A level Biology B · Edexcel 9BI0 · Topic 10B
Energy flow and productivity
Key points
- Producers convert light energy into chemical energy by photosynthesis; only a small percentage of light reaching a plant is captured.
- Gross primary productivity (GPP): total rate of energy fixed by plants. Net primary productivity: NPP = GPP − R (energy lost in respiration). NPP is available for growth and consumers.
- At each trophic level, energy is lost by respiration, in faeces and urine, and in parts not eaten, so food chains are short.
- Efficiency of energy transfer = (energy at higher level ÷ energy at lower level) × 100.
- Productivity is measured in units such as kJ m year, and biomass by dry mass.
- Farming increases efficiency by limiting movement and heat loss of livestock and removing pests and competitors.
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Almost all energy in an ecosystem enters through photosynthesis. As it passes from one organism to the next, most of it is lost, which limits the length of food chains and explains why eating lower down the chain feeds more people.
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