Transfer of Energy

Transfer of Energy

• Energy flows from the sun to the first trophic level (producers) in the form of light
• Producers convert light energy into chemical energy
  • This occurs during photosynthesis, when producers convert carbon dioxide and water into glucose and oxygen
• Producers use this glucose (during respiration) to produce their own biomass
  • Biomass is a store of chemical energy
• When primary consumers consume (eat) producers, they break down the biomass of the producer (digestion) and use the chemical energy to increase or sustain their own biomass
• When secondary consumers consume (eat) primary consumers, they break down the biomass of the primary consumer (digestion) and use the chemical energy to increase or sustain their own biomass, and so on
• In this way, as chemical energy is transferred from one trophic level to the next, biomass is also transferred

Losses of energy (leading to losses of biomass)

• Not all energy is transferred from one trophic level to the next
• Approximately, only 10% of the energy of each trophic level is passed on to the next
• This is why food chains are rarely made up of more than six trophic levels – the total amount of energy available eventually becomes too small to support another trophic level
• As some of the energy transferred is needed by higher trophic levels to generate biomass, the gradual loss of energy up the food chain means that biomass also decreases the higher up the food chain you go

Energy is lost at each trophic level

Biomass is also lost at each trophic level

• These losses of energy (and therefore biomass) occur for several reasons:
  • Organisms rarely eat every part of the organism they are consuming – some of the biological material of plants and animals may be inedible (eg. many predators do not consume the bones of their prey)
  • Not all the ingested material is digested and absorbed, some is egested as faeces
  • Energy is used for movement
  • Energy is used to generate heat
  • Energy is used for metabolic processes
  • Some absorbed material is lost as waste:
    • Carbon dioxide and water are waste products of respiration
    • Water and urea are the waste products in the urine, which is produced when proteins are broken down

Calculating the efficiency of energy and biomass transfers

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

Worked Example

Figure 1 shows:

• A food chain with four trophic levels
• The total biomass of the organisms at each trophic level

Calculate the efficiency of biomass transfer from the first to the
second trophic level.

Give your answer to 3 significant figures.

Use the equation:

Exam Tip

This is a complicated concept but by learning the main ways in which energy is lost between trophic levels, you will be able to answer most questions on this topic.

Make sure you read the question carefully and tailor your answer to the specific organism you are being asked about – e.g. plants do not produce urine or faeces so you could not give this as one of the ways in which they use energy that cannot be passed on!