13.3 Nutrient cycles

Question 1

What is the common sequence for all nutrient cycles

Answer 1

. Nutrient is taken up by producers (eg plants) as simple inorganic molecules

. The producer incorporates the nutrient into complex organic molecule

. When the producer is eaten this nutrient passes into consumers

. Then it passes along food chain and these animals are eaten by other consumers

. Sabrobiotic microorganisms break down the nutrients when they die and release it into its original simple form

. So cycle is complete

Question 2

Why do organisms need nitrogen

Answer 2

To manufacture nitrogen containing compounds in our bodies such as DNA, RNA, ATP, FAD, NAD etc

Question 3

How do plants take up nitrogen

Answer 3

Nitrogen makes up 78% of air but can’t be used directly by plants

In the form of nitrate ions (NO3-) from the soil by active transport to the roots

This is where nitrogen enters the living component in ecosystem

Question 4

How are nitrate ion concentrations restored in natural ecosystems

Answer 4

Nitrate ions are very soluble and can easily welch through the soil so they pass by plant roots

So nitrogen containing compounds are recycled to maintain the concentration

Question 5

How is nitrate ion concentration maintained in agricultural ecosystems

Answer 5

Add fertilisers to the soil to increase the concentration

Question 6

What are the 4 main stages of nitrogen cycle

Answer 6

. Ammonification
. Nitrification
. Nitrogen fixing
. Denitrification

All of these involve saprobiotic microorganisms

Question 7

Describe the nitrogen cycle

Answer 7

. Nitrogen in atmosphere
. Nitrogen fixing occurs with free bacteria living in soil which turn it into ammonium ions (NH4-)

.Through nitrification, these NH4- ions are turned into NO2- ions (nitrite) then are turned to NO3- ions (nitrate).
This process is OXIDATION so energy is released

. These are absorbed into ammonia containing molecules eg proteins in producers

. Also, Nitrogen fixing bacteria living on bean plants in symbiotic relationship make the nitrogen into ammonium containing molecules in producers

. Through feeding and digestion, these enter consumers making up their amine groups in amino acids etc

. Then when plants and animals die, saprobionts break them down into nitrogen containing compounds eg amino acids, nucleic acids etc

. Finally through ammonification, these compounds are made into ammonium ions again.

Simultaneously to this, there is denitrifying bacteria that return the nitrate ions to the atmosphere

Question 8

What do nitrifying bacteria require

What do farmers do to help this

Answer 8

They need oxygen to respire

Farmers can turn around the soil to make air pockets so oxygen can enter and be used by these bacteria.
It is helpful because more nitrifying bacteria means more nitrate ions for plants to grow

Question 9

Two types of nitrogen fixation

Answer 9

. Free living nitrogen fixing bacteria live in soil and reduce gaseous nitrogen to ammonia, which they then use to make into nitrate ions to be absorbed into plants to make amino acids
Nitrogen rich compounds are released from them when they die and decay

. Mutualistic bacteria live in a symbiotic relationship with legume plants . Eg they obtain carbohydrates from the plant, and the plant gains nitrate ions directly to make ammonium molecules.

Question 10

What is denitrification

Answer 10

When soil becomes waterlogged, and has a low concentration of oxygen, fewer aerobically nitrifying and nitrogen fixing bacteria are found.

There is an increase in anaerobic denitrifying bacteria.

These convert soil nitrates into gaseous nitrogen, which reduces the availability of nitrogen containing compounds for plants.

Question 11

What is phosphorus present in

Answer 11

Component of ATP, phospholipids, and nucleic acids

So it is constantly recycled

Question 12

Where is the main reservoir of phosphorus found

Answer 12

It is in mineral form rather than in the atmosphere:

Exists mainly as phosphate ions PO4 3- in the form of sedimentary rock deposits.
These originate from seas but are brought to the surface by geological uplifting of rocks

Question 13

How are phosphate ions released from sedimentary rocks

What are the next stages of the cycle

Answer 13

. Weathering and erosion help phosphate ions to be dissolved
. Also use of fertilisers

. Available for absorption by plants which incorporate them into their biomass
. These are then passed onto animals through feeding and digestion

Question 14

Once phosphate ions are in animals, where do they go

Answer 14

. They can be excreted so return as dissolved phosphate ions in lakes and soils

. Or they become parts of remains eg Guano, bones and shells

Through deposition these can form sedimentary rocks again

Question 15

What do Mycorrhizae do in nutrient cycles

Answer 15

. Certain types of fungi associate with the roots of plants
. Increase surface area for absorption of water and minerals by extending plant roots
.They act as a sponge to hold water and mineral ions near the roots

So plant can better resist drought, and take up inorganic ions more regularly

Question 16

What type of relationship is it between Mycorrhizae and plants

Answer 16

Mutualistic: The plant provides carbohydrates, amino acids etc for the fungi

Question 17

How does crop rotation help crop yields increase

Answer 17

. Different plants absorb different mineral ions and nutrients from the soil
. Different plants have different pathogens