Nutrient cycles

Question 1

How does energy enter the ecosystem?

Answer 1

• the energy enters an ecosystem as sunlight and is lost as heat
• this heat cannot be recycled
• the flow of energy through an ecosystem is therefore in one direction, (linear)
• provided the sun continues to supply energy to the earth, this is not a problem

Question 2

What is the sequence of the nutrient cycle?

Answer 2

• 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 the producers and consumers die their complex molecules are broken down by saprobiontic microorganisms that release the nutrient in its original simple form, the cycle is then complete

Question 3

What is the nitrogen cycle?

Answer 3

• 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, there are very few organisms that can use nitrogen gas directly
• plants take up most of the nitrogen they require in the form of nitrate ions from the soil
• these ions are absorbed using active transport by the roots
• this is where nitrogen enters the living component of the ecosystem
• animals obtain nitrogen-containing compounds by eating and digesting plants
• nitrate ions are very soluble and easily leach (wash) 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 ecosystems, the concentration of soil nitrate can be further increased by the addition of fertilisers
• when plants and animals die, the process of decomposition begins, in a series of steps by which microorganisms replenish the nitrate ions by decomposition is most important because, in natural ecosystems, there are very few nitrate ions available from other sources

Question 4

What are the main stages of the nitrogen cycle?

Answer 4

• ammonification
• notification
• nitrogen fixation
• gentrification
• each stage involves saprobiontic microorganisms

Question 5

What is ammonification?

Answer 5

• the production of ammonia from organic nitrogen-containing compounds like urea from the breakdown of excess amino acids and proteins
• saprobiontic microorganisms release ammonia, from organic matter which forms ammonium ions in soil

Question 6

What is nitrification?

Answer 6

• plants use light energy to produce organic compounds
• some bacteria, however, obtain their energy from chemical reactions involving inorganic ions
• like the oxidation of ammonium ions to nitrate ions
• which is carried out by nitrifying bacteria which require oxygen
• farmers should keep soil structure light and aerated by ploughing
• good drainage prevents the air spaced from being blocked with water
• this increases productivity

Question 7

Which stages does conversion occur in during nitrification?

Answer 7

• oxidation of ammonium ions to nitrate ions

- oxidation of nitrate ions to nitrate ions

Question 8

What is nitrogen fixation?

Answer 8

• nitrogen gas is converted into nitrogen-containing compounds
• it can be carried out industrially or naturally when lightning passes through the atmosphere
• the most important form of nitrogen fixation is carried out by microorganisms
• free-living nitrogen-fixing bacteria
• mutuality nitrogen-fixing bacteria

Question 9

What is free-living nitrogen-fixing bacteria?

Answer 9

• these bacteria reduce gaseous nitrogen to ammonia, which they then use o manufacture amino acids
• nitrogen-rich compounds are released from them when they die and decay

Question 10

What is mutualistic nitrogen-fixing bacteria?

Answer 10

• these bacteria live in nodules on the roots of plants such as peas and beans
• they obtain carbohydrates from the plant and the plant acquired amino acids from the bacteria

Question 11

What is gentrification?

Answer 11

• when soils become waterlogged and have a low oxygen concentration there are fewer aerobic nitrifying and nitrogen-fixing bacteria are found and more anaerobic denitrifying bacteria
• these convert soil nitrates into gaseous nitrogen which reduces the availability of nitrogen for plants
• so the soils on which crops grow must therefore be kept well aerated to prevent this

Question 12

What is the phosphorus cycle?

Answer 12

• phosphorus is an important biological element as it is a component of ATP, phospholipids and nucleic acids
• so life depends on it being constantly recycled
• in the phosphorus cycle the main reservoir is in mineral form as phosphate ions in rock deposits
• they originate in the sea 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
• the phosphate ions pass into animals which feed on the plants
• on the death of plants and animals, decomposes such as certain bacteria and fungi break them down releasing phosphate ions into the water or soil
• some phosphate ions remain in parts of animals, such as bones or shells, that are very slow to breakdown
• these are then transported by streams and rivers into lakes and oceans where they form sedimentary rocks thus completing the cycle

Question 13

What is the role of mycorrhizae in nutrient cycles?

Answer 13

• mycorrhizae act like extensions of the plant’s root system and vastly increase the total surface area like a sponge and so holds water and minerals in the neighborhood of the roots
• this enables the plant to better resist drought and to take up inorganic ions like phosphate ions more readily
• the relationship is mutualistic as plant benefits from improved water and ion uptake while the fungus receives organic compounds such as sugars and amino acids from the plant

Question 14

How do nutrients enter the ecosystem?

Answer 14

• nutrients, by contrast, do not have an extraterrestrial source
• there is limited availability of nutrient ions in a usable form
• it is important therefore that elements such as carbon, nitrogen and phosphorus are recycles
• the flow of nutrients within an ecosystem is not linear, but mostly cyclic