GPP and NPP
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of photosynthesis and food chains. You can test your knowledge on these below.
What is photosynthesis and where does it occur?
It’s the process by which green plants convert light energy into chemical energy in the form of organic compounds. It takes place in chloroplasts.
What are producers in a food chain?
Producers are autotrophic organisms (like green plants) that synthesise organic compounds from CO₂ and water using light energy.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains GPP and NPP or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What is GPP?
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The sun is the original source of energy for most ecosystems.
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Gross Primary Productivity is the total amount of chemical energy converted from light energy by photosynthesis in a given area and time.
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It represents all the energy fixed by autotrophs (photosynthesising plants) before any is lost through respiration.
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Even though Gross Primary Productivity (GPP) refers to the energy captured by producers via photosynthesis, it is not the same as the total energy entering the ecosystem. Here's why:
- Only a Small Fraction of Solar Energy Is Used
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Only around 1–3% of the total solar energy that hits plant leaves is converted into GPP.
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Most sunlight is:
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Reflected by clouds, dust, or plant surfaces.
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Not of the correct wavelength to be absorbed by chlorophyll.
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Transmitted through leaves without being absorbed by chlorophyll.
What is NPP?
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Net Primary Productivity is the energy available to the next trophic level (consumers). It is the amount of energy that is stored in biomass.
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It is calculated by subtracting the energy lost as heat through respiratory losses (R) from the GPP: NPP = GPP - R
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Energy is lost from respiration for processes such as cell division, active transport or metabolic reactions.
- NPP is crucial because it determines the energy available for growth and reproduction in herbivores and other consumers.
Units
- Both GPP and NPP are typically measured in: kJ m-2 year-1 (kilojoules per square metre per year)
- Remember EAT = Energy per Area per Time
Ecological Significance
- GPP shows the total photosynthetic productivity of a producer community.
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NPP reflects how much energy is stored in biomass and therefore supports food chains.
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Typically, 40–60% of GPP is lost due to respiration, meaning only about 40–60% becomes NPP.
Key Terms
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GPP: Total energy fixed by producers via photosynthesis.
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NPP: GPP minus respiratory losses; energy available to the next trophic level.
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Respiratory losses (R): Energy used by producers for metabolic processes.
No answer provided.
Exam Tips
Use correct units (kJ m⁻² yr⁻¹) in calculations.
Avoid saying “energy produced”; instead, use “energy transferred” or “converted”.
Always link NPP to energy available to consumers, as this is often the focus of application questions.
Don’t overlook the link between the absorption spectrum and the light energy that is not used in GPP.
No answer provided.
Describe the difference between gross primary productivity and net primary productivity. (3 marks)
- GPP is the total amount of energy fixed by producers during photosynthesis.
- NPP is the energy remaining after respiratory losses have been subtracted from GPP.
- NPP is the energy available to primary consumers (herbivores).
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!