Topic 4 Energy Transfer

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Energy losses in food chains

How much light energy is used in photosynthesis?

  • 1-3%


  • Some is reflected
  • some is absorbed by atmosphere
  • some light is the wrong wavelength
  • sometimes light misses a chlorophyll

What happens to the light energy that is used in photosynthesis?

  • its turned into organic compounds e.g. glucose
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Food Chains

A food chain shows the flow of energy

Producers start the food chain. They are autotrophic (Builds us organic compounds from simple molecules)

They are photosythetic organisms and build up complex organic compounds in photosythesis

Consumers are hetrotrophic, this means they rely on an external source of organic compounds i.e. they take in organic compounds

Animals are consumers

Primary consumers eat producers

secondary consumers eat primary consumers

tetiary consumers eat secondary consumers

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Energy transfer between trophic levels

Gross Primary Product (GPP) is the energy converted to organic molecules (-feaces)

Net Primary Product (NPP) is the energy which is stored as new biomass (GPP- Respiration)

The NPP is passed on to next trophic level i.e. the primary consumer

there are large amounts of losses in this link:

  • large amounts of the plant may be indigestible
  • not all of the plant may be eaten
  • some energy is lost in excretion
  • energy lost in respiration

The NP (GP- R) is available to the next trophic level


food chains are usually no longer than 4-5 tropic levels as theres not enough energy to support a breeding population at a higher tropic level

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Calculating the efficiency of energy transfer



Percentage efficiency = Energy available after transfer      x100

                                      Energy available before transfer

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Pyramids of number

Bars are drawn which are proportional to the numbers of organisms present at each trophic level e.g.

Grass-----------------> Rabbit-------------------->Fox

There will be higer number of grass plants than rabbits and higher number of rabbits than foxes


  • size of organism isnt taken into account and this results in an inverted pyramid
  • it is also difficult to represent very large numbers accurately on the same scale in a food chain
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Pyramids of Biomass

These are more reliable than  pyramids of number and show the biomass at each trophic level rather than the number of organisms which overcomes the problems of size

Points to consider

  • Fresh mass can be measured but is unreliable as the water content varies so it is important to use dry mass
  • Dry mass involves killing organisms so the samples tend to be small and this will also affect reliability
  • Biomass only represents the situation at the given point in time and doesnt take into acocunt any changes at different times of the year
  • Pyramid may still be inverted due to fluctuations

e.g. Maine ecosystems where phytoplankton which are producers could at any point have less biomass than primary consumers. Over the course of the whole year the biomass of the phytoplankon would exceed that of the primary consumers

Biomass at any one point in time is called the standing crop

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Pyramids of energy

These are the most accurate representation and are always pyramid shaped

Consist of utilised energy in a given area in a given time so the bars represent the energy at each level

Producer bar represents the light energy converted into chemical energy in photosynthesis

The bars representing consumers are more reliable than biomass as biomass could be equal but different compounds may store different amounts of energy  e.g. 1g of fat stores twice as much energy as 1g of carbohydrate

  • Units are- Kj m-2 year-1
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