# 5.3.5 Net Primary Production

### Net Primary Production

• During photosynthesis, primary producers (such as plants and algae) convert light energy into chemical energy in biological molecules
• The amount of this chemical energy that is stored in plant biomass in a given area is known as the gross primary production (GPP)
• However, not all of the products from photosynthesis are used to form new plant biomass
• Some products of photosynthesis are used as respiratory substrates
• This means they are used up in respiration to produce energy
• Some of the energy is lost as heat
• In fact, approximately 50% of gross primary production (GPP) is lost to the environment as heat when plants respire
• This is known as respiratory loss (R)
• 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 the energy that is available to a plant for growth and reproduction. it is the energy that is stored in the plant’s biomass (after respiratory losses have been taken into account)
• NPP is also important because it represents the energy that is available to organisms in 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

#### Exam Tip

‘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.

### Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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