Transfers of energy and matter — IB Diploma Biology HL

Revision notes

9 short notes, in the order of the specification. Each one in short:

1. Ecosystems as open systems and the source of energy

   Ecosystems are open systems: both energy and matter can enter and exit them. In a closed system only energy can pass in and out. Sunlight is the principal source of energy that sustains most ecosystems, captured by photosynthesis. Exceptions include ecosystems in caves and below the depth of light penetration in oceans, which are sustained by chemical energy.

2. Food chains, food webs and decomposers

   Chemical energy flows through food chains as each consumer feeds on the organism at the previous stage. Food chains and food webs represent feeding relationships in a community, with arrows showing the direction of transfer of energy and biomass. Decomposers obtain their energy from carbon compounds in dead organic matter: faeces, dead parts of organisms and dead whole organisms.

3. Autotrophs and heterotrophs

   Autotrophs are organisms that use external energy sources to synthesise carbon compounds from simple inorganic substances. Photoautotrophs use light; chemoautotrophs, such as iron-oxidising bacteria, use energy from oxidation reactions. Heterotrophs use carbon compounds obtained from other organisms to synthesise the carbon compounds they need. Both release energy by oxidising carbon compounds in cell respiration.

4. Trophic levels and energy pyramids

   Practical

   A trophic level is the position of an organism in a food chain: producer, primary consumer, secondary consumer or tertiary consumer. Many organisms have a varied diet and occupy different trophic levels in different food chains. An energy pyramid shows the energy flowing through each trophic level per unit area per year, with bars drawn to scale.

5. Energy losses between trophic levels

   Energy availability falls at each successive stage in a food chain because large amounts of energy are lost between trophic levels. Not all of an organism is eaten, not all that is eaten is digested, and much energy is converted to heat in cell respiration. These losses restrict ecosystems to a small number of trophic levels.

6. Primary and secondary production

   Primary production is the accumulation of carbon compounds in biomass by autotrophs. Secondary production is the accumulation of carbon compounds in biomass by heterotrophs. Both are measured as mass of carbon per unit area per unit time, usually g m⁻² yr⁻¹. Secondary production is lower than primary production because carbon compounds are lost as carbon dioxide and water in respiration.

7. The carbon cycle, carbon sinks and combustion

   Carbon is recycled in ecosystems by photosynthesis, feeding and respiration. An ecosystem is a carbon sink if photosynthesis exceeds respiration, giving a net uptake of carbon dioxide, and a carbon source if respiration exceeds photosynthesis. Combustion of biomass, peat, coal, oil and natural gas releases carbon dioxide; human activities have greatly increased combustion rates.

8. The Keeling Curve

   The Keeling Curve is the record of atmospheric carbon dioxide concentration measured at Mauna Loa, Hawaii, since 1958. Each year CO₂ falls in the northern summer, when photosynthesis exceeds respiration, and rises in winter, when respiration exceeds photosynthesis. The long-term rise, from about 315 ppm to over 420 ppm, is caused mainly by combustion of fossil fuels.

9. Interdependence of autotrophs and heterotrophs and recycling of elements

   Aerobic respiration depends on atmospheric oxygen produced by photosynthesis, and photosynthesis depends on atmospheric carbon dioxide produced by respiration. These huge annual fluxes make this a major interaction between autotrophs and heterotrophs. All chemical elements required by living organisms, not just carbon, are recycled in ecosystems, with decomposers playing a key role.