 What better way than to start the day with a protein-packed, healthy breakfast? And this protein is digested in our body into simplest amino acids, right? Our body utilizes whatever part it requires and excretes out the remaining part. Now the most critical in the excretion of these amino acids are the nitrogen part of it. Well, here I have shown just three different amino acids but our body utilizes 20 different kinds of them. And again, it is not just the amino acids from the proteins that creates the nitrogenous waste. We also consume another macromolecule which also has nitrogen in it. Can you guess? Well, it's the nucleic acids that make up our DNA and RNA, right? They also have nitrogen in them. And they will be present in the cells and nuclei of all the plants and animals that we consume, right? Well, now you may think, why is just getting rid of nitrogen such a big deal? I mean, there are other components in this molecule like carbon, hydrogen, oxygen. Why is nitrogen creating such a fuss? Well, the answer is that nitrogen forms ammonia inside the animal's body and which is a highly, highly toxic compound. So getting rid of it from the body is very crucial. And if we accumulate that in our body, it has the potential to damage the brain cells as well. And that's the reason we are going to learn about the different strategies. Different animal groups have adopted over the course of evolution to get rid of their nitrogenous waste. So as we discussed, nitrogen turns into ammonia inside the body. Now, ammonia is highly toxic. It is extremely toxic. So the animal cannot store them inside the body. It has to be immediately sent out of the body as soon as it is formed, okay? And to send them out of the body, they have to be diluted in water. And how much water does ammonia require to be sent out? Well, one gram of ammonia requires around 300 to 500 ml of water. Now, how much water do you consume per day? One liter, two liter, or a maximum of four liters, right? Which is 4,000 ml of water. And out of that 4,000 ml, one gram will require 500 ml of water. That means all the water that you will consume will be utilized to send out the ammonia that is produced inside the body. And there will be no water left for other biological needs, right? So excreting ammonia is only seen in animals that has abundance of water. It is only present in the aquatic animals. And how do they remove ammonia? Mostly by diffusion. And that's the reason it requires very less energy. If we talk about the examples, there are unicellular organisms like amoeba, sponges and hydra. They excrete ammonia by means of diffusion, okay? But in tadpoles, earthworms or turtles, it is seen that they are excreted in the form of urine. But in bony fishes, it is excreted in the form of ammonia mines. Now, evolution didn't favor this kind of excretion, okay? Because as the life transitioned into the terrestrial environment, there was less water for the ammonia to be directly excreted out. And second, since it's very toxic, it has to be immediately removed from the body. And the terrestrial animals, like you and me, we can't effort to sit in the toilet the whole day. So evolution favored the conversion of ammonia into urea inside the body. And urea is much, much less toxic. That means we can afford to keep them in our body for some time, okay? And also, the urea which is formed, it needs less water to be sent out of the body. One gram requires just 50 ml. So that's the reason it is a very good fit for the terrestrial life. But it also comes with a cost, okay? Forming urea inside the body from ammonia requires ATP, so it needs energy. So in humans or mammals, it is the liver in which the ammonia gets converted into urea. But in cartilaginous fishes, it is most body cells that are capable of converting ammonia to urea. It's not just a liver, okay? But in mammals, it is only the liver where the ammonia is converted into urea. So let's look at a few examples. And talking about the examples, we humans fit here because we excrete our nitrogenous waste in the form of urea. Sharks, rays, kangaroos, whales, camels, most adult frogs, etc. Okay? So if we talk about the amphibians, you can see that the early life, that is the dead pole stage, it excretes ammonia, whereas the adult frog, it excretes urea. So this shows that the excretion of nitrogen is habitat dependent. When there is enough water, it is excreting ammonia, but when it transitioned to land and had less water in its surrounding, it transitioned into urea excretion. And this is how the body of any animal manages the toxic nitrogen waste inside it. You can call it the body's best balancing act. But again, converting to urea was not enough. Animals like birds and reptiles could not survive if they had to eliminate 50 ml water for one gram of nitrogenous waste. So in animals like them, ammonia is converted into uric acid. Now if you have ever got a chance to look carefully at the lizard poop, you would have seen a whitish part and a dark part in it. The dark part is the fecal matter, whereas the whitish part is the uric acid, or you can call it the lizard ure. And unlike the urine of us humans, it is not liquid, it is in a paste form, because very less water was utilized to send it out of the body. And that's why it has that consistency. And again, there are other benefits of uric acid. It is the least toxic and excreting one gram requires just one ml of water. And that's why it is just the right fit for animals that stay in air condition. And you know, all good things in life comes with a cost. So all this good qualities of uric acid of being less toxic, requiring less water will come with more cost. It needs a lot of ATP and therefore it requires the maximum amount of energy. And examples are birds, lizards, snakes, etc. Now the animals that excrete ammonia are called ammonotelic animals and the process of excretion is called ammonotelism. But the animals are called ammonotelic animals and the ones who excrete urea, that is you and me, we are called as ureotelic animals and the process is called ureotelism. And the ones who excrete uric acid are called uricotelic organisms and the process is called uricotelism. And these are the three different strategies used by the living world to excrete nitrogen.