Nutrient cycles (A-level only) Flashcards
The role of saprobionts in decomposition
Saprobionts secrete enzymes onto dead matter and break down large molecules into soluble ones by extracellular digestion so that they can be absorbed by the saprobiont.
In particular, saprobionts break down organic matter that contains nitrogen which releases ammonium ions that move on to the next stage of the cycle (nitrification).
4 Stages of the nitrogen cycle
Nitrogen Fixation
Ammonification
Nitrification
Denitrification
Nitrogen fixation
This is where nitrogen fixing bacteria convert nitrogen gas into ammonia, which forms ammonium ions (NH4+).
An example of this kind of bacteria is Rhizobium, which is found in the root nodules of leguminous plants. Rhizobium are mutualistic:
The bacteria provide the plant with nitrogen compounds
The plant provides the bacteria with carbohydrates
Ammonification
Organic material is broken down by saprobiotic bacteria to release ammonium ions back to the soil.
Plants use nitrates to form proteins and nucleic acids etc.
Consumers will digest the plant material to obtain this.
This nitrogenous organic material (dead plants, dead animals, waste) is turned into ammonia by saprobionts, which goes on to form ammonium ions.
Nitrification
Ammonium ions that are released from ammonification are converted into nitrates by bacteria in the soil.
Nitrifying bacteria (e.g. Nitrosomonas) firstly convert NH4+ ions to nitrites.
Another bacteria (e.g. Nitrobacter) then convert nitrites to nitrates.
Dentrification
Denitrifying bacteria convert nitrates in the soil back into atmospheric nitrogen (N2).
This takes place in anaerobic conditions (e.g waterlogged soils).
What are most of the sugars made by plants used for?
Most of the sugars synthesised by plants are used as respiratory substances and the rest are used to make biological molecules which form the biomass of the plant
2 stages of phosphorus cycle
Uptake of phosphate ions
Breakdown of phosphate ions
The role of mycorrhizae in facilitating the uptake of water and inorganic ions by plants
Mycorrhizae are fungi that form symbiotic relationships with the roots of plants and help them absorb inorganic ions and water from the soil by associating with the roots using hyphae.
Uptake of phosphate ions
Phosphate ions are assimilated by plants because of the symbiotic relationship between the plant roots and mycorrhizae.
Mycorrhizae help to increase the rate of phosphate ion uptake.
Breakdown of phosphate ions
As phosphate ions are transferred through the food chain, they are lost as waste products or when an organism dies.
Saprobionts decompose waste and dead organisms through extracellular digestion.
Phosphate ions are released into the soil and can be recycled for reuse in the cycle.
Agricultural impacts on soil
Agriculture leads to depleted levels of nutrients in the soil (e.g. nitrogen and phosphates).
This is because when crops are removed for harvest, they are not decomposed and the nutrients are not returned to the soil for reuse.
Removing livestock for slaughter also decreases nutrient levels because they are not replaced by decomposition.
Fertilisers
Fertilisers can be used to replace the nutrients (e.g nitrogen and phosphate) lost through agriculture.
Increasing nutrient levels by fertilisers allows farming to continue without negative impacts on the crop or livestock quality.
Atrificial fertilisers
Artificial fertilisers:
Artificial fertilisers (e.g. ammonium nitrate) are inorganic.
Artificial fertilisers are produced specifically for replacing nutrients.
Natural fertilisers
Natural fertilisers:
Natural fertilisers (e.g. composted food waste, manure) are organic.
Natural fertilisers are taken from organic matter and used to replace nutrients.
