nutrient cycles

Question 1

phosphorus cycle

Answer 1

released into soil + water - weathering as phosphate ions
- phosphate compounds from sedimentary rocks leach

phosphate ions taken up by plants - roots or absorbed by water by algae - transferred to consumers by feeding

phosphate ions - waste products and dead organisms - release into soil or water
- saprobiont decomposition

phospholipid+3H2O breaks ester bond - glycerol, fatty acid and a phosphate

Question 2

nitrogen cycle

Answer 2

nitrogen fixation
- atmospheric nitrogen gas converted nitrogen-containing compounds
- nitrogen-fixing bacteria (nitrogen to ammonia)
forming ammonium ions -> plants

ammonification
- proteins and nucleic acids in dead organic matter converted to ammonium ions by saprobionts

nitrification
- ammonium ions converted (oxidised) into nitrite ions by nitrosomonas
- nitrite ions converted to nitrate ions by nitrobacter
- bacteria are chemo-autotrophs
- bacteria outcompete with plants for ammonium ions - plants only have access to nitrate ions

• faster in aerobic conditions

denitrification
- nitrate ions reduced to nitrogen gas by denitrifying bacteria - faster in anaerobic

Question 3

name the general stages in the phosphorus cycle

Answer 3

weathering

runoff

assimilation

decomposition

uplift

Question 4

why is the phosphorus cycle a slow process

Answer 4

phosphorus has no gas phase, so there is no atmospheric cycle

most phosphorus is stored as PO4 3- in rocks

Question 5

explain the significance of phosphorus to living organisms

Answer 5

plants convert inorganic phosphate into biological molecules eg DNA, ATP, NADP

phosphorus is passed to consumers via feeding

Question 6

what happens during uplift

Answer 6

sedimentary layers from oceans (formed by the bodies of aquatic organisms) are brought up to land over many years

Question 7

how does mining affect the phosphorus cycle

Answer 7

speeds up uplift

Question 8

name the 4 main stages of the nitrogen cycle

Answer 8

nitrogen fixing

ammonification

nitrification

denitrification

Question 9

why cant organisms use nitrogen directly from the atmosphere

Answer 9

N2 is very stable due to strong covalent triple bond

Question 10

what happens during atmospheric fixation of nitrogen

Answer 10

high energy of lightning breaks N2 into N

N reacts with oxygen to form NO2 -

NO 2 - dissolves in water to form NO3 -

Question 11

outline the role of bacteria in nitrogen fixation

Answer 11

mutualistic nitrogen-fixing bacteria in nodules of legumes and free-living bacteria in soil

use the enzyme nitrogenase to reduce gaseous nitrogen into ammonia

Question 12

outline the role of bacteria in ammonification

Answer 12

saprobiots feed on and decompose organic waste containing nitrogen (eg proteins, urea)

NH3 released

NH3 dissolves in water in the soil to form NH4+

Question 13

outline the role of bacteria in nitrification

Answer 13

2-step process carried out by saprobionts in aerobic conditions

Question 14

outline the role of bacteria in denitrification

Answer 14

anaerobic denitrifying bacteria convert soil nitrates back into gaseous nitrogen

Question 15

explain the significance of nitrogen to living organisms

Answer 15

plant roots uptake nitrates via active transport and use them to make biological compounds

• amino acids
• NAD / NADP
• nucleic acids

Question 16

outline the role of mycorrhizae

Answer 16

mutualistic relationship between plant and fungus

increases surface area of root system = increases uptake of water and mineral ions

Question 17

give 3 benefits of planting a different crop on the same field each year

Answer 17

nitrogen-fixing crops eg legumes make soil more fertile by increasing soil nitrate content

different crops have different pathogens

different crops use different proportions of certain ions

Question 18

name the 2 categories of fertiliser and state the purpose of using fertiliser

Answer 18

organic - decaying organic matter and animal waste

inorganic - minerals from rocks, usually containing nitrogen, phosphorus, potassium

used to increase gross productivity for higher yield

Question 19

at a certain point, using more fertiliser no longer increases crop yield - why

Answer 19

a factor unrelated to the concentration of mineral ions limits the rate of photosynthesis, so rate of growth cannot increase any futher

Question 20

outline 2 main environmental issues caused by the use of fertilisers

Answer 20

leaching - nitrates dissolves in rainwater and runoff into water sources

eutrophication - water source becomes putrid as a result of algal bloom

Question 21

what happens during eutrophication

Answer 21

• Starts with leaching of N-rich and P-rich chemicals (eg. fertilisers and sewage) into water bodies such as ponds, lakes, rivers and seas. (ie, increased availability to make proteins and nucleic acids).

• Algae grow rapidly (increase in population size of algae). Increased competition for resources, and increased respiration.

• Therefore, concentration of oxygen in water decreases.

• Animals etc die, therefore more decomposition by bacteria and fungi / saprobionts leading to further decrease in oxygen concentration.

• Leading to more death and further depletion of oxygen…

• Eventually leads to everything in the pond dying.

Question 22

how can the risk of eutrophication be reduced

Answer 22

sewage treatment marshes on farms

pumping nutrient-enriched sediment out of water

using phosphate-free detergent

using less fertiliser in farming land so less is leached by rainfall ect

increasing aeration of water bodies eg cleaning ponds and lakes regularly - less eutrophication in running water than in stagnant water