Net Primary Production (AQA A Level Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
Net Primary Production
During photosynthesis, primary producers (such as plants and algae) convert light energy into chemical energy in biological molecules
Gross primary production (GPP) can be defined as the amount of chemical energy stored in the carbohydrates within plants (during photosynthesis)
In other words, the quantity of energy stored in the glucose produced represents the gross primary production
Net primary production (NPP) refers to the amount of energy available to herbivores in the plant’s biomass after plant respiratory losses
Of total energy trapped in glucose during photosynthesis, 90% of this energy will be released from glucose to create ATP for the plant
The plant uses most of its glucose to fuel active cellular processes
A significant percentage of the energy originally captured will not be used to create new plant cells and therefore will not be able to be passed on to herbivores via eating
The chemical energy that is leftover in a plant after respiratory loss is known as the net primary production (NPP)
Net primary production can therefore be calculated using the equation below:
NPP = GPP - R
This means that NPP, like GPP, is expressed in units of energy per unit area or volume, for example:
Using area: J m–2 (joules per square metre)
Using volume: J m–3 (joules per cubic metre)
NPP is important because it represents the energy that is available to organisms at higher trophic levels in the ecosystem, such as primary consumers (herbivores and omnivores) and decomposers
Net primary production (NPP) is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account
Net primary productivity
Net primary productivity is expressed using units of energy/mass per unit area per unit time
Time must be included as it is a rate
For example
Mj m–2 y-1 (megajoules per square metre per year)
kg km-2y-1 (kilograms per square kilometre per year)
Examiner Tips and Tricks
‘Net’ means ‘overall’ (i.e after any necessary deductions have been taken into account) so net primary production refers to the overall chemical energy available after the necessary deductions (in this case energy lost through respiration) have been taken into account. Again, make sure you are consistent with your units - if an exam question gives GPP and R in joules (per unit area), you should give the NPP in joules (per unit area) too, unless the question specifically asks otherwise.Note the difference between production and productivity! One is a rate, the other isn't.
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