Ecosystems

• Plants synthesise organic compounds

  1. From atmospheric or aquatic carbon dioxide

  2. Most sugars used as respiratory substrates

  3. Rest used to make other biological molecules (e.g. cellulose)

  4. Biological molecules form plant biomass
• Biomass


  Total dry mass of tissue or mass of C measured over a given time in a specific area. Units: area = gm-2, volume = gm-3
• How chemical energy store in dry mass is estimated

  1. Using calorimetry

  2. Energy released = specific heat capacity of water x volume of water (cm3) x temperature increase of water

  3. Bomb calorimetry preferable to reduce heat loss to surroundings

  4. Can ensure all water removed from sample before weighing by heating & reweighing until mass constant
• Pyramid of biomass


  • Diagram that shows the biomass at each trophic level
  • Preferable to pyramid of numbers as the shape may be skewed since a small number of producers can support many consumers
• Biomass decreases along a food chain


  • Some energy lost in nitrogenous waste (urine) & faeces
  • Some organisms not consumed and energy lost to surroundings as heat
• 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 & energy transfer to other trophic levels after respiratory losses
• Net production of consumers


  N = I - (F + R); where I: chemical energy from ingested food, F: energy lost as faeces & urine, R: respiratory losses
• Length of food chains is limited due to energy loss at each trophic level
• Primary & secondary productivity


  Rate of primary or secondary production, respectively, measured as biomass in a given area in a given time e.g. kJ ha–1 year–1
• Common farming practices used to increase the efficiency of energy transfer

  • Exclusion of predators
  • Reducing Respiratory losses within human food chain - Artificial heating
• The Sun is the source of all energy in ecosystems with photosynthetic organisms using this to produce their own food (autotrophs)
• Heterotrophs


  Organisms that can't synthesise their own food
• Most of the Sun's energy not converted to organic matter as: Most solar energy is absorbed by the atmosphere or reflected by clouds, Photosynthetic pigments cannot absorb some wavelengths of light, Energy lost as heat during respiration/photosynthesis