The nitrogen cycle | Energy and matter in biological systems | High school biology | Khan Academy

Nitrogen often gets less attention than carbon or oxygen, but nitrogen is very important to life as well. Like carbon and oxygen, it cycles through our biosphere.

Now, one thing that may be surprising about nitrogen, if you haven't studied it much, is that it is very, very common in our atmosphere. Seventy-eight percent 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—are not 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 bacterium in this soil. I'll draw it a little bit bigger so you could see it. So, prokaryotes right over here, there's the bacterium, and certain types of bacteria, as well as prokaryotes, are capable of fixing nitrogen.

So, what they're able to do is take that N2 and turn it into a form that is more usable by complex organisms like plants. This is the bacterium right over here; that's the bacterium—that's just a little circular strand of DNA. I could draw other things; I could make it more complicated, but let me just do it like this. The bacterium is able to fix that N2 and convert it to ammonia (NH3), and it's this ammonia that is really useful for plants and other complex organisms.

So, as 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. These are examples of organic molecules that you will find in plants and in many different types of organisms.

So, this right over here is an amino acid. You 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 macromolecule.

So, nitrogen is essential for life, but the step of fixing that nitrogen is done by bacteria, which can then produce the ammonia, which is then usable by plants, and then by eating the plants, things like you and me can get that nitrogen into our systems.

Now, it's not just a one-way street. We’re not just going from the nitrogen in the atmosphere to fixing it by prokaryotes and then that getting used and turned into ammonia to be utilized by higher organisms. Eventually, all of it would get depleted as an organism dies.

Let's say this is a dead organism. It could be a bacterium, but I'll do a higher organism—a multicellular 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, there are many different types of bacteria. Even though I might draw them looking kind of the same, let's say this is another bacterium in orange.

As these bacteria digest these plants, they're able to 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 could take them back to ammonia. So, we could have bacteria that take us back to ammonia, or eventually, by converting the nitrogen into nitrites and nitrates, we could go back to that molecular nitrogen, that N2, which then gets released back into the atmosphere.

Or 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.

Nitrogen doesn't get as much attention as carbon or oxygen, but it is essential for life. In fact, when you look at plants and growing plants, thinking about fertilizer is key. Fertilizer is something that, 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 in it. In another video, we're going to talk about phosphorus, and that phosphorus and nitrogen and their availability in the soil is often a rate-limiting factor for plants. You know that because if you add more nitrogen or that phosphorus, or add more ammonia to that soil, then you’re going to see that the plants are going to grow faster.