Transfers Along a Food Chain (Edexcel IGCSE Biology: Double Science)

• Light energy from the sun enters the first trophic level of food chains when producers convert light energy into chemical energy
  • Light energy is transferred to chemical energy in the form of carbon-containing compounds within the biomass of producers
• Energy and chemical substances are then transferred through food chains when:
  • primary consumers eat producers, breaking down the carbon compounds within their biomass and using the energy and substances released to build new biomass within their own tissues
  • secondary consumers eat primary consumers, again breaking down the molecules that make up their biomass and using the energy and substances to build their own tissues
  • this process is repeated at each trophic level
• Some energy and substances are lost to the environment between trophic levels

Energy and chemical substance are transferred through food chains when organisms consume other organisms

• When organisms consume other organisms, not all of the stored energy is passed to the next trophic level; only around 10% of the energy available at each trophic level is converted into biomass at the next level
  • This explains why food chains are short; the energy available eventually becomes too small to support another trophic level

Energy is lost at each trophic level of a food chain

Explaining energy loss from food chains

• Energy may be lost from food chains because consumers are not able to digest and absorb all of the chemical energy stored in food organisms
  
  • Organisms rarely eat every part of the organism they are consuming, e.g. many predators do not consume the bones, fur, teeth or claws of their prey, so energy stored in uneaten body parts does not pass to the consumer
  • Not all the ingested material is digested and absorbed, some is egested as faeces, e.g. plant material can be difficult to digest and passes through the digestive system of herbivores
• Once a consumer has digested and absorbed its food, not all of the energy will be converted to biomass; energy may be unavailable for building biomass due to:
  • heat loss during respiration
  • transfer into waste products, e.g.
    
    • carbon dioxide and water are waste products of respiration
    • urea is a waste product of protein breakdown

Energy may be unavailable to the next trophic level when it is lost as heat during respiration, when undigested food is egested, and when energy is transferred to waste products

Calculating the efficiency of energy or biomass transfer

• You may be asked to calculate the efficiency of energy and biomass transfers between trophic levels using percentages