Topic 5.3 - Energy & Ecosystems

Question 1

Describe how biomass is formed in plants (3)

Answer 1

1) During photosynthesis, plants make organic compounds from atmospheric or aquatic CO2

2) Most sugars synthesised are used by the plant as respiratory substrates

3) Rest used to make other groups of biological molecules (e.g. carbs, lipids, proteins) -> form biomass

Question 2

How can biomass be measured?

Answer 2

Mass of carbon or dry mass of tissue per given area

Question 3

Describe how dry mass of tissue can be measured (3)

Answer 3

1) Sample dried in an oven e.g. at 100C (avoid combustion)

2) Sample weighed and reheated at regular intervals

3) Until mass remains constant (all water evaporated)

Question 4

Explain why dry mass is more representative than fresh (wet) mass

Answer 4

Water volume in wet samples will vary but will not affect dry mass

Question 5

Describe how the chemical energy stored in dry biomass can be estimated (3)

Answer 5

Using calorimetry:

1) Known mass of dry biomass is fully combusted (burnt)

2) Heat energy released heats a known volume of water

3) Increase in temperature of water is used to calculate chemical energy of biomass

Question 6

Explain how features of a calorimeter enable valid measurement of heat energy released (3)

Answer 6

1) Stirrer -> evenly distributes heat energy (in water)

2) Air / insulation -> reduces heat loss & gain to and from surroundings

3) Water -> has a high specific heat capacity

Question 7

What is gross primary production? (GPP) (1+1)

Answer 7

Chemical energy store in plant biomass, in a given area or volume, in a given time

(Total energy transferred into chemical energy from light energy during photosynthesis)

Question 8

What is net primary production (NPP)

Answer 8

Chemical energy store in plant biomass after respiratory losses to environment taken into account

Question 9

State the formula for NPP

Answer 9

NPP = GPP - R

(R = Respiratory losses to the environment)

Question 10

Explain the importance of NPP in ecosystems (2)

Answer 10

1) NPP is available for plant growth and reproduction

2) NPP is also available to other trophic levels in the ecosystem, such as herbivores and decomposers

Question 11

State the units used for primary or secondary productivity

Answer 11

kJ ha^-1 year^-1 (unit for energy, per unit area, per year)

Question 12

Why are these units used for primary and secondary productivity? (2)

Answer 12

1) Per unit area -> takes into account that different environments vary in size
- Standardising results to enable comparison between environments

2) Per year -> takes into account effect of seasonal variation (temperature etc..)
- More representative and enables comparison between environments

Question 13

Explain why most light falling on producers is not used in photosynthesis? (3)

Answer 13

1) Light is reflected or wrong wavelength

2) Light misses chlorophyll / chloroplasts

3) CO2 concentration or temperature is a limiting factor

Question 14

State the formula for net production of consumers (N)

Answer 14

N = I - (F + R)

I = Chemical energy store in ingested food
F = Chemical energy lost to the environment in faeces and urine

Question 15

State the formula for efficiency of energy transfer

Answer 15

Energy or biomass available after transfer / energy or biomass available before transfer

(x100 for %)

Question 16

Explain why energy transfer between trophic levels is inefficient (4)

Answer 16

1) Heat energy is lost via respiration

2) Energy lost via parts of organism that aren’t eaten (e.g. bones)

3) Energy lost via food not digested -> lost as faeces

4) Energy lost via excretion e.g. urea in urine

Question 17

Explain how crop farming practices increase energy transfer efficiency (1+3, 1)

Answer 17

1) Simplifying food webs to reduce energy / biomass losses to non-human food chains e.g.

• Herbicides kill weeds -> less competition (e.g. for light) so more energy to create biomass
• Pesticides kill insects -> reduce loss of biomass from crops
• Fungicides reduce fungal infections -> more energy to create biomass

2) Fertilisers e.g. nitrates to prevent poor growth due to the lack of nutrients

Question 18

Explain how livestock farming practices increase energy transfer efficiency (1+4)

Answer 18

Reducing respiratory losses within a human food chain (so more energy to create biomass)

• Restrict movement and keep warm -> less energy lost as heat from respiration
• Slaughter animal while still growing / young, when most of their energy is used for growth
• Treated with antibiotics -> prevent loss of energy due to pathogens
• Selective breeding to produce breeds with higher growth rates