Nutrient Cycle

Question 1

How do saprobionts digest material externally?

Answer 1

• secrete extra cellular enzymes onto their food

Question 2

Why do living organisms need nitrogen?

Answer 2

• to be able to make amino acids and proteins needed for growth and repair (most plants only take up nitrogen in the form of ammonia or nitrate as it is unreactive)

Question 3

Nitrogen fixing bacteria

Answer 3

• fix nitrogen gas from the air into ammonia which dissolves in water to form ammonium ions which can be oxidised to form nitrate ions

Question 4

Saprobionts

Answer 4

• decomposes protein/DNA in dead remains and waste products, releasing ammonium ions

Question 5

Nitrifying bacteria

Answer 5

• convert ammonium ions to nitrites, then nitrates

Question 6

Denitrifiying bacteria

Answer 6

• convert nitrates back into nitrogen gas

Question 7

Where do nitrogen fixing bacteria live?

Answer 7

• in soil or in the root nodules of leguminous plants

Question 8

What does a plant gain from having the nitrogen fixing bacteria and what would the bacteria get from the plant?

Answer 8

• ammonium ions that form after bacteria fix nitrogen can be used by the plant to make amino acids and in return, the plant provides bacteria with a constant source of organic compounds like glucose for respiration

Question 9

What does lightning provide?

Answer 9

• energy for nitrogen to react in the air to form oxides which are carried (fixed) into soils by rainwater forming nitric acid which gets converted to nitrates

Question 10

What happens when soil becomes waterlogged or too acidic?

Answer 10

• oxygen levels drop causing denitrifying bacteria to thrive as they work in anaerobic conditions and nitrates get converted to nitrogen gas which plants can’t use so the cycle is interrupted and soil fertility is reduced

Question 11

What does digging drainage ditches and ploughing do?

Answer 11

• both add oxygen to the soil which reduces denitrification rates and encourages more nitrate to form in the soil by nitrifying bacteria

Question 12

Why is nitrate easier to absorb than ammonium ions?

Answer 12

• nitrate ions are more water soluble

Question 13

Describe how the action of microorganisms in the soil produces a source of nitrates for crop plants?

Answer 13

• protein is formed into ammonium compounds by saprobionts
• ammonium into nitrite and into nitrate by nitrifying bacteria
• nitrogen into ammonium by nitrogen fixing bacteria in the soil

Question 14

Why does applying high concentrations of fertiliser to the soil reduce plant growth (refer to water potential)?

Answer 14

• fertiliser salts dissolve and lowers water potential of the soil so roots have higher water potential
• water moves by osmosis out of the roots into the soil so the plant does not have enough water

Question 15

What are mycorrhizal fungi?

Answer 15

• fungi that has mutualistic association with roots forming an extension designed to increase surface area for absorption of water by osmosis
• acts like a sponge to hold water to avoid drought and makes it easier to take up ions like phosphates

Question 16

State the differences between the phosphorus and nitrogen cycle

Answer 16

• phosphorus cycle has no atmospheric involvement whereas nitrogen cycle has nitrogen gas in the atmosphere
• phosphate in phosphorus cycle has low solubility in water whereas nitrate in nitrogen cycle is highly soluble in water
• phosphorus cycle is slower due to the above two points whereas nitrogen cycle is faster due to the above points
• in phosphorus cycle phosphorus is very reactive and never exists naturally in its elemental form whereas in nitrogen cycle nitrogen gas is unreactive and exits naturally in its elemental form

Question 17

First step of eutrophication

Answer 17

• extra nutrient added from leaching causes algae and other surface plants to grow very rapidly (algal bloom)

Question 18

Second step of eutrophication

Answer 18

• this blocks out light making it limiting factor and reduces photosynthesis so rooted plants die providing more nutrients

Question 19

Third step of eutrophication

Answer 19

• saprobiontic bacteria multiply as they feed on dead algae

Question 20

Forth step of eutrophication

Answer 20

• as bacteria multiply they respire aerobically a lot more so take up oxygen decreasing oxygen content and a biological oxygen demand

Question 21

Fifth step of eutrophication

Answer 21

• less oxygen for fish to respire so migrate or die and body produces more nutrient for bacteria so process continues

Question 22

What is leaching?

Answer 22

• some fertiliser washed from the soil by rain into the nearest pond, lake or river

Question 23

Purpose of fertilisers

Answer 23

• increase efficiency of energy transfer along food chains, improving productivity so crops grow better and are cheaper

Question 24

Why do we need fertilisers?

Answer 24

• replenishes minerals removed from soil when crops are harvested or animals eat them to prevent them becoming a limiting factor of plant growth and to help sustain ecosystem

Question 25

What do natural fertilisers consist of?

Answer 25

• dead and decaying remains of plants and animals
• animal waste (manure, slurry, bone meal and urea)

Question 26

Why may artificially made fertilisers not be used?

Answer 26

• expensive
• contaminate soil, water, air
• lead to pH imbalance of soil
• increases pests and affect health
• contributes to climate change

Question 27

Why would overuse of nitrogen fertiliser lead to reduced species diversity?

Answer 27

• nitrogen rich soils favour rapidly growing species like grasses and nettles which outcompete others which would die reducing species diversity, reducing number of habitats and number of different food sources for other organisms

Question 28

Causes of eutrophication

Answer 28

• leaching of nitrate and phosphates from fertiliser
• raw sewage
• manure washed out of farmlands

Question 29

What is the overall effect of eutrophication?

Answer 29

• decreased oxygen levels in water leading to death of aquatic organisms and increase in pathogen bacteria

Question 30

Suggest a practical for measuring algal growth

Answer 30

• using colorimeter
• measure the absorbency of light
• zero the colorimeter

Question 31

Why use logarithmic scale?

Answer 31

• values have large range

Question 32

Evaluate why farmers should not use fertilisers to increase yields when growing plants

Answer 32

• Should not use: prevents development of mycorrhizae, mycorrhizae help plants defend themselves, mycorrhizae help plants take up nitrates and phosphates (increasing crop yield)
• should use: increases gross primary production so increases yield, most soil is poor in phosphate so without fertiliser, plant might not get enough phosphate