🩷 3.5 - 3.5.4 Nutrient cycles (A-level only)

Question 1

What is a natural ecosystem?

Answer 1

An ecosystem which has not been damaged by human activity. Nutrients are recycled through the food webs, but human activity often disrupts the cycling of nutrients.

Question 2

What are two examples of microorganisms which are an important part of food webs?

Answer 2

Bacteria and fungi.

Question 3

What are many bacteria and fungi called in food webs?

Answer 3

Saprobionts.

Question 4

What are saprobionts?

Answer 4

A type of decomposer which feeds on the remains of dead plants and animals and on their waste products (faeces, urine), breaking them down.

Question 5

What do saprobionts allow?

Answer 5

Important chemical elements in the remains to be recycled.

Question 6

What do saprobionts secrete?

Answer 6

Enzymes and digest their food externally, then absorb the nutrients they need. This is known as extracellular digestion.

Question 7

What happens during saprobionts extracellular digestion?

Answer 7

Organic molecules are broken down into inorganic ions.

Question 8

What is obtaining nutrients from dead organic matter using extracellular digestion known as?

Answer 8

Saprobiotic nutrition.

Question 9

What relationships are known as mycorrhizae?

Answer 9

Symbiotic relationships formed by fungi with the roots of plants.

Question 10

The fungi are made up of long, thin strands called …

Answer 10

… hyphae, which connect to the plants roots.

Question 11

What do the hyphae increase?

Answer 11

The surface area of the plants root system, helping the plant to absorb ions from the soil that are usually scarce (e.g. phosphorus). They also increase the uptake of water by the plant.

Question 12

What do the fungi in turn obtain?

Answer 12

Organic compounds, such as glucose, from the plant.

Question 13

What does the nitrogen cycle show?

Answer 13

How nitrogen is recycled in ecosystems and how nitrogen is converted into a usable form and then passed on between different living organisms and the non-living environment.

Question 14

Why do plants and animals need nitrogen?

Answer 14

To make proteins and nucleic acids (DNA and RNA).

Question 15

Even though the atmosphere is made up of about 78% nitrogen gas, plants and animals can’t use it, why?

Answer 15

It is not in the right form - they need bacteria to convert it into nitrogen-containing compounds first.

Question 16

What does the nitrogen cycle include?

Answer 16

Food chains (nitrogen is passed on when organisms are eaten).

Question 17

What are the four different processes that involve bacteria in the nitrogen cycle?

Answer 17

1) Nitrogen fixation.
2) Ammonification.
3) Nitrification.
4) Denitrification.

Question 18

What happens during the first part of the nitrogen cycle, nitrogen fixation?

Answer 18

Nitrogen Gas in the atmosphere is turned into nitrogen-containing compounds. Biological nitrogen fixation is carried out by bacteria. The bacteria turns nitrogen into ammonia which forms ammonium ions in solution that can be then used by plants. The bacteria are found inside root nodules of leguminous plants. They form a mutualistic relationship with the plants (provide the plant with nitrogen compounds and the plant provides them with carbohydrates).

Question 19

What happens during the second part of the nitrogen cycle, ammonification?

Answer 19

Nitrogen compounds from dead organisms are turned into ammonia by saprobionts, which goes on the form ammonium ions.
Animal waste (urine and faeces) also contains nitrogen compounds. These are also turned into ammonia by saprobionts and go on to form ammonium ions.

Question 20

What happens during the third part of the nitrogen cycle, nitrification?

Answer 20

Ammonium ions in the soil are changed into nitrogen compounds that can then be used by plants (nitrates). Nitrifying bacteria change ammonium ions into nitrites and then other nitrifying bacteria changes nitrites into nitrates.

Question 21

What happens during the forth part of the nitrogen cycle, denitrification?

Answer 21

Nitrates in the soil are converted into nitrogen gas by denitrifying bacteria - they use nitrates in the soil soil to carry out respiration and produce nitrogen gas. This happens under anaerobic conditions (no oxygen).

Question 22

What are two other ways nitrogen gets into an ecosystem?

Answer 22

By lightening (which fixes atmospheric nitrogen) or by artificial fertilisers (they are produced from atmospheric nitrogen on an industrial scale in the Haber process).

Question 23

What does the phosphorus cycle show?

Answer 23

How phosphorus is passed through an ecosystem.

Question 24

In what cycle is phosphorus passed through the food web in?

Answer 24

The phosphorus cycle.

Question 25

Why do plants and animals need phosphorus?

Answer 25

To make biological molecules such as phospholipids (which make ups cell membranes), DNA and ATP.

Question 26

Where is phosphorus found?

Answer 26

In rocks and dissolved in the oceans in the form of phosphate ions.

Question 27

Phosphate ions dissolved in water in the soil can be assimilated by plants and other producers. What does this mean?

Answer 27

Absorbed and then used to make more complex molecules.

Question 28

What is the process of the phosphorus cycle?

Answer 28

1) Phosphate ions in rocks are released into the soil by weathering.
2) Phosphate ions are taken into the plants through the roots.
3) Phosphate ions are transferred through the food chain as animals eat the planet and are in turn eaten by other animals.
4)Phosphate ions are lost from the animals in waste products.
5) When plants/animals die, saprobionts break down the organic compounds, releasing phosphate ions into the soil for assimilation by plants. These microorganisms also release the phosphate ions from urine and faeces.
6) Weathering of rocks also releases phosphate ions into seas, lakes and rivers which is taken up by aquatic producers, such as algae and passed also the food chain to birds.
7) The waste products contain a high proportion of phosphate ions and a significant amount is returned to soils and often used a a natural fertiliser.

Question 29

What are mycorrhizae in the phosphorus cycle used for?

Answer 29

To greatly increase the rate at which phosphorus can be assimilated.

Question 30

When crops are harvested, what are lost? How?

Answer 30

Nutrients.
When crops are harvested, they are removed from the field where they were grown rather than being allowed ti die and decompose there. This means that the mineral ions they contain, are not returned to the soil by decomposers in the nitrogen or phosphorus cycle.

Question 31

How are mineral ions such as phosphates and nitrates apart from when plants are harvested, lost from the system?

Answer 31

When animals or animal producers are removed from the land. Animals eat grass and other plants, taking in their nutrients. When they are taken elsewhere, for example for slaughter or to a different field, the nutrients are not replaced through their remains or waste products.

Question 32

Why are fertilisers added to soils? Why?

Answer 32

To replace lost nutrients, so more energy from the ecosystem can be used for growth, increasing the efficiency of energy transfer.

Question 33

Fertilisers can be two types. What are these?

Answer 33

Artificial or natural.

Question 34

What are artificial fertilisers?

Answer 34

They are inorganic which means they contain pure chemicals as powders or pellets.

Pure chemicals such as ammonium nitrate.

Question 35

What are natural fertilisers?

Answer 35

They are organic which means they include manure, composed vegetables, crop residues and sewage sludge.

Crop residues are the parts which are left over after the harvest.

Question 36

How does using fertilisers sometimes cause environmental issues?

Answer 36

Sometimes more fertilisers are added than the plants need or are able to use at a particular time which can lead to the fertilisers leaching into waterways. This can lead to eutrophication.

Question 37

What is leaching?

Answer 37

It is when water-soluble compounds in the soil are washed away. This can be by rain or irrigation systems. They are often washed into nearby ponds and rivers.

Question 38

When is leaching more likely to occur?

Answer 38

If the fertiliser is applied just before a heavy rainfall.

Question 39

Inorganic ions in chemical fertilisers are relatively soluble. What does this mean?

Answer 39

It means that excess minerals that are not used immediately are more likely to leach into waterways.

Question 40

Why is leaching less likely in natural fertilisers?

Answer 40

The nitrogen and phosphorus are still contained in organic molecules that need to be decomposed by microorganisms before they can be absorbed by plants. This means that their release into the soil for uptake by plants is more controlled and therefore, leaching is less likely.

Question 41

Why is the leaching of phosphates less likely than the leaching of nitrates?

Answer 41

Because phosphates are less soluble in water.

Question 42

What do fertilisers also change the balance of in the soil? How is this a negative which can raise environmental issues?

Answer 42

Nutrients.
It is a negative because too much of a particular nutrient can cause crops and other plants to die therefore, causing environmental issues.

Question 43

What is eutrophication caused by?

Answer 43

Excess nutrients.

Question 44

What is the process of eutrophication?

Answer 44

1) Mineral ions leach from fertilised fields stimulate the rapid growth of algae in ponds and rivers.
2) Large amounts of algae block light from reaching the plants below.
3) The plants die eventually because they are unable to photosynthesise enough.
4) Bacteria feed on the dead plant matter. The increased numbers of bacteria reduce the oxygen concentration in the water by carrying out aerobic respiration.
5) Fish and other aquatic organisms die because there is not enough dissolved oxygen.