5.3 Energy and Ecosystems

• Describe how biomass is formed in plants

  • ● 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 (eg. carbs, lipids & proteins) → form biomass
• How can biomass be measured?

  Mass of carbon or dry mass of tissue per given area
• Describe how dry mass of tissue can be measured

  1. Sample dried in an oven eg. at 100oC (avoid combustion)
  2. Sample weighed and reheated at regular intervals until mass remains constant (all water evaporated)
• Explain why dry mass is more representative than fresh (wet) mass

  Water volume in wet samples will vary but will not affect dry mass.
• Describe how the chemical energy stored in dry biomass can be estimated

  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

  
• Explain how features of a calorimeter enable valid measurement of heat energy released

  ● Stirrer → evenly distributes heat energy (in water)

  ● Air / insulation → reduces heat loss & gain to & from surroundings

  ● Water → has a high specific heat capacity
• What is gross primary production (GPP)?

  • ● 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
• What is net primary production (NPP)?

  Chemical energy store in plant biomass after respiratory losses to environment taken into account
• State the formula for NPP

  NPP = GPP - R

  R = respiratory losses to the environment
• Explain the importance of NPP in ecosystems

  • ● NPP is available for plant growth and reproduction
  • ● NPP is also available to other trophic levels in the ecosystem, such as herbivores and decomposers
• What is primary or secondary productivity?

  The rate of primary or secondary production, respectively.
• State the units used for primary or secondary productivity

  $kJ ha^-1 year^-1$ (unit for energy, per unit area, per year)
• Explain why these units for primary or secondary productivity are used

  1. ● Per unit area → takes into account that different environments vary in size
  2. ○ Standardising results to enable comparison between environments
  3. ● Per year → takes into account effect of seasonal variation (temperature etc.) on biomass
  4. ○ More representative and enables comparison between environments
• Explain why most light falling on producers is not used in photosynthesis

  • ● Light is reflected or wrong wavelength
  • ● Light misses chlorophyll / chloroplasts / photosynthetic tissue
  • ● CO2 concentration or temperature is a limiting factor
• State the formula for net production of consumers (N)

  N = I - (F + R)

  I = the chemical energy store in ingested food

  F = the chemical energy lost to the environment in faeces and urine
• State the formula for efficiency of energy transfer

  (Energy or biomass available after transfer / energy or biomass available before transfer) x 100 if a %
• Explain why energy transfer between trophic levels is inefficient

  • ● Heat energy is lost via respiration
  • ● Energy lost via parts of organism that aren’t eaten (eg. bones)
  • ● Energy lost via food not digested → lost as faeces
  • ● Energy lost via excretion eg. urea in urine
• Explain how crop farming practices increase efficiency of energy transfer

  1. ● Simplifying food webs to reduce energy / biomass losses to non-human food chains eg.
  2. ○ Herbicides kill weeds → less competition (eg. for light) so more energy to create biomass
  3. ○ Pesticides kill insects (pests) → reduce loss of biomass from crops
  4. ○ Fungicides reduce fungal infections → more energy to create biomass
  5. ● Fertilisers e.g. nitrates to prevent poor growth due to lack of nutrients
• Explain how livestock farming practices increase efficiency of energy transfer

  1. ● Reducing respiratory losses within a human food chain (so more energy to create biomass):
  2. ○ Restrict movement and keep warm → less energy lost as heat from respiration
  3. ○ Slaughter animal while still growing / young, when most of their energy is used for growth
  4. ○ Treated with antibiotics → prevent loss of energy due to pathogens
  5. ○ Selective breeding to produce breeds with higher growth rates