At the heart of both the phosphorus and nitrogen cycles are microorganisms such as bacteria. These key nutrients pass from plants to animals through ingestion and digestion (eating!) in a very straightforward way, but what happens after those organisms die is a far more interesting tale.
The recycling of these nutrients amongst many others enables their transfer between living and non-living elements on the planet: water, land, air and populations in different places and of different trophic levels.
Phosphorus is an essential element present in DNA, ATP, etc. and originates from the Earth’s rocks which slowly erode into water bodies.
Thereon, it is absorbed through the water plants use in their photosynthesis, and becomes part of their biomass which can be spread via ingestion to organisms higher up the food web.
Phosphorus makes its way back into the soil and later into water bodies through the action of decomposers which break down the plant and animal matter. Since phosphorus is present in fertilisers used to supplement plants, this too is subject to running off into bodies of water.
Finally, a phenomenon known as geological uplift shifts rocks from the water bodies back up into landmass.
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, N2 which of course is what most of the air is made of. In the ground, N is found in ammonia (NH3), nitrite (NO2-) and nitrate(NO3-).
Find it hard to distinguish the formulae for nitrite and nitrate? Needn’t be! A is large (3-) and i is little (2-), former’s nitrate, latter nitrite.
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 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.
Upon their death, saprobiotic bacteria decompose the remains and produce ammonia which then undergoes nitrification to NO2- and NO3- by nitrifying bacteria.
Denitrifying bacteria turn the N in nitrates into nitrogen gas again, so the cycle may begin once more!
Summary: nitrogen fixation –> assimilation –> ammonification –> nitrification –> denitrification
As briefly touched on before, the nitrogen contained in fertilisers used to enhance plant growth can deviate from the expected cycle when fertilisers end up in the wrong location…
This is how fertilisers can reach further areas than planned, by the action of rain and irrigation.
The fertiliser is therefore washed away, out of the control of the plant grower.
Eutrophication is the process of artificial or natural chemicals reaching bodies of water and changing their ecosystem.
Fertilisers will cause the aquatic organism phytoplankton to grow aggressively and cover the surface of the water.
It’s super beautiful, but all the organisms within the body of water are being deprived of oxygen, causing hypoxia. While certain species die and others thrive, the balance of the ecosystem is shifted dramatically. This can have unprecedented and unpredictable effects on the wider community.
This qualification is designed to harness students' interest in the world of living things.
The four units each take an overarching topic such as diversity or disease. These topics allow students to see links between concepts to bring the subject to life.
Students must carry out 10 required practical activities in the course. The practicals are not constrained, allowing each school to choose resources and equipment that best suit their circumstances.
Practicals in this specification include investigations into enzymes and photosynthesis as well as using equipment such as microscopes and potometers. Exam questions will be asked on the practicals, but there is no coursework or practical exam.
A range of question styles are used, such as short answer questions, calculations and longer extended writing questions, allowing students to demonstrate the skills required for university study for subjects such as biology, medicine or dentistry.
This qualification is available for teaching now in Bahrain, China, Egypt, Indonesia, Jordan, Kuwait, Malaysia, Oman, Qatar, Saudi Arabia, Singapore, Thailand and the United Arab Emirates.
Unit 1: The diversity of living organisms (AS)
Unit 2: Biological systems and disease (AS)
Unit 3: Populations and genes (A2)
Unit 4: Control (A2)
Unit 5: Synoptic paper (A2)
- Candidates may re-sit a unit any number of times.
- The best result for each unit will count towards the final qualification.
- Candidates who wish to repeat a qualification may do so by re-sitting one or more units.
The diversity of living organisms
- Biological molecules
- Cells and cell structure
- Biochemical reactions in cells are controlled by enzymes
- Transport into and out of cells
- Gas exchange and the transport of oxygen in living organisms
- DNA, genes and chromosomes
- Protein synthesis
- Genetic diversity may arise by meiosis
- Species and taxonomy
- Biodiversity within a community
Biological systems and disease
- The causes of disease: pathogens, lifestyle and genes
- Digestion and absorption
- HIV as an example of a human disease caused by a virus
- The defensive functions of mammalian blood
- The circulation of blood and the structure of the mammalian heart
- Heart disease may be associated with specific risk factors
- Mass transport systems in plants
- The role of aphids in spreading plant viruses
- Cell division by binary fission and mitosis
- Mutation and cancer
Populations and genes (A-level only)
- The effect of biotic and abiotic factors on populations
- Energy transfer through ecosystems
- Nutrient cycles
- Allele frequencies in populations
- Evolution may lead to speciation
Control (A-level only)
- Control systems involve stimulus and response
- Nerve impulses and synaptic transmission
- Skeletal muscles as effectors
- Control systems in plants
- Homeostasis and negative feedback
- Hormones and the control of blood glucose concentration
- Control of heart rate
- Regulation of transcription and translation
- Recombinant DNA technology
Download the complete OxfordAQA International AS / A-level Biology specification (PDF, 760 KB).
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Science taster webinar recordings
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Get all the help you need to undertake practicals with your class. This guide harmonises the rules and guidance for A-level Biology, Chemistry and Physics, and includes a full glossary of terms.
Subject specific vocabulary
Ensure that everyone is using scientific language correctly with this list of word definitions used in our international A-level science specifications and exam papers (based on ‘The Language of Measurement’ produced by the Association of Science Education).
Specimen Papers and Mark Schemes
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