Syllabus Edition

First teaching 2024

First exams 2026

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Energy Flow in Ecosystems (DP IB Environmental Systems & Societies (ESS)) : Revision Note

Energy Flow in Ecosystems

  • Ecosystems rely on a constant supply of energy and matter to maintain their structure and function

    • Energy is essential for driving biological processes, while matter cycles through the ecosystem, being reused and recycled

  • Ecosystems are considered open systems, meaning they exchange both energy and matter with their surroundings

    • Energy enters ecosystems primarily from the sun, entering as sunlight and being converted into chemical energy by producers through photosynthesis

      • This energy is then transferred between trophic levels as organisms consume one another, with some energy lost as heat at each transfer

      • Decomposers break down organic matter, releasing energy and returning nutrients to the environment

    • Matter, such as nutrients and water, flows into and out of ecosystems through various processes like decomposition, nutrient cycling and precipitation

The first law of thermodynamics

  • Energy exists in many different forms, including light energy, heat energy, chemical energy, electrical energy and kinetic energy

  • The way in which energy behaves within systems can be explained by the laws of thermodynamics

    • There are two laws of thermodynamics

  • The first law of thermodynamics is as follows:

Energy can neither be created nor destroyed, it can only be transformed from one form to another

  • This is also known as the principle of conservation of energy

    • It means that the energy entering a system equals the energy leaving it

    • It means that as energy flows through ecosystems, it can only change from one form to another

  • The transfer of energy in food chains within ecosystems demonstrates the principle of conservation of energy:

    • Energy enters the system (the food chain or food web) in the form of sunlight

    • Producers convert this light energy into biomass (stored chemical energy) via photosynthesis

    • This chemical energy is passed along the food chain, via consumers, as biomass

    • All energy ultimately leaves the food chain, food web or ecosystem as heat energy

The second law of thermodynamics

  • The second law of thermodynamics states that:

Energy transfers in ecosystems are inefficient

  • This is because energy transfers in any system are never 100% efficient

  • The second law of thermodynamics explains the decrease in available energy within ecosystems:

    • In a food chain, energy is transformed from a more concentrated (ordered) form (e.g. light energy from the Sun), into a more dispersed or disordered form (heat energy lost by organisms)

    • Initially, light energy from the Sun is absorbed by producers

      • However, even at this initial stage, energy absorption and transfer by producers is inefficient

      • This is due to reflection, transmission (light passing through leaves) and inefficient energy transfer during photosynthesis

    • The energy that is converted to plant biomass is then inefficiently transferred along the food chain due to respiration and the production of waste heat energy

      • In ecosystems, the biggest losses occur during cellular respiration

      • When energy is transformed, some must be degraded into a less useful form, such as heat

    • As a result of these inefficient energy transfers, food chains are often short (they rarely contain more than five trophic levels)

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies Content Creator

Alistair graduated from Oxford University 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 & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Bridgette Barrett

Reviewer: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.

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