Keywords: IB Biology Topic C4.2, Energy Flow, Trophic Levels, Food Chains, Food Webs, Pyramid of Energy, 10% Rule, Carbon Cycle, Methanogenesis, Peat Formation, Fluxes and Sinks.
Welcome to the economy of nature: Topic C4.2 Energy Flow and Matter Cycles. In the new IB Biology syllabus, this unit focuses on the 'Bio-Logic' of thermodynamics—specifically, why energy can only be used once while matter can be used forever. You must understand that every time energy moves from a plant to a herbivore, a massive amount is lost as heat, which limits the length of food chains.
This unit is a staple for Paper 1A (MCQs) and Paper 2 data-analysis. You are expected to interpret energy pyramids (units: \,m^{-2}\,y^{-1}$) and explain the carbon cycle in terms of fluxes (processes) and sinks (storage). The curriculum now emphasizes the specific chemical transformations of carbon, such as methanogenesis and the formation of fossil fuels.
Before we look at the diagrams, remember this fundamental distinction: Sunlight is the 'income' that keeps the planet running, but Carbon is the 'currency' that makes up the bodies of organisms. Energy is a one-way street; Matter is a roundabout.
Energy enters most ecosystems as sunlight. It is converted into chemical energy (glucose) by producers and then passed along trophic levels. However, energy cannot be recycled.
The Bio-Logic: Energy is "dissipated" (Option B). Because you lose roughly 90% at every step, a fifth-level consumer would require a massive territory just to find enough calories to survive. The "energy tax" is simply too high.
A pyramid of energy is a quantitative representation of the energy available at each trophic level. Unlike pyramids of numbers or biomass, pyramids of energy can **never** be inverted.
The Approach: Energy is a rate of flow. You need to know how much energy passed through that level over a year. Option C is the only one that includes energy ($kJ$), area ($m^{-2}$), and time ($yr^{-1}$).
Carbon moves between the atmosphere, biosphere, and lithosphere. You must know the specific chemical processes (Fluxes) that move carbon.
The Bio-Logic: Peat forms when saprotrophs cannot work (Option B). They need oxygen to respire; without it, they can't break down dead plants. The carbon stays "trapped" in the ground, eventually becoming coal over millions of years.
A **Sink** is a place where carbon is stored (e.g., the ocean, forests, fossil fuels). A **Flux** is the rate of transfer between sinks.
The Logic: Fossil fuels are a sink. Burning them creates a flux (Option A) that moves carbon that has been buried for millions of years back into the atmosphere as $CO_2$.
The IBO loves to compare these two. Use this checklist:
Final Summary: Topic C4.2 is about the balance of the biosphere. Energy limits the 'height' of life (trophic levels), while the carbon cycle determines the 'breadth' of life (biomass). Master the 10% rule and the conditions for peat formation, and you will be well-prepared for any ecology paper.
Click the black box to reveal the answers!