Efficiency of Energy Transfer: Extended (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))

Extended Tier Only

• Energy is transferred through food chains when organisms consume the tissues of other organisms
• The transfer of energy between trophic levels is not efficient
  • Not all of the energy received by an organism is transferred to stored energy in the tissues, but may instead be lost to the environment; energy lost in this way is not available to the next trophic level, e.g.
    
    • Movement requires energy
    • Respiration generates heat which is lost from the body to the environment
    • Processing and excreting metabolic waste, e.g. urea in urine, requires energy
  • Not all of the energy stored in the tissues of an organism is available to consumers at the next trophic level, e.g. because:
    • Energy may be stored in body parts that are not eaten, e.g. bones and fur of animals, or the roots of plants, so does not pass to the next trophic level
    • Energy may be stored in parts of a consumed organism that cannot be easily digested, so energy stored in undigested waste, i.e. faeces, does not pass to the next trophic level
• Energy loss at each trophic level explains why food chains are rarely more than 5 organisms long; at higher trophic levels organisms would need to eat an unrealistic volume of food to gain enough energy to survive

Energy is lost at each trophic level

Energy transfer in a human food chain

• Humans are omnivores, obtaining energy from both plants and animals, and this gives us a choice of what we eat
• These choices, however, have an impact on what we grow and how we use ecosystems
• Think of the following food chains both involving humans:

wheat  →  cow   →   human

wheat  →   human

• Given what we know about energy transfer in food chains, it is clear that if humans eat wheat there is much more energy available to them than if they eat the cows that eat the wheat
• This is because energy is lost from the cows, so there is less available to pass on to humans
• Therefore, it is more energy efficient for humans to be the herbivores rather than the carnivores
  • In reality, we often feed animals on plants that we cannot eat, e.g. grass, or that are too widely distributed for us to collect, e.g. algae