Topic 5—B: Energy Transfer and Nutrient Cycles- 3. Nutrient Cycles in Natural Ecosystems

Question 1

What is a natural ecosystem?

Answer 1

• One that hasn’t been changed by human activity

Question 2

What happens in natural ecosystems?

Answer 2

• Nutrients such as nitrogen and phosphorus are recycled through food webs but human activity often disrupts this

Question 3

What two things do Saprobionts do?

Answer 3

• They feed on the remains of dead plants and animals and on their waste products e.g. (faeces and urine) breaking them down. This makes Saprobionts a type of decomposer and it allows important chemical elements in the remains and waste to be recycled
• They secrete enzymes and digest their food externally, then absorb the nutrients they need (known as extracellular digestion). During this process, organic molecules are broken down into organic ions

Question 4

What is Saprobiotic nutrition?

Answer 4

• Obtaining nutrients from dead organic matter and animal waste using extracellular digestion

Question 5

Mycorrhizae

Answer 5

• These are relationships which are caused by some fungi forming symbiotic relationships with the roots of plants

Question 6

What are the fungi made up of?

Answer 6

• Long, thin strands called hyphae which connect to the plants roots
• The hyphae greatly increase the surface area of the plant’s root system, helping the plant to absorb ions from the soil that are usually scarce. E.g. (phosphorus)
• Hyphae also increase the uptake of water by the plant
• In turn, the fungi obtain organic compounds, such as glucose, from the plant

Question 7

Why do plants and animals need nitrogen?

Answer 7

• To make proteins and nucleic acids (DNA & RNA)

Question 8

What do plant and animals need to convert nitrogen gas in the atmosphere to nitrogen-containing compounds?

Answer 8

Bacteria

Question 9

What does the nitrogen cycle show?

Answer 9

• It shows how nitrogen is converted into a useable form and then passed on between different living organisms and the non-living environment

Question 10

What does the nutrient cycle include?

Answer 10

• Food chains as nitrogen is passed on when organisms are eaten
• 4 different processes that involve bacteria
  . Nitrogen fixation
  . Ammonification
  . Nitrification
  . Denitrification

Question 11

Nitrogen fixation

Answer 11

• It is when nitrogen gas in the atmosphere is turned into nitrogen-containing compounds.
• It’s carried out by bacteria such as Rhizobium
• It turns nitrogen into ammonia which goes on to form ammonium ions in solution that can be used by plants
• Rhizobium are found inside root nodules (growths on the roots) of leguminous plants. E.g. (peas, beans and clover)
• They form a mutualistic relationship with the plants- they provide the plant with nitrogen compounds and the plant provides them with carbohydrates
• Other nitrogen-fixing bacteria are found in the soil

Question 12

Ammonification

Answer 12

• It is when nitrogen compounds from dead organisms are turned into ammonia by Saprobionts, which goes on to form ammonium ions.
• Animal waste (urine and faeces) also contains nitrogen compounds so these are also turned into ammonia by Saprobionts and go on to form ammonium ions.

Question 13

Nitrification

Answer 13

• When ammonium ions in the soil are changed into nitrogen compounds that can be used by plants (nitrates)
• First nitrifying bacteria called nitrosomonas change ammonium ions into nitrites.
• Then other nitrifying bacteria called nitrobacter change nitrites into nitrates

Question 14

Denitrification

Answer 14

• When nitrates in the soil are converted into nitrogen gas by denitrifying bacteria- they use nitrates in the soil to carry out respiration and produce nitrogen gas.
• Happens under anaerobic conditions e.g. waterlogged soils
• Other ways that nitrogen gets into an ecosystem are by lightning (which fixes atmospheric nitrogen into nitrogen oxides) or by artificial fertilisers (produced from atmospheric nitrogen on an industrial scale in the Haber process)

Question 15

Why do plants and animals need phosphorus?

Answer 15

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

Question 16

Where is phosphorus found?

Answer 16

• It is found in rocks and dissolved in the oceans in the form of phosphate ions

Question 17

What can be assimilated?

Answer 17

• Phosphate ions dissolved in water in the soil can be assimilated (absorbed and then used to make more complex molecules) by plants and other producers.

Question 18

What does the phosphorus cycle show?

Answer 18

• It shows how phosphorus is passed through an ecosystem

Question 19

Phosphorus cycle

Answer 19

• Phosphate ions in rocks are released into the soil by weathering
• Phosphate ions are taken into the plants through the roots. Mycorrhizae greatly increases the rate at which phosphorus can be assimilated
• Phosphate ions are transferred through the food chain as animals eat the plants and are in turn eaten by other animals
• Phosphate ions are lost from the animals in waste products
• When plants and animals die, Saprobionts are involved in breaking 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
• Weathering of rocks also releases phosphate ions into seas, lakes and rivers
• This is taken up by aquatic producers such as algae and passed along the food chain to birds
• The waste produced by sea birds is known as guano and contains a high proportion of phosphate ions
• Guano returns a significant amount of phosphate ions to soils and is often used as a natural fertiliser.

Question 20

Importance of nitrogen cycle

Answer 20

• Air is 78% nitrogen, however plants and animals can’t obtain nitrogen through gas exchange
• Nitrogen gas (N2) contains a triple bond which is hard to break
• Microorganisms are needed to convert nitrogen gas into nitrogen containing substances that plants and animals can absorb.