Nutrient cycles (A-level only)
AQA
A-Level
Nutrients are recycled within ecosystems, whereas energy flows through and is lost. Nitrogen and phosphorus follow this pattern: producers absorb inorganic ions from soil or water, building them into organic molecules. Elements pass along food chains, and microorganisms return them to the inorganic pool when organisms die. Nitrogen enters mainly as nitrate, absorbed by active transport into roots, and is used to make amino acids, proteins, ATP and DNA. It has a vast atmospheric reservoir of nitrogen gas, fixed mainly by nitrogen-fixing bacteria and archaea, though lightning also contributes. Phosphorus has no significant gaseous stage; its reservoir is rock, which weathers to release phosphate ions into soil water. Phosphate is needed for phospholipids, ATP, DNA and RNA.
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Objectives
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Exam Tips
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Pitfalls
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Key Terms
36
Mark Points
Subtopics in this area
Nutrients are recycled within natural ecosystems, exemplified by the nitrogen cycle and the phosphorus cycle.
Microorganisms play a vital role in recycling chemical elements such as phosphorus and nitrogen.
The role of saprobionts in decomposition.
The role of mycorrhizae in facilitating the uptake of water and inorganic ions by plants.
The role of bacteria in the nitrogen cycle in sufficient detail to illustrate the processes of saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification.
The use of natural and artificial fertilisers to replace the nitrates and phosphates lost by harvesting plants and removing livestock.
The environmental issues arising from the use of fertilisers including leaching and eutrophication.
Nutrient cycles (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Contrast the one-way flow of energy with the recycling of nitrogen and phosphorus in an ecosystem.
- Name the inorganic ion by which plants take up each of nitrogen and phosphorus, and the transport mechanism used.
- Describe two named organic molecules that require nitrogen and two that require phosphorus.
- Predict what happens to nutrient availability in an ecosystem from which decomposers are removed.
- Name four groups of soil microorganism and state the conversion each one carries out.
- Explain why sterilising the soil is a valid control in an experiment on a single soil organism.
- Describe how extracellular digestion by saprobionts releases ammonium and phosphate ions.
- Predict the effect on plant growth of soil that has been sterilised and not re-inoculated.
- Describe saprobiotic nutrition as a sequence of secretion of enzymes, hydrolysis and absorption of products.
- Name two hydrolytic enzymes a saprobiont secretes and the polymer each one acts on.
- Explain how saprobiont activity returns ammonium and phosphate ions to the soil.
- Predict the effect of waterlogging or low temperature on the rate of decomposition, and justify it.
- Explain how fungal hyphae increase the uptake of water and inorganic ions by a plant root.
- Describe what each partner gains in a mycorrhizal association.
- Use overlapping or non-overlapping standard deviations to decide whether a difference in yield is significant.
- Evaluate a mycorrhizae experiment by identifying limitations such as a single species, sterile soil or greenhouse conditions.
- Sequence the nitrogen cycle from dead organic matter through to nitrate, naming the bacteria responsible for each step.
- Explain why nitrification requires an aerated soil and denitrification an anaerobic one.
- Describe nitrogen fixation in terms of nitrogenase, the reduction of nitrogen gas and the ATP cost.
- Predict the effect of ploughing or draining a field on the nitrate content of its soil, and justify it.
- Explain why harvesting a crop depletes soil nitrate and phosphate whereas a natural ecosystem does not.
- Compare natural and artificial fertilisers in terms of how quickly their ions become available.
- Name a molecule requiring nitrogen and one requiring phosphorus, and link each to growth.
- Justify keeping fertiliser application constant as a controlled variable in a plant growth investigation.
- Describe the sequence of eutrophication from fertiliser application to the death of fish, in the correct causal order.
- Explain why nitrate leaches more readily than phosphate from agricultural soil, referring to the solubility of nitrate and to the adsorption of phosphate.
- Identify which organisms consume the dissolved oxygen during eutrophication, and why.
- Suggest two farming practices that reduce leaching, and justify each one.
Marking Points
Key points examiners look for in your answers
- Explaining that nutrients are recycled whereas energy flows through and is lost
- Identifying that producers absorb the nutrient as a named inorganic ion, such as nitrate or phosphate
- Naming an organic molecule the element is built into, such as protein, DNA or ATP
- Describing how decomposers or saprobionts return the element to the inorganic pool
- Identifying the reservoir, such as the atmosphere for nitrogen and rock for phosphorus
- Naming a functional group of soil microorganism, such as saprobionts, nitrogen-fixing, nitrifying or denitrifying bacteria, and stating the conversion it carries out.
- Describing saprobionts digesting dead material extracellularly using secreted hydrolytic enzymes and absorbing the products, releasing ammonium and phosphate ions.
- Identifying the release of a named inorganic ion, such as ammonium, nitrite, nitrate or phosphate, and linking it to the correct process.
- Explaining that sterilising soil kills or removes other microorganisms, so the test organism is the only variable changed and any ions released can be attributed to it.
- Explaining that sterilising soil removes the microorganisms that release mineral ions, so plants in sterile, un-inoculated soil show reduced growth.
- Distinguishing the four nitrogen-cycle processes by their substrate and product: ammonification, nitrification, nitrogen fixation and denitrification.
- one mark for saprobionts feeding on dead organisms, faeces or urine
- one mark for secreting enzymes onto the material, that is extracellular digestion
- one mark for naming a hydrolytic enzyme, or the hydrolysis of a named polymer
- one mark for absorbing the soluble products across the cell surface membrane
- one mark for releasing ammonia or ammonium ions, and phosphate, back into the soil
- one mark for the increased surface area of hyphae increasing uptake of water and inorganic ions
- one mark for the plant supplying the fungus with organic compounds in return, making the relationship mutualistic
- one mark for describing a significant difference, or no significant difference, with the treatment named rather than just saying the results are significant
- one mark for using overlapping standard deviations to show no significant difference, or non-overlapping to show a significant one
- one mark for a valid limitation, such as only one mycorrhizal species or one crop studied, or sterile soil and greenhouse rather than field conditions
- one mark for ammonification as the release of ammonia or ammonium ions from dead material, faeces and urine
- one mark for nitrification as ammonium to nitrite and nitrite to nitrate by nitrifying bacteria
- one mark for nitrogen fixation as the reduction of atmospheric nitrogen to ammonia
- one mark for denitrification as the reduction of nitrate to nitrogen gas under anaerobic conditions
- one mark for linking aeration of the soil to more nitrification and less denitrification
- one mark for the idea that harvesting a crop or removing livestock takes ions out of the field inside the biomass
- one mark for fertilisers replacing the nitrate and phosphate lost
- one mark for a named use of the ion, such as nitrate for amino acids and proteins, or phosphate for ATP, DNA and phospholipids
- one mark for a valid difference between natural and artificial fertilisers, such as rate of release or precision of composition
- one mark for stating that fertiliser must be kept constant as a controlled variable in a growth experiment
- one mark for leaching as the washing of soluble nitrate through the soil into watercourses
- one mark for the extra nitrate or phosphate no longer limiting algal growth, so an algal bloom forms
- one mark for the bloom blocking light so that submerged plants die
- one mark for saprobiotic bacteria multiplying as they decompose the dead material
- one mark for aerobic bacterial respiration reducing dissolved oxygen, so fish and aerobic organisms die
Examiner Tips
Expert advice for maximising your marks
- 💡Nutrient cycles appear on the published list of topics for the 'importance of cycles in biology' essay, so keep a labelled diagram of each in your notes.
- 💡Name the ion, not the element: nitrate and phosphate, absorbed by active transport.
- 💡When asked why a mineral is needed, answer with molecules and then a consequence, for example nitrate to amino acids to enzymes to growth.
- 💡In experiments using sterilised soil, the control mark is for removing other fungi and bacteria so the test organism is the only variable changed; the test organism itself is the independent variable.
- 💡Learn the four nitrogen-cycle processes with the ion at each end: ammonification, nitrification, nitrogen fixation, denitrification.
- 💡Species names are never required, but the role and the ion always are.
- 💡The word examiners look for is extracellular: enzymes are secreted onto the material, then the products are absorbed.
- 💡Name at least one enzyme and one ion released; a general 'they break down dead things' rarely scores more than one mark.
- 💡Saprobiont, saprobiotic and saprophyte are all accepted terms, so use whichever you remember.
- 💡In evaluation questions, keep conclusions drawn from the data separate from limitations of the method; the mark scheme caps how many limitation marks you can earn.
- 💡Quote the treatment or the letter from the table when stating a difference, for example greater in Q than in P.
- 💡Overlapping error bars mean the difference is probably not significant; the phrase 'not due to chance' is accepted for significant.
- 💡Draw the cycle as a loop of ions, nitrogen gas to ammonium to nitrite to nitrate and back to nitrogen gas, and label the bacteria on each arrow.
- 💡Anything that waterlogs soil favours denitrification and blocks nitrification; say so explicitly when explaining a poor yield.
- 💡Answer 'why does a crop need nitrate' with molecules first, amino acids, proteins, nucleotides, ATP and chlorophyll, then growth.
- 💡Comparison questions need a stated difference on both sides, for example organic releases ions slowly whereas inorganic dissolves at once.
- 💡In experiments, say the fertiliser is kept the same so that it is not a second variable affecting growth.
- 💡Write eutrophication as a chain of cause and effect with at least five links; a list of terms scores poorly.
- 💡Name the organisms doing the respiring as aerobic saprobiotic bacteria, not the algae.
- 💡If a question is about experimental design rather than pollution, resist writing about eutrophication because it earns nothing there.
Common Mistakes
Pitfalls to avoid in your exam answers
- Saying energy is recycled, when only matter is recycled and energy flows through the ecosystem.
- Claiming plants absorb nitrogen gas from the air, which is actually fixed by nitrogen-fixing microorganisms or lightning.
- Giving phosphorus an atmospheric stage, when its reservoir is rock and sediment.
- Naming only proteins as the use of nitrogen, forgetting nucleotides, ATP and chlorophyll.
- Describing uptake as diffusion when nitrate and phosphate enter root cells by active transport.
- Writing that decomposers 'break things down' with no mention of extracellular enzymes or absorption. Correction: state that saprobionts secrete hydrolytic enzymes, digest material outside the cell and absorb the soluble products.
- Treating nitrifying and nitrogen-fixing bacteria as the same organisms doing the same job. Correction: nitrifying bacteria oxidise ammonium to nitrite then nitrate; nitrogen-fixing bacteria reduce atmospheric nitrogen to ammonia.
- Saying sterilising soil removes 'harmful bacteria', which is not enough without a named group or role. Correction: state that sterilisation removes other microorganisms so the test organism is the only variable changed.
- Assuming microorganisms affect only nitrogen, when they release phosphate from dead material too. Correction: saprobionts also release phosphate ions from dead matter.
- Describing the microorganisms as making food for the plant rather than releasing mineral ions. Correction: they release inorganic ions such as ammonium, nitrate and phosphate, which plants absorb.
- saying saprobionts eat or ingest dead material, when they digest it outside the cell and absorb the products
- treating saprophyte as though it meant a plant, when the organisms are bacteria and fungi
- describing decomposition as rotting without naming enzymes, hydrolysis or the ions released
- confusing saprobionts with mycorrhizae, which are mutualistic with living roots rather than feeding on dead tissue
- omitting the absorption step, so the products of digestion never enter the organism
- writing that 'the results are significant' with no statement of what is greater, lower or different, which is rejected
- saying mycorrhizae give the plant minerals, rather than increasing the surface area for absorption
- omitting the mutualism, so the fungus's gain of sugars from the plant is never mentioned
- concluding that mycorrhizae always increase yield, when the data show no significant difference without water shortage
- treating a large sample size as proof of a significant difference rather than as improved reliability
- swapping nitrification and nitrogen fixation, which are different processes carried out by different bacteria
- writing that nitrifying bacteria convert ammonium straight to nitrate, missing the nitrite intermediate
- placing denitrification in aerobic conditions, when it occurs in waterlogged, anaerobic soil
- saying fertiliser feeds the plant, as though it were a food rather than a source of mineral ions
- claiming natural fertiliser acts immediately, when saprobionts must decompose it first
- explaining only nitrate and ignoring phosphate, which the specification names alongside it
- answering that fertiliser increases photosynthesis without saying which molecule the ion is needed for
- assuming more fertiliser always means more yield, when uptake saturates and surplus is leached
- writing about leaching and eutrophication in questions about controlling a variable, where examiners explicitly ignore them
- saying the algae use up all the oxygen, when the oxygen is consumed by the decomposing bacteria
- leaving out the light-blocking step, so there is no reason given for the plants dying
- confusing leaching with surface run-off; leaching is downward movement through the soil
- claiming eutrophication is caused only by nitrate, ignoring phosphate from fertiliser and sewage