Energy flows once and is lost as heat. Matter cycles forever. Two laws that shape every ecosystem on Earth.
Every sub-topic below feeds at least one of these questions.
Transfers of Energy and Matter.
The required syllabus content for C4.2, in order. Each card is one lesson-sized checkpoint.
Food chains and food webs
Trophic levels: producers, consumers, decomposers
Detritivores vs decomposers; saprotrophic nutrition
NPP = GPP − Respiration | Units: kJ m⁻² yr⁻¹ or g dry mass m⁻² yr⁻¹
Energy transfer efficiency; the ~10% rule
Reasons for energy loss between trophic levels
Count of organisms at each level.
Carbon cycle: photosynthesis, respiration, decomposition
Combustion, fossilisation; carbon sinks and sources
How Saprotrophic Nutrition Works
Nitrogen cycle: fixation, nitrification, denitrification, ammonification ★HL
Leaching, eutrophication and soil fertility ★HL
IB Biology DP | C4.2 Transfers of Energy and Matter | SL & HL
Food chains run from producers up through consumers. Food webs are networks of interconnected chains. Arrows show direction of energy flow, not predation.
| Level | Role | Example |
|---|---|---|
| TL 1 · Producer | Autotroph — fixes CO₂ by photosynthesis | Oak tree |
| TL 2 · Primary consumer | Herbivore | Caterpillar |
| TL 3 · Secondary consumer | Carnivore eating herbivores | Blue tit |
| TL 4+ · Tertiary consumer | Apex predator | Sparrowhawk |
Both break down dead material — but differently. Together they're essential for nutrient cycling.
Earthworms, woodlice, dung beetles. Ingest dead matter; digest it inside their gut (intracellular). Fragment large pieces into smaller ones, increasing surface area for decomposers.
Saprotrophic. Secrete enzymes onto dead matter (extracellular); absorb soluble products. Release mineral ions (NH₄⁺, PO₄³⁻) directly into the soil.
Producers fix energy from sunlight. Some they use themselves for respiration; the remainder is available to consumers.
NPP = GPP − R
GPP = gross primary productivity (total energy fixed). R = energy lost to respiration. NPP = net primary productivity = energy available to consumers. Units: kJ m⁻² yr⁻¹ or g dry mass m⁻² yr⁻¹.
Example: Forest GPP = 8,500 kJ m⁻² yr⁻¹; respiration = 3,200 kJ m⁻² yr⁻¹; NPP = 5,300 kJ m⁻² yr⁻¹. ~62% of GPP becomes available to consumers.
Only a small fraction of energy at one trophic level reaches the next. Most is lost as heat from respiration.
Range 5–20% across ecosystems.
The biggest single loss. Lost forever to space.
Undigested material never absorbed; goes to decomposers, not the next level.
Organisms that die without being eaten — feed decomposers, not predators.
This is why food chains are short — by the 4th or 5th trophic level, there's barely any energy left to support large populations.
Pyramids of numbers, biomass, and energy each tell a different story about an ecosystem.
Can be inverted — one large tree supports millions of insects. Least informative because organism size is ignored.
Can be inverted in some systems — phytoplankton reproduce so fast that their standing biomass is less than zooplankton biomass at any instant.
Always upright. Energy must decrease at each level — second law of thermodynamics. The most accurate representation.
Four processes drive the carbon cycle. Two add CO₂ to the atmosphere; two remove it. Plus geological-timescale sinks.
Producers fix atmospheric CO₂ into glucose using light energy. Major removal pathway. Performed by plants, algae, cyanobacteria.
All organisms release CO₂ as they oxidise glucose for ATP. Continuous, day and night.
Saprotrophs break down dead organic matter, releasing CO₂ and mineral nutrients. The recycling channel.
Burning fossil fuels (coal, oil, gas) releases carbon fixed over millions of years. The major anthropogenic driver of rising atmospheric CO₂.
Dead organisms buried under heat and pressure for millions of years → coal, oil, natural gas. A geologically slow carbon sink. Oceans, forests and peat bogs are biological carbon sinks — they absorb more CO₂ than they release on contemporary timescales.
If you can't define one of these in a sentence, that's where to revise next.