Nutrient Cycles (A Level only) (AQA A Level Biology): Flashcards
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In a nutrient cycle, how are elements recycled through an ecosystem?
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In a nutrient cycle, how are elements recycled through an ecosystem?
Elements are incorporated into biological molecules in the tissues of living organisms, then released back into the environment when decomposers break down dead or waste matter.
What is nitrogen fixation?
The conversion of nitrogen gas (N~2~) in the atmosphere into ammonium compounds by nitrogen-fixing bacteria.
Where are nitrogen-fixing bacteria found?
They can be free-living in the soil, or live within the root nodules of legume plants.
What is ammonification?
The process in which saprobionts (bacteria and fungi) return nitrogen from dead and waste matter to the soil as ammonia, which forms ammonium ions.
Which bacteria carry out the two steps of nitrification?
Nitrosomonas bacteria convert ammonium ions into nitrites.
Nitrobacter bacteria then convert nitrites into nitrates.
What is denitrification?
The process in which denitrifying bacteria use nitrates in the soil for respiration, producing nitrogen gas that returns to the atmosphere.
Under what conditions does denitrification occur?
In anaerobic conditions, such as in waterlogged soil.
True or False?
Plants absorb nitrogen as nitrate ions, which are used to build plant proteins.
True.
Ammonium compounds are converted into nitrates, which are absorbed by plants and used to build plant proteins.
Phosphorus in rocks is released into the soil and water as ions due to .
Phosphorus in rocks is released into the soil and water as phosphate ions due to weathering.
Why do plants and animals require phosphorus?
For the production of phospholipids, nucleic acids (DNA and RNA) and ATP.
How are phosphate ions returned to the soil or water in the phosphorus cycle?
Phosphate ions in waste products and dead organisms are released during decomposition by saprobionts.
Why do plants need mineral elements in addition to the products of photosynthesis?
Many biological molecules cannot be produced from the products of photosynthesis alone and require additional chemical elements, e.g. nitrogen, phosphorus and magnesium.
Which mineral elements do plants require for proteins, ATP and chlorophyll?
Proteins and nucleic acids require nitrogen.
Phospholipids, nucleic acids and ATP require phosphorus.
Chlorophyll requires magnesium.
In what form do plants absorb these elements from the soil?
As mineral ions, e.g. nitrate ions or phosphate ions, which are actively absorbed from the soil.
How can the effect of minerals on plant growth be investigated?
By growing plants under controlled conditions and selectively removing individual minerals from the nutrient solution.
What is the control solution in this investigation?
A nutrient solution with all minerals present, against which plantlets grown in mineral-deficient solutions are compared.
The roots of each plantlet are pushed through a hole in foil so they are submerged in the nutrient solution.
The roots of each Bryophyllum plantlet are pushed through a hole in aluminium foil so they are submerged in the nutrient solution.
Give examples of qualitative measures of plantlet growth.
The colour of the plantlets and their leaves, or how wilted the plantlet has become.
Give examples of quantitative measures of plantlet growth.
The height or mass of the plantlets, or the length and width of their leaves.
True or False?
The test tubes should be placed in different locations to test the effect of the minerals.
False.
The test tubes should be placed in the same location, so that light and temperature are controlled and only the mineral content differs between tubes.
Give an example set of nutrient solutions used to test individual minerals.
Solution 1 = all minerals present (control).
Solution 2 = nitrate absent.
Solution 3 = phosphate absent.
Solution 4 = magnesium absent.
Solution 5 = calcium absent.
What are saprobionts?
Organisms that decompose dead and waste organic matter, e.g. fungi and bacteria.
Describe how saprobionts decompose a food source.
Enzymes are secreted onto the food source.
Extracellular digestion occurs.
Nutrients are absorbed.
Why is decomposition by saprobionts important for other organisms?
Not all products of extracellular digestion are absorbed, so some mineral ions remain in the soil where they can be absorbed by plants.
Why is it not enough to describe the role of saprobionts as simply 'decomposition'?
You must state that they release mineral ions, such as nitrates and phosphates, back into the soil.
What are mycorrhizae?
Symbiotic relationships between fungi and the roots of plants.
How do mycorrhizae benefit a plant?
Fungal filaments (hyphae) increase the surface area of the root system, increasing absorption of water and inorganic ions.
What do the fungi receive in a mycorrhizal relationship?
Carbon compounds, e.g. glucose, from the plant.
What is the role of nitrogen-fixing bacteria in mineral cycling?
They convert atmospheric nitrogen gas into nitrogen-containing compounds, such as ammonia.
What is the role of nitrifying bacteria in mineral cycling?
They convert ammonium ions in the soil into nitrates.
What is the role of denitrifying bacteria in mineral cycling?
They use nitrates during respiration, releasing nitrogen gas.
Saprobionts carry out by converting nitrogen compounds in dead and waste matter into .
Saprobionts carry out ammonification by converting nitrogen compounds in dead and waste matter into ammonia.
Why are fertilisers needed in agricultural ecosystems?
Crops and livestock are harvested and removed, so the nutrients in their biomass are not returned to the soil; fertilisers replace the lost mineral ions.
What are natural fertilisers?
Fertilisers made from organic matter, such as manure, compost, crop residues or sewage.
What are artificial fertilisers?
Fertilisers made from inorganic matter as powders or pellets containing chemical compounds, e.g. ammonium nitrate.
Give two advantages of natural fertilisers.
They release nutrients slowly, so nutrients are less likely to be leached into rivers and lakes.
They contain organic matter, improving soil structure and water retention.
Give two advantages of artificial fertilisers.
Nutrients are concentrated and easy to apply.
Precise nutrient content allows controlled dosing.
Artificial fertilisers are highly in water, so they can be out of the soil into rivers and lakes.
Artificial fertilisers are highly soluble in water, so they can be leached out of the soil into rivers and lakes.
What is leaching?
The process in which excess mineral ions dissolved in soil water are carried by rainwater into nearby rivers, lakes or streams.
When is leaching more likely to occur?
After heavy rainfall, and when using artificial fertilisers because their inorganic ions are highly soluble.
Why is leaching less likely with natural fertilisers?
Their organic matter must first be decomposed by microorganisms before the minerals become water-soluble.
Describe the process of eutrophication.
Mineral ions enter the water, causing rapid growth of algae at the surface (an algal bloom).
The algae block sunlight, preventing aquatic plants below from photosynthesising.
The plants, and eventually the algae, die and dead organic matter accumulates.
Bacteria decompose the dead matter, respiring aerobically and using up dissolved oxygen.
Oxygen levels fall and aquatic animals such as fish and insects can no longer survive.
True or False?
During eutrophication, bacteria decomposing dead matter increase the oxygen concentration of the water.
False.
The bacteria respire aerobically as they decompose the dead matter, which uses up dissolved oxygen and causes oxygen levels to fall.
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