International A Level Biology / Environment and climate change

Gross and net primary productivity revision notes

Edexcel International A Level Biology (YBI11), chapter 16: Environment and climate change, specification point 5.9. Free lesson.

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Gross primary productivity (GPP) is the chemical energy made by photosynthesis, while net primary productivity (NPP) is what remains after plant respiration. NPP shows how much energy is stored in new plant biomass and can become available to other organisms.

How light energy becomes plant biomass

GPP is the rate at which plants convert light energy into chemical energy in organic molecules by photosynthesis. Only a small fraction of light falling on leaves is used. Some light misses the chloroplasts, passes through the leaf or is reflected. Plants respire all the time, using some of the energy and releasing it as heat. What remains is stored in new plant biomass as NPP. Primary means producers, the first trophic level. NPP supports plant growth and reproduction, and provides energy for consumers and decomposers.

How to calculate NPP

Use the relationship between the total energy made by photosynthesis and the energy used by plant respiration. GPP, respiration and NPP are all rates. They are usually measured in kJ m⁻² year⁻¹. NPP can also be measured as dry biomass in g m⁻² year⁻¹, because water content varies. GPP is the total chemical energy made by photosynthesis. R is the energy used in plant respiration. NPP is the energy left in new plant biomass.

How to calculate a percentage

To find the percentage of GPP used in respiration, compare R with the total GPP. The percentage left as NPP is the remainder.

Why NPP differs between ecosystems

NPP depends on how strongly limiting factors affect photosynthesis. Tropical rainforest has one of the highest NPP values on land, while Arctic tundra has one of the lowest. Warmth increases respiration as well as photosynthesis, so a high GPP does not guarantee a high NPP. Rainforests are warm all year, so photosynthesis enzymes work quickly. Tundra cold slows them down. Rainforests receive intense light all year. Tundra light is weak, with weeks of winter darkness. Rainforests receive heavy rainfall. Much of the tundra's soil water is frozen and unavailable to plants. Rainforests grow…

NPP belongs to producers

NPP describes energy stored in new plant biomass, so it applies to producers, not animals. Animals can store energy in their own biomass, but this is not called NPP. For example, hibernating bears use energy stored in fat, built up by feeding before winter.