7.17 Biomass transfer and efficiency
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Give three reasons why not all the biomass eaten by an animal becomes new biomass in that animal.
Mark scheme
Any three from:
- egested as faeces (not all digested / absorbed) (1)
- excreted as urea / CO2 / water (1)
- used in respiration (1)
- for movement / heat (1)
- some parts not eaten (bones) (1)
Model answer
Some is egested in faeces, some is excreted as urea and carbon dioxide, and much of the glucose is used in respiration for movement and keeping warm.
A field of grass contains 5000 kg of biomass. The cows that eat it gain 400 kg of biomass.
Calculate the efficiency of biomass transfer.
Mark scheme
- 400 ÷ 5000 × 100 (1)
- = 8% (1)
Model answer
efficiency = 400 ÷ 5000 × 100 = 8%.
Explain why food chains rarely have more than five trophic levels.
Mark scheme
- biomass / energy is lost at each level (1)
- not enough left to support further levels (1)
Model answer
So much biomass is lost at each trophic level that there is not enough left to support more levels.
Explain why mammals and birds have a lower efficiency of biomass transfer than fish or insects.
Mark scheme
- mammals / birds keep their body temperature constant (1)
- so more glucose used in respiration (for heat) (1)
Model answer
Mammals and birds use a lot of glucose in respiration to keep their bodies warm, so less is turned into new biomass.
About what percentage of the light energy falling on a plant is transferred into biomass?
Mark scheme
about 1%
Model answer
About 1%.
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