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5.3.5Energy Transfers In and Between Organisms (A Level only)

Net Primary Production — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 17 of 30 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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5.3.5 Net Primary Production — AQA A Level Biology 7402 notes: Define net primary production, calculate it from GPP and respiration and explain its significance for consumers.Net primary production (NPP) is the chemical energy store left in plant biomass after respiratory losses …

  • NPP = GPP − R
  • Exam-style question 1
  • Exam-style question 2
  • Respiration loss (R)
  • Climax community
  • Eutrophication

Section 1 of 8Definition and equation

Net primary production (NPP) is the chemical energy store left in plant biomass after respiratory losses have been subtracted from gross primary production. It is the energy actually available to the next trophic level (herbivores) and to decomposers.

NPP = GPP − R

where R is the energy lost through plant respiration.

Definition and equation
NPP is expressed in the same units as GPP, kJ m-2 year-1.
For example, if a woodland has a GPP of 20 000 kJ m-2 year-1 and respiration losses of 8 000 kJ m-2 year-1, NPP is 12 000 kJ m-2 year-1.

Section 2 of 8NPP as growth

NPP represents the energy stored in new plant tissue, so it is closely linked to growth and an increase in biomass.

1
In a year, NPP is partitioned among leaves, wood, roots and reproductive structures.
2
Some is lost by leaf fall and some is eaten by herbivores.
3
In a community in which NPP is large, a large number and mass of consumers can be supported.
4
When respiration is high relative to GPP,
  • such as in warm conditions where enzymes work fast but light is limited, NPP is low and the plant grows slowly.

Net production can be measured by the rate of increase in dry mass per unit area, giving a practical way to estimate NPP by repeated harvesting of plots. Dry mass values are converted to energy using calorimetry.

Section 3 of 8Comparing ecosystems and crops

The ratio NPP/GPP varies.

Comparing ecosystems and crops
Typically, plants use about 20–60% of GPP in respiration.
Young, fast-growing vegetation has a high ratio; mature forests have a lower ratio because they carry a large amount of non-photosynthetic living tissue that respires but does not photosynthesise.
In a stable climax community, NPP is largely balanced by losses to decay and consumption, so total biomass changes little.

In agriculture, the aim is to maximise NPP in the harvested parts. This is done with fertilisers, irrigation, pest control, selective breeding, and growing in a way that keeps the leaf canopy dense through the season.

Section 4 of 8Link to consumers and the carbon balance

Because consumers rely on NPP, the amount of energy available limits the biomass at higher levels.

Link to consumers and the carbon balance
When NPP rises, as in a nutrient-enriched lake, the food web can support more consumers (although eutrophication has damaging side-effects).
On a global scale, NPP is the net carbon uptake by vegetation; if respiration by all organisms and combustion exceed it, atmospheric CO2 increases.
TermMeaningRelationship
GPPTotal energy fixedStarting value
REnergy lost in plant respirationSubtracted
NPPEnergy stored in new biomassGPP − R

Section 5 of 8Worked example and exam approach

Suppose a crop field has a GPP of 25 000 kJ m-2 year-1 and the crop respires 9 500 kJ m-2 year-1.

Worked example and exam approach
NPP = 25 000 − 9 500 = 15 500 kJ m-2 year-1.
If the farmer harvests the grain, which contains 6 200 kJ m-2 year-1, the harvested share of NPP is 6 200 / 15 500 × 100 = 40%.
The rest remains as stems, leaves and roots which may be returned to the soil or burned.

When asked to explain why NPP differs between sites, link the factor to either GPP or R.

Worked example and exam approach
A cooler site may have a lower GPP because enzyme-controlled reactions are slower, but also a lower R, so NPP might fall less than GPP.
A site with a dense canopy has a higher GPP but also a larger respiring mass.
When asked how a farmer can raise NPP, give a measure and link it to the equation: fertiliser increases chlorophyll and enzyme production so GPP rises; weed control reduces competition so more resources are available; pest control keeps leaf area intact.

Remember the key principle that the harvestable energy of a crop is a fraction of NPP, and that selective breeding aims to increase the share of NPP stored in the edible parts, not only the total.

Section 6 of 8Exam-style questions

  1. A grassland has GPP of 18 000 kJ m-2 year-1 and NPP of 10 800 kJ m-2 year-1. Calculate the energy lost in respiration and the percentage of GPP used in respiration. [3]3 marks

  2. Explain why net primary production is a better indication of the energy available to herbivores than gross primary production. [2]2 marks

Section 7 of 8Key terms

TermMeaning
Net primary productionEnergy stored in producer biomass after plant respiration is deducted from GPP.
Respiration loss (R)Chemical energy used by plants for life processes and lost as heat.
Climax communityStable community at the end of succession with little net change in biomass.
EutrophicationEnrichment of water with nutrients, causing algal growth and oxygen depletion.
PartitioningAllocation of production to different plant organs.

Section 8 of 8Exam technique

1
Subtracting consumer respiration from GPP. Only the producer's respiration is subtracted to get NPP.
2
Writing NPP = GPP + R.
3
Leaving out units.
4
Assuming all NPP is eaten; some goes to decomposers or is stored in perennial tissue.

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