What you need to know
- 5.9(i)Relationship between GPP, NPP and plant respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. (R).
- 5.9(ii)Calculate net primary productivityNPP: the rate at which energy is stored in plant biomass after respiratory losses. NPP = GPP − R..
Trophic-transfer percentages are 5C 1. Consumer respiration is not the R in NPP = GPP − R. Units stay as given; do not convert unless asked.
By the end, you should be able to…
- Define GPP, NPP and R.
- Rearrange NPP = GPP − R.
- Explain why NPP is the energy available to primary consumerAn organism that gets its energy by eating other organisms..
Build your understanding
Three linked quantities
gross primary productivityGPP: the rate at which plants store energy in organic molecules by photosynthesis, per unit area per year. (GPP) is the total chemical energy fixed by photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen. in producerAn organism, usually a plant, that makes its own organic molecules by photosynthesis. Producers form the first trophic level. in a given time, usually also for a given area. Units you will meet include kJ m⁻² year⁻¹. GPP is not “greenness” and not a chlorophyllThe green pigment in chloroplasts. It absorbs mainly red and blue light. reading from a satellite.
Producers respire. That plant respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. is R in this topic. net primary productivityNPP: the rate at which energy is stored in plant biomass after respiratory losses. NPP = GPP − R. (NPP) is GPP minus R: NPP = GPP − R. NPP is the energy stored in new producer biomassThe mass of living material, often measured as dry mass per unit area. — the store that can be eaten, die into decomposers, or accumulate as wood and litter.
Calculate, then interpret
If GPP is 28 000 kJ m⁻² year⁻¹ and R is 9 500 kJ m⁻² year⁻¹, NPP = 18 500 kJ m⁻² year⁻¹. If you are given GPP and NPP, R = GPP − NPP. Adding R would invent biomass that does not exist.
NPP is always less than GPP in a living producer communityAll the populations of different species living in one habitat at the same time. because respiration is not zero. A student who writes NPP = GPP + R has reversed the energy account.
Who gets NPP
Primary consumerAn organism that gets its energy by eating other organisms. eat producer biomass. That biomass holds NPP, not the whole of GPP. Saying “herbivores use GPP” skips plant respiration. Decomposers also use dead producer tissue, which is still NPP that was not eaten while alive.
Light, carbon dioxide, water and temperature can change GPP, as in 5A and Core Practical 10. A hotter night can raise R and therefore cut NPP even if daytime photosynthesis is unchanged. That link foreshadows Q10Temperature coefficient: the rate at (T + 10) °C divided by the rate at T °C. in 5C 8; you do not need the formula yet.
NOT: NPP is not GPP plus energy from soil minerals. Minerals are not an energy input in this equation.
- Worked values: GPP 28 000 and R 9 500 give NPP 18 500 kJ m⁻² year⁻¹
- If GPP and NPP are given, R = GPP − NPP
- NPP is less than GPP because respiration is not zero
- Herbivores use NPP, not the whole of GPP
- A warmer night can raise R and cut NPP even if daytime GPP is unchanged
Two traps this diagram blocks
- Do not write NPP = GPP + R.
- Do not describe GPP as a measure of greenness.
From the diagram alone
Question
GPP is 28 000 kJ m⁻² year⁻¹ and plant respiration is 9 500 kJ m⁻² year⁻¹. Calculate NPP and explain why herbivores do not receive the GPP figure. [3]
Show a model answer
NPP = 28000 − 9500 = 18 500 kJ m⁻² year⁻¹. The 9 500 kJ has been used in plant respiration, so it is not stored in biomass that a herbivore can eat.
| Symbol | Meaning | Typical mistake |
|---|---|---|
| GPP | Energy fixed by photosynthesis | Calling a leaf-area index GPP |
| R | Plant respiration in this equation | Adding cow respiration into R |
| NPP | GPP − R; biomass available to consumers | Treating NPP as larger than GPP |
Make your reasoning visible
Write the equation before the numbers
NPP = GPP − R, then substitute.
Keep the units
kJ m⁻² year⁻¹ stay kJ m⁻² year⁻¹.
Key explanations
- 28 000 − 9 500 = 18 500 kJ m⁻² year⁻¹ in the worked example.
- Herbivores and decomposers use NPP. They do not receive the energy the plant has already respired.
Avoid this misconception
The claim: NPP is GPP plus the energy plants get from the soil.
The correction: NPP is GPP minus plant respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP.. Minerals from soil are not an energy input in 5.9.
If a wood fixes 28 000 kJ of energy per square metre each year, why can herbivores eat so much less?
Gross primary productivity (GPP) is the energy fixed by photosynthesis. The producers respire some of it (R). What is left, the net primary productivity (NPP), becomes new biomass.
What the model shows
A producer month by month, with GPP in and R out, and the annual account NPP = GPP − R.
What to look for
Months when NPP is negative, and how warmer nights and cloudier summers change GPP, R and NPP.
What it tells us
NPP = GPP − R: 28 000 − 9 500 = 18 500 kJ m⁻² year⁻¹. NPP is the energy available to consumers and decomposers.
Be careful what you claim
Fictional ecosystems and an illustrative monthly pattern. R here is the producers’ respiration only.
Try it in the model: drag the Month slider from summer into winter.
GPP, respiration and NPP
Follow a fictional ecosystem through the year: light fixed as GPP, energy respired as R, and what is left as NPP. Warm the nights or cloud the summer and see which term changes.
What you see
- Transfers: a food chain with the energy stored in each trophic level, then two bars showing where one level’s energy goes: not eaten, egested, excreted, respired as heat or stored in the next level. The graph is a pyramid of energy.
- NPP: a producer in the month you choose, with GPP arriving as light and R leaving as heat, an annual bar of GPP split into R and NPP, and a month-by-month graph.
How to use it
- Transfers: choose a food chain and a transfer (or click an arrow), and choose whether to divide the first transfer by NPP or GPP.
- NPP: choose an ecosystem and a month, change the light and night warming, and choose whether to find NPP or R.
What it shows
- How to calculate the efficiency of energy or biomass transfer between trophic levels (5.10).
- Why transfers are incomplete and food chains are short.
- The relationship between GPP, NPP and plant respiration: NPP = GPP − R (5.9(i)).
Limits of the model
- All ecosystems and figures are fictional, chosen to match the lesson’s worked examples. Real values vary a lot from year to year and place to place.
- Each consumer level is treated as eating only the level below. Real food webs have omnivores and decomposer chains.
- Respiration follows a simple Q₁₀ of 2, with half of it at night. The monthly split is illustrative.
Guided tour · step 1 of 9
In Ecosystem, just below these steps, drag Month (Jan–Dec) to Jun or Jul, or click a bar on the graph in the simulator.
With full leaves and long days GPP is large, much larger than R, so NPP is large and the trees grow.
Ecosystem
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Define GPP.
Show suggested answer
The total chemical energy fixed by photosynthesis in producers in a given time (and usually area).
Practice question
Retrieval and explanation · suggested answer
GPP = 42 000 kJ m⁻² year⁻¹ and NPP = 25 000 kJ m⁻² year⁻¹. Calculate R.
Show suggested answer
R = GPP − NPP = 17 000 kJ m⁻² year⁻¹.
Practice question
Retrieval and explanation · suggested answer
Why is NPP always less than GPP in a living crop?
Show suggested answer
Producers respire some of the energy they fix, so R is greater than zero and NPP = GPP − R is smaller than GPP.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two fields have the same GPP. Field A has higher night temperatures and a lower NPP. Explain, stating one assumption.
Show a hint
R is plant respiration.
Explore the reasoning
Higher night temperature can raise plant respiration R. If GPP is unchanged, NPP = GPP − R falls. Assume water and pests are not the hidden difference, and that R in the equation is still producer respiration only.
Recall this lesson
Lesson recall questions
Question 1 of 5: Define gross primary productivity (GPP).
GPP, NPP and plant respiration
Practice question
Retrieval and explanation · suggested answer
Define gross primary productivity (GPP).
Lesson: 5C 2: Net Primary Productivity
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A green map is not GPP
5C · Net Primary ProductivityNASA Earth Observatory chlorophyll maps estimate photosynthetic pigment in surface water as a proxy for phytoplankton. Pigment is not GPP, and it is not NPP. Crop and ocean…Specification 5.9(i)Read & use the scienceClose article
Why this matters
NASA Earth Observatory chlorophyll maps estimate photosynthetic pigment in surface water as a proxy for phytoplankton. Pigment is not GPP, and it is not NPP. Crop and ocean scientists still have to measure or model how much of the fixed carbon is respired. Treat satellite greenness as enrichment, not a 5.9 data table. Clouds, coloured dissolved matter and water depth can change the signal too. Ground or ship measurements help check a satellite estimate before it is converted into a statement about carbon fixed by organisms.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.9(i) understand the relationship between gross primary productivity (GPP), net primary productivity (NPP) and plant respiration (R)
Use the science
If a satellite map is greener this year, can you conclude NPP has risen?
Explore a possible response
No. Greenness tracks pigment, not the GPP−R account. Respiration, mixed pixels and depth all still matter.
Explore the source
Measuring a planet’s net production
5C · Net Primary ProductivityNASA scientists combine satellite observations with surface data to map net primary production. The word net matters: it is carbon fixed by photosynthesis after the producer’s own respiration…Specification 5.9(i) · 5.9(ii)Read & use the scienceClose article
Why this matters
NASA scientists combine satellite observations with surface data to map net primary production. The word net matters: it is carbon fixed by photosynthesis after the producer’s own respiration is subtracted. A greener landscape may have more leaf area, yet its NPP need not increase if respiration, drought or temperature also changes. The satellite map is a modelled estimate, checked against field measurements, not a direct reading of every leaf’s biochemical rate. In a school calculation, GPP minus R gives NPP. In research, estimating all three terms across a forest or ocean introduces uncertainty.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.9(i) understand the relationship between gross primary productivity (GPP), net primary productivity (NPP) and plant respiration (R)
- 5.9(ii) be able to calculate net primary productivity
Use the science
If two forests have equal GPP but one has higher plant respiration, which has lower NPP and why?
Explore a possible response
The forest with higher plant respiration has lower NPP because NPP = GPP − R.
Explore the source
- NASA satellites measure Earth’s metabolismNASA Earth Observatory