Main content
Course: Biology archive > Unit 28
Lesson 7: Biogeochemical cycles- Intro to biogeochemical cycles
- Biogeochemical cycles overview
- The water cycle
- The water cycle
- The carbon cycle
- The carbon cycle
- The nitrogen cycle
- The nitrogen cycle
- The phosphorus cycle
- Phosphorus cycle
- Eutrophication and dead zones
- Biogeochemical cycles
© 2024 Khan AcademyTerms of usePrivacy PolicyCookie Notice
The nitrogen cycle
The nitrogen cycle is a crucial process that converts atmospheric nitrogen into a form that plants and other organisms can use. This process, known as nitrogen fixation, is primarily carried out by bacteria. Nitrogen is essential for life, forming a key component in amino acids, ATP, and DNA. The cycle also includes the return of nitrogen to the atmosphere through decomposition. Created by Sal Khan.
Want to join the conversation?
- When we breathe in air, do we also breathe in the nitrogen in the atmosphere and do we take in this nitrogen? Or do we just exhale it? Is the only source of nitrogen for humans from our food? :)(26 votes)
- No, we cannot obtain usable nitrogen through breathing. Nitrogen has to be "fixed“ into a usable form first before it is consumed by heterotrophs(21 votes)
- Is there anything else that can capture Nitrogen aside from Bacteria?(8 votes)
- Yes there are. Organisms called Archaea also capture nitrogen and fix them. Hope this helps😁(14 votes)
- If bacteria are the prime fixers of nitrogen, what are some ideas on how the nitrogen was fixed which was used to make protoDNA or protoRNA in pre-bacteria times?(7 votes)
- https://www.youtube.com/watch?v=NNijmxsKGbc
Turns out that lighting can break apart and recombine compounds that were in the early earth to form amino acids. This is most likely what happened back then.
If you are interested in how life started, watch How the Universe Works S04 EP5: The Dawn of Life.
Hope I helped a bit! :)(7 votes)
- Like nitrogen has many advantages, so is there any disadvantage of nitrogen also?(7 votes)
- That's a great question! You've probably figured this out by now, seeing how long ago you asked, but I figured I'd give you an answer anyway. An overabundance of nitrogen in an area can cause eutrophication (which is often caused by fertilizers). Algaes feed on the nutrients, causing rapid growth in a body of water, such as a bond. And as the algaes grow, they use up all the oxgen within the water, essentially killing all the other living organisms within it. Hope that helps!(1 vote)
- In nitrogen fixation, are the nitrogen fixing bacteria releasing ammonia directly into the soil, or are they producing ammonia, converting it to amino acids and assimilating this into their biomass so that the nitrogen is only released into the soil when they die and are decomposed by saprobionts?(5 votes)
- Good question!
There are many different types of nitrogen fixation (most of which are not well studied) — my answer focuses on the system where Rhizobia (a genus of bacteria) associate with a group of plants known as legumes (e.g.s beans, clovers, alfalfa).
The Rhizobia are encased in a structure known as a nodule on the roots of legumes.
The nodule protects the bacteria and serves as a site of exchange of small molecules including nutrients between the host plant and the bacteria.
The plant supplies food for the bacteria and in return receives both ammonia and nitrogen containing amino acids.
For more details see this figure (and the accompanying article):
https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.0030192
There are also many free-living nitrogen fixing bacteria and they will primarily incorporate the nitrogen they fix into amino acids, which as you suggested will be released to the environment when they die.
Since, nitrogen fixation requires large amount of energy free-living bacteria will not normally release ammonia into the environment.
You can also learn more about nitrogen fixation in general from the following:
•https://en.wikipedia.org/wiki/Nitrogen_fixation
•https://aces.nmsu.edu/pubs/_a/A129/
•https://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419
Does that help?(1 vote)
- What does he mean in the video when he says"fixing"?(3 votes)
- I believe he meant used / transform into a form where sth can use.1:36supports my claim.3:10is concluded by3:17, which also supports my claim.(4 votes)
- I get that we can't use the nitrogen we breathe, but are there any large organisms that do?(3 votes)
- On our scale, no. Nitrogen gas in the air is inert and doesn't react easily, so it just exists there. Nitrogen has to be somehow absorbed into water or soil before being used by anything.(3 votes)
- Why does our atmosphere contain a lot of nitrogen?(3 votes)
- In the our galaxy nitrogen is the 7th most abundant element and the 5th most in our solar system.(2 votes)
- If nitrogen makes up 72% of air ,then how much of oxygen and carbon makes up air?(3 votes)
- oxygen 20.9476%
Carbon dioxide 0.0314%
https://www.thoughtco.com/chemical-composition-of-air-604288(2 votes)
- Why can't plants directly fix nitrogen from air?(0 votes)
- Because they lack the enzyme dinitrogenase, which will help assimilate the gaseous dinitrogen (N2) in the air(8 votes)
Video transcript
- [Voiceover] Nitrogen
often gets less attention than carbon or oxygen, but nitrogen is very important to life as well. And like carbon and oxygen, it
cycles through our biosphere. Now, one thing that's maybe
surprising about nitrogen, if you haven't studied it much, is that it is very, very
common in our atmosphere. 78% of our atmosphere
is molecular nitrogen, so it's in the form of N2. So this right over here
is molecular nitrogen. You have two nitrogen atoms
covalently bonded to each other. Now, unlike carbon, which can
be directly fixed by plants, in the carbon cycle video we talk about how autotrophs like plants
can take light energy and use that to fix carbon from the air into a solid form and store that energy in those carbon-carbon bonds, nitrogen cannot be directly fixed by complex organisms, like plants. Instead, the key actor that
fixes the nitrogen from the air, so you have all of these N2
molecules in the air here, the actors here aren't plants, but prokaryotes, like bacteria. So let me draw some soil here. And the bacteria could be in
a bunch of different places. But you could have a
bacteria in this soil. I'll draw them a little bit
bigger so you can see it. Some prokaryotes right over
here, there's the bacteria. And certain types of bacteria are capable, and prokaryotes are
capable of fixing nitrogen. So what they're able
to do, is they're able to take that N2 and turn it into a form that is more usable by
complex organisms like plants. So this is the bacteria, right over here. So that's the bacteria. That's this little circular strand of DNA, I could draw other-- I could make it more complicated, but let me just do it like this. Bacteria is able to fix that N2 and take it to ammonia, NH3. And it's this ammonia, it's this ammonia, that is really useful for plants and other complex organisms. So this right over here. So as the plant in the
video on the carbon cycle we talk about how plants fix carbon, carbon makes up a large
part of organic molecules, but many important organic molecules also need nitrogen. And these are examples
of organic molecules that you will find in plants, and you'll find them in many
different types of organisms. So this right over here is an amino acid, amino acid, we see the nitrogen right over there. This right over here is
our good old friend, ATP, adenosine triphosphate,
the quick-store of energy in biological systems. You see the nitrogen in
blue right over here. This is the famous DNA, deoxyribonucleic acid. And you see the nitrogens
throughout this macro molecule. So nitrogen is essential for life, but the step of fixing that nitrogen that's done by bacteria, which
can then produce the ammonia, which is then usable by plants, and then by eating the
plants, beings like you and me can get that nitrogen into our systems. Now, it's not just a one-way street. That is, we're just
going from the nitrogen in the atmosphere, and it
gets fixed by prokaryotes, and then that gets used-- that gets turned into ammonia and gets used by higher organisms. Because eventually all of
it would, would get deleted. As an organism dies, as an organism dies, let's say this is a dead organism, it could be a bacteria, but I'll go with a higher organism,
a multi-cellular one, this is a dead plant here, I don't want to draw dead
animals, it's more morbid. So let's say this is a dead plant there. And when it gets decomposed, and there are many
different types of bacteria, even though I might draw them
kind of looking the same, let's say this is another
bacteria in orange. As these bacteria digest these plants, they're able to, they could
take some of the nitrogen and break them down into
nitrites and nitrates, these are molecules
involving a nitrogen bonded to two or three oxygens, and they can take them back to ammonia. So we can go back, so
we can have a bacteria that take us back to ammonia, or eventually by converting the nitrogen into nitrites and
nitrates, we could go back we could go back to that
molecular nitrogen, that N2. That then gets released
back into the atmosphere. So, or it could get, even, you know, it could get stored in other ways as well. But, in general, as you
see here, this pattern. That we have these elements
that are essential for life. They don't just disappear
or form out of nowhere, they're constantly being
recycled in our biosphere. And nitrogen doesn't get as much attention as carbon or oxygen, but
it is essential for life. In fact, when you look at especially plants and growing plants, and you think about fertilizer. Fertilizer is something
that you should think if you add it to a plant,
it's going to grow more. So without it, it kind of limits
how fast a plant can grow, a lot of fertilizer is
going to have nitrogen, and in other video, we're going
to talk about phosphorous. And that phosphorous and nitrogen and their availability in the soil, is often a rate-limiting
factor for plants. And you know that, because
if you add more nitrogen or that phosphorous, it will
add more ammonia to that soil, you're going to, the plants
are going to grow faster.