13.3 - Nutrient Cycles

Question 1

energy enters an ecosystem as ________ and is lost as ____.

Answer 1

1) sunlight
2) heat

Question 2

Can energy lost via heat loss be recycled

Answer 2

No
—> therefore the flow of energy through an ecosystem is in one direction and linear

Question 3

Describe the basic sequence all nutrient cycles follow

Answer 3

• the nutrient is taken up by producers (plants) as simple inorganic molecules
• the producer incorporates the nutrient into complex organic molecules
• when the producer is eaten, the nutrient passes into consumers (animals)
• It then passes along the food chain when these animals are eaten by other consumers.
• When the producers and consumers die, their complex molecules are broken down by saprobiontic microorganisms (decomposers) that release the nutrient in its original simple form. The cycle is then complete.

Question 4

Describe the basic sequence all nutrient cycles follow

Answer 4

• the nutrient is taken up by producers (plants) as simple inorganic molecules
• the producer incorporates the nutrient into complex organic molecules
• when the producer is eaten, the nutrient passes into consumers (animals)
• It then passes along the food chain when these animals are eaten by other consumers.
• When the producers and consumers die, their complex molecules are broken down by saprobiontic microorganisms (decomposers) that release the nutrient in its original simple form. The cycle is then complete.

Question 5

Living organisms require a source of nitrogen from which to manufacture ____, ___ ____ and ____ ____-____ _____.

Answer 5

1) proteins
2) nucleic acids
3) other nitrogen-containing compounds

Question 6

What % of the atmosphere is nitrogen

Answer 6

78%

Question 7

Plants take up most of the nitrogen they require in the form of….

Answer 7

• Nitrate ions
• absorbed by the roots via active transport

Question 8

How do animals obtain nitrogen

Answer 8

obtaining nitrogen-containing compounds by eating and digesting plants.

Question 9

Why are nitrate ions able to be leached (washed away)

Answer 9

They’re soluble

Question 10

What is a natural ecosystem

Answer 10

One that hasn’t been changed by human activity

Question 11

How are nutrients recycled in natural ecosystems

Answer 11

• done through the food webs
• human activity may disrupt this (e.g. fertilisers)

Question 12

What is the importance of bacteria and fungi

Answer 12

• some are saprobionts (a type of decomposer)
• they feed on the remains of dead plants and animals and on their waste products (faeces or urine), breaking them down
• this allows important chemical elements in the remains to be recycled

Question 13

How do saprobionts work

Answer 13

• secrete enzymes and digest their food externally, then absorb the nutrients they need (extracellular digestion)
• during this process, organic molecules are broken down into inorganic ions
• the process of obtaining nutrients from dead organic matter using extracellular digestion is known as ‘saprobiotic nutrition’

Question 14

Describe the significance of fungi forming symbiotic relationships with plant roots

Answer 14

• some fungi form symbiotic relationships with plant roots. The relationships are known as mycorrhizae

Question 15

Describe the characteristics of mycorrhizae

Answer 15

• the fungi are made up of long, thin strands called hyphae, which connect to the plant’s roots
• it greatly increases the SA 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 uptake of water by the plant
• in return, the fungi obtain organic compounds, such as glucose from the plant

Question 16

What is the need for the nitrogen cycle

Answer 16

• 78% of atmosphere is nitrogen gas, but plants and animals can’t use it in that form
• they need bacteria to convert it into ‘nitrogen-containing compounds’
• the nitrogen cycle is how this is done in 4 different process that involve bacteria:
  1) nitrogen fixation
  2) Ammonification
  3) Nitrification
  4) Dentrification

Question 17

What is the need for the nitrogen cycle

Answer 17

• 78% of atmosphere is nitrogen gas, but plants and animals can’t use it in that form
• they need bacteria to convert it into ‘nitrogen-containing compounds’
• the nitrogen cycle is how this is done in 4 different process that involve bacteria:
  1) nitrogen fixation
  2) Ammonification
  3) Nitrification
  4) Dentrification

Question 18

Describe nitrogen fixation

Answer 18

• it is when nitrogen gas in the atmosphere is turned into nitrogen-containing compounds
• biological nitrogen fixation is carried out by bacteria (such as Rhizobium)
• they turn nitrogen —> ammonia, which goes on to form ammonium ions in solution that can be used by plants
• they’re found inside root nodules of leguminous plants (e.g. peas, beans and clover)
• they form a mutualistic relationship with the plants. They provide nitrogen compounds and the plant provides carbohydrates

Question 19

Describe ammonification

Answer 19

• when nitrogen compounds found 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. These are also turned into ammonia by saprobionts and go on to form ammonium ions

Question 20

Describe nitrification

Answer 20

• is 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 —> nitrites
• then other nitrifying bacteria called ‘Nitrobacter’ change nitrites —> nitrates

Question 21

Describe dentrification

Answer 21

• 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
• this happens under anaerobic conditions (where there’s no oxygen) e.g. waterlogged soil

Question 22

How might soil nitrates be increased in agricultural practises to increase yield

Answer 22

• Ploughing helps to aerate the soil
• so prevents the build-up of denitrifying bacteria that can reduce the level of soil nitrates

Question 23

What is phosphorus used for

Answer 23

• phospholipids
• DNA
• ATP
• Bones (calcium phosphate)
• Phosphocreatine (used in anaerobic evens)

Question 24

Where is phosphorus found

Answer 24

Found in:
- rocks
- dissolved in the oceans in the form of phosphate ions
—> the ions dissolved in water can be assimilated by plants and other producers

Question 25

Describe the phosphorus cycle

Answer 25

1) phosphate ion in rocks are released into the soil by weathering
2) phosphate ions are taken into the plants through the roots. Mycorrhizae greatly increase the rate at which phosphorus can be assimilated
3) phosphate ions are transferred through the food chain as animals eat the plants and are in turn eaten by other animals
4) phosphate ions are lost from the animals in waste products
5) when plant/animal dies, saprobionts are involved in breaking down the organic compounds, releasing phosphate ions into the soil for assimilation by plants. They also release the phosphate ions from urine and faeces
6) weathering of rocks also releases phosphate ions into seas/lakes/rivers. This is taken up by aquatic producers (such as algae) and passed along the food chain to birds
7) 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 (particularly in coastal areas). It’s often used as a natural fertiliser

Question 26

Fill in the Gaps:

A few organisms can convert nitrogen gas into compounds useful to other organisms in a process known as (1). These organisms can be free-living or live in a relationship with certain (2). Most plants obtain their nitrogen by absorbing (3) from the soil through their (4) by active transport. They then convert this to (5), which is passed to animals when they eat the plants. On death, (6) break down these organisms, releasing (7), which can then be oxidised to form nitrite ions by (8) bacteria. Further oxidation by the same type of bacteria forms (9) ions. These ions may be converted back to atmospheric nitrogen by the activities of (10) bacteria.

Answer 26

1 nitrogen fixation
2 plants
3 nitrate ions
4 root hairs
5 proteins / amino acids
6 saprobionts
7 ammonia
8 nitrifying
9 nitrate
10 denitrifying