5.4 Nutrient cycles Flashcards
What is the nitrogen cycle important
Since animals and plants cannot obtain nitrogen through gas exchange
What can’t animals and plants obtain nitrogen through gas exchange
Since nitrogen gas contains a triple bond so it requires a lot of energy to break
What is nitrogen so essential
Needed for:
- Proteins
- ATP
- Nucleic acids
What does nitrogen fixing mean
Converting nitrogen gas into a nitrogen containing compound
Where does nitrogen fixing occur
In the root nodules
Why does nitrogen fixing happen in the root nodules
Because there are bacteria in the nodules called nitrogenase are able to convert nitrogen gas into ammonium or nitrates
What type of plants contain nitrogenase bacteria in their nodules
Legumes
How is ammonia absorbed into the plant after being made at the nodules
In the form ammonium ions in water
Once ammonium ions are in the plant what are they used for
Amino acids
ATP
Nucleic acids
What 2 things can either happen to a plant during the nitrogen cycle
Either:
- Eaten by herbivores
- Death
Once a herbivore has eaten a plant how does the nitrogen get put back into the nitrogen cycle , 2 ways
- By death of the animal
- By excretion of urine
What microbe converts the dead organic matter into ammonia during the nitrogen cycle
Saprobiants
What is the name of the process where dead organic matter is converted into ammonia
Ammonification
During nitrification, what is ammonia firstly converted into
Nitrites , NO2-
After ammonia has been converted into nitrites what are the nitrites then converted into
Nitrates, NO3-
What type of bacteria converts ammonia into nitrites and nitrates
Nitrifying bacteria
What is the name of the process by which nitrates are converted back into nitrogen gas
Denitrification
If nitrates aren’t denitrified back into nitrogen gas, how else go they rejoin the nitrogen cycle
By absorption through the soil and into plants
What type of reaction is the convertion of nitrogen gas to ammonia
Reduction reaction
How do nitrogen fixing bacteria respire
Aerobically
How do saprobionts respire
Aerobically
How do nitrifying bacteria respire
Aerobically
How do denitrifying bacteria respire
Anaerobically
How does nitrogen in the atmosphere get into proteins in plants
Nitrogen fixing
How does proteins in plants pass on their nitrogen to proteins in animals
Feeding
How does the proteins in animals become proteins in dead organic matter
Decomposition
How does proteins in animals become nitrogenous waste (urea)
Excretion
How does proteins in plants become proteins in dead organic matter
Decomposition
How does nitrogenous wastes (urea) become ammonia in the soil, and what does this
Ammonification by saprobiants
How does proteins in dead organic matter become ammonia in soil
Ammonification
How does ammonia in soil become nitrites in soil
Nitrification
How do nitrites in the soil become nitrates in the soil
Nitrification
How do nitrates in the soil enter proteins in plants
Absorption
How do nitrates in the soil re-enter the atmosphere
Denitrification
Why do plants need nitrogen
To manufacture proteins, nucleic acids and other nitrogen-containing compounds
Only few plants can use nitrogen gas directly, so what form of nitrogen do plants commonly use and how is it uptaken
Nitrate ions from soil that are absorbed by active transport by the roots
How do animals obtain nitrogen-containing compounds
By eating and digesting plants that have absorbed the nitrates
Are nitrate ions soluble
Yes
Are nitrate ions easily leached
Yes, they are easily leeched (wash) through the soil
In a natural ecosystem how are the nitrate concentrations restored after the nitrates have been leeched away beyond the reach of the roots
Largely by the recycling of nitrogen-containing compounds
In an agricultural ecosystem, how are the nitrate concentrations restored / increased
They are increased by adding fertilisers
In a natural ecosystem how are the nitrogen-containing compounds recycled
When plants and animals die, the process of decomposition begins, where microorganisms replenish the nitrate concentrations in the soil - releasing nitrate ions
What are the 4 stages in the nitrogen cycle
- Ammonification
- Nitrification
- Nitrogen fixing
- Denitrification
How do saprobiontic microorganisms convert nitrogen-containing compounds into ammonia
By feeding on faeces and dead organic matter - releasing ammonia
When ammonia is released by the saprobionts, what does it then form
Ammonium ions in the soil - when nitrogen returns to the non-living components of the ecosystem
During nitrification what are the 2 steps from ammonia ions to nitrate ions, stating what type of reaction it is
- Oxidation of ammonium ions to nitrite ions
- Oxidation of nitrite ions to nitrate ions
How do nitrifying bacteria obtain their energy
Using the energy released by the oxidation reactions converting ammonium ions to nitrate ions
Why do farmers need to keep soil structure light and well aerated by ploughing, and have a good drainage system
To create soil with air spaces so nitrifying bacteria can carry out the oxidation reactions with sufficient oxygen
What are the 2 types of microorganisms that carry out nitrogen fixation
- Free-living nitrogen-fixing bacteria
- Mutualistic nitrogen-fixing bacteria
How do free-living nitrogen-fixing bacteria fix nitrogen
By reducing gaseous nitrogen into ammonia
How do mutualistic nitrogen-fixing bacteria fix nitrogen
By living in root nodules of plants such as beans and peas
How is the relationship between nitrogen-fixing bacteria in the root nodules and the plant mutualistic
The plant receives nitrogen containing compounds and the bacteria receive carbon containing compounds
What condition is the soil usually in when there are more denitrifying bacteria than nitrifying
Waterlogged, low oxygen concentration
How could you identify plants that are growing in nitrogen poor soils
Light green with smaller leaves and stunted growth
Why is nitrogen in the atmosphere not directly available to plants
Since the N2 has a triple bond making it very stable since it’s hard to break the bonds
How does deforestation affect the nitrogen cycle
- Nitrates will be leeched out of the soil
- Soil erosion, so reducing soil quality
How does crop rotation improve soil fertility
It increases the fertility if it includes legueous plants
Livestock farming creates large volumes of animal waste. How would this affect the nitrogen cycle
More dead organic matter so greater concentration of ammonia in soil so more nitrates available
What would happen if a farmer used too much fertiliser, what adverse affects can this have on the environment
Washed into rivers, eutrophication and leeching into surrounding water bodies
Why are animals not essential to the nitrogen cycle
They only contribute to the dead organic matter
How does harvesting of crops affect the nitrogen cycle
It reduces the soils fertility
Why is it necessary for farmers to plough their fields
To minimise the denitrifying bacteria so more nitrogen compounds remain in the soil
Explain how grass, non-legume, obtains the nitrogen it needs
Nitrates or ammonium ions
Name 3 biological molecules that contain phosphorus
- ATP
- Phospholipids
- Nucleic acids
In the carbon and nitrogen cycle the main reservoir of each element is in the atmosphere, what is the main reservoir of phophorus
In mineral form rather than in the atmosphere
In what form does phosphorus exist in
Phosphate ions , PO4^3-
What is the name of the process by which the sedimentary rocks containing phosphate ions are brought to the surface after being under the sea
Geographical uplifting
What processes help the phosphate ions within the sedimentary rock be dissolved
Weathering and erosion
How to the phosphate ions get into animals from the rocks
The ions are dissolved meaning they can be absorbed by plants and animals then feed on the plants
In animals what happens to excess phosphate ions
They are excreted and may accumulate in waste materials
When plants or animals die what happens to the phosphate
Certain bacteria and fungi break down the dead organic matter releasing the phosphate ions into the water or soil, however some of the ions remain in parts of the animal such as bones or shells that are very slow to breakdown
What happens to the phosphate ions released after decomposition or ions that very dissolved out of rocks but not absorbed
They’re transported by streams and rivers into lakes and oceans where they form sedimentary rocks thus completing the cycle
What are the roles of mycorrhizae in nutrient cycles
- Act like extensions of the root system and vastly increase the surface area for the absorption of water and minerals
- It also acts as a sponge and so holds water and minerals in the neighbourhood of the roots
How does mycorrhizae acting like a sponge benefit the plant
It holds water and minerals in the neighbourhood of the roots and enables the plant to better resist drought and to take up inorganic ions more readily
How does mycorrhiza play a part in the nutrient cycles, particularly the phosphate cycle
It improves the uptake of relatively scarce ions
What type of relationship is between mycorrhizae and the plant
Mutualistic
Why is the relationship between mycorrhizae and the plant mutualistic
The plant benefits from improved water and mineral ion uptake while the mycorrhiza receives organic compounds such as sugars and amino acids from the plant
Why do farmers add fertilisers to their fields
To increase the mineral ions concentrations in the soil
Why are fertilisers necessary for farmers to use to keep up the high demand for crops
Because food production is intensive since its concentrated on specific areas of land that are repeatedly used - this leads to large demand on the soil since mineral ions are continually taken up by the crops so the minerals are being removed from the soil
Why does harvesting mean that the farmer will need to use fertilisers
Since normally the plants/ animals will die and become dead organic matter however when harvested the organisms are removed before they die so that area will have fallen concentrations of many minerals
What are the 2 types of fertilisers
- Natural (organic) fertilisers
- Artificial (inorganic) fertilisers
What does natural/ organic fertiliser consist of
Dead and decaying remains of plants and animals as well as animal wastes such as manure and bone meal
How are artificial/ inorganic fertilisers made
They’re mined from rocks and deposits and then converted into different forms and blended together to give the appropriate balance of minerals for a particular crop
In artificial/ inorganic fertiliser there are almost always compounds containing what 3 elements
- Nitrogen
- Phosphorus
- Potassium
Explain why fertilisers are needed in an argicultural ecosystem
Crops are grown repeatedly and intensively in the same area of land. Mineral ions are taken up by the crops, which are transported and consumed away from the land. The mineral ions they contain are not returned to the same area of land and so the levels in the soil are reduced, which can limit the rate of photosynthesis. Fertilisers need to be applied to replace them if photosynthesis/ productivity is to be maintained
Suggest a reason why, after a certain point, the addition of more fertiliser no longer improves the productivity of a crop
Some other factor is limiting photosynthesis, e.g. light, carbon dioxide, and only the addition of this factor will increase photosynthesis and hence productivity
Distinguish between natural and artificial fertilisers
Natural fertilisers are organic and come from living organisms in the form of dead remains, urine or faeces. Whereas artificial fertilisers are inorganic and are mixed from rocks and deposits
What are 2 advantages of using natural fertilisers
- There’s a gradual release of minerals ions
- Decreases likelihood of leeching
What are the advantages of using artificial fertilisers
- They provide a more concentrated source of minerals
- A lower volume is needed compared to natural fertilisers resulting in lower costs
- Can be blended in known composition
What are the 2 main benefits of using fertilisers
- Replaces mineral ions removed in harvest
- Increases crop productivity
What are 3 disadvantages of using fertilisers
- Reduces species diversity
- Leaching
- Eutrophication
What is the role of saprobionts in decomposition
They convert dead organic matter into mineral ions as a result of extracellular digestion, releasing mineral ions
What is the role of mycorrhizae
To increase the surface area of the plants roots for absorption of water and mineral ions
Use of artificial fertilisers can cause eutrophication, explain how
Nutrients leaching into water body which increase algal growth so decrease light which decreases photosynthesis, so less oxygen in the water so more death
Explain why applying high concentrations of nitrate to the soil can cause crops to wilt
Water potential in soil is decreased so water moves out by osmosis
Explain why leguminous plants can grow in soils with low concentrations of nitrate ions
Bacteria reduces the nitrogen gas into ammonia and then ammonium ions can be absorbed
Nitrogenase catalyses the reduction of nitrogen during nitrogen fixation. The reaction
requires 16 molecules of ATP for each molecule of nitrogen that is reduced.
When ammonia inhibits nitrogenase activity, nitrogen-fixing bacteria may benefit.
Explain how. (2 marks)
- Less/no ATP/energy required/used
OR
More ATP/energy available; - ATP/energy can be used for
growth/synthesis/replication
OR
Lower (rate of) respiration required
OR
ATP for phosphorylation;
