism excretes ammonia (most toxic, most water). Ureotelism excretes urea (moderate). Uricotelism excretes uric acid (least toxic, least water). The type an animal uses mainly depends on how much water is available in its habitat.
Ammonotelic: many bony fishes, aquatic amphibians and aquatic insects. Ureotelic: mammals (including humans), many terrestrial amphibians and marine fishes, and the frog. Uricotelic: reptiles, birds, land snails and insects such as the cockroach.
Because it depends on water availability. Aquatic animals have unlimited water, so they can flush out toxic but soluble ammonia easily. Land animals must conserve water, so they invest energy to make the safer, water-saving urea or uric acid instead.
Uric acid is better for saving water. It is almost insoluble and leaves the body as a near-dry pellet or paste, which is why flying birds and desert reptiles are uricotelic. Urea still needs a fair amount of water dissolved in urine.
No. In ammonotelic bony fish, ammonia is highly soluble and diffuses out mainly as ammonium ions across the gill surfaces, so the kidneys play no significant role in removing ammonia in these fish.
