[3.5.3] Energy & Ecosystems

Question 1

Describe how biomass is formed in plants.

Answer 1

• During photosynthesis, plants make organic (carbon) compounds from atmospheric or aquatic CO₂.
• Most sugars synthesised are used by the plant as respiratory substrates.
• 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.

Answer 3

1. Sample dried in an oven e.g. 100°C (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 represenantive 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.

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 temeprature 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.

Answer 6

• Stirrer -> evenly distributes heat energy (in water).
• Air/insulation -> reduces heat loss & gain to & from surroundings.
• Water -> has a high specific heat capacity.

Question 7

What is gross primary production (GPP)?

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 enivronment 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.

Answer 10

• NPP is available for plant growth and reproduction.
• NPP is also available to other trophic levels in the ecosystem, such as herbivores and decomposers.

Question 11

What is primary or secondary productivity?

Answer 11

• The rate of primary or secondary production, respectively.

Question 12

State the units used for primary or secondary productivity.

Answer 12

KJ ha⁻¹ year⁻¹ (unit for energy, per unit area, per year)

Question 13

Explain why these units for primary or secondary productivity are used.

Answer 13

• Per unit area -> takes into account that different envrionments vary in size.
  
  • Standardising results to enable comparison between environments.
• Per year -> takes into account effect of seasonal variation (temperature etc.) on biomass.
  
  • More representative and enables comparison between environments.

Question 14

Explain why most light falling on prodcuers is not used in photosynthesis.

Answer 14

• Light is reflected or wrong wavelength.
• Light misses chlorophyll / chloroplasts / photosynthetic tissue.
• CO₂ concentration or temperature is a limiting factor.

Question 15

State the formula for net production of consumers (N).

Answer 15

N = I - (F + R)
I = the chemical energy store in ingested food.
F = the chemical energy lost to the environment in faeces and urine.

Question 16

State the formula for efficiency of energy transfer.

Answer 16

Energy or biomass available after transfer / energy or biomass available before transfer (x100 if %)

Question 17

Explain why energy transfer between trophic levels is inefficient.

Answer 17

• Heat energy is lost via respiration.
• Energy lost via parts of organism that aren’t eaten (e.g. bones).
• Energy lost via food not digested -> lost as faeces.
• Energy lost via excretion e.g. urea in urine.

Question 18

Explain how crop farming practices increase energy transfer efficiency.

Answer 18

• Simplifying food webs to reduce energy / biomass losses to non-human food chains. For example:
  
  • Herbicides kill weeds -> less competition (e.g. for light) so more energy to create biomass.
  • Pesticides kill insects (pests) -> reduce loss of biomass from crops.
  • Fungicides reduce fungal infections -> more energy to create biomass.
• Fertilisers e.g. nitrates to prevent poor growth due to lack of nutrients.

Question 19

Explain how livestock farming practices increase energy transfer efficiency.

Answer 19

• 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.