I. Amino Acid Breakdown
A. Leads to ammonia which is toxic & must be excreted as ammonia, uric acid or urea
A. Ammonia can be excreted across epithelial tissues as NH3 or NH4+
B. NH3 passively diffuses across diverse epithelial tissues
C. NH4+ requires special transporters
D. Used by most fish, simple invertebrates, molluscs, sharks because they live in water
III. Uric Acid
A. Uric acid is excreted in a very concentrated form with little need for water
B. Energy is needed to produce uric acid, whereas little energy is needed for ammonia waste
C. Uric acid is less toxic than ammonia
D. Used by birds, reptiles, insects. Bird uric acid is 3000x more concentrated in its excrement than in its blood.
A. Less toxic than ammonia, but much energy is needed to produce it
B. Made in the liver, then released in the blood, most species then eliminate it in urine (some through gills)
C. Used by all mammals, some larval bony fish. Mammal urea is 25x more concentrated in the urine than in the blood.
V. Nitrogen waste variations –some animals, like the lungfish, can change their nitrogen waste depending on the surroundings. For example in surrounding water they excrete ammonia. However, when water levels decrease and they burrow into the mud and form a mucus cocoon, they convert to urea excreters.