Energy Flow

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Food chains

- Pyramids of biomass are charts that show the mass of living organisms at each step in a food chain.

- Energy is lost moving up in a food chain, and this limits the length of the chain.


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Food Webs

When all food chains in a habitat are joined together they form a food web. An example of a food web (

- Although it looks complex, it is just several food chains joined together

- What if one population decreases? Fewer slugs means more grass for the rabbits and insects therefore, an increase in population of insects and rabbits. However, the thrushes would end up eating more insects, so the population of insects could decrease in the process.

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Energy Transfer

- Energy is transferred along food chains, but not all of the energy available can be absorbed by organisms.

- The amount of available energy decreases from one stage to the next.

- Some of the available energy goes into growth and the production of offspring. This energy becomes available to the next stage, but most of the available energy is used up in other ways. For example:

- Energy released by respiration is used for movement and other life processes, and is eventually lost as heat to the surroundings

- Energy is lost in waste materials, such as faeces.

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Calculating Energy Efficiency

This bullock has eaten 100 kJ of stored energy in the form of grass, and excreted 63 kJ in the form of faeces, urine and gas. The energy stored in its body tissues is 4 kJ. So how much has been used up in respiration?

Energy balance in a bullock. 100 kJ eaten, 63 kJ excreted, and 4 kJ transferred to body tissue (

The energy released by respiration = 100 - 63 - 4 = 33 kJ

Only 4 kJ of the original energy available to the bullock is available to the next stage, which might be humans. The efficiency of this energy transfer is:

efficiency = 4 ÷ 100 × 100 = 4%

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A pyramid of biomass is a chart, drawn to scale, showing the biomass at each stage in a food chain. The bars become narrower as you reach the top. This pyramid of biomass is for the food chain:

oak tree → caterpillar → blue *** → sparrowhawk

Note that you do not need to draw the organisms, but you must draw your pyramid of biomass to scale. Each bar should be labelled with the name of the organism.

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Pyramids Of Biomass

Each stage in a food chain or pyramid of biomass is called a trophic level. It can be difficult to make a pyramid of biomass because:

- There may be problems measuring dry biomass

- An organism may belong to more than one trophic level, so it cannot easily be represented by one bar.

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