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Nitrogen Fixation: Significance to Plants and Humans

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  • 0:05 Nitrogen Fixation
  • 1:03 Nitrogen-Fixing Bacteria
  • 2:30 Significance of…
  • 3:57 Significance to Humans
  • 4:58 Lesson Summary
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Lesson Transcript
Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

Almost 80% of our atmosphere is nitrogen, but we can't use it. We will look at how this unusable nitrogen is converted into a form we can use and why nitrogen is important to plants and humans.

Nitrogen Fixation

You may have learned about the water cycle or the carbon cycle before. These cycles move substances - water or carbon - throughout the environment and organisms living there. The process is fairly simple. However, the movement of nitrogen through living and nonliving things is a bit more complex.

The nitrogen cycle is more complex than other biogeochemical cycles because most organisms cannot use the nitrogen found in the atmosphere - even though almost 80% of the air is nitrogen! In the atmosphere, nitrogen is found as N2. This N2 is not usable by most plants and animals. In order for plants and humans to get the nitrogen they need, the nitrogen must be converted into ammonia - NH3. The conversion of this atmospheric nitrogen to the usable form of ammonia is known as nitrogen fixation. Nitrogen fixation can be defined as the process of creating ammonia from atmospheric nitrogen. Without this process, most plants and animals would not have the nitrogen needed to live.

Nitrogen-Fixing Bacteria

Most commonly, nitrogen fixation is carried out by bacteria. Now, this isn't all bacteria, but some specific forms. You may think of the bacteria that get you sick, but the bacteria that make ammonia from atmospheric nitrogen are found near plant roots and won't make you sick.

This nitrogen-fixing bacteria is most commonly found near the roots of legumes. Legumes are plants such as soybeans and garden peas. The bacteria and the plants have a relationship that is beneficial for both. You may remember from ecology that when two species live together, it's known as symbiosis. There are several types of this close, long-term interaction between two different species. In the case of nitrogen-fixing bacteria and legumes, both species benefit from the interaction, meaning that it is mutualism. The nitrogen-fixing bacteria benefit because they have a safe place to live amongst the roots of the plants, and the legumes benefit because they basically have an unlimited source of useable nitrogen. The plant does use some energy in order to provide a good home for the bacteria, but this expense is well worth it for the plant because of the nitrogen produced by the bacteria.

The specific name of bacteria found in most legumes is Rhizobium bacteria. The bacteria contain a specific enzyme - nitrogenase - that breaks the bonds between the atmospheric nitrogen. Once the bonds are broken, the bacteria can then 'fix' the nitrogen into ammonia. The plants - and then animals - can then use this fixed form of nitrogen.

Significance of Nitrogen to Plants

Now that we know that nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia, we need to look at why this matters. So what if plants can't use the nitrogen in the air? What do they really need nitrogen for anyway?

We know that plants need elements such as carbon, hydrogen and oxygen for photosynthesis, but nitrogen is actually also needed for this process as well. Chlorophyll, the green pigment found in the chloroplasts of plant cells, is responsible for photosynthesis. One of the elements that helps make chlorophyll is nitrogen. Without nitrogen, plants would not be able to perform photosynthesis.

Diagram of an amino acid
Diagram of Amino Acid

Plants also need nitrogen because it is found in all living cells. Having the proper amount of nitrogen helps with plant growth. Plants that have the right amount of nitrogen will have more seed and fruit production than those plants that do not have enough nitrogen. This nutrient also improves the quality of the leaves.

Lastly, plants need nitrogen specifically for proteins. Proteins are molecules that are responsible for many life processes. In plants, proteins can be used as enzymes - which help control the rate of reactions and control metabolic activities.

If plants do not have enough nitrogen, they will not be able to function properly. Because of this, nitrogen is found in many fertilizers. You may have seen ads for fertilizers that show plants that were given the fertilizer and plants that were not given the fertilizer. Generally, the plants that receive the fertilizer - and therefore plenty of nitrogen - have much bigger and more vibrant flowers than the ones that did not get this nutrient boost.

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