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IB DP Biology — C: Interaction and Interdependence | SL | C4.2 Transfers of Energy and Matter
📋 What You Will Learn
- Identify the trophic levels in a food chain and explain how energy flows through an ecosystem
- Explain why energy transfer between trophic levels is inefficient and calculate ecological efficiency
- Describe the carbon and nitrogen cycles, including all key processes and the organisms involved
- Analyse data on biomass pyramids, energy budgets, and biogeochemical cycles to draw conclusions
- Evaluate the impact of human activity on energy flow and nutrient cycling in ecosystems
🔍 Guiding Question How do ecosystems maintain the flow of energy and the cycling of matter, and what are the consequences when these processes are disrupted?
🌍 Why This Matters Every meal you eat represents a chain of energy transfers stretching back to the sun. The rising concentration of atmospheric CO₂ — now exceeding 420 ppm — is a direct consequence of disrupted carbon cycling. Understanding how energy flows and matter cycles through ecosystems is not abstract theory; it is the foundation of climate science, food security, and conservation biology.
The Ecosystem: A System of Flows
Here is the fundamental problem every ecosystem must solve: energy is constantly being lost, but matter cannot be lost — it must be recycled. These two facts drive everything. Energy enters ecosystems from the sun and exits as heat, never to return. Matter, by contrast, cycles endlessly — the carbon in your body may once have been in a dinosaur's bone, a coral reef, or the atmosphere above an ancient ocean.
An ecosystem is not just a collection of organisms. It is a network of energy flows and matter transfers, held together by feeding relationships, decomposition, and the chemistry of the atmosphere and soil. To understand it, you need to think in two currencies simultaneously: energy (which flows in one direction) and matter (which cycles).
📖 Key Definition — Ecosystem An ecosystem is a community of living organisms (biotic components) interacting with each other and with the non-living (abiotic) components of their environment, functioning together as a system.
📖 Key Definition — Trophic Level A trophic level is a feeding position in a food chain or food web. Producers occupy trophic level 1; primary consumers occupy trophic level 2; secondary consumers occupy trophic level 3; and so on.
📖 Key Definition — Producer A producer (autotroph) is an organism that synthesises organic molecules from inorganic sources using an external energy source — typically sunlight (photoautotrophs) or chemical energy (chemoautotrophs).
📖 Key Definition — Consumer A consumer (heterotroph) is an organism that obtains energy and nutrients by ingesting other organisms. Herbivores are primary consumers; carnivores feeding on herbivores are secondary consumers.
📖 Key Definition — Decomposer Decomposers are organisms (primarily bacteria and fungi) that break down dead organic matter by extracellular digestion, releasing inorganic nutrients back into the environment and completing biogeochemical cycles.
Energy Flow Through Ecosystems
Imagine you are a blade of grass in a meadow. The sun delivers energy to your leaves, and you convert roughly 1–3% of it into chemical energy stored in glucose. A grasshopper eats you. A frog eats the grasshopper. A hawk eats the frog. At every step, most of the energy that entered the previous organism is gone — lost as heat, used in respiration, excreted, or locked in parts that are