5.4 Nutrient Cycles Flashcards
Explain the role of saprobionts in recycling chemical elements
● Decompose (break down) organic compounds eg. proteins / urea / DNA in dead matter / organic waste ● By secreting enzymes for extracellular digestion (saprobiotic nutrition)
● Absorb soluble needed nutrients and release minerals ions eg. phosphate ions
Explain the role of mycorrhizae
Mycorrhizae = symbiotic association between fungi and plant roots
● Fungi (hyphae) act as an extension of plant roots to increase surface area of root system ● To increase rate of uptake / absorption of water and inorganic ions
● In return, fungi receive organic compounds eg. carbohydrates
Give examples of biological molecules that contain nitrogen
Amino acids / proteins or enzymes / urea / DNA or RNA / chlorophyll / ATP or ADP / NAD or NADP
Describe the role of bacteria in nitrogen fixation
● Nitrogen gas (N2) converted into ammonia (NH3), which forms ammonium ions (NH4+) in soil
● By nitrogen-fixing bacteria (may be found in root nodules)
Describe the role of bacteria in ammonification
● Nitrogen-containing compounds eg. proteins
● Converted to ammonia, which forms ammonium ions in soil
● By saprobionts - secrete enzymes for extracellular digestion
Describe the role of bacteria in nitrification
● Ammonium ions in soil converted into nitrites then nitrates, via a two-step oxidation reaction ○ For uptake by plant root hair cells by active transport
● By nitrifying bacteria in aerobic conditions (oxygen)
Describe the role of bacteria in denitrification
● Nitrates in soil converted into nitrogen gas (reduction)
● By denitrifying bacteria in anaerobic conditions (no oxygen, eg. waterlogged soil)
Suggest why ploughing (aerating) soil increases its fertility
● More ammonium converted into nitrite and nitrate / more nitrification / more (active) nitrifying bacteria
● Less nitrate converted to nitrogen gas / less denitrification / fewer (active) denitrifying bacteria
Give examples of biological molecules that contain phosphorus
Phospholipids / DNA or RNA / ATP or ADP / NADP / TP or GP / RuBP
Describe the phosphorus cycle
- Phosphate ions in rocks released by erosion
- Phosphate ions taken up by producers and incorporated into their biomass ○ Rate of absorption increased by mycorrhizae
- Phosphate ions transferred through food chain eg. as herbivores eat producers
- Some phosphate ions lost from animals in waste products (excretion)
- Saprobionts decompose organic compounds eg. DNA in dead matter releasing phosphate ions
Explain why fertilisers are used
● To replace nitrates lost when plants are harvested and livestock are removed ○ Those removed from soil and incorporated into biomass can’t be released back into the soil through decomposition by saprobionts
● So improve efficiency of energy transfer → increase productivity / yield
Describe the difference between artificial and natural fertilisers
Natural :
● Organic, eg compost,
● Ions released during decomposition by saprobionts
Artificial:
- Contain inorganic compounds of nitrogen, phosphorus and potassium
Explain the key environmental issue arising from use of fertilisers
● nitrates dissolve in water, leading to leaching of nutrients into lakes
● This leads to eutrophication :
1. Rapid growth of algae in pond so light blocked
2. So submerged plants die as they cannot photosynthesise
3. So saprobionts decompose dead plant matter, using oxygen in aerobic respiration
4. So less oxygen for fish to aerobically respire, leading to their death
Explain the key advantage of using natural fertiliser over artificial fertiliser
● Less water soluble so less leaching → eutrophication less likely
● Organic molecules require breaking down by saprobionts → slow release of nitrate / phosphate etc.
