Read Nitrogen Waste by Toxicity and Route
The chemistry of a nitrogenous waste determines how much water an animal needs and which organs handle it.
Nitrogenous Wastes is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Compare the waste forms
Ammonia is highly toxic and water-soluble, so aquatic animals can eliminate it rapidly with abundant water. Urea is less toxic and is produced by the liver from ammonia. Uric acid conserves water and is common in birds, reptiles and insects.
Connect organs and blood
Liver damage impairs ammonia-to-urea conversion. Urea is high in hepatic venous blood after production and low in renal venous blood after kidney removal. Aquatic amphibians are ammonotelic; adult frogs and humans ureotelic; pigeons, lizards and cockroaches uricotelic.
Compare nitrogenous wastes
Ammonia is the most toxic major nitrogenous waste, costs the least energy to synthesise, and requires the most water for safe excretion. Urea is intermediate; uric acid conserves water but costs more energy. Read a table across toxicity, synthesis cost, and water loss together.
OlympIQ exam lens
- ✓Ammonia: toxic; high water requirement.
- ✓Liver converts ammonia to urea.
- ✓Ammonotelic aquatic amphibians; ureotelic adult frog/humans; uricotelic pigeon/lizard/cockroach.
Do not classify every amphibian the same way: aquatic larval/adult forms may differ.
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