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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.

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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.