Energy + ecosystems

• How do plants use the sugars from photosynthesis

  • Primarily as respiratory substrates
  • To synthesise other biological molecules e.g. cellulose
• Biomass


  Total dry mass of tissue or mass of carbon measured over a given time in a specific area
• Units for biomass


  • When an area is being sampled: gm-2
  • When a volume (e.g. a pond) is being sampled: gm-3
• How can the chemical energy store in dry biomass be estimated

  1. Using calorimetry

  2. Energy released = specific heat capacity of water x volume of water (cm3) x temperature increase of water
• Why is bomb calorimetry preferable to simple calorimetry

  Reduces heat loss to surroundings
• How could a student ensure that all water had been removed from a sample before weighing

  Heat the sample and reweigh it until the mass reading is constant
• Gross primary production (GPP)


  Total chemical energy in plant biomass within a given volume or area
• Net primary productivity (NPP)

  Total chemical energy available for plant growth, plant reproduction and energy transfer to other trophic levels after respiratory losses
• Mathematical relationship between GPP and NPP

  NPP = GPP - R, where R represents respiratory losses
• Why is most of the Sun's energy not converted to organic matter
• How can the net production of consumers be calculated

  N = I - (F + R), where I: chemical energy from ingested food, F: energy lost as faeces and urine, R: respiratory losses
• Why does biomass decrease along a food chain
• Primary and secondary productivity

  • Rate of primary or secondary production
  • Biomass in a specific area over a given time period e.g. kJ ha–1 year–1
• Farming practices used to increase the efficiency of energy transfer

  • Exclusion of predators: no energy lost to other organisms in food web
  • Artificial heating: reduce energy lost to maintain constant body temperature
  • Restriction of movement
  • Feeding is controlled at the optimum
• General equation for % efficiency

  energy converted to a useful form (J) x 100 / total energy supplied (J)
• Why the length of food chains is limited
• Pyramid of biomass


  Diagram that shows the biomass at each trophic level
• Why a pyramid of biomass is preferable to a pyramid of numbers