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The nitrogen cycle
Review your understanding of the nitrogen cycle in this free article aligned to AP standards.
Key points
- The element nitrogen is essential for life. DNA, RNA, and protein molecules all contain nitrogen atoms.
- The largest reservoir of nitrogen on Earth is the atmosphere, with nitrogen gas (
- The nitrogen cycle is the cyclic movement of nitrogen in different chemical forms between living organisms and the environment. The steps of the nitrogen cycle are described below.
- Nitrogen fixation: During this step, atmospheric nitrogen gas is fixed, or converted into a form that can be used by plants and animals. Two natural processes are responsible for most nitrogen fixation.
- In the atmosphere, energy from lightning helps recombine the atoms in nitrogen gas and oxygen gas (
- In the soil, nitrogen-fixing bacteria use enzymes to combine nitrogen gas with hydrogen gas (
- Fixed nitrogen can be taken up by plants and incorporated into biological molecules in the plants’ tissues. This allows nitrogen to move up the food chain as plants are eaten by animals, who are themselves eaten.
- Ammonification: Nitrogen-containing biological molecules may stay in organisms’ tissues, or be broken down and released as waste. As dead organisms and waste decompose, ammonia and ammonium ions are returned to the environment through ammonification. This process is carried out by certain bacteria and fungi.
- Nitrification: During nitrification, groups of nitrifying bacteria convert the products of ammonification into nitrites (
- Denitrification: During denitrification, denitrifying bacteria convert nitrites and nitrates back into nitrogen gas, returning nitrogen to the atmosphere.
- The nitrogen cycle tends to occur faster than other major nutrient cycles because living, active bacteria control many of the chemical transformations of nitrogen.
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Can a plant ever fully grow? I mean a plant is always growing, grass for example, every time you cut it it regrows and continues to grow until it breaks itself then it just starts the prosses over again. trees also never stop growing. So can a plant ever truly fully grow?(5 votes)
Due to the seasons and type of plants (ex. perennials, annuals, biennials, etc.), each plant has a "life-span", if you will. That being said, each species has a specific growing period and an average range of height that it will reach. Perhaps grown indoors and under very specific care plants could continue growing, but the seasons, temperatures, resources available (light, water, soil, air quality) along with the nature of each plant ultimately determine the growing span and height.(7 votes)
Why can't nitrogen be directly used by plants? Why does it have to be converted into ammonia first?(5 votes)
It can't because plants are not strong enough to break the strong triple-bond of nitrogen. The bacteria and lightning are strong enough.(2 votes)
What does the plant on the right represent, I do not understand how the plant is using nitrates?(2 votes)- The plant absorbs the nitrates from the soil. Bacteria then use denitrification (process of reducing nitrates back to molecular nitrogen/N2) in order to convert the nitrates to N2. The N2 is released back to the atmosphere, and the process will start again -- bacteria will convert the N2 to ammonia, other bacteria will convert the ammonia into nitrites and nitrates, and then denitrifying bacteria will convert it back to N2 again! (The plants are taking in the ammonia, nitrites, and nitrates all the while. The bacteria needed are present in the soil.)(5 votes)
How are nitrates, nitrites, and ammonia different? How are they used differently by plants and animals?(4 votes)
I can answer the nitrites vs. nitrates part. Nitrates are relatively harmless until broken down into nitrites. In nitrification, nitrites are transformed into nitrates. Some scientists say, nitrates in the human body could be carcinogenic. They are typically in high concentration in red meat. I would encourage to do your own research from here, as I am not a scientist with a PhD or anything fancy.(0 votes)
- Fixed nitrogen can be taken up by plants and incorporated into biological molecules in the plants’ tissues but where would it go into there is no opening in the tissue so therefore how would it enter and then do it thing.(1 vote)
Tissues are made of cells. Cells are made of macromolecules (carbohydrates, DNA, RNA, proteins, and fats). Macromolecules are made of molecules like NH4-, NO3-, and NO3-. Molecules are very small compared to tissue, so there doesn't need to be a hole for molecules to be incorporated into tissue.
Plants have stomata, which are pores in the leaves and stems. It is through the stomata that plants "breathe" in gases like N2. A stomate is made of specialized cells and is found on the plants external tissue. Gasses like N2, CO2, and O2 can pass through cell membranes freely, and do not require transporters.(6 votes)
- i don't understand during this step atmospheric nitrogen gas is fixed or converted into a form that can be used by plants and animals.(3 votes)
How long does it take a plant to fully grow.(0 votes)
You have google(5 votes)
- how do plants use nitrates?(1 vote)
- Why is ammonium so important for plants that it has to be on the ground for them to survive(1 vote)
- How are they used differently by plants and animals?(1 vote)