Nitrogenous Wastes: Definition, Forms & Interrelationships

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  • 0:01 Nitrogenous Waste
  • 1:23 Forms of Nitrogenous Waste
  • 2:33 Waste & Environment
  • 5:21 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

The natural processes of creating energy always result in some sort of wasteful byproduct. In this lesson we are going to look at how organisms produce and dispose of nitrogenous wastes and see how this relates to those organisms' evolutionary history.

Nitrogenous Waste

Nature is full of beautiful and majestic phenomena. Sunsets, the soft snow of a crisp winter morn, blooming flowers in springtime. Bird poop. Okay, some parts of nature aren't that majestic, but they're still important. Life exists because energy can be transferred between molecules, but this process is never perfect and almost always results in something being wasted. Now, if an organism wastes anything, it's because it simply can't use it, and that means this waste product is probably harmful.

Now, living creatures that don't create their own energy by processes like photosynthesis extract much of it from consuming protein molecules. When the amino acids within the proteins are broken up for energy, the leftover bits form the molecule ammonia, three hydrogen atoms bonded to a nitrogen, which is a very toxic substance. So, the organism needs to get rid of it. Nitrogen-based waste formed from metabolic processes, or nitrogenous waste can be expelled in a series of ways. It's not exactly something to make a motivational poster about, but it's still as important as anything else in nature's majesty.

Forms of Nitrogenous Waste

Alright, so what are the basic forms of nitrogenous waste? First is simply ammonia, a nitrogenous waste with a single nitrogen molecule. Like I said, ammonia is very toxic, but flushing it out of the body takes a lot of water. However, since this is such a simple molecule, it doesn't take a lot of effort to make and expel from the system--just lots and lots of water.

The next form of nitrogenous waste is urea, a nitrogenous waste molecule with two nitrogen atoms. Urea is less toxic than ammonia, so it can stay in the body a little longer and takes less water to flush out. But, it also requires more energy to produce.

The third major form of nitrogenous waste is uric acid, a nitrogenous molecule with four nitrogen atoms. This is the most complex of the common nitrogenous wastes and requires the most energy for a cell to produce, but it also gets the most nitrogen out of the body, is not toxic, and requires very little water to get it out of the body.

Waste and Environment

So, why do living things need three different kinds of nitrogenous waste? Why can't all organisms that produce this kind of waste just rely on ammonia? Well, remember how ammonia is really toxic and needs lots of water to get rid of? How many creatures have constant, and I do mean constant, access to that much water? The only animals that can continually flush this much ammonia out of their systems are those that live in water, things like jellyfish and bony fish. Since these creatures are constantly surrounded by water, they are able to continually expel ammonia.

This is not true of other creatures. Mammals, for example, aren't generally surrounded by water all of the time, so we can't constantly get rid of ammonia, and it would build up in our systems and poison us. So, mammals package and dilute ammonia as urea. This is less toxic, so it can stay in the system long enough for the body to build up enough water to flush it out.

Let me ask, what happens when you drink lots of water? Urine is the main way that animals get rid of urea. This system had to develop because mammals evolved on dry land and couldn't constantly flush ammonia out of their systems. Even marine mammals, like dolphins, process nitrogenous wastes as urea, thanks to the shared evolutionary ancestry with land mammals.

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