Animals have different ways of getting rid of toxic nitrogen waste. Aquatic creatures simply dump ammonia into the water, while land animals convert it to safer forms like urea or uric acid to save water.

The urea cycle in mammals transforms ammonia to urea in liver cells. This process involves five key enzymes working in the mitochondria and cytoplasm. The resulting urea is then excreted through urine by the kidneys.

Nitrogenous Waste Elimination and Urea Cycle

Ammonia elimination in animals

• Aquatic animals
  • Excrete ammonia directly into surrounding water
  • High solubility of ammonia in aquatic environments allows for efficient removal
  • Large volume of water dilutes toxic effects of ammonia (oceans, lakes, rivers)
• Terrestrial animals
  • Convert ammonia into less toxic compounds to conserve water
    • Urea synthesized by mammals is less toxic than ammonia
    • Uric acid produced by birds, reptiles, and insects is least toxic form
  • Conservation of water crucial in terrestrial environments to prevent dehydration (deserts, savannas)
  • Preventing water loss through excretion of concentrated nitrogenous wastes

Nitrogenous waste across species

• Mammals
  • Urea primary nitrogenous waste product
    • Soluble in water allowing excretion in urine
    • Requires more water for excretion compared to uric acid
  • Urea less toxic than ammonia enables storage in bladder
• Birds and reptiles
  • Uric acid main nitrogenous waste product
    • Insoluble in water allows excretion with minimal water loss
    • Can be excreted as semi-solid paste with feces (bird droppings)
  • Adaptation to conserve water in dry habitats and during egg development
  • Uric acid least toxic nitrogenous waste compared to ammonia and urea

Urea cycle steps and enzymes

• Occurs in liver hepatocytes to convert ammonia to urea
• Ammonia converted to urea through series of enzymatic reactions
• Key steps and enzymes:
  1. Carbamoyl phosphate synthetase I (CPS-I)
     • Combines ammonia and bicarbonate to form carbamoyl phosphate
     • Occurs in mitochondrial matrix
  2. Ornithine transcarbamylase (OTC)
     • Catalyzes reaction between carbamoyl phosphate and ornithine to form citrulline
     • Occurs in mitochondrial matrix
  3. Argininosuccinate synthetase (ASS)
     • Combines citrulline and aspartate to form argininosuccinate
     • Occurs in cytosol
  4. Argininosuccinate lyase (ASL)
     • Cleaves argininosuccinate into arginine and fumarate
     • Occurs in cytosol
  5. Arginase
     • Hydrolyzes arginine to form urea and regenerate ornithine
     • Occurs in cytosol
• Ornithine regenerated to continue urea cycle
• Urea released into bloodstream and excreted by kidneys in urine

Nitrogen Metabolism and Excretion

• Nitrogen metabolism involves the breakdown and conversion of nitrogenous compounds in the body
• Amino acid catabolism produces ammonia as a byproduct, which must be eliminated
• The excretory system plays a crucial role in removing nitrogenous wastes from the body
• Hepatic function is essential for nitrogen metabolism and waste elimination
  • The liver performs detoxification of harmful substances, including ammonia conversion to urea