5.4 Nutrient Cycles

Question 1

What biological molecules contain nitrogen?

Answer 1

DNA
RNA
ATP
amino acids/protein

Question 2

The 4 stages of the nitrogen cycle are:

Answer 2

1. ammonification
2. nitrification
3. denitrification
4. nitrogen fixation

Question 3

Ammonification definition

Answer 3

The production of ammonia from organic nitrogen-containing compounds (e.g. urea, proteins, nucleic acids)

Question 4

Ammonification in the nitrogen cycle

Answer 4

Saprobionts feed on faeces and dead organic material, releasing ammonia into the soil

Question 5

Nitrification

Answer 5

When nitrifying bacteria convert/oxidise ammonium ions into nitrate ions

Question 6

Features of nitrification

Answer 6

1. REQUIRES OXYGEN!
2. Two-stage reaction (ammonium ions to NO2- and then NO3-)

Question 7

Denitrification

Answer 7

When anaerobic denitrifying bacteria convert soil nitrates into gaseous nitrogen.

Question 8

Why is denitrification bad?

Answer 8

It reduces the availability of nitrogen-containing compounds (nitrate ions) in the soil for plants

Question 9

When does denitrification happen?

Answer 9

When soils become waterlogged and have a low oxygen concentration

Question 10

Nitrogen fixation

Answer 10

When nitrogen-fixing bacteria convert nitrogen gas into nitrogen-containing compounds

Question 11

What are the two types of nitrogen-fixing bacteria?

Answer 11

1. Free-living nitrogen-fixing bacteria
2. Mutualistic nitrogen-fixing bacteria

Question 12

What do free-living nitrogen-fixing bacteria do?

Answer 12

They reduce gaseous nitrogen to ammonia, which they then use to manufacture amino acids

Question 13

What do mutualistic nitrogen-fixing bacteria do?

Answer 13

They live in nodules on the roots of leguminous plants (peas & beans). They obtain carbohydrates from the plant, and the plant acquires amino acids from the bacteria

Question 14

In what form is phosphorous found in living things?

Answer 14

ATP
DNA
RNA
Phospholipids

Question 15

In what form is phosphorous found in the abiotic environment?

Answer 15

Phosphate ions in sedimentary rocks & dissolved in water

Question 16

Where is usually a good starting point for the phosphorous cycle, and what is the first step?

Answer 16

Phosphate ions in the sea are brought to the surface by the geological uplifting of rocks

Question 17

Phosphorous cycle step 2

Answer 17

Weathering and erosion of sedimentary rock dissolves phosphate ions, making them available for absorption by plants.
The phosphorous is incorporated into ATP, phospholipids, nucleic acids etc

Question 18

Phosphorous cycle step 3

Answer 18

Phosphate ions pass onto animals, which feed on the plants

Question 19

Phosphorous cycle step 4

Answer 19

When plants and animals die, decomposers (saprobiotic bacteria & fungi) break them down, releasing phosphate ions into the water/soil

Question 20

Phosphorous cycle step 5

Answer 20

Some phosphate ions remain in parts of animals, such as bones or shells, that are very slow to break down

Question 21

Phosphorous cycle step 6

Answer 21

Phosphate ions in excreta, released by decomposition and dissolved out of rocks, are transported by streams and rivers into lakes and oceans where they form sedimentary rock

Question 22

What are mycorrhizae?

Answer 22

Associations/a mutualistic relationship between plant roots and fungi

Question 23

How does mycorrhizae work?

Answer 23

Fungi act like extensions of the plant’s root system & increase total SA for uptake of water and inorganic ions
Mycorrhiza acts like a sponge: holds water & inorganic ions in the neighbourhood of the roots

Question 24

How do plants benefit from mycorrhizae?

Answer 24

Improved water & inorganic ion uptake.
Plants can resist drought and take up inorganic ions more readily: important for scarce ions eg. phosphate ions

Question 25

How do fungi benefit from mycorrhizae?

Answer 25

Receive organic compounds eg. sugars & amino acids from the plant

Question 26

What do fertilisers do (what is their main job?)

Answer 26

To offset the loss of mineral ions in the soil

Question 27

Natural fertilisers

Answer 27

aka. organic fertilisers
Consist of dead & decaying remains of plants & animals as well as animal wastes eg. manure

Question 28

Artificial fertilisers

Answer 28

aka. inorganic fertilisers
Mined from rocks & deposits and then converted into different forms and blended together to give the appropriate balance of minerals for a particular crop

Question 29

How do fertilisers increase productivity?

Answer 29

More nitrates: plants likely to develop earlier, grow taller & have greater leaf area. This increases rate of photosynthesis and improves crop productivity.
More phosphates: more phospholipids

Question 30

3 problems with nitrogen-containing fertilisers

Answer 30

1. Reduced species diversity
2. Leaching
3. Eutrophication

Question 31

How do nitrogen-containing fertilisers cause reduced species diversity?

Answer 31

Nitrogen-rich soils favour growth of grasses, nettles & other rapidly growing species.
These out-compete many other species, which die as a result.

Question 32

What is leaching?

Answer 32

The process by which nutrients are removed from the soil

Question 33

Why is leaching a problem?

Answer 33

Leached nitrate ions end up in watercourses (e.g. streams and rivers), that in turn may drain freshwater lakes.
They will be harmful to humans if the river/lake is a source of drinking water.

Question 34

Eutrophication steps

Answer 34

1. Nitrate ion concentration of water grows due to leaching: algae population increases
2. Algae mostly grow at the surface: this causes an algal bloom
3. Surface algae absorb light & prevent it from reaching below surface
4. Light becomes limiting factor for growth of plants and lower depths: die
5. Population of saprobiotic bacteria grow: increased demand for oxygen for aerobic respiration
6. Oxygen conc in water reduced, aerobic organisms eg fish die
7. Population of anaerobic organisms rise: further decompose dead material, releasing more nitrates & toxic wastes