Syllabus Edition

First teaching 2023

First exams 2025

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Primary & Secondary Production (SL IB Biology)

Revision Note

Naomi H

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Naomi H

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Primary Production

  • During photosynthesis, autotrophs can convert light energy into chemical energy stored in biological molecules
    • Organisms that do this are known as producers 
  • The accumulation of carbon compounds in the biomass of autotrophs is known as primary production
    • Biomass accumulates when organisms grow and reproduce
  • Primary production occurs more quickly in some biomes than in others
    • Biomes that have more hours of sunlight, optimum temperatures and higher levels of rainfall will allow photosynthesis to occur at a higher rate, leading to faster primary production
  • E.g. tropical forest biomes have high levels of sunlight, rainfall, and ideal temperatures, so primary production occurs very quickly
rainforest-productivity-photo

CC BY-SA 4.0, via Wikimedia Commons

Tropical forest biomes have very high rates of primary production due to the high rate at which photosynthesis can occur

Measuring the rate of primary production

  • The rate at which producers convert light energy into chemical energy is expressed in units of biomass, per unit area (or volume), per unit time, e.g.
    • Using area: g m–2 yr-1 (biomass, per square metre, per year)
    • Using volume: g m–3 yr-1 (biomass, per cubic metre, per year)
      • Volume would be used when calculating rate of production in aquatic habitats

Secondary Production

  • Organisms that gain carbon compounds by ingesting other organisms are known as heterotrophs
  • When heterotrophs ingest other organisms, the chemical energy in the biomass of these organisms is transferred to the heterotrophs and stored in their own biomass
  • This process by which biomass is stored in the tissues of heterotrophs is known as secondary production
  • However, not all the energy stored in the carbon compounds consumed by heterotrophs is transferred to new biomass
    • Carbon is lost in the form of carbon dioxide as a waste product of respiration
    • Energy is lost during the excretion of other metabolic waste, such as water and urea
  • Due to the subtraction of respiratory losses, the rate of secondary production is always lower than the rate of primary production in an ecosystem
    • The rate of secondary production is therefore calculated by subtracting respiratory losses from the stored energy ingested by a heterotroph

Examiner Tip

The terminology here can be confusing, so be sure that you are happy with the following main ideas:

  • Primary production is the accumulation of biomass in autotrophs while secondary production is the accumulation of biomass in heterotrophs
  • The rate of production, i.e. how quickly or slowly production is taking place, is usually measured in g m-2 yr-1
  • Secondary production is lower than primary production due to energy losses at each trophic level

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Naomi H

Author: Naomi H

Expertise: Biology

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.