Nutrient cycles Flashcards
How does energy in an ecosystem flow?
Energy enters the ecosystem as sunlight and is lost as heat, which cannot be recycled.
This is in a linear direction.
Nutrients however, do not have an extra-terrestrial source, so there is a limited availability of nutrient ions in a usable form.
It is important that elements carbon, nitrogen and phosphorus are recycled, so the flow is cyclic.
What is the sequence of nutrient cycles?
The nutrient is taken up by producers as simple, inorganic molecules.
The producer incorporates the nutrient into complex organic molecules.
When the producer is eaten, the nutrient passes into consumers.
It then passes along the food chain when these animals are eaten by other consumers.
When they die, their complex molecules are broken down by saprobiontic microorganisms that release the nutrient in its original simple form.
What is the role of saprobionts?
They ensure that nutrients are released for reuse.
Without them, nutrients would remain locked up as part of complex molecules that cannot be taken up and used again by plants.
What is nitrogen?
Living organisms require a source of nitrogen from which to manufacture proteins, nucleic acids and other nitrogen-containing compounds.
Although 78% of the atmosphere is nitrogen, few organisms can use the gas directly.
How does nitrogen enter the ecosystem?
Plants take up most of the nitrogen in nitrogen ions (NO^-3), from the soil.
These are absorbed using active transport, by the roots.
Animals obtain nitrogen compounds by eating and digesting plants.
How is nitrogen replenished?
Nitrate ions are very soluble and easily leach through the soil, beyond the reach of plant roots.
In natural ecosystems, the nitrate concentrations are restored largely by the recycling of nitrogen-containing compounds.
In agricultural systems, the concentration can be further increased by the adding fertilisers.
How is nitrogen replenished through death?
When plants and animals die, the process of decomposition begins, by which microorganisms replenish the nitrate concentrations in the soil.
This is most important because, in natural ecosystems, there are very few nitrate ions available from other sources.
What is ammonification?
The production of ammonia from organic nitrogen-containing compounds.
In nature, these compounds include urea (breakdown of amino acids) and proteins, nucleic acids and vitamins (in dead organisms and faeces).
Saprobiontic microorganisms - fungi and bacteria, feed on faeces and dead organisms, releasing ammonia, which then forms ammonium ions in the soil.
How nitrogen returns to the non-living component of the ecosystem.
What is nitrification?
Some bacteria obtain their energy from chemical reaction involving inorganic ions.
One reaction is the conversion of ammonium ions to nitrate ions.
This is an oxidation reaction and so releases energy.
What is nitrifying bacteria?
These free-living soil microorganisms carry out nitrification.
This conversion occurs by:
Oxidation of ammonium ions to nitrite ions (NO2^-)
Oxidation of nitrite ions to nitrate ions (NO3^-)
How can nitrifying bacteria work better?
They require oxygen to carry out these conversions and so they require a soil that has many air spaces.
To raise productivity, it is important for farmers to keep soil structure light and well aerated by ploughing.
Good drainage also prevents the air spaces from being filled with water and so prevents air being force out of the soil.
What is nitrogen fixation?
The process by which nitrogen gas is converted into nitrogen-containing compounds.
It can be carried out industrially and also occurs naturally when lightning passes through the atmosphere.
The most important form is carried out by microorganisms.
What are the main two types of bacteria in nitrogen fixation?
Free living nitrogen fixing bacteria - these bacteria reduce gaseous nitrogen to ammonia, which they then use to manufacture amino acids. Nitrogen-rich compounds are released from them when they die and decay.
Mutualistic nitrogen-fixing bacteria - they live in nodules on the roots of plants such as peas and beans. They obtain carbohydrates from the plant and the plant acquires amino acids from the bacteria.
What is denitrification?
When soils become waterlogged, and have a low oxygen concentration, the type of microorganisms present changes.
Fewer aerobic nitrifying and nitrogen-fixing bacteria are found, and there is an increase in anaerobic denitrifying bacteria.
These convert soil nitrates into gaseous nitrogen, which reduces the availability of nitrogen-containing compounds for plants.
For land to be productive, the soils on which crops grown must therefore be kept well aerated to prevent the build-up of denitrifying bacteria.
What is phosphorus?
An important biological element as it is a component of ATP, phospholipids and nucleic acids.
Life therefore depends on it being constantly recycled.
How does the phosphorus cycle differ from other cycles?
In the carbon and nitrogen cycles the main reservoir of each element is in the atmosphere.
In the phosphorus cycle however, the main reservoir is in mineral form - it lacks a gaseous phase all together.
How does phosphorus exist?
Phosphorus exists mainly as phosphate ions (PO4^3-) in the form of sedimentary rock deposits.
These have their origins in the seas but are brought to the surface by the geological uplifting of rocks.
The weathering and erosion of these rocks helps phosphate ions to become dissolved and so available for absorption by plants which incorporate them into their biomass.
These phosphate ions pass into animals which feed on the plants.
How does phosphorus return to the oceans, lakes and soils?
Excess phosphate ions are excreted by animals and may accumulate in waste material such as guano formed from the excretory products of some sea birds.
On the death of plants and animals, decomposers such as certain bacteria and fungi break them down into the water or soil.
Some phosphate ions remain in parts of animals, such as bones or shells, that are very slow to break down.
Phosphate ions in excreta, released by decomposition and dissolved out of rocks, are transported by streams and rivers into lakes and oceans where they form sedimentary rocks thus completing the cycle.
What is a summary of the phosphate cycle?
sedimentation ——————> phosphate in rocks
I I
Phosphate ions in plants <–absorption—- Dissolved ions in oceans, lakes and soils. <—— erosion and use of fertilisers
I feeding and digestion I excretion
Phosphate ions in animals ————————————excretion and decomposition——-> ions in wastes and remains
