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Guiding questions

• How can flows of energy and matter through ecosystems be modelled?
• How do human actions affect the flow energy and matter, and what is the impact on ecosystems?

SL and HL knowledge statements

2.2.1 Ecosystems are sustained by supplies of energy and matter.

2.2.2 The first law of thermodynamics states that as energy flows through ecosystems, it can be transformed from one form to another but cannot be created or destroyed.

2.2.3 Photosynthesis and cellular respiration transform energy and matter in ecosystems.

2.2.4 Photosynthesis is the conversion of light energy to chemical energy in the form of glucose, some of which can be stored as biomass by autotrophs.

2.2.5 Producers form the first trophic level in a food chain.

2.2.6 Cellular respiration releases energy from glucose by converting it into a chemical form that can easily be used in carrying out active processes within living cells.

2.2.7 Some of the chemical energy released during cellular respiration is transformed into heat.

2.2.8 The second law of thermodynamics states that energy transformations in ecosystems are inefficient.

2.2.9 Consumers gain chemical energy from carbon (organic) compounds obtained from other organisms. Consumers have diverse strategies for obtaining energy-containing carbon compounds.

2.2.10 Because producers in ecosystems make their own carbon compounds by photosynthesis, they are at the start of food chains. Consumers obtain carbon compounds from producers or other consumers, so form the subsequent trophic levels.

2.2.11 Carbon compounds and the energy they contain are passed from one organism to the next in a food chain. The stages in a food chain are called trophic levels.

2.2.12 There are losses of energy and organic matter as food is transferred along a food chain.

2.2.13 Gross productivity (GP) is the total gain in biomass by an organism. Net productivity (NP) is the amount remaining after losses due to cellular respiration.

2.2.14 The number of trophic levels in ecosystems is limited due to energy losses.

2.2.15 Food webs show the complexity of trophic relationships in communities.

2.2.16 Biomass of a trophic level can be measured by collecting and drying samples.

2.2.17 Ecological pyramids are used to represent relative numbers, biomass or energy of the trophic levels in an ecosystem.

2.2.18 Non-biodegradable pollutants, such as polychlorinated biphenyl (PCB), dichlorodiphenyltrichloroethane (DDT) and mercury, cause changes to ecosystems through the processes of bioaccumulation and biomagnification.

2.2.19 Non-biodegradable pollutants are absorbed within microplastics, which increases their transmission in the food chain.

2.2.20 Human activities, such as burning fossil fuels, deforestation, urbanization and agriculture, have impacts on flows of energy and transfers of matter in ecosystems.

Knowledge statements for HL only

2.2.21 Autotrophs synthesize carbon compounds from inorganic sources of carbon and other elements. Heterotrophs obtain carbon compounds from other organisms.

2.2.22 Photoautotrophs use light as an external energy source in photosynthesis. Chemoautotrophs use exothermic inorganic chemical reactions as an external energy source in chemosynthesis.

2.2.23 Primary productivity is the rate of production of biomass using an external energy source and inorganic sources of carbon and other elements.

2.2.24 Secondary productivity is the gain in biomass by consumers using carbon compounds absorbed and assimilated from ingested food.

2.2.25 Net primary productivity is the basis for food chains because it is the quantity of carbon compounds sustainably available to primary consumers.

2.2.26 Maximum sustainable yields (MSYs) are the net primary or net secondary productivity of a system.

2.2.27 Sustainable yields are higher for lower trophic levels.

2.2.28 Ecological efficiency is the percentage of energy received by one trophic level that is passed on to the next level.

2.2.29 The second law of thermodynamics shows how the entropy of a system increases as biomass passes through ecosystems.

Practical activities

1. 2.2.3 Application of skills: Create system diagrams from a set of data of ecosystems showing transfers and transformations of energy and matter.
2. 2.2.10 Application of skills: Create a food chain from given data.
3. 2.2.14 Application of skills: Work out the efficiency of transfer between trophic levels.
4. 2.2.15 Application of skills: Create a food web from given data.
5. 2.2.17 Application of skills: Create pyramids of numbers, biomass and energy from given data.
6. 2.2.17 Application of skills: Follow experimental procedures on how to find biomass and energy from biological samples (plant material only).
7. 2.2.25 Application of skills: Use laboratory and field techniques for measuring primary and secondary productivity and work out GP and NP from data.
8. Add from existing PSOW booklet.

Possible engagement opportunities

• from official syllabus. Turn into CAS suggestions connected to UN SDGs.

Happy learning!