Question

which excretiory product requires large amount of energy for its elimination

Answer 1

The excretory product requires large amount of energy for its elimination is the kidney.

Answer 2

A common fact is that Ammonia, Urea and Uric acid are the most common excreted metabolites and their removal needs varying amount of water, highest for ammonia (hence suited only for aquatic animals) and lowest for uric acid (suited for desert animals). Why is this so? Is it because of different solubility of these compounds in water or their toxicity? If it is the latter, how would excess water reduce the toxicity of a compound? This is a bit over-simplistic question, but I want to understand the tradeoffs involved in selecting the mode of excretion, ammonotelic, ureotelic, uricotelic.