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

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

70 original IB-style questions on C4.2, written from the 2025 guide: 32 multiple-choice, 24 short-answer, 7 data-based, 3 extended-response part, 3 drawing, 1 labelling. Below is a 21-mark standard-level practice paper built from them, ready to hand out as a class quiz or homework, or to sit 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

  • 32 multiple-choice
  • 24 short-answer
  • 7 data-based
  • 3 extended-response part
  • 3 drawing
  • 1 labelling
  • 0 higher level only

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

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The practice paper

Take it on screen → Build a fresh paper Paper code BbB-EAAAAAAIABQAO0Qh
Biology · topic quiz
Standard level · topic practice, not an exam format
30 minutes21 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
Name:
1.

Peat that formed over thousands of years is drained and burned in a power station. Which statement describes the effect on the carbon cycle?

[1]
  1. Carbon from a long-term store is released as CO₂
  2. Carbon moves from the air into a long-term store
  3. The carbon can never re-enter the atmosphere
  4. Combustion destroys the carbon atoms entirely
2.

Which organism in the kelp forest food web is a primary consumer?

[1]
tertiary consumer secondary consumers primary consumers producers orca sea otter rockfish sea urchin kelp crab zooplankton kelp phytoplankton
  1. Kelp
  2. Sea urchin
  3. Sea otter
  4. Orca
3.

How many different food chains in the tundra food web include the arctic hare?

[1]
secondary consumersprimary consumersproducerslichensgrasses and sedgeslemmingarctic harecaribousnowy owlarctic foxwolf
  1. 1
  2. 2
  3. 3
  4. 4
4.

Experiments show that adding iron to iron-poor ocean surface waters stimulates phytoplankton blooms. What is the expected short-term effect on the carbon cycle?

[1]
  1. Photosynthesis rises, so more CO₂ is taken up
  2. CO₂ uptake falls as the phytoplankton respire
  3. Ocean pH rises as CO₂ is released from water
  4. The ocean becomes a source as decomposition rises
5.

Explain why heat is released to the environment by all organisms, whether they are autotrophs or heterotrophs, so that the temperature inside a compost heap or a beehive is often higher than that of the surrounding air.

[3]
6.

Dense communities of animals live around hydrothermal vents on the deep ocean floor, where no light penetrates. Explain how producers can supply energy to these communities without light.

[3]
7.

Explain the roles of decomposers in the recycling of chemical elements in ecosystems.

[3]
8.

In a grassland, the biomass gained by the producers each year greatly exceeds the total biomass gained by all the animals.

(a)State what is meant by secondary production.[1]
(b)Explain why secondary production is lower than primary production in the same ecosystem.[2]
9.

Draw a diagram of the carbon cycle. Show carbon pools as boxes and the processes that transfer carbon between them as labelled arrows, including the atmosphere, the oceans, producers, consumers, decomposers and fossil fuels.

[5]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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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. C4.2.19 [1]
  • A: combustion oxidizes the carbon compounds in biomass/peat/fossil fuels to CO₂, returning to the atmosphere carbon that had been removed from circulation;
2. C4.2.10 [1]
  • B: the sea urchin feeds directly on kelp, a producer; the otter eats urchins (secondary consumer) and the orca eats otters and rockfish (tertiary consumer);
3. C4.2.4 [1]
  • C: the hare eats only grasses and sedges and is eaten by the snowy owl, the arctic fox and the wolf, giving three chains: grasses → hare → owl, grasses → hare → fox and grasses → hare → wolf;
4. C4.2.18 [1]
  • A: iron is the limiting nutrient in such waters; fertilization boosts photosynthetic CO₂ drawdown, part of which sinks, the basis of proposed (and controversial) geoengineering schemes;
5. C4.2.13 [3 max]
  • all organisms, autotrophs and heterotrophs, release energy by oxidizing carbon compounds in cell respiration;
  • energy transfers are not 100 % efficient, so some chemical energy is converted to heat, OWTTE;
  • heat is produced when ATP is made in cell respiration, because only part of the energy released from glucose is transferred to ATP;
  • heat is also produced when ATP is used in cells, e.g. for active transport, muscle contraction or synthesis of macromolecules;
  • heat cannot be converted back into chemical energy / cannot be used by organisms to do work, so it is lost to the environment;
  • autotrophs release heat even though they capture light energy, because they respire the carbon compounds they make;

Accept 'energy is lost as heat' for the second point. Do not accept 'heat is produced by photosynthesis'.

6. C4.2.7 [3 max]
  • the producers are chemoautotrophic bacteria/archaea;
  • they obtain energy by oxidizing (reduced) inorganic substances released from the vents e.g. hydrogen sulfide / iron(II);
  • the (exothermic) oxidation reactions release energy that is used to fix CO₂ into carbon compounds;
  • these carbon compounds feed the consumers (directly or through symbiosis), supporting the community, OWTTE;
7. C4.2.22 [3 max]
  • decomposers (saprotrophic bacteria/fungi) digest dead organic matter / wastes externally (secreting enzymes);
  • they release/absorb the products, breaking complex carbon compounds down;
  • elements (C, N, P etc.) are returned to the soil/air in inorganic forms (e.g. CO₂, ammonium/nitrate);
  • (so) producers can reabsorb the elements, cycling continues / nutrients are not locked up in dead matter, OWTTE;
8. C4.2.16
  • (a) [1]
    • the accumulation of biomass / production of carbon compounds by heterotrophs/consumers;
  • (b) [2 max]
    • consumers depend on the carbon compounds made by producers, so secondary production is limited by primary production;
    • not all of the producers' biomass is eaten / not all food eaten is digested and assimilated;
    • much of the assimilated carbon is lost as CO₂ (and heat) in cell respiration, so only a small fraction becomes consumer biomass, OWTTE;
9. C4.2.17 [5]
  • carbon dioxide in the atmosphere drawn as a pool, with photosynthesis as an arrow from the atmosphere to producers and cell respiration as arrows from producers, consumers and decomposers back to the atmosphere, labelled;
  • feeding shown as arrows from producers to consumers (and between consumers), and death/egestion as arrows from producers and consumers to decomposers/dead organic matter, labelled;
  • fossil fuels (peat, coal, oil, gas) drawn as a pool formed by fossilization/incomplete decomposition of dead organic matter, with combustion (of fossil fuels or biomass) as an arrow returning carbon dioxide to the atmosphere, labelled;
  • exchange of carbon dioxide between the atmosphere and the oceans shown as arrows in both directions (dissolving and release), with dissolved carbon dioxide/hydrogen carbonate taken up by marine producers, labelled;
  • an additional pool or process correctly shown, e.g. limestone/calcium carbonate formed from shells and coral, or methane released by methanogens in anaerobic conditions and oxidized to carbon dioxide in the atmosphere, labelled, with all arrows in the correct direction;

Accept any clear layout. Do not credit an arrow from consumers directly to producers or photosynthesis drawn as releasing carbon dioxide.

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