Originally posted by curioucity
Why is it that animals cannot do a set of solid-waste-free metabolisms? plants' metabolism always dump gas at the very most......
Well, the feces excreted by animals represents that material, mostly, that they did not want to absorb into their bodies in the first place.
If we consider the waste products of cellular metabolism, the main waste products are carbon dioxide and water. Plants release these into the air as gases, or use some of that "metabolic water" for their own purposes. A human, for example, will get rid of the carbon dioxide in the same form for the most part. The metabolic water can be conserved for use, or if in excess, removed from the blood by the kidneys.
I think that maybe your question is related to different fates of the products of nitrogen metabolism in plants vs animals. Again, considering a human vs a plant; A human is unable to store any significant amount of the protein that they consume. Excess amino acids cannot just be stored the way that one can store excess fat or glucose. So, your liver (mostly) takes the excess amino acids ( and nucleotides and any nitrogenous substance for that matter) and cuts off the amino group. This makes ammonia, which is very toxic. So, the ammonia is made into urea, which is nice and water soluble. Your kidneys can remove the urea and dump it out in the urine. One of the main reasons to pee is to get rid of this excess nitrogen, mostly in the form of urea. Birds and reptiles get rid of most of their excess nitrogen as uric acid, a solid. This is a way to conserve water. Uric acid is much of that white stuff in bird poop
But, plants can't pee. I once read an interesting essay on this topic titled "Why Trees Don't Urinate On Dogs". They do have to deal with excess nitrogen and ammonia, but they can't easily excrete it. So, they attach it to carbon compounds and hold onto it in some form that won't cause them any metabolic problems. This is why plants have such interesting nitrogen compounds, all of those amines (e.g. caffeine, nicotine, morphine, cocaine, quinine, etc). Some plants have come up with amines that serve their purposes very well. Amines, being basic, typically taste bitter, a way to discourage animals from eating them. Other amines might make an animal sick, or give it hallucinations. Still other amines kill or inhibit the growth of microbes, a way of warding off parasites. This is why so many medicinal substances come from plants.
Hope this helps.
Chuck