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Energy transfer and nutrient recyclingEdexcel A-Level Biology B: Flashcards

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Define gross primary productivity.

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Define gross primary productivity.
The rate at which plants fix energy by photosynthesis.
How is net primary productivity calculated?
NPP = GPP − R, where R is energy lost in plant respiration.
What does NPP represent?
The energy stored in new plant biomass, available to consumers and decomposers.
What is the formula for efficiency of energy transfer?
(Energy at the higher level ÷ energy at the lower level) × 100.
Producers 5000 and primary consumers 450 kJ. What is the efficiency?
450 ÷ 5000 × 100 = 9%.
Why is only about 10% of energy transferred between levels?
Much is lost as heat in respiration, in faeces and urine, and some is not eaten.
Why are food chains short?
Energy is lost at each level, so too little remains to support more levels.
How does restricting movement in farm animals increase efficiency?
Less energy is lost in respiration, so more goes into growth.
What is a saprobiont?
A bacterium or fungus that feeds on dead organic matter by extracellular digestion.
What is extracellular digestion?
Enzymes are secreted onto food outside the cell and the soluble products are absorbed.
What is ammonification?
Conversion of organic nitrogen compounds to ammonium ions.
What do nitrifying bacteria do?
Convert ammonium ions to nitrite and then nitrate.
Why does peat form in waterlogged soil?
Little oxygen and acidic, cold conditions slow the saprobionts, so dead matter accumulates.

Exam questions on Energy transfer and nutrient recycling

  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.
    Calculate the NPP of the grassland as a percentage of its GPP.2 marks
  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.
    Calculate the efficiency of energy transfer from the primary consumers to the secondary consumers. Give your answer to one decimal place.2 marks
  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.
    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
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).