Nutrient cycles

Question 1

What are the common features of all nutrient cycles?

Answer 1

• Nutrients are taken up by producers (plants) as simple inorganic molecules
• Producer incorporates nutrient into complex inorganic molecules
• When producers are eaten, nutrients passes into consumers
• Passes along the food chain + animals eaten by other consumers
• When producers + consumers die, their complex molecules are broken down by saprobionts which release nutrients
  (Then, the cycle repeats)

Question 2

Why do plants need nitrogen?

Answer 2

To manufacture proteins, nucleic acids + other nitrogen-containing compounds

Question 3

Nitrate ions are very soluble. What does this mean when it is in soil?

Answer 3

They easily leach (wash) through the soil, beyond the reach of plant roots

Question 4

How can concentration of soil nitrate be further increased?

Answer 4

By the addition of fertilisers

Question 5

What is assimilation?

Answer 5

When nitrates are taken up by the roots

Question 6

Name the four main stages of the nitrogen cycle in order

Answer 6

Ammonification, nitrification, denitrification + nitrogen fixation (nitrates made by nitrification can either be absorbed by roots or denitrification + nitrogen fixation can take place - produce ammonium ions)

Question 7

Describe the process of ammonification

Answer 7

Saprobionts break down dead organisms + release nitrogen contained as NH3 and are converted to NH4+ (ammonia to ammonium ions)

Question 8

Describe the process of nitrification

Answer 8

-It is carried out by nitrifying bacteria
- Ammonium ions are converted to nitrite ions
- Nitrite ions are then converted to nitrate ions
(Both of these are oxidation reactions)

Question 9

Describe the process of nitrogen fixation

Answer 9

• It is carried out by nitrogen fixing bacteria

- Nitrogen gas is converted into ammonium ions

Question 10

Describe the two ways in which nitrogen fixation can occur

Answer 10

1. Happens naturally during lightning
2. Carried out by bacteria
   - Mutualistic (nitrogen fixing) bacteria that live inside roots nodules of leguminous plants
   - Free living nitrogen fixing bacteria in the soil

Question 11

Why do the plants and the bacteria have a mutualistic relationship?

Answer 11

They both benefit one another:

The plant provides bacteria with carbohydrates and the bacteria provide plants with nitrates

Question 12

What do the free living nitrogen fixing bacteria do?

Answer 12

Nitrogen gas can be reduced to ammonia (which can then be used to make amino acids)

Question 13

Describe the process of denitrification

Answer 13

• Nitrates in soil are converted to nitrogen gas by denitrifying bacteria
• This requires anaerobic conditions

Question 14

Why does denitrification occur?

Answer 14

• It happens when the soil becomes waterlogged
• There is low O2 concentration so there are fewer aerobic bacteria (nitrifying + nitrogen fixing bacteria) and more anaerobic denitrifying bacteria

Question 15

Why is denitrification bad and how can you prevent denitrifying bacteria from building-up?

Answer 15

• Denitrification reduces availability of nitrogen-containing compounds for plants
• You can prevent build-up of denitrifying bacteria by keeping the soil well aerated (make sure there is plenty of O2 available)

Question 16

What important biological elements need phosphorus (or phosphate ions)?

Answer 16

ATP, phospholipids + nucleic acids

Question 17

Where is phosphorus found (and in what form)?

Answer 17

Phosphorus exists mostly as phosphate ions in the form of sedimentary rock deposits

Question 18

Define weathering

Answer 18

The break down of rocks on the Earth’s surface

Question 19

Define erosion

Answer 19

The act of taking broken down rocks away

Question 20

What is guano? Why does it contain lots of phosphate ions?

Answer 20

Seabirds faeces. They have lot of phosphate ions from the fish that they eat from the sea - the sea contains phosphate ions that are be dissolved out of rocks

Question 21

What is uplifting?

Answer 21

Rocks that were underground being forced/brought up the Earth’s surface

Question 22

How do animals get phosphate ions?

Answer 22

They eat the plants that absorb the phosphorus ions from the soil

Question 23

How do phosphate ions form sedimentary rocks?

Answer 23

The phosphate ions from guano and decomposition remains are transported by streams and rivers into lakes and oceans where they form sedimentary rocks - this completes the cycle

Question 24

How can plants get phosphate ions?

Answer 24

• Guano contains lots of phosphate ions (from the fish that seabirds eat) which the soil can absorb
• Decomposition of dead organisms leaving remains e.g bones + shells releasing phosphate ions into the water/soil

Question 25

What is mycorrhizae?

Answer 25

Mycorrhizae are associations between certain types of fungi and the roots of plants

Question 26

How/why does mycorrhizae play an important part in nutrient cycles?

Answer 26

• Fungi increase SA for absorption of water + minerals

- Helps increase uptake of scarce ions e.g phosphate ions

Question 27

Why is the relationship between the fungi and plant described as mutualistic?

Answer 27

Both the plant and the fungi benefit:

• Plants get water + nutrients from fungi
• Fungi get organic compounds (e.g amino acids & sugars) from plants