What you’ll learn20 learning objectivesChoose one objective for a focused lesson, or study the complete topic.2.2.1Ecosystem sustenance• Sustained by supplies of energy and matter• Open systems with exchangesSyllabus objective2.2.2First law of thermodynamics• Energy transformed but not created or destroyed• Transformations: light to chemical, chemical to heatSyllabus objective2.2.3Energy and matter transformation• Photosynthesis and cellular respirationSyllabus objective2.2.4Photosynthesis• Light energy → chemical energy (glucose)• Stored as biomass by autotrophsSyllabus objective2.2.5Producers• First trophic level in food chain• Plants, algae, photosynthetic bacteriaSyllabus objective2.2.6Cellular respiration• Releases energy from glucose• Converts to usable chemical formSyllabus objective2.2.7Heat generation• Some energy transformed to heat during respiration• Not 100% efficientSyllabus objective2.2.8Second law and energy loss• The second law of thermodynamics states that energy transformations in ecosystems are inefficient• The second law of thermodynamics relates to the quality of energy, and that when energy is transformed, some must be degraded into a less useful form, such as heat• In ecosystems, the biggest losses occur during cellular respiration• The second law of thermodynamics explains why energy transfers are never 100% efficientSyllabus objective2.2.9Consumer energy sources• Consumers gain chemical energy from carbon (organic) compounds obtained from other organisms• Consumers have diverse strategies for obtaining energy-containing carbon compounds• Include, with examples, herbivores, detritivores, predators, parasites, saprotrophs, scavengers and decomposersSyllabus objective2.2.10Producers and food chains• 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• In a food chain, organic matter flows from primary producers to primary consumers to secondary consumers, and so onSyllabus objective2.2.11Food chains and trophic levels• 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• Traditionally, decomposers are not included in food chains as they typically gain carbon compounds from a variety of sources• However, consider the role of decomposers in energy transformations in food websSyllabus objective2.2.12Food-chain energy losses• There are losses of energy and organic matter as food is transferred along a food chain• Not all the food available to a given trophic level is harvested: of what is harvested, not all is consumed; of what is consumed, not all is absorbed; of what is absorbed• There is, therefore, never 100% transference of organic matter from one trophic level to the nextSyllabus objective2.2.13Gross and net productivity• 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• Consider: values of both GP and NP from given data• Losses due to cellular respiration are typically greater in consumers than in producers due to more energy-requiring activitySyllabus objective2.2.14Limits on trophic levels• The number of trophic levels in ecosystems is limited due to energy losses• Energy released by cellular respiration and lost as heat by organisms is unavailable to organisms in higher trophic levels• Because of this and other energy losses, typically 10% or less of the energy flowing to a trophic level is available to the next level, limiting the length of food chains• Avoid the common misconception that organisms at higher trophic levels must eat more food to get enough energySyllabus objective2.2.15Food webs• Show complexity of trophic relationships• Arrows indicate energy flow direction• Species may feed at multiple trophic levelsSyllabus objective2.2.16Biomass measurement• Dry mass of samples ≈ organic matter mass• Energy measured by combustionSyllabus objective2.2.17Ecological pyramids• Represent numbers, biomass, or energy of trophic levels• Pyramids of energy: kJ m⁻² year⁻¹Syllabus objective2.2.18Non-biodegradable pollutants• PCB, DDT, mercury• Bioaccumulation: increase over time in organisms• Biomagnification: increase along food chainSyllabus objective2.2.19Microplastics• Absorb non-biodegradable pollutants• Increase transmission in food chainSyllabus objective2.2.20Human activity impacts• Burning fossil fuels, deforestation, urbanization, agriculture• Reduce primary productivity, disrupt food websSyllabus objective