Nutrient Cycles (A Level Only)
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レッスンノート
Nutrient Cycles: Key Principles
- In a functioning ecosystem, the elements that living organisms need are constantly recycled.
- These elements are incorporated into biological molecules within the tissues of living organisms.
- They are released back into the environment when decomposers break down dead or waste matter.
- Examples of nutrient cycles include the nitrogen cycle and the phosphorus cycle.
The Nitrogen Cycle
- Nitrogen is present as N₂ gas in the atmosphere and within biological molecules, e.g. proteins, in the tissues of living organisms.
- Nitrogen fixation: N₂ gas is converted into ammonium compounds by nitrogen-fixing bacteria; these bacteria can be free-living in the soil, or may live within root nodules of legume plants.
- Ammonium compounds are converted into nitrates, which are then absorbed by plants and used to build plant proteins.
- Ammonification: nitrogen from living organisms is returned to the soil in the form of ammonia by the action of saprobionts such as bacteria and fungi; this ammonia forms ammonium ions in the soil.
- Nitrification: ammonium ions in the soil are converted into nitrates by nitrifying bacteria; Nitrosomonas bacteria convert ammonium ions into nitrites, and Nitrobacter bacteria then convert nitrites into nitrates.
- Denitrification: denitrifying bacteria use nitrates in the soil for respiration; this occurs in anaerobic conditions, such as in waterlogged soil, and produces nitrogen gas, which returns to the atmosphere.
The Phosphorus Cycle
- Plants and animals require phosphorus for production of, e.g. phospholipids, nucleic acids (DNA and RNA), and ATP.
- Phosphorus in rocks is released into the soil and into water sources in the form of phosphate ions (PO₄³⁻) due to weathering.
- Phosphate ions are taken up from the soil by plants, 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 be taken up and used again by producers, or may be trapped in sediments that may turn into phosphorus-containing rock once again.
Investigating Minerals & Plant Growth
- Many biological molecules in plants cannot be produced from the products of photosynthesis alone, but require additional chemical elements, e.g. proteins and nucleic acids require nitrogen, phospholipids, nucleic acids and ATP require phosphorus, and chlorophyll requires magnesium.
- Plants obtain these elements in the form of mineral ions, e.g. nitrate ions or phosphate ions, that they actively absorb from the soil.
- The effect of minerals on plant growth can be investigated by growing plants under controlled conditions and selectively removing minerals.
- Apparatus includes Bryophyllum plantlets, nutrient solutions containing a combination of nitrates, phosphates, magnesium and calcium, measuring cylinder, test tubes, test tube rack, and aluminium foil.
- Method: fill test tubes with set volumes of each nutrient solution; cover with foil and create a small hole; push plantlet roots through the hole so roots are submerged; place test tubes in the same location; after a set period, observe and take qualitative and quantitative measures of growth.
- Qualitative measures could include colour of plantlets and leaves, or how wilted the plantlet has become; quantitative measures could include height or mass of plantlets, or length and width of leaves.
Microorganisms & Recycling Minerals
- Microorganisms play a vital role in recycling chemical elements, such as phosphorus and nitrogen, in ecosystems; examples include saprobionts in decomposition and mycorrhizae in plant roots.
- Saprobionts are organisms that decompose dead and waste organic matter; examples include fungi and bacteria.
- Decomposition by saprobionts releases chemical elements, such as phosphorus and nitrogen, from within dead and waste material.
- The process of decomposition by saprobionts: enzymes are secreted onto a food source, extracellular digestion occurs, and nutrients are absorbed.
- Not all products of extracellular digestion are absorbed by saprobionts; some mineral ions remain in the soil where they can be absorbed by plants.
- Mycorrhizae are symbiotic relationships between fungi and the roots of plants; fungi form long, thin filaments known as hyphae, which connect with plant roots.
- The hyphae effectively increase the surface area of the root systems of the plants, increasing absorption of water and inorganic ions; in return the fungi receive carbon compounds, e.g. glucose, from the plant.
Roles of Microorganisms in Mineral Cycling
- Saprobionts: decompose dead and waste matter via extracellular digestion, making inorganic ions available to other organisms; carry out ammonification by converting nitrogen compounds in waste and dead matter into ammonia.
- Nitrogen-fixing bacteria: convert atmospheric nitrogen gas into nitrogen-containing compounds, such as ammonia.
- Nitrifying bacteria: convert ammonium ions in soil into nitrates.
- Denitrifying bacteria: use nitrates during respiration, releasing nitrogen gas in the process.
- Mycorrhizal fungi: increase surface area of root systems, helping plants to absorb water and mineral ions from soil; form symbiotic relationships with plant roots.
Fertilisers: Natural & Artificial
- In natural ecosystems, decomposition by saprobionts recycles nutrients from waste and dead organisms back into the soil.
- In agricultural ecosystems, crops and livestock are harvested and removed, so the nutrients contained in their biomass are not returned to the soil; this disrupts nutrient cycles and can lead to reduced soil fertility.
- To maintain productivity, fertilisers are used to replace lost mineral ions.
- Natural fertilisers are made from organic matter, such as manure, compost, crop residues, and sewage.
- Advantages of natural fertilisers: release nutrients slowly over time, so nutrients are less likely to be washed away into rivers and lakes after rain; contain organic matter, so can improve soil structure and water retention.
- Disadvantages of natural fertilisers: nutrients are less concentrated, so large amounts needed; nutrient content is variable and harder to control.
- Artificial fertilisers are made up of inorganic matter in the form of powders or pellets that contain chemical compounds, e.g. ammonium nitrate.
- Advantages of artificial fertilisers: nutrients are concentrated and easy to apply; precise nutrient content allows controlled dosing.
- Disadvantages of artificial fertilisers: are highly soluble in water so can be leached out of soil into rivers and lakes when it rains; do not improve soil structure.
Environmental Issues Caused by Fertilisers
- Fertilisers are often applied in larger quantities than crops require; as a result, excess mineral ions may remain dissolved in the soil water.
- These mineral ions can be carried by rainwater into nearby rivers, lakes or streams; this is known as leaching.
- Leaching is more likely to occur after heavy rainfall and when using artificial fertilisers, as the inorganic ions are highly soluble.
- Leaching is less likely to occur with natural fertilisers as organic matter must be decomposed by microorganisms before minerals become water-soluble.
- Leaching can lead to eutrophication, which occurs as follows: mineral ions enter water bodies, causing rapid growth of algae at the surface (an algal bloom).
- Algae block sunlight, which prevents aquatic plants below the surface from photosynthesising.
- These plants, and eventually the algae, begin to die and dead organic matter accumulates.
- Bacteria decompose the dead matter, respiring aerobically and using up the oxygen dissolved in the water.
- Oxygen levels fall and aquatic animals such as fish and insects can no longer survive.
スライド
練習問題
無料プレビュー — 59問中8問。すべて見るには登録を。
1.Which of the following is a nutrient cycle found in ecosystems?
Easy- AThe nitrogen cycle
- BThe oxygen cycle
- CThe carbon cycle
- DThe water cycle
2.In the nitrogen cycle, which process converts nitrogen gas into ammonium compounds?
Easy- ANitrogen fixation
- BNitrification
- CAmmonification
- DDenitrification
3.Which bacteria convert ammonium ions in the soil into nitrites?
Easy- ANitrosomonas
- BNitrobacter
- CRhizobium
- DDenitrifying bacteria
4.Denitrification in the nitrogen cycle is carried out by bacteria that:
Medium- AUse nitrates in the soil for respiration in anaerobic conditions
- BConvert ammonium ions into nitrates
- CFix atmospheric nitrogen into ammonia
- DBreak down dead organic matter into ammonia
5.Which of the following is NOT a form in which phosphorus is required by living organisms?
Medium- ACalcium phosphate in bones
- BPhospholipids
- CNucleic acids
- DATP
6.Saprobionts obtain nutrients from dead organisms by:
Medium- ASecreting enzymes for extracellular digestion and absorbing the products
- BEngulfing dead matter by phagocytosis
- CPhotosynthesising using light energy
- DConverting atmospheric nitrogen into ammonia
7.Mycorrhizae are best described as:
Medium- ASymbiotic relationships between fungi and plant roots
- BParasitic relationships between bacteria and plant roots
- CFree-living nitrogen-fixing bacteria in the soil
- DDecomposers that break down dead organic matter
8.Which of the following is a disadvantage of using artificial fertilisers compared with natural fertilisers?
Medium- AThey are highly soluble and can be leached into rivers and lakes
- BThey release nutrients too slowly
- CThey contain too much organic matter
- DThey do not contain enough nutrients
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