2.3 Flows of energy and matter

Significant ideas:

  • Ecosystems are linked together by energy and matter flows.
  • The Sun’s energy drives these flows, and humans are impacting the flows of energy and matter both locally and globally.

Knowledge and understanding:

  1. As solar radiation (insolation) enters the Earth’s atmosphere, some energy becomes unavailable for ecosystems as this energy is absorbed by inorganic matter or reflected back into the atmosphere.
  2. Pathways of radiation through the atmosphere involve a loss of radiation through reflection and absorption as shown in figure 4.

    ESS Figure 4

    Figure 4: Pathways of radiation through the atmosphere (Image source: IB Diploma Programme Environmental Systems and Societies Guide, First examinations 2017)

  3. Pathways of energy through an ecosystem include:
    • conversion of light energy to chemical energy
    • transfer of chemical energy from one trophic level to another with varying efficiencies
    • overall conversion of ultraviolet and visible light to heat energy by an ecosystem
    • re-radiation of heat energy to the atmosphere.
  4. The conversion of energy into biomass for a given period of time is measured as productivity.
  5. Net primary productivity (NPP) is calculated by subtracting respiratory losses (R) from gross primary productivity (GPP).
    • NPP = GPP – R
  6. Gross secondary productivity (GSP) is the total energy or biomass assimilated by consumers and is calculated by subtracting the mass of fecal loss from the mass of food consumed.
    • GSP = food eaten – fecal loss
  7. Net secondary productivity (NSP) is calculated by subtracting respiratory losses (R) from GSP.
    • NSP = GSP – R
  8. Maximum sustainable yields are equivalent to the net primary or net secondary productivity of a system.Matter also flows through ecosystems linking them together. This flow of matter involves transfers and transformations.
  9. The carbon and nitrogen cycles are used to illustrate this flow of matter using flow diagrams. These cycles contain storages (sometimes referred to as sinks) and flows, which move matter between storages.
  10. Storages in the carbon cycle include organisms and forests (both organic), or the atmosphere, soil, fossil fuels and oceans (all inorganic).
  11. Flows in the carbon cycle include consumption (feeding), death and decomposition, photosynthesis, respiration, dissolving and fossilization.
  12. Storages in the nitrogen cycle include organisms (organic), soil, fossil fuels, atmosphere and water bodies (all inorganic).
  13. Flows in the nitrogen cycle include nitrogen fixation by bacteria and lightning, absorption, assimilation, consumption (feeding), excretion, death and decomposition, and denitrification by bacteria in water-logged soils.
  14. Human activities such as burning fossil fuels, deforestation, urbanization and agriculture impact energy flows as well as the carbon and nitrogen cycles.

Applications and skills:

  • Analyse quantitative models of flows of energy and matter.
  • Construct a quantitative model of the flows of energy or matter for given data.
  • Analyse the efficiency of energy transfers through a system.
  • Calculate the values of both GPP and NPP from given data.
  • Calculate the values of both GSP and NSP from given data.
  • Discuss human impacts on energy flows, and on the carbon and nitrogen cycles.


  • Human impacts on the flows of energy and matter occur on a global scale.

Theory of knowledge:

  • The Sun’s energy drives energy flows, and throughout history there have been “myths” about the importance of the Sun—what role can mythology and anecdotes play in the passing on of scientific knowledge?


  • Introduction to the atmosphere (6.1)
  • Introduction to water systems (4.1)
  • Introduction to soil systems (5.1)
  • Human population carrying capacity (8.4)