the nitrogen cycle Flashcards
why is nitrogen vital for all living organisms?
nitrogen is vital for all living organisms, since it is an essential component of nucleic acids. the two nucleic acids you need to know about are DNA & RNA.
what are other organic compounds which always contain nitrogen?
other organic compounds which always contain nitrogen include amino acids, proteins, ATP & ADP
although nitrogen makes up nearly ___ of the earth’s atmosphere, it is a scarce resource for living organisms.
nitrogen shortage can lead to …
the difficulty arises from the fact that …
although nitrogen makes up nearly 80% of the earth’s atmosphere, it is a scarce resource for living organisms.
nitrogen shortage can lead to lack of growth (organism is unable to make proteins which are needed for growth) and even death.
the difficulty arises from the fact that nitrogen gas is inert (unreactive) and therefore useless to most living organisms when it is in the gaseous state.
what are the four main stages of the nitrogen cycle?
- nitrogen fixation
- ammonification and the role of saprobionts in decomposition
- nitrification
- denitrification
explain the process of the nitrogen cycle
- Nitrogen Fixation
nitrogen fixation is carried out by nitrogen fixing bacteria. they convert the nitrogen into ammonia using a reduction reaction catalysed by the enzyme nitrogenase. some of these bacteria live in special swellings on the roots, called root nodules, of plants called legumes. the bacterium receives water and sugars from the plant and in return supplies the plant with ammonia. the ammonium ions can be used to make proteins and other nitrogen containing organic compounds which are required by the plant, such as nucleic acids. this mutually beneficial relationship between the bacteria and the plant is an example of symbiosis. types of legumes include Clover, beans, peas, alfalfa. there are also free living nitrogen fixing bacteria in the soil. The ammonia that they produce dissolves in water in the soil to form ammonium ions.
- Decomposition and Ammonification
nitrogen is passed through food chains in the form of organic compounds. when organisms die, excrete urea or egest faeces, decomposers such as bacteria and fungi convert the organic nitrogen locked up these organic compounds into ammonia. this is converted into ammonium compounds in the soil, so returning the nitrogen to the abiotic phase. these organisms are collectively called saprobiotic microorganisms or saprobionts. saprobiotic microorganisms are fungi/bacteria that perform extracellular digestion. they secrete enzymes on to the detritus so the nitrogen-containing compounds (e.g. DNA/proteins/urea) are digested externally. they absorb some of the products of digestion and use them, but some (e.g. ammonia) are released into the environment.
- Nitrification
this process is carried out by bacteria called nitrifying bacteria. these bacteria transform ammonium ions into nitrates. this releases energy which they use for the synthesis of organic molecules such as carbohydrates. for this reason, these bacteria are described as autotrophs. green plants are also called autotrophs. the difference between these two types of autotrophs is that green plants use light energy to synthesise organic matter (= PHOTOautotrophs) and some bacteria oxidise chemical compounds (e.g. nitrites) to release energy they need to synthesise organic matter (= CHEMOautotrophs). there are two stages of nitrification, each carried out by different types of nitrifying bacteria.
1. The oxidation of ammonium ions to nitrite by nitrifying bacteria
2. The oxidation of nitrites to nitrates by nitrifying bacteria
plants take up nitrogen from the soil in the form of nitrates through their roots by active transport. this is how most of the nitrogen returns to the biotic phase.
- Denitrification
this is the loss of nitrates from the soil under anaerobic conditions (often in water-logged soil). nitrates are converted to nitrogen gas. it is due to reduction reactions carried out by denitrifying bacteria. aerating the soil will reduce this process. the inorganic nitrogen is taken up by plants from the soil, largely in the form
of nitrates which is then used to synthesise nitrogen-containing organic molecules such as proteins or DNA. these move into other organisms when they eat the plants, and digest and absorb the organic compounds (nitrogen flow through food webs).
summarise the role of each of the following bacteria in the nitrogen cycle
- nitrogen fixing bacteria
- saprobiotic bacteria
- nitrifying bacteria
- denitrifying bacteria
- nitrogen fixing bacteria
reduce nitrogen gas in atmosphere to ammonia, which dissolves to form ammonium ions - saprobiotic bacteria
decompose dead/waste material and convert nitrogen-containing organic matter into ammonia - nitrifying bacteria
oxidise nitrites to nitrates (chemoautotrophs) - denitrifying bacteria
convert nitrate ions in the soil to nitrogen gas in the atmosphere
what is crop rotation?
crop rotation is where farmers grow different crops in a field at different times. legumes (e.g. clover) are often incorporated into the rotation. they will be grown for part of the year, then ploughed into the soil and left to decay.
how does crop rotation improve crop yield?
nitrogen-fixing bacteria in the clover convert nitrogen gas to ammonia. the clover uses the N in ammonia to make proteins etc. when left to decay, saprobiotic bacteria will convert the proteins in dead clover to ammonia, and nitrifying bacteria will convert it to nitrates.
different crops grown after the legumes will then absorb different nutrients from different soil depths (due to different requirements and different root lengths).
different crops will have different pest species. those from the previous years crop are unlikely to affect the new crop.
