Energy transfer and nutrient recyclingEdexcel A-Level Biology B: Subtopic test

1. 1
   In a grassland, the gross primary productivity (GPP) of the plants is 20 000 kJ m⁻² year⁻¹. The plants lose 8 000 kJ m⁻² year⁻¹ as heat in respiration.

   (a)

   What is the net primary productivity (NPP) of the grassland?

   [1 mark]

   • A28 000 kJ m⁻² year⁻¹
   • B8 000 kJ m⁻² year⁻¹
   • C2.5 kJ m⁻² year⁻¹
   • D12 000 kJ m⁻² year⁻¹

   (b)

   Why is NPP, rather than GPP, the energy available to primary consumers and decomposers?

   [1 mark]

   • ASome of the GPP is used by the plants in respiration and is lost as heat, so it is not stored in new biomass
   • BPrimary consumers cannot digest the energy fixed in photosynthesis
   • CThe plants use all of the GPP to make seeds
   • DNPP includes the energy in the sunlight that is not absorbed

   (c)

   Calculate the NPP of the grassland as a percentage of its GPP.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The energy stored in the biomass at three trophic levels of a lake food chain was measured over one year. Producers: 31 200 kJ m⁻² year⁻¹. Primary consumers: 3 380 kJ m⁻² year⁻¹. Secondary consumers: 405 kJ m⁻² year⁻¹.

   (a)

   What is the efficiency of energy transfer from the producers to the primary consumers?

   [1 mark]

   • A9.2%
   • B10.8%
   • C89.2%
   • D1.1%

   (b)

   Which is a reason why not all of the energy in the producers is transferred to the primary consumers?

   [1 mark]

   • AEnergy is destroyed as it passes along the food chain
   • BThe primary consumers make their own energy by photosynthesis
   • CSome of the producer biomass is not eaten or cannot be digested
   • DThe producers use all of their energy in photosynthesis

   (c)

   Calculate the efficiency of energy transfer from the primary consumers to the secondary consumers. Give your answer to one decimal place.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A farmer rears beef cattle in two ways. Group X graze freely outdoors on pasture. Group Y are kept in heated barns with little room to move and are fed on concentrated feed. After 12 months, the cattle in group Y have gained more body mass per unit of food energy than those in group X.

   (a)

   Suggest three reasons why the cattle in group Y convert more of their food energy into body mass than those in group X.

   [3 marks]

   (b)

   One hectare of wheat produces 8 000 MJ of edible energy per year. Using the idea that about 10% of energy passes from one trophic level to the next, explain why this hectare of wheat could support more people if they eat the wheat directly than if the wheat is fed to cattle first. Include a calculation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A compost heap of dead plant material, kept moist and aerated, warms up and shrinks in volume over a few weeks. In a nearby bog the soil is waterlogged, acidic and cold, and dead plant material accumulates as peat with very little decay.

   (a)

   Explain the role of microorganisms in the recycling of nutrients in the compost heap.

   [6 marks]

   (b)

   Explain why little decay occurs in the bog, and suggest the effect on the productivity of the plants growing there.

   [6 marks]

   Total for question 4: 12 marks

End of questions