5.4 Nutrient Cycles

Question 1

What are the 4 biological processes that occur in the cycling of nitrogen?

Answer 1

• nitrogen fixation
• ammonification
• nitrification
• dentrification

Question 2

What is the nitrogen cycle?

Answer 2

• all life requires nitrogen containing compounds (proteins and nucleic acids)
• air which is 79% nitrogen gas, is the major reservoir of nitrogen
• but most organisms cannot use nitrogen in this form as nitrogen gas is INERT (unreactive)

Question 3

What is Nitrogen Fixation?

Answer 3

• the nitrogen molecule is inert. To break it apart so that its atoms can combine with other atoms requires the input of substantial amounts of energy
• Nitrogen fixation is the process of converting N2 into NH3/NH+4

Question 4

What are the 3 processes responsible for nitrogen fixation?

Answer 4

• biological fixation by certain microbes as nitrogen fixing bacteria
  >atmospheric fixation by lightning
  >industrial fixation - Haber process
• Free living bacteria
• Symbiotic (mutualistic) bacteria in root nodules of legumes

Question 5

What is nitrification?

Answer 5

• ammonia can be taken up directly by plants - usually through their roots
• However, most of the ammonia produced by decay is converted into nitrates
• This is accomplished in 2 steps:
  > Bacteria OXIDISE the NH3 to nitrites NO2-
  > Bacteria OXIDISE the nitrites to nitrates (NO3-)
• Both types of bacteria are known as nitrifying bacteria

Question 6

What is denitrification?

Answer 6

• reduces nitrates to nitrogen gas, thus replenishing the atmosphere
• thus it closes the nitrogen cycle

Question 7

What is bacteria?

Answer 7

• bacteria are the agents. They live deep in the soil and in aquatic sediments where conditions are anaerobic
• They use nitrates as an alternative to oxygen for respiration, known as denitrifying bacteria

Question 8

What is ammonification?

Answer 8

• the nitrogen containing compounds made by plants enter and pass through food chains
• At each tropic level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions
• The final beneficiaries of these materials are microorganisms of decay
• They break down the molecules in excretions and dead organisms into ammonia or ammonium ions

Question 9

What are 2 biological molecules that contain nitrogen?

Answer 9

amino acids, proteins, DNA, RNA

Question 10

What are 2 biological molecules that contain phosphorus?

Answer 10

Phospholipids, ATP,DNA, RNA

Question 11

What is the simple sequence of the nitrogen cycle?

Answer 11

• At any one time the nutrient will be in the atmosphere
• The nutrient will also be in living tissue, passed from producers to consumers
• It will be broken down by decomposers
• It can also be in solution (aquatic environment or water in the soil)

Question 12

What is the phosphorus cycle?

Answer 12

• phosphate ions in rocks (mainly PO4^3-) released by chemical weathering and washed into soils by rain
• these ions are then absorbed by root hair cells of plants and used in biological molecules such as ATP, DNA, RNA
• Consumers eat plants and animals containing phosphate rich compounds which are digested and absorbed in the small intestine as nucleotides or phosphate ions
• organism wastes or dead bodies are digested by saprobiotic bacteria, releasing phosphate ions back into the soil where they can be taken up by plants

Question 13

What is decomposition?

Answer 13

• Invertebrates such as worms, slugs and woodlice break down large waste material into smaller pieces by feeding on it (detritivores)
• True decomposers are known as saprobionts, these are mainly fungi and bacteria
• Extracellular enzymes are released onto waste material and the products of hydrolysis are absorbed

Question 14

What is Mycorrhizae and ion uptake?

Answer 14

• as well as being responsible for breaking down dead material micro-organisms also have a role in assisting the uptake of inorganic ions and water
• The root system of most plants have fungi called mycorrhizae this is an example of mutualism
• The fungus consists of thread called hyphae these hyphae increase the surface area of the roots and therefore allow more effective uptake of ions and water

Question 15

What are the types of fertilisers?

Answer 15

• Natural (organic) fertilisers
• Artificial (inorganic) fertilisers

Question 16

What are examples of natural fertilisers?

Answer 16

dead and decaying plants, animal waste (manure)

Question 17

What are examples of artificial fertilisers?

Answer 17

Mined from rocks and blended to give the right mix of nutrients. NPK - nitrogen, phosphorus, potassium

Question 18

What is leaching?

Answer 18

• fertilisers containing nitrogen, phosphorus and potassium, found in the soil are washed away into nearby water supplies

Question 19

What is eutrophication?

Answer 19

death of aquatic life as a result of leaching

Question 20

What are the positives of organic fertilisers?

Answer 20

• add structure to the soil in the form of humus
• Nutrients not readily leached from the soil

Question 21

What are the negatives of organic fertilisers?

Answer 21

• offensive smells
• may be difficult to spread
• mineral release slow

Question 22

What are the positives of inorganic fertilisers?

Answer 22

• clean chemicals that lack the odour of organic manure
• nutrients released rapidly into the soil

Question 23

What are the negatives of inorganic fertiisers?

Answer 23

• readily leached from the soil
• increased risk of eutrophication

Question 24

What is the process of eutrophication?

Answer 24

The algae eats the nitrate causing an algal bloom. The algae covers the rivers, lake and ponds so sunlight cannot pass through. Plants cannot photosynthesise so die. Microorganisms decompose them but use up all the oxygen making the water anoxic. Therefore the aquatic life can’t respire so die

Question 25

What are the general stages of the phosphorus cycle?

Answer 25

• weathering
• runoff
• assimilation
• decomposition
• uplift

Question 26

Why is the phosphorus cycle a slow process?

Answer 26

• phosphorus has no gas phase, so there is no atmospheric cycle
• most phosphorus is stored as PO4^3- in rocks

Question 27

What happens during weathering and runoff?

Answer 27

phosphate compounds from sedimentary rocks leach into surface water and soil

Question 28

Explain the significance of phosphorus to living organisms?

Answer 28

• plants convert inorganic phosphate into biological molecules
• phosphorus is passed to consumers via feeding

Question 29

What happens during uplift?

Answer 29

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

Question 30

How does mining affect the phosphorus cycle?

Answer 30

speeds up uplift

Question 31

Outline the role of bacteria in nitrogen fixation

Answer 31

• 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 32

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

Answer 32

• nitrogen fixing crops e.g. legumes make soil more fertile by increasing soil nitrate content
• different crops have different pathogens
• different crops use different proportions of certain ions

Question 33

At a certain point, using more fertiliser no longer increases crop yield. Why?

Answer 33

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