5.4 Nutrient cycles

What is the role of saprobionts in recycling chemical elements?

• They decompose organic compounds in dead matter and organic waste.

• They secrete enzymes for extracellular digestion (saprobiotic nutrition).

• They absorb necessary soluble nutrients and release mineral ions (e.g., phosphate ions).

What are mycorrhizae?

• A symbiotic association between fungi and plant roots.

What is the role of the fungi in mycorrhizae?

• The fungal hyphae act as an extension of plant roots, increasing the surface area of the root system.

• This increases the rate of uptake/absorption of water and inorganic ions.

What do the fungi receive in return from the plant?

• Organic compounds, such as carbohydrates.

Give examples of biological molecules that contain nitrogen.

• Amino acids / proteins (enzymes)

• Urea

• DNA or RNA

• Chlorophyll

• ATP or ADP

• NAD or NADP

What is the first key stage of the nitrogen cycle?

• Nitrogen fixation by nitrogen-fixing bacteria.

Where are nitrogen-fixing bacteria sometimes found?

• In root nodules of leguminous plants (e.g., peas, beans).

What happens during nitrogen fixation?

• Nitrogen gas (N₂) is converted into ammonia (NH₃), which forms ammonium ions (NH₄⁺) in the soil.

• By nitrogen fixing bacteria

What is the second key stage of the nitrogen cycle?

• Ammonification by saprobionts.

What is the role of bacteria in ammonification?

• Nitrogen-containing compounds (e.g., proteins, urea) from dead organisms/waste are broken down.

• They are converted into ammonia, which forms ammonium ions in the soil.

• This is done by saprobionts, which secrete enzymes for extracellular digestion.

What is the role of bacteria in nitrification?

• Ammonium ions in the soil are converted into nitrites, then nitrates, via a two-step oxidation reaction.

• This is done by nitrifying bacteria in aerobic conditions (with oxygen).

Why is nitrification important for plants?

• The nitrates produced can be taken up by plant root hair cells via active transport.

What is the role of bacteria in denitrification?

• Nitrates in the soil are converted into nitrogen gas via reduction.

• This is done by denitrifying bacteria in anaerobic conditions (e.g., waterlogged soil).

Why does ploughing (aerating) soil increase its fertility?

• It increases the conversion of ammonium into nitrite and nitrate (more nitrification).

• It provides more oxygen for nitrifying bacteria.

• It reduces the conversion of nitrate into nitrogen gas (less denitrification) by reducing anaerobic conditions.

Give examples of biological molecules that contain phosphorus.

• Phospholipids

• DNA or RNA

• ATP or ADP

• NADP

• Triose phosphate (TP) or Glycerate phosphate (GP)

• Ribulose bisphosphate (RuBP)

What is step 1 of the phosphorus cycle?

• Phosphate ions in rocks are released into soils or oceans by erosion or weathering.

What is step 2 of the phosphorus cycle?

• Phosphate ions are taken up by producers (plants/algae) and incorporated into their biomass.

• The rate of uptake is increased by mycorrhizae.

What is step 3 of the phosphorus cycle?

• Phosphate ions are transferred through the food chain when herbivores eat producers.

What is step 4 of the phosphorus cycle?

• Some phosphate ions are lost from animals in waste products (excretion).

What is step 5 of the phosphorus cycle?

• Saprobionts secrete extracellular enzymes to decompose organic compounds (e.g., DNA) in dead matter and organic waste, releasing phosphate ions.

Why are fertilisers used in agriculture?

• To replace nitrates and phosphates lost when plants are harvested and livestock are removed.

• These ions, now incorporated into biomass, cannot be released back into the soil through decomposition.

• This improves the efficiency of energy transfer, increasing productivity and yield.

What is the main difference between natural and artificial fertilisers?

• Natural fertilisers are organic (e.g., manure, compost, sewage). Their ions are released during decomposition by saprobionts.

• Artificial fertilisers contain inorganic compounds of nitrogen, phosphorus, and potassium.

What is the key environmental issue caused by fertiliser use?

• Phosphates and nitrates dissolve in water and leach into lakes, rivers, and oceans.

• This leads to eutrophication.

What is step 1 of the eutrophication process?

• Rapid growth of algae in a pond or river (an algal bloom).

What is step 2 of the eutrophication process?

• The algal bloom blocks light from reaching submerged plants.

What is step 3 of the eutrophication process?

• Submerged plants die because they cannot photosynthesise.

What is step 4 of the eutrophication process?

• Saprobionts decompose the dead plant matter, using oxygen in aerobic respiration.

What is step 5 of the eutrophication process?

• Oxygen levels in the water decrease.

• Fish and other aquatic organisms die due to a lack of oxygen for aerobic respiration.

What is a key advantage of using natural fertiliser over artificial fertiliser?

• Natural fertilisers are less water-soluble, so less leaching occurs, making eutrophication less likely.

• The organic molecules require breaking down by saprobionts, leading to a slow release of nitrates and phosphates.