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Nitrogen, Nitrogen Cycle and Excretion of Nitrogen

by Kevin Ahern, PhD
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    00:01 In the lectures on amino acid metabolism, I've talked about how amines are moved from one molecule to another, and how ammonia is a toxic byproduct of their metabolism.

    00:11 With this lecture, I'll talk about how those amines and how that ammonia are combined and prepared for excretion.

    00:18 Nitrogen in the body, as we have seen, comes in various forms.

    00:22 Balance is absolutely critical.

    00:24 Nitrogen is an essential nutrient, but it’s also one that two high levels can be toxic.

    00:30 The body must be able to handle the nitrogen arising from amino acid metabolism.

    00:35 But nitrogen is also needed for the synthesis of other molecules.

    00:38 This includes the nucleotides, ATP, GTP, CTP, UTP, dATP, dCTP, dGTP and dTTP.

    00:47 Nitrogen is also needed for making of non-protein amino acids.

    00:51 These include ornithine, citrulline and sarcosine as shown in the screen.

    00:56 And there are yet other nitrogen containing compounds that are necessary for synthesis in the body.

    01:01 This includes choline, various vitamins and carnitine.

    01:07 Now, the nitrogen cycle is a very important cycle in our environment.

    01:10 Nitrogen is a -- the nitrogen in our atmosphere must be reduced.

    01:14 And there are nitrifying organisms at the very deepest parts of our soil that grab that nitrogen and convert it into the amines that are useful for higher organisms.

    01:23 That over all process is shown on the slide.

    01:28 Now, nitrogen must be excreted if the balance is to be properly maintained.

    01:33 As I've noted, amino acids through transamination make nitrogen mobile.

    01:38 It's very easy to move nitrogen from one amino acid to a keto acid and vice versa.

    01:45 The toxicity of ammonia means that the nitrogen that's produced as a byproduct of amino acid metabolism must be handled properly.

    01:54 In the excretion process, there are various strategies that are employed by different organisms.

    02:00 Organisms for example that are ammonotelic - excrete ammonia.

    02:04 This includes fish and this is why you have to periodically clean your aquarium.

    02:08 Uricotelic organisms excrete uric acid and these include birds.

    02:14 Ureotelic organisms include human beings.

    02:16 These excrete urea and it also includes most vertebrates and a few invertebrates.

    02:22 Now, when we look at the structure of these molecules, we see of course that ammonia only contains one nitrogen but has the advantage that it's water soluble.

    02:31 It has the disadvantage of course that it's toxic.

    02:34 Uric acid is a very good way to get rid of a lot of nitrogen.

    02:37 It has four nitrogens within it but suffers from the disadvantage that it is not very water soluble.

    02:44 Urea is the sort of the perfect compromise of the two.

    02:47 It contains two nitrogens for excretion and it's very water soluble.

    02:52 The ammonia that's produced by amino acid catabolism is used in the urea cycle.

    02:58 And the uric acid that's produced by purine catabolism is a byproduct of the breakdown of nucleotides.

    03:05 And finally, urea is produced by the urea cycle.


    About the Lecture

    The lecture Nitrogen, Nitrogen Cycle and Excretion of Nitrogen by Kevin Ahern, PhD is from the course Amino Acid Metabolism.


    Included Quiz Questions

    1. Its balance is critical.
    2. It is needed for the synthesis of sugars and glycogen.
    3. It is present only in the nitrous form.
    4. All of the answers are true.
    5. None of the answers are true.
    1. The ureotelic organisms include most vertebrates.
    2. The ammonotelic organisms are primarily birds.
    3. The uricotelic organisms secrete urea.
    4. All of the answers are true.
    5. None of the answers are true.

    Author of lecture Nitrogen, Nitrogen Cycle and Excretion of Nitrogen

     Kevin Ahern, PhD

    Kevin Ahern, PhD


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