Biology  by Bradford
support
IB Biology 2025 · Theme C · Interaction and interdependence

C4.2 Transfers of energy and matter. Practice questions with markscheme.

83 original IB-style questions on C4.2, written from the 2025 guide: 38 multiple-choice, 31 short-answer, 7 data-based, 3 extended-response parts, 3 drawing, 1 labelling. This subtopic has no additional higher level content, so the 22-mark practice paper below serves SL and HL alike. Hand one out as a class quiz or homework, or sit it yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

22 statements at SL and HL.

  1. C4.2.1SL / HL Ecosystems as open systems in which both energy and matter can enter and exit
  2. C4.2.2SL / HL Sunlight as the principal source of energy that sustains most ecosystems
  3. C4.2.3SL / HL Flow of chemical energy through food chains
  4. C4.2.4SL / HL Construction of food chains and food webs to represent feeding relationships in a community
  5. C4.2.5SL / HL Supply of energy to decomposers as carbon compounds in organic matter coming from dead organisms
  6. C4.2.6SL / HL Autotrophs as organisms that use external energy sources to synthesize carbon compounds from simple inorganic substances
  7. C4.2.7SL / HL Use of light as the external energy source in photoautotrophs and oxidation reactions as the energy source in chemoautotrophs
  8. C4.2.8SL / HL Heterotrophs as organisms that use carbon compounds obtained from other organisms to synthesize the carbon compounds that they require
  9. C4.2.9SL / HL Release of energy in both autotrophs and heterotrophs by oxidation of carbon compounds in cell respiration
  10. C4.2.10SL / HL Classification of organisms into trophic levels
  11. C4.2.11SL / HL Construction of energy pyramids
  12. C4.2.12SL / HL Reductions in energy availability at each successive stage in food chains due to large energy losses between trophic levels
  13. C4.2.13SL / HL Heat loss to the environment in both autotrophs and heterotrophs due to conversion of chemical energy to heat in cell respiration
  14. C4.2.14SL / HL Restrictions on the number of trophic levels in ecosystems due to energy losses
  15. C4.2.15SL / HL Primary production as accumulation of carbon compounds in biomass by autotrophs
  16. C4.2.16SL / HL Secondary production as accumulation of carbon compounds in biomass by heterotrophs
  17. C4.2.17SL / HL Constructing carbon cycle diagrams
  18. C4.2.18SL / HL Ecosystems as carbon sinks and carbon sources
  19. C4.2.19SL / HL Release of carbon dioxide into the atmosphere during combustion of biomass, peat, coal, oil and natural gas
  20. C4.2.20SL / HL Analysis of the Keeling Curve in terms of photosynthesis, respiration and combustion
  21. C4.2.21SL / HL Dependence of aerobic respiration on atmospheric oxygen produced by photosynthesis, and of photosynthesis on atmospheric carbon dioxide produced by respiration
  22. C4.2.22SL / HL Recycling of all chemical elements required by living organisms in ecosystems

In the bank for C4.2

  • 38 multiple-choice
  • 31 short-answer
  • 7 data-based
  • 3 extended-response parts
  • 3 drawing
  • 1 labelling
  • 0 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

Make your own

The practice paper

No additional higher level content in this subtopic, so one paper serves both · 22 marks
Take it online → Build a fresh paper Paper code BbB-EAAAAAAIABQAO0Qh
Biology · topic quiz
Standard level · topic practice, not an exam format
35 minutes22 marks

Paper code: this paper was generated, so the code is its recipe. Enter it at biologybybradford.com/exam-maker to rebuild this exact paper and its markscheme.

Covers C4.2 Transfers of energy and matter · SL
Name:
1.

The food web shows feeding relationships in a freshwater pond. Which statement about the stickleback is correct?

[1]
pondweedalgaepond snailwater fleamayfly nymphdiving beetle larvasticklebackheron
  1. It is a secondary consumer in every food chain.
  2. It occupies the second and third trophic levels.
  3. It occupies the third and fourth trophic levels.
  4. It is a tertiary consumer in every food chain.
2.

The diagram shows a food web in a forest stream. Dead leaves fall into the stream from trees on the banks and are partly broken down by bacteria and fungi before caddisfly larvae eat them.

algae on rocksdead leaves (detritus)mayfly nymphcaddisfly larvastonefly nymphbrook trout
(a)Identify the trophic level of the stonefly nymph.[1]
(b)Explain why the dead leaves are not a producer in this food web, although energy enters the web through them.[2]
(c)Explain why decomposers such as the bacteria and fungi are not usually shown in food chains.[2]
3.

Atmospheric CO₂ has been measured continuously at the Mauna Loa observatory, Hawaii, since the 1950s. Within each year the concentration oscillates, falling through the northern-hemisphere summer and rising through winter; the size of this annual swing is the seasonal amplitude. The table shows two related datasets: the mean CO₂ concentration and seasonal amplitude for each decade, alongside the global mean temperature anomaly (the difference from the 20th-century average temperature) for the same decade.

DecadeMean CO₂ concentration / ppmSeasonal amplitude / ppmGlobal temperature anomaly / °C
1970s3285.00.00
1980s3455.3+0.18
1990s3605.5+0.32
2000s3795.9+0.51
2010s4016.2+0.72
(a)Calculate the mean rate of increase of CO₂ concentration between the 1970s and the 2010s, in ppm per decade.[2]
(b)Describe the relationship between mean CO₂ concentration and global temperature anomaly shown by the data.[2]
(c)Using the data, deduce whether the annual oscillation or the long-term trend accounts for the change in CO₂ concentration between the 1970s and the 2010s.[2]
(d)Suggest a reason for the increase in seasonal amplitude across the decades.[2]
(e)Discuss whether these data alone demonstrate that rising CO₂ causes the rise in temperature.[2]
(f)Combustion released more CO₂ over these decades than the rise in atmospheric concentration accounts for. Suggest what happened to the remainder.[2]
4.

Outline the processes that transfer carbon between organisms and the atmosphere in the carbon cycle.

[4]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
Rebuild or edit this exact paper (and its markscheme): biologybybradford.com/exam-maker?code=BbB-EAAAAAAIABQAO0Qh

Show the markscheme

Markscheme BbB-EAAAAAAIABQAO0Qh

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. C4.2.10 [1]
  • C: eating water fleas (primary consumers) makes the stickleback a secondary consumer (third trophic level), while eating diving beetle larvae (secondary consumers) makes it a tertiary consumer (fourth level);
2. C4.2.5, C4.2.10, C4.2.12
  • (a) [1]
    • secondary consumer / third trophic level;
  • (b) [2]
    • the leaves are dead organic matter, not living organisms that fix energy;
    • the energy in them was captured by photosynthesis in the trees (the producers) outside the stream, OWTTE;
  • (c) [2]
    • they feed on dead organic matter from every trophic level, so cannot be placed at one level in a chain;
    • they release the energy in dead matter (as heat by respiration) rather than passing it on to a consumer / they recycle nutrients rather than energy, OWTTE;
3. C4.2.20, D4.3
  • (a) [2]
    • (401 − 328) ÷ 4 (decades);
    • = 18.25 ≈ 18 ppm per decade;

    Accept 18–18.3. Award ECF for correct method.

  • (b) [2]
    • (strong) positive correlation: temperature anomaly rises as CO₂ rises;
    • across the whole period / consistently in every decade (about +0.1 °C per ~10 ppm), OWTTE;
  • (c) [2 max]
    • the long-term trend: the mean rose by 73 ppm, over ten times the seasonal amplitude (5.0 to 6.2 ppm);
    • the annual oscillation cancels out within each year: the summer drawdown by photosynthesis is matched by the winter release by respiration/decomposition, so it causes no net change;
    • (so) the rise comes from an additional net input of CO₂, (mainly) from combustion (of fossil fuels), OWTTE;
  • (d) [2]
    • higher CO₂ (and a longer/warmer growing season) increases summer photosynthesis / plant growth;
    • (so) the summer drawdown (and matching winter release) both become larger, OWTTE;
  • (e) [2 max]
    • no, correlation does not (by itself) establish causation;
    • a third factor could (in principle) drive both / the direction of causation is not shown by the correlation;

    Beyond this subtopic. Award if given; the marks above can be earned without it.

    • D4.3.1 (however) the known greenhouse mechanism plus experimental/physical evidence supports causation, the data are consistent with it, OWTTE;
  • (f) [2 max]
    • it was taken up by carbon sinks;
    • (by) ecosystems in which photosynthesis exceeds respiration (e.g. growing or regrowing forests), giving a net uptake of CO₂;
    • CO₂ dissolved in the oceans / was fixed by marine producers (phytoplankton), OWTTE;

    Content pivot.

4. C4.2.17 [4 max]
  • photosynthesis fixes CO₂ (from air/water) into carbon compounds in producers;
  • feeding passes carbon compounds along food chains;
  • cell respiration (by all organisms) returns CO₂ to the atmosphere;
  • decomposition: saprotrophs respire carbon from dead matter, releasing CO₂;
  • combustion of biomass/fossil fuels releases CO₂;
  • (some carbon is stored long-term in) fossil fuels / peat / carbonate rocks (removed from circulation), OWTTE;

More in Theme C · Interaction and interdependence

All 40 IB Biology subtopics →  ·  Command terms →  ·  AP Biology units →  ·  Open the exam maker →