In most freshwater fish, nitrogenous waste is primarily excreted as ammonia, which is highly soluble in water and is toxic at low concentrations. In terrestrial mammals, ammonia is converted to urea before it is excreted. Urea is also highly soluble in water but is less toxic than ammonia at low concentrations. Which of the following best explains why freshwater fish do not convert ammonia to urea for excretion?
- A The metabolic pathways of fish do not normally involve nitrogen consumption.
- B The dilution of ammonia by direct excretion into freshwater conserves energy.
- C Ammonia is concentrated in tissues, where it is stored prior to excretion.
- D The nitrogen in ammonia is recycled for use in protein and nucleotide synthesis.
