3.5.4 | NUTRIENT CYCLES

Question 1

give examples of biological molecules that contain nitrogen

Answer 1

amino acids/proteins
enzymes
urea
DNA/RNA
ATP/ADP
NAD/NADP

Question 2

describe the role of bacteria in nitrogen fixing

Answer 2

nitrogen gas (N₂) converted into ammonia (NH₃) in soil by nitrogen-fixing bacteria

Question 3

describe the role of bacteria in ammonification

Answer 3

nitrogen-containing compounds from dead organisms/waste are broken down/decomposed
converted to ammonia (NH₃) which forms ammonium ions (NH₄⁺) in the soil
by saprobionts

Question 4

describe the role of bacteria in nitrification

Answer 4

ammonium ions in the soil converted to nitrites (NO₂⁻) then nitrates (NO₃⁻) for uptake by plants via active transport
by nitrifying bacteria in aerobic conditions

Question 5

describe the role of bacteria in denitrification

Answer 5

nitrates (NO₃⁻) in the soil converted to nitrogen gas (N₂) by denitrifying bacteria in anaerobic conditions

Question 6

suggest why ploughing/aerating soil increases fertility

Answer 6

more ammonium converted into nitrite and nitrate/more nitrification
less nitrate converted to nitrogen gas/less denitrification

Question 7

describe the role of saprobionts in recycling chemical elements

Answer 7

decompose organic compounds by secreting enzymes for extracellular digestion
absorb soluble needed nutrients and release mineral ions

Question 8

explain the role of mycorrhizae

Answer 8

fungi act as an extension of plant roots to increase surface area of root system to increase root uptake/absorption of water and inorganic ions
in return fungi receive carbohydrates

Question 9

give examples of biological molecules that contain phosphorous

Answer 9

phospholipids
DNA/RNA
ATP/ADP
NADP
TP/GP
RuBP

Question 10

describe the phosphorous cycle……………………..

Answer 10

phosphate ions in rocks released by erosion/weathering
phosphate ions taken up by producers/plants/algae and incorporated into their biomass
- rate of absorption increased by mycorrhizae
phosphate ions transferred through food chain

Question 11

explain the key enviromental issue arising from use of fertilisers

Answer 11

phosphates/nitrates dissolve in water, leading to leaching of nutrients into lakes/rivers/oceans this leads to eutrophication
- rapid growth of algae in pond/river (algal bloom) so light blocked
- submerged plants die as they cannot photosynthesise
- saprobionts decompose dead plant matter, using oxygen in aerobic respiration
- less oxygen for fish to aerobically respire leading to their death

Question 12

describe the difference between natural and artificial fertilisers

Question 13

explain the key advantage of using natural fertiliser over artifical fertiliser

Answer 13

less water soluble so less leaching so eutrophication is less likely
organic molecules require breaking down by saprobionts so slow release of nitrates/phosphates etc.

Question 14

why can ammonia and nitrate not be referred to as biological molecules

Answer 14

they do not contain carbon