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Nitrogen Cycle: Fixation and Ammonification (A-level Biology)

Nitrogen cycle: fixation and ammonification, nitrogen cycle, overview of the nitrogen cycle.

Nitrogen is fundamental to living organisms to create proteins and nucleic acids (DNA and RNA). Plants and animals can’t use nitrogen gas – the form that nitrogen is found in the atmosphere. It must be converted into nitrates by bacteria first.

The nitrogen cycle demonstrates the movement of nitrogen from the atmosphere into the soil, where is it utilised by bacteria and plants.

There are four main biological processes in the nitrogen cycle:

Nitrogen Fixation

• Ammonification
• Nitrification

This is where nitrogen fixing bacteria convert nitrogen gas into ammonia, which forms ammonium ions (NH 4 + ).

An example of this kind of bacteria is Rhizobium , which is found in the root nodules of leguminous plants. Rhizobium are mutualistic:

• The bacteria provide the plant with nitrogen compounds
• The plant provides the bacteria with carbohydrates

2. Ammonification

Organic material is broken down by saprobiotic bacteria to release ammonium ions back to the soil.

• Plants use nitrates to form proteins and nucleic acids etc.
• Consumers will digest the plant material to obtain this.
• This nitrogenous organic material (dead plants, dead animals, waste) is turned into ammonia by saprobionts, which goes on to form ammonium ions.

The nitrogen cycle is a continuous process that involves the transformation of nitrogen between different forms in the environment, including the atmosphere, soil, water, and living organisms. It is an important biogeochemical cycle that is essential for life on Earth.

Nitrogen fixation is the process of converting atmospheric nitrogen gas (N2) into forms that can be used by living organisms, such as nitrates (NO3-) and ammonia (NH3). This process is performed by certain bacteria in the soil, including nitrogen-fixing bacteria, and by lightning.

Ammonification is the process of converting organic nitrogen compounds, such as proteins, into ammonia (NH3) through the action of decomposers, such as bacteria and fungi. This process is an important step in the nitrogen cycle because it releases nitrogen in a form that can be taken up by plants.

The nitrogen cycle supports life on Earth by providing plants and other organisms with the nitrogen they need to grow and produce essential organic compounds, such as proteins and DNA. The nitrogen cycle also helps regulate the amount of nitrogen in the environment, ensuring that it is not depleted or accumulated in harmful amounts.

Human activities, such as the use of synthetic fertilizers, the burning of fossil fuels, and deforestation, can have a significant impact on the nitrogen cycle. These activities can alter the balance of nitrogen in the environment, leading to the overproduction of nitrogen compounds and the release of harmful pollutants, such as nitrogen oxides.

Bacteria play a critical role in the nitrogen cycle by performing key steps in the process, such as nitrogen fixation and ammonification. Nitrogen-fixing bacteria, such as Rhizobia, convert atmospheric nitrogen into forms that can be used by plants, while decomposer bacteria and fungi convert organic nitrogen compounds into ammonia.

Understanding the nitrogen cycle is important in biology because it provides a framework for understanding the role of nitrogen in the environment and how it affects the growth and survival of plants and other organisms. This knowledge is crucial for managing the nitrogen cycle and ensuring the health of ecosystems and the sustainability of life on Earth.

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Edexcel B 1: Biological Molecules

Importance of meiosis (a-level biology), transfer rna (a-level biology), messenger rna (a-level biology), features of the genetic code (a-level biology), introducing the genetic code (a-level biology), genes and protein synthesis (a-level biology), inorganic ions (a-level biology), properties of water (a-level biology), structure of water (a-level biology), synthesising proteins from dna (a-level biology), edexcel b 10: ecosystems, nitrogen cycle: nitrification and denitrification (a-level biology), the phosphorus cycle (a-level biology), introduction to nutrient cycles (a-level biology), reducing biomass loss (a-level biology), sources of biomass loss (a-level biology), transfer of biomass (a-level biology), measuring biomass (a-level biology), net primary production (a-level biology), gross primary production (a-level biology), edexcel b 2: cells, viruses, reproduction, types of variation (a-level biology), chromosome mutations (a-level biology), cell division by meiosis (a-level 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Nutrient Cycles ( AQA A Level Biology )

Revision note.

Biology & Environmental Systems and Societies

Nutrient Cycles

• These nutrients are then returned to the environment when organisms produce waste or die and decompose
• This is due to the waste products and dead organisms being digested (decomposed) by microorganisms
• The products of this decomposition are available to plants as nutrients in the soil
• These plants can then sustain organisms in higher trophic levels (consumers)
• This means these nutrients are constantly being cycled in ecosystems

The nitrogen cycle

• The phosphorous cycle
• The nitrogen cycle shows how nitrogen is recycled in ecosystems
• Plants and animals require nitrogen in order to produce proteins and nucleic acids (DNA and RNA)
• Instead, they rely on certain bacteria to convert the nitrogen gas into nitrogen-containing compounds , which can be taken up by plants
• The nitrogen cycle shows this conversion, as well as how the nitrogen in the nitrogen-containing compounds is then passed between trophic levels or between living organisms and the non-living environment
• There are four key processes in the nitrogen cycle that are carried out by different types of bacteria
• Atmospheric nitrogen gas is converted into nitrogen-containing compounds
• This biological nitrogen fixation is carried out by nitrogen-fixing bacteria such as Rhizobium
• The bacteria convert nitrogen into ammonia, which forms ammonium ions (in solution) that can then be used by plants
• These nitrogen-fixing bacteria are found inside the root nodules (small growths on the roots) of leguminous plants such as peas, beans and clover
• The bacteria have a symbiotic (mutually beneficial) relationship with these plants - the bacteria provide the plants with nitrogen-containing compounds and the plants provide the bacteria with organic compounds such as carbohydrates
• Nitrogen compounds in waste products (e.g. urine and faeces) and dead organisms are converted into ammonia by saprobionts (a type of decomposer including some fungi and bacteria)
• This ammonia forms ammonium ions in the soil
• The ammonium ions in the soil are converted by nitrifying bacteria into nitrogen compounds that can be used by plants, known as nitrates
• Initially, nitrifying bacteria such as Nitrosomonas convert ammonium ions into nitrites
• Different nitrifying bacteria such as  Nitrobacter then convert these nitrites into nitrates
• Denitrifying bacteria use nitrates in the soil during respiration
• This process produces nitrogen gas , which returns to the atmosphere
• This process occurs in anaerobic conditions (when there is little or no oxygen available, such as in waterlogged soil)

The phosphorus cycle

• The phosphorus cycle shows how phosphorus is recycled in ecosystems
• Plants and animals require phosphorus in order to produce certain biological molecules such as phospholipids (for cell membranes), nucleic acids (DNA and RNA) and ATP
• Phosphorus in rocks is slowly released into the soil and into water sources in the form of phosphate ions (PO₄³⁻) by the process of weathering (the slow breaking down and erosion of rocks over time)
• Phosphate ions are taken up from the soil by plants through their roots or absorbed from water by algae
• Phosphate ions are transferred to consumers during feeding
• Phosphate ions in waste products and dead organisms are released into the soil or water during decomposition by saprobionts
• The phosphate ions can now be taken up and used once again by producers or may be trapped in sediments that, over very long geological time periods may turn into phosphorus-containing rock once again

You do not need to learn the Latin names of specific types of nitrogen-fixing bacteria or nitrifying bacteria included in the notes above. You just need to know the processes that these groups of bacteria are responsible for carrying out and why these processes are important in the context of the nitrogen cycle.

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Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Nitrogen Cycles (WJEC)

Introduction nitrogen nitrogen-fixing bacteria and lightning mycorrhizae saprobiotic bacteria denitrifying bacteria summary, introduction.

Nitrogen is an important element in organisms, taking part in one of their most important building blocks – amino acids . That’s not to mention DNA itself…

In the nitrogen cycle there are two stages of N presence: the atmosphere and the ground. Whenever N is in the atmosphere it’s in the form of nitrogen gas, N 2 which of course is what most of the air is made of. In the ground, N is found in ammonia (NH 3 ) , nitrite (NO 2- ) and nitrate (NO 3- ) .

Find it hard to distinguish the formulae for nitrite and nitrate? Needn’t be! A is l a rge (3-) and i is l i ttle (2-), former’s nitr a te, latter nitr i te.

Both nitrogen-fixing bacteria and lightning can take the nitrogen gas in the air and fix it into the soil, where plants take it up (nitrate assimilation) and pass it on through the trophic levels to other organisms.

Mycorrhizae

Mycorrhizae are symbiotic associations between fungi and plant roots which benefit the fungus in terms to access to carbohydrates , and benefit the plant by improving nutrient uptake from the soil.

This is especially beneficial to plants in nutrient poor soil. Fungi may be able to better extract nutrients such as nitrogen from the soil on behalf of the plant due to a smaller diameter of protruding extensions that can explore more soil as well as being able to chemically bind target nutrients.

  S a probiotic bacteria 

Upon their death, saprobiotic bacteria decompose the remains and produce ammonia which then undergoes nitrification to NO2- and NO3- by nitrifying…

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Nitrogen cycle (AQA A-level biology)

Subject: Biology

Age range: 16+

Resource type: Lesson (complete)

Last updated

15 August 2024

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This lesson guides students through the stages of the nitrogen cycle, focusing on the vital roles performed by microorganisms in this cycle. The detailed PowerPoint and accompanying resources are part of the 1st lesson in a series of 3 lessons which have been planned to cover point 5.4 (nutrient cycles) of the AQA A-level biology specification.

The lesson begins by challenging students to recall two monomers containing nitrogen that were met in topic 1, allowing them to recognise that this chemical element is a key component of nucleotides in DNA and amino acids, which are needed to synthesise proteins. Moving forwards, they will learn that despite the high % of nitrogen in the Earth’s atmosphere, it cannot be used directly by plants, and therefore plants need a supply of “fixed” nitrogen. A diagram is constantly updated and displayed as new information is introduced and this supports their understanding. The students will discover that microorganisms are involved in nitrogen fixation, decomposition and ammonification, nitrification, and denitrification. As each of these biological actions is introduced, time is spent considering key details and understanding checks are used to allow the students to assess their progress. There are also several prior knowledge checks, where students are encouraged to make links to content met in topics 1 - 4. Answers to all questions are embedded into the PowerPoint.

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Topic 5: Energy transfers in and between organisms (AQA A-level Biology)

Normally the first topic to be taught in the second year of the AQA A-level Biology course, topic 5 contains some very important biological processes which include photosynthesis, respiration and energy transfer between organisms. All 15 lessons included in this bundle are highly detailed and have been planned at length to ensure that students remain motivated and engaged whilst being constantly challenged on their current understanding. Links to previously-covered topics are also made throughout the lessons. The following specification points are covered in these lessons: TOPIC 5.1 * The light-dependent reaction of photosynthesis * The use of reduced NADP and ATP from the light-dependent reaction in the light-independent reaction * The light-independent reaction of photosynthesis * Environmental factors that limit the rate of photosynthesis TOPIC 5.2 * Respiration produces ATP * Glycolysis as the first stage of aerobic and anaerobic respiration * The conversion of pyruvate to lactate or ethanol in the anaerobic pathways * The link reaction and the Krebs cycle * Synthesis of ATP by oxidative phosphorylation * Other respiratory substrates TOPIC 5.3 * Gross primary production and net primary production * The net production of consumers * Farming practices designed to increase the efficiency of energy transfer TOPIC 5.4 * The role of microorganisms in the nitrogen cycle If you would like to sample the quality of the lessons in this bundle, then download the chloroplast structure, anaerobic respiration, oxidative phosphorylation and GPP lessons as these have been uploaded for free

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AS and A-level Biology

• Specification
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Specification at a glance

• 3.1 Biological molecules
• 3.3 Organisms exchange substances with their environment
• 3.4 Genetic information, variation and relationships between organisms
• 3.5 Energy transfers in and between organisms (A-level only)
• 3.6 Organisms respond to changes in their internal and external environments (A-level only)
• 3.7 Genetics, populations, evolution and ecosystems (A-level only)
• 3.8 The control of gene expression (A-level only)
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These qualifications are linear. Linear means that students will sit all the AS exams at the end of their AS course and all the A-level exams at the end of their A-level course.

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Core content

• 1 Biological molecules
• 3 Organisms exchange substances with their environment
• 4 Genetic information, variation and relationships between organisms
• 5 Energy transfers in and between organisms (A-level only)
• 6 Organisms respond to changes in their internal and external environments (A-level only)
• 7 Genetics, populations, evolution and ecosystems (A-level only)
• 8 The control of gene expression (A-level only)

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