5.4 Nutrient Cycles

Question 1

What are saprobionts?

Answer 1

Microorganisms that feed on the remains of dead plants and animals (including faeces and urine) and break down the remains

Question 2

What are mycorrhizae?

Answer 2

The symbiotic relationship between fungi and the roots of plants. The fungi has hyphae which increases the surface area of the roots for mineral and water absorption. The fungi obtain organic compounds such as glucose from the plant.

Question 3

What are the four steps of the nitrogen cycle?

Answer 3

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

Question 4

Why do plants and animals need nitrogen (for what molecules)?

Answer 4

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

Question 5

Explain the process of the four stages of the nitrogen cycle.

Answer 5

1. Nitrogen fixation - nitrogen gas in the atmosphere is turned into nitrogen-containing compounds by nitrogen-fixing bacteria. These compounds can be uptaken by plants.
2. Ammonification - Nitrogen compounds from dead organisms or waste are turned into ammonium ions by saprobionts.
3. Nitrification - Ammonium ions are changed into nitrites then nitrates which can be taken up by plants again.
4. Denitrification - Nitrates in the soil are converted to nitrogen gas by denitrifying bacteria. This happens under anaerobic conditions e.g. waterlogged soil.

Question 6

Describe the process of the phosphorus cycle

Answer 6

• Phosphate ions are released into the soil by weathering.
• Phosphate ions are transferred into plants by the roots with the help of mycorrhizae and then can be transferred to animals when they eat the plants.
• Phosphate ions are lost when animals die and saprobionts break down the organic compounds releasing phosphate ions into the soil for plants.
• Weathering of rocks also releases phosphate ions into seas, lakes and rivers which can be taken up by aquatic producers (e.g. algae) and then taken by birds and return to the soil when saprobionts break down bird faeces.

Question 7

Why do plants and animals need phosphate ions?

Answer 7

To make biological molecules such as phospholipids, DNA and ATP.

Question 8

Why do farmers use fertilisers?

Answer 8

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

Question 9

What are the two types of fertilisers?

Answer 9

• Artificial - inorganic and contain pure chemicals
• Natural - organic and include manure, composted vegetables etc.

Question 10

What are the environmental issues concerning fertilisers?

Answer 10

Sometimes too much fertiliser is applied and they may leach into waterways. Leaching is when water-soluble compounds in the soil are washed away. This can lead to eutrophication.

Question 11

Explain the process of eutrophication

Answer 11

1. Mineral ions leach from fertilised fields stimulate the rapid growth of algae in ponds and rivers.
2. Large amounts of algae blocks light from reaching the plants below.
3. The plants will die because they are unable to photosynthesise
4. Saprobiotic bacteria feed on the the dead plant matter and can replicate more. The increased number of bacteria reduces the oxygen concentration in the water as they aerobically respire.
5. Fish die because there isn’t enough dissolved oxygen.