Nitrogen Fixation - Online Tutor, Practice Problems & Exam Prep

Nitrogen is vital for life, forming essential components of nucleic acids and proteins. Plants absorb nitrogen primarily as ammonium (NH₄⁺) and nitrates (NO₃^-), obtained through nitrogen fixation by bacteria. Legumes host rhizobia in root nodules, facilitating this process. Mycorrhizae, symbiotic fungi, enhance nutrient absorption, including nitrogen. The nitrogen cycle illustrates these interactions, emphasizing the energy-intensive nature of nitrogen fixation, which requires 16 ATP to convert one molecule of N₂ into ammonia. Understanding these processes is crucial for grasping ecosystem dynamics and nutrient cycling.

Nitrogen Cycle

Nitrogen is an essential nutrient to living organisms. The reason for this is it's a required element in both nucleic acids and proteins. So pretty much the basics need this stuff. And for us animals, we just have to eat something that has nitrogen in it. Done deal, end of story. Plants don't have it as easy though. They have to obtain their nitrogen through some more complex means. Now, nitrogen actually cycles through ecosystems in a very creatively termed chemical cycle called the nitrogen cycle. And in this process, nitrogen is actually converted through a variety of chemical forms. The most abundant form that plants will see nitrogen in is N2 in the atmosphere, this atmospheric gaseous nitrogen. It's actually almost 80% of the atmosphere. It's technically about 78%. We'll just call it 80, keep it nice and even, or nice round numbers. Thing is plants can't absorb this nitrogen. Sucks for them. They can't absorb this gaseous N2. They actually are going to rely on this process that bacteria and archaea will carry out called nitrogen fixation, which is the conversion of that gaseous N2 into this usable NH3 ammonia form of nitrogen.

Now, plants are actually going to absorb mostly ammonium. Right? Which is the protonated form of this. You know, ammonia is a weak base, so, you know, after it's chemically converted it's going to pick up a proton turn into ammonium, most likely. And they also will absorb these nitrates, which will, come as a product of various downstream reactions after the bacteria and archaea convert the gaseous nitrogen into ammonia. So, these are the two main forms absorbed by plants, but it's important to note that not every plant is going to rely on this process. In fact, carnivorous plants, for example, use carnivory to supplement their nitrogen intake. Yeah. I mean if you ever wondered why do plants, which make their own food. Right? Plants make their own sugar. Why would they need to eat these other organisms? Well, it's actually to obtain more nitrogen. That's why they eat animals. Right? Animals that are made up of meat, and therefore have a bunch of nitrogen in them.

Now, some plants like epiphytes actually don't contact soil at all. They have to live on other plants, and they actually absorb all their water and nutrients from the air, the rain, and just like the debris that's around them. So it's important to remember that life has so much variety; there's always going to be exceptions, but nitrogen fixation, in a large part, is what plants are going to rely on in order to obtain their nitrogen. You can see this nice chart here shows how nitrogen cycles through ecosystems, in the course of the nitrogen cycle, and you might notice that decomposers, right, decomposers actually also contribute to ammonium. In fact, decomposers will also provide some sources of nitrogen to plants. We'll talk a little bit more about that later. Now this process, this nitrogen fixation, is super energy-intensive, you know, some species of bacteria and archaea. It's a really energy intensive process. It's mainly facilitated by this multi enzyme complex called nitrogenase, another creative name for you. And this complex is going to reduce gaseous nitrogen into ammonia. So, it basically facilitates nitrogen fixation.

Now, as I said, this process is extremely energy intensive, and in fact, to convert one molecule of N2 into two molecules of NH3 requires 8 high energy electrons and a whopping 16 ATP, which you can see behind my head there. So this is a massively energy intensive process. Especially when you consider that these guys are not just converting one molecule, they're going to be converting tons and tons of molecules into ammonia, making it usable to plants. So a lot of energy is required. Fortunately, they can lean on plants a bit to get some of that energy. We'll talk about that when we flip the page.

Nodules

While some nitrifying bacteria are content to live in the soil and pass off the fruits of their labor to their fellow plants, some want to take this step further and will actually perform nitrogen fixation inside plant roots. Mostly, this occurs in plants of the legume family. Legumes are flowering plants with the Latin name Fabaceae, and they carry these rhizobia bacteria in what are called nodules in their roots. Now, rhizobia are gram-negative soil bacteria that perform nitrogen fixation, and they do so in the roots of these legumes. Technically, that makes them endophytes, or organisms that live inside other plants. Usually, they are fungi or bacteria. Nodules are swollen nodes or lumps in the roots where these bacteria have infected the plants. And make no mistake, even though this relationship is beneficial for both organisms, this is technically a bacterial infection in the plant, and that's why the swelling occurs. I mean, if you've ever seen a swollen lump in a tree stump or something like that, that's also a site of infection. That's in part how plants will respond to infections, and that is why these swollen lumps look like infections because they are infections. But it's not, you know, an unwanted infection. In fact, plants are inviting these bacteria in by releasing these chemicals into the soil called flavonoids. And these actually signal the rhizobia, who in turn release their own signal, which are called nod factors, and these nod factors will actually come into contact with the root hairs.

So, I'm going to jump out of the image here, and just draw right here, so let's pretend, here we have rhizobia bacteria. The flavonoids will be released by the root hair, and they're going to make their way to the rhizobia over here. And in turn, she is going to release her nod factors, and that is going to stimulate the root hair and cause a morphological change that will actually allow the bacteria to enter into the cortex, the inner part of the plant, through what's called an infection thread. So basically, the rhizobia here, who I'm just going to put 'rr' for now, she is going to take advantage of that morphological change, enter the root hair, go through the root hair cell, and make her way into the cortex. So this is supposed to be a cortex cell here, and this of course is a root hair. So, once inside the actual plant cortex, these rhizobia are going to set up shop. You can see an actual image of what this looks like here. All of these dark lumps are bacteria living in the plant cell, and they're going to get along swimmingly actually. They have a mutualistic relationship because the plant provides carbohydrates and protection to the bacteria. Right? And the bacteria are going to provide usable nitrogen. And remember earlier I said that the energy demands of performing nitrogen fixation are very great, but these plants, right, in this mutualistic relationship, they're helping the bacteria out with those energy demands. They're saying, hey, look, you're giving us nitrogen, here's some carbohydrate that you'll be able to convert into ATP, and then you can use that ATP to make the nitrogen usable for us. So it's like, you know, you scratch my back I'll scratch yours.

Now, in addition, plants will also produce this molecule called leghemoglobin. And hopefully, you can pick out that word, hemoglobin, in there. Remember, that is the oxygen-binding molecule found in red blood cells. Well, this is like a special kind of hemoglobin in a sense, this leg hemoglobin. It still binds oxygen, but the reason it's binding this oxygen is actually to protect nitrogenase, that enzyme that's going to carry out the chemical reaction that turns gaseous nitrogen into ammonia; it's going to protect that enzyme complex from oxygen poisoning. So it's just one more way in which plants are actually helping out these rhizobia bacteria.

Now, I also mentioned that these bacteria weren't the only way that plants can obtain nitrogen. And I mentioned detritivores, and of course, when you think detritivores, hopefully, you're thinking fungi. Obviously, bacteria play a role in the breakdown of organic matter as well. Most certainly not going to discount how important bacteria are, but these fungi will form, remember, mycorrhizae, those fungus roots, those associations between the fungal hyphae and the plant roots. And in this association, they will actually be able to provide nutrients for the plants, like nitrogen and phosphorus—both super important. Right? Both going to wind up components of nucleic acids, for example. Now the hyphae increase the surface area for absorption, so they're going to help plants absorb nutrients better. They also, you know, the fungus also will break down organic matter, which is going to free up these elements for absorption. Right? They're going to help break down the structures that they're part of, making them easier for plants to absorb. And we're actually going to see two types of mycorrhizae. There's ectomycorrhizae, which I have pictures of here. And basically, this is mycorrhizae where the hyphae wrap around plant cells but don't actually penetrate into plant cells. So these are not endophytes. Our arbuscular mycorrhizae, on the other hand, are endophytes. They're going to actually penetrate their hyphae into the cortical cells of plant roots. And I don't have a picture of that here, but, you know, you can imagine, looking at this image here for example, that one of these purple hyphae would extend into a plant cell in arbuscular mycorrhizae. Here, what I actually have pictures of are ectomycorrhizae. So this is a more zoomed-out image. These brown twiggy-looking things are plant roots, and all this white gunk all over it, that kind of looks like mold, is the fungal part of the mycorrhizae. So you can see, you know, with the naked eye, basically, that close association of plant and fungus. Here we have some more zoomed-in pictures like right here, and especially right here. Here we're looking at a fungal sheath around the root, and here in this image that's behind my head, you can actually see the hyphae wrapped around plant cells. These dark spots are going to be the actual cells of the plant, and all this white gunk in-between them is the hyphae, so that's the fungus part of it. So this close association not only helps plants absorb nutrients but can also help provide some nitrogen and phosphorus for plants, and also don't forget that there's just a bunch of other fungi in the soil, free-living, that's also breaking down organic matter and contributing to the amount of free nitrogen available in the soil. That's all I have for this lesson. See you guys next time.

Here’s what students ask on this topic:

What is nitrogen fixation and why is it important for plants?

Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into ammonia (NH₃), a form usable by plants. This process is crucial because nitrogen is an essential component of nucleic acids and proteins, which are vital for plant growth and development. Plants cannot directly absorb atmospheric nitrogen, so they rely on nitrogen-fixing bacteria, such as rhizobia, to convert N₂ into ammonia. This ammonia is then further processed into ammonium (NH₄⁺) and nitrates (NO₃^-), which plants can absorb through their roots. Without nitrogen fixation, plants would be unable to obtain the nitrogen necessary for their metabolic processes.

How do rhizobia bacteria contribute to nitrogen fixation in legumes?

Rhizobia bacteria form a symbiotic relationship with legumes by infecting their root nodules. The plant releases flavonoids into the soil, attracting rhizobia, which respond by releasing nod factors. These nod factors cause morphological changes in the root hairs, allowing the bacteria to enter the plant's cortex through an infection thread. Inside the root nodules, rhizobia convert atmospheric nitrogen (N₂) into ammonia (NH₃) using the enzyme nitrogenase. This process is energy-intensive, requiring 16 ATP molecules for each N₂ molecule converted. The plant provides carbohydrates and protection to the bacteria, while the bacteria supply the plant with usable nitrogen, facilitating mutualistic benefits.

What role do mycorrhizal fungi play in plant nitrogen absorption?

Mycorrhizal fungi form symbiotic associations with plant roots, enhancing nutrient absorption, including nitrogen and phosphorus. There are two main types: ectomycorrhizae, which wrap around plant cells, and arbuscular mycorrhizae, which penetrate plant cells. The fungal hyphae increase the surface area for nutrient absorption and help break down organic matter, releasing nitrogen and phosphorus for plant uptake. This relationship benefits plants by improving their nutrient acquisition, while the fungi receive carbohydrates from the plant. Mycorrhizal fungi thus play a crucial role in supporting plant nutrition and growth.

Why is the nitrogenase enzyme important in nitrogen fixation?

Nitrogenase is a multi-enzyme complex essential for nitrogen fixation, the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃). This enzyme facilitates the reduction of N₂ to NH₃, a form that plants can use. The reaction is highly energy-intensive, requiring 8 high-energy electrons and 16 ATP molecules for each N₂ molecule converted. Nitrogenase is sensitive to oxygen, so leguminous plants produce leghemoglobin to bind oxygen and protect the enzyme from oxygen poisoning. Without nitrogenase, the conversion of atmospheric nitrogen into a usable form for plants would not be possible, severely limiting plant growth and development.

How do carnivorous plants obtain nitrogen?

Carnivorous plants obtain nitrogen by capturing and digesting insects and other small animals. Unlike most plants that rely on nitrogen fixation, carnivorous plants grow in nitrogen-poor environments, such as bogs and acidic soils. They have evolved specialized structures, like pitcher traps and sticky leaves, to capture prey. The captured organisms are broken down by digestive enzymes, releasing nitrogen and other nutrients, which the plant then absorbs. This adaptation allows carnivorous plants to supplement their nitrogen intake and thrive in environments where nitrogen is scarce.