IB Biology · Theme C: Interaction and interdependence · SL and HL
C4.2 Transfers of energy and matter
A one-page summary of C4.2 Transfers of energy and matter, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What is the reason matter can be recycled in ecosystems but energy cannot?
How is the energy that is lost by each group of organisms in an ecosystem replaced?
What C4.2 covers
Ecosystems, energy flow and feeding relationships
- C4.2.1An ecosystem lets everything cross its border
- C4.2.2 · NOSSunlight runs most ecosystems — but not all
- C4.2.3Energy moves one feeding step at a time
- C4.2.4Real communities are webs, not lines
- C4.2.5Death still has energy left in it
Autotrophs, heterotrophs and releasing energy
- C4.2.6Building matter costs energy
- C4.2.7Two very different power sources
- C4.2.8Someone else's molecules, rebuilt
- C4.2.9Every living thing respires
Trophic levels, energy pyramids and production
- C4.2.10The same species, different levels
- C4.2.11 · SkillsDrawing energy to scale
- C4.2.12Three leaks, one ~10% left
- C4.2.13Heat escapes twice, not once
- C4.2.14Why food chains run out of levels
- C4.2.15Measuring how fast life builds up
- C4.2.16Consumers build biomass more slowly
The carbon cycle, sinks, sources and recycling
- C4.2.17 · SkillsDrawing carbon's loop
- C4.2.18 · C4.2.19When an ecosystem tips the balance
- C4.2.20A yearly breath, visible in the data
- C4.2.20The rise underneath the wiggle
- C4.2.21Two processes, each other's supplier
- C4.2.22Not just carbon comes back around
C4.2 Transfers of energy and matter: summary
Open systems & energy flow
- Ecosystems are open systems: energy and matter both cross the boundary; sunlight is the principal, but not only, energy source (a generalization, with cave/deep-ocean exceptions).
- Chemical energy flows one-way through food chains/webs; decomposers recycle dead matter's remaining energy and nutrients.
Autotrophs & heterotrophs
- Autotrophs fix CO2 using light (photoautotrophs) or oxidation reactions (chemoautotrophs) — both energy-requiring.
- Heterotrophs digest others' carbon compounds and reassemble them; both groups respire, losing heat twice (making and using ATP).
Trophic levels & pyramids
- Trophic level depends on the food chain considered; pyramids of energy are drawn to scale and never invert.
- Roughly 90% of energy is lost at each transfer (uneaten, egested, respired), restricting food chains to a handful of levels.
Production
- Primary production = autotroph biomass accumulation (g m⁻² yr⁻¹); secondary production = heterotroph biomass accumulation, always lower.
- Biomes vary hugely in primary production, driven mainly by light, warmth and water availability.
Carbon cycle & balance
- Carbon cycles via photosynthesis, feeding and respiration; an ecosystem is a sink or source depending which process dominates.
- Combustion releases long-stored carbon; the Keeling Curve shows an annual wiggle riding on a rising long-term trend.
Recycling & interdependence
- Photosynthesis and respiration are mutually dependent, exchanging O2 and CO2 in huge annual fluxes.
- All elements life needs — not just carbon — are recycled by decomposers, including nitrogen, phosphorus and sulfur.
Key terms
- Ecosystem
- A community of organisms and their physical environment, functioning as an open system exchanging both energy and matter.
- Autotroph
- An organism that uses an external energy source to fix carbon dioxide into the organic carbon compounds it needs.
- Heterotroph
- An organism that obtains carbon compounds from other organisms and uses them to build its own required compounds.
- Trophic level
- An organism's feeding position in a specific food chain — producer, primary, secondary or tertiary consumer.
- Primary production
- The rate at which autotrophs accumulate carbon compounds in biomass, usually measured in g m⁻² yr⁻¹.
- Secondary production
- The rate at which heterotrophs accumulate carbon compounds in biomass — always lower than primary production.
- Carbon sink
- A system where photosynthesis exceeds respiration, giving a net uptake of atmospheric CO2.
- Carbon source
- A system where respiration or combustion exceeds photosynthesis, giving a net release of CO2.
- Decomposer
- A heterotroph that obtains energy from dead organic matter, releasing heat and recycling inorganic nutrients.
Sample exam questions
Three of the 68 multiple-choice questions for C4.2. Try each one before opening the answer.
Question 1. An ecosystem is best described as an open system because:
- Only energy can enter and leave; matter is fixed inside permanently
- Both energy and matter can enter and exit — for example sunlight enters and heat leaves, while nutrients and organisms can move in or out
- Neither energy nor matter can cross its boundary
- Matter enters continuously but energy is generated internally and never leaves
Show the answer
Answer: B. An ecosystem is an open system: energy enters mainly as sunlight and leaves mainly as heat, while matter (water, nutrients, migrating organisms, wind-blown seeds, etc.) can also cross the ecosystem boundary in both directions.
Question 2. Which statement correctly distinguishes an open system from a closed system, as used to describe ecosystems?
- In a closed system, both energy and matter can enter or leave; in an open system, neither can
- There is no meaningful difference between the two terms
- A closed system allows matter but not energy to cross its boundary
- In a closed system, only energy can enter or leave; in an open system, both energy and matter can enter or leave
Show the answer
Answer: D. A closed system exchanges energy with its surroundings but not matter (e.g. a sealed, transparent terrarium exposed to light). An open system, like a real ecosystem, exchanges both energy and matter with its surroundings.
Question 3. 'Sunlight sustains most ecosystems' is an example of a generalization (a law-like statement) in science. Which feature of laws does this best illustrate?
- Laws describe an observed pattern and can be used to make predictions, but do not themselves explain the underlying mechanism, and they may have exceptions
- Laws provide a mechanistic explanation for why the pattern occurs
- Laws are always true without any exceptions
- Laws replace the need for further observation
Show the answer
Answer: A. Laws in science (unlike theories) are generalized rules of thumb describing observed patterns; they let scientists predict energy flow in a new ecosystem, but the statement 'sunlight sustains most ecosystems' does not explain the mechanism and comes with known exceptions (caves, deep ocean).
Linking questions
Questions that connect C4.2 to other parts of the course, the kind that come up in Paper 2.
- Cell respiration (C1.2) is the process by which both autotrophs and heterotrophs release energy from carbon compounds, and its inefficiency is the direct cause of heat loss and reduced energy availability between trophic levels. Explain this connection. (see C1.2)
- Photosynthesis (C1.3) is the entry point for both carbon and energy into most ecosystems. Explain how the biochemistry of photosynthesis sets an upper limit on primary production and, ultimately, on the total energy available to consumers. (see C1.3)
- Food webs and trophic level classification (C4.2.4, C4.2.10) build directly on the populations and communities studied in C4.1 — for example predator-prey relationships and interspecific competition occur between organisms that also occupy specific trophic levels. Explain how energy flow constrains the structure of the communities described in C4.1. (see C4.1)
- Rising atmospheric CO₂, tracked by the Keeling Curve (C4.2.20), is a major driver of climate change (D4.3). Explain how disruption of the carbon sink/source balance described in this topic contributes to the enhanced greenhouse effect. (see D4.3)
Practise C4.2
Study notes, every question and full markschemes for C4.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.