Section 5 - Energy transfer in and between organisms: 13. Energy and Ecosystems

Question 1

What are producers

Answer 1

Photosynthetic organisms that manufacture organic material through the use of light energy

Question 2

What are consumers

Answer 2

Organisms that obtain energy by feeding on other organisms rather than using sunlight directly
- Primary: Eats producers
- Secondary: Eats Primary consumer
- Tertiary: Eats Secondary consumer

Question 3

What are Saprobionts

Answer 3

Organisms that break down dead/waste organic matter, releasing minerals to be reused
(Decomposers - Fungi, bacteria, etc.)

Question 4

What is a food chain

Answer 4

Linear feeding relationship with each stage described as a trophic level and the arrows representing a transfer of energy

Question 5

What is a food web

Answer 5

The more realistic representation of more complex feeding relationships, joining together multiple food chains in an ecosystem.

Question 6

What is biomass

Answer 6

The total mass of all living material in a specific area at a given time
- Measured using ‘Dry biomass’, as varying water levels would alter the value
(However, this does mean the organism has to be dead)
- Calculated as dry biomass per square meter (gm^-2)

Question 7

What is calorimetry and how is it carried out

Answer 7

Process used to estimate the chemical energy store in dry biomass:
- Dry material is weighed, before being durned in pure O(2) in a sealed chamber (bomb)
- Bomb is surrounded by water, which increases in temperature due to the combustion
- Using the specific heat capacity for water, as well as the volume and change in temperature, the energy released can be calculated (KJ kg^-1)

Question 8

Why is most light energy not converted into organic matter by producers

Answer 8

Plants only convert ~1-3% of the available energy, because:
- >90% of the sun’s energy is reflected away or absorbed by the atmosphere
- Not all light wavelengths can be absorbed
- Light may not hit the chlorophyll
- Factors (eg. low CO(2)) may limit the rate of photosynthesis

Question 9

What is the ‘Gross Primary Production’ (GPP) for producers

Answer 9

The total quantity of chemical energy stored in plant biomass in a given area/volume in a given time

Question 10

What is the ‘Net Primary Production’ (NPP) for producers

Answer 10

The quantity of chemical energy stored after respiratory losses (~20-50%) have been taken into account

∴ NPP = GPP - Respiratory losses

Question 11

Why is only a small % of the NPP transferred up the trophic levels to the consumer

Answer 11

Only 10-20% is transferred because:
- Some of the organism isn’t consumed
- Some of the biomass is not digested (faeces)
- Some energy is lost in excretory material
- Some energy is lost to the environment as heat during respiration

Question 12

What is the Net Production of energy (N) for consumers

Answer 12

N = I - (F+R)

I: Chemical energy stored in food
F: Energy loss (Faeces and urine)
R: Energy loss (Respiration)

Question 13

How does the inefficiency of energy transfer up trophic levels effect food chains

Answer 13

Total available energy decreases up the food chain
- ∴ Most food chains have only 4-5 trophic levels
- ∴ The total biomass of organisms (in a certain area) at higher trophic levels is less

Question 14

What are some examples of farming practices that increase the efficiency of energy transfer, so improve yield

Answer 14

• Restriction of animals movement, to reduce respiratory losses
• Environment kept warm to reduce energy loss as heat in animals
• Controlled feeding for max growth
• Exclude predators, so there is no loss of energy to other organisms
  (However, pesticides can impact natural cycles to potentially reduce future yield)

Question 15

What is a nutrient cycle

Answer 15

A circular loop, recycling elements and ions in a constant cycle, with no external source

Question 16

What is the basic sequence of all nutrient cycles

Answer 16

1) Nutrients are taken up by producers as simple inorganic molecules
2) The producers incorporate the nutrients into complex organic molecules
3) Nutrients are passed to the consumer when the producer is eaten, carrying nutrients further up the food chain
4) When organisms die, or excrete waster, the complex molecules within them are broken down by saprobiontic organisms
5) Nutrients are released as simple, inorganic molecules, to be used again

Question 17

What are the 4 main stages of the Nitrogen cycle

Answer 17

• Nitrogen fixation
• Nitrification
• Ammonification
• Denitrification

Question 18

What is ‘Nitrogen fixation’ in the Nitrogen cycle

Answer 18

Process where nitrogen gas is converted into nitrogen containing compounds (ammonium ions)
- Can be carried out by ‘free-living nitrogen fixing bacteria’:
- Present in the soil
- Reduces gaseous nitrogen to form ammonia
- Nitrogen compounds are released when they die
- Can be carried out by ‘Mutualistic nitrogen fixing bacteria’:
- Live in the nodules on legume roots
- Reduces gaseous nitrogen to form ammonia
- Plant provides carbs, bacteria provides amino acids (mutualistic)
- Nitrogen fixing can also occur when lightning passes through the atmosphere

Question 19

What is ‘Nitrification’ in the Nitrogen cycle

Answer 19

The process that converts ammonium ions to nitrate ions through oxidation reactions
- Carried out by ‘nitrifying bacteria’ (free-living soil microorganisms)
- Occurs in 2 stages:
- Oxidation of ammonium ions to Nitrite ions (NO(2)^-)
- Oxidation of Nitrite to Nitrate ions (NO(3)^-)

Question 20

What is ‘Ammonification’ in the nitrogen cycle

Answer 20

Process that produces ammonium ions from organic nitrogen-containing compounds (waste material)
- After nitrates have been absorbed and passed up the food chain, nitrogen containing compounds are present within the organisms (Proteins, nucleic acids, vitamins, etc.)
- When waste is excreted (urea/faeces) or an organism dies, the compounds are broken down by saprobiontic microorganisms.
- This releases ammonia, forming ammonium ions to go back into the soil

Question 21

What is denitrification

Answer 21

The process where soil nitrates (that haven’t yet been absorbed) are converted back into gaseous nitrogen
- Carried out by ‘Denitrifying bacteria’
- In waterlogged soil, there is less O(2)
∴ Fewer Aerobic nitrifying/nitrogen-fixing bacteria, and more anaerobic denitrifying bacteria
∴ Farmland soil must be well aerated to remain productive, to avoid denitrification>nitrifying/nitrogen-fixation

Question 22

What is the main process of the phosphorous cycle

Answer 22

1) Phosphate ions are stored in sedimentary rocks
- Originate in the sea, but bought to the surface by geological movement
- Weathering and erosion release ions which become dissolved in water systems, then in soil

2) Phosphate ions are absorbed by plants
- Incorporated into plant biomass
- Ions pass up the food chain by feeding
- ∴ phosphate ions are present in waste material and dead organic matter

3) Ions within waste are released as saprobionts break down the material
- Ions can be released into the water or the soil to be used again
- Some ions remain in bones or shells, which break down slowly

4) Ions are redeposited in sedimentary rocks
- Sedimentation: Ions dissolved in water ways result in rock formation
- Deposition: Ions that remain in waste (eg. bones) lead to the formation of sedimentary rocks

Question 23

What is Mycorrhizae

Answer 23

A symbiotic association between plants and fungi

Question 24

What is the role of Mycorrhizae in nutrient cycles

Answer 24

• Fungi increases the surface area for absorption, holding water and minerals near to the roots
• Allows plants to be more drought resistant and take up inorganic ions more readily
• Relationship is mutualistic:
  
  • Plant has improved water/mineral uptake
  • Fungi receives organic compounds from the plant (eg. amino acids)

Question 25

Why does intensive food production increase the need for fertilisers

Answer 25

• Crops are continually grown and harvested in the same area of land
• Crops are removed, so minerals aren’t returned to the soil
• ∴ Conc. of minerals in the soil gradually decreases
• Introduces a limiting factor for plant growth, decreasing yield
• ∴ Fertilisers must be used to increase the fertility of the soil

Question 26

What are natural (organic) fertilisers

Answer 26

Dead and decaying remains of plants/animals, and animal waste
(eg. manure, slurry, bone meal)

Question 27

What are artificial (inorganic) fertilisers

Answer 27

Mined from rocks and deposits, converted into different forms/blends with the appropriate mineral balance.
(eg. NPK fertilisers)

Question 28

Why are nitrogen-containing fertilisers important for increasing crop yield

Answer 28

• Nitrogen is required to make ATP, DNA, amino acids, etc
  (∴ Required for plant growth)
• Higher nitrogen levels cause plants to develop earlier and grow taller
• Nitrogen increases the rate of photosynthesis, as it causes more leaves to be present, increasing the yield

(Fertiliser use has increased UK agricultural food production by 100% since 1955)

Question 29

How do Nitrogen-containing fertilisers reduce species diversity

Answer 29

• Nitrogen rich soil favours rapidly growing species (eg. grass)
• ∴ These species outcompete the others
• ∴ Nitrogen-containing fertilisers will result in a decline in slower growing species, impacting the balance of the ecosystem

Question 30

How does leaching occur, and why does the use of Nitrogen-containing fertilisers increase the impact of this

Answer 30

• Leaching = process by which nutrients are removed from the soil.
• Rain water will dissolve the soluble ions and wash them away (out of reach of plants)
• May enter water sources, which can have negative impacts if Nitrogen level is high:
  
  • May be harmful in drinking water
  • May cause Eutrophication

Question 31

What is the process of Eutrophication

Answer 31

• When leaching occurs, the nutrient concentration increases in water bodies
• Nitrate ions cause more plant growth (no longer limiting factor) resulting in algal blooms on the water surface
• Algae absorbs light and prevents it reaching lower depths
• This causes algae and plants at lower depths to die
• This increases the rate of decomposition by saprobionts, decreasing O(2) levels, as it is used up by these microorganisms
• This results in the death of many aerobic organisms (eg. fish)
• Positive feedback: death of aerobic organisms decreases competition, so more anaerobic organisms survive, so break down more organic matter to release more nutrients.