Energy and Ecosystems

• Describe how Biomass is formed in plants:

  • During photosynthesis, plants make organic (carbon) compounds from atmosphere or aquatic CO2
  • Most sugars synthesised are used by the plant as respiratory substrates
  • Rest used to make other groups of biological molecules such as carbohydrates, lipids and 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 e.g. at 100°c
  2. Sample weighed and reheated at regular intervals until mass remains constant
• 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:

  • Use a Calorimeter

  1. Known mass of dry mass is fully combusted
  2. Heat energy released heats a known volume of water
  3. Increase in temperature of water is used to calculate chemical energy store of biomass
• Explain how features of a Calorimeter enable valid measurement of heat energy released:

  • Stirrer -> Evenly distributes heat energy
  • Air/Insulation -> Reduces heat loss and gain 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?

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

  NPP = GPP - Respiratory Loss
• 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
• State the units used for primary or secondary productivity:

  $kJ\ ha^{-1}year^{-1}$
• Explain why the units for primary and secondary productivity are used:

  • Per unit area -> takes into account that different environments vary in size

  -Standardising results to enable comparison between environments

  • Per year -> takes into account effect if seasonal variation on biomass

  -More representative and enables comparisson
• Explain why most light falling on producers is not used in photosynthesis:

  • Light is reflected or wrong wavelength
  • Light misses chlorophyll
  • CO2 concentration or temperature is a limiting factor
• State the formula for NPP in animals:

  NPP = I - (F+R)
• State the formula for efficiency of energy transfer:?

  Energy or biomass after transfer / energy or biomass before transfer
• Explain why energy transfer between trophic levels in inefficient:

  • Heat energy loss 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
• Explain how crop farming practices increases efficiency of energy transfer:

  • Simplifying food webs to reduce biomass losses to non-human food chains

  -Herbicides kill weeds -> Less competition so more energy to create biomass

  -Pesticides kill insects -> reduces 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
• Explain how livestock farming practices increase efficiency of energy transfer:

  • Reducing respiratory losses with a human food chain

  -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