IB DP Biology: Energy flow: food chains, trophic levels and energy pyramids (DP Biology) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover the first part of sub-topic C4.2, Transfers of energy and matter (statements C4.2.1 to C4.2.14), from the IB Biology guide, first assessment 2025. All of it is SL and HL. Learn the content in the study guide and the revision notes first. Course hub: IB DP Biology. Checklist: IB DP Biology checklist.

Questions

1. Distinguish between an open system and a closed system. [2]

2. A hydrothermal vent community lives deep in the ocean where no light reaches. Explain how the community obtains its energy. [3]

3. A salt marsh has these feeding relationships. Snails and mud shrimp both eat microalgae. Shore crabs eat snails and mud shrimp. Redshanks (wading birds) eat mud shrimp and shore crabs. Gulls eat shore crabs.

(a) Construct the food web, listing every arrow in the form “food → feeder”. [3] (b) State the trophic level of the redshank in two different food chains from this web. [2]

4. State three types of material that provide decomposers in a woodland with carbon compounds. [3]

5. (a) Distinguish between a photoautotroph and a chemoautotroph. [2] (b) Explain why a lettuce plant releases carbon dioxide in the dark. [2]

6. A fungus grows on a fallen log. Describe how it obtains and uses carbon compounds from the wood. [3]

7. The table shows energy stored in biomass in a mudflat over one year.

Trophic level	Energy (kJ m⁻² year⁻¹)
Microalgae	18 500
Snails	1 665
Crabs	233
Wading birds	24.5

(a) Calculate the percentage of energy transferred at each of the three steps. [3] (b) Calculate the energy in microalgae that is not passed to the snails. [1] (c) Suggest why the transfer from snails to crabs is higher than the transfer from microalgae to snails. [2]

8. Caterpillars on a shrub ingested 820 kJ m⁻² year⁻¹. They lost 295 kJ in faeces and 15 kJ in urine, and released 410 kJ in respiration.

(a) Calculate the energy added to caterpillar biomass. [2] (b) Calculate the percentage of the ingested energy released through respiration. [1] (c) Explain what happens to the energy in the faeces. [2] (d) Explain why the heat released cannot be used by other organisms to make carbon compounds. [1]

9. A badger eats berries, voles that eat grass, and shrews that eat beetles that eat leaves.

(a) State the trophic level of the badger when it eats each of these three foods. [3] (b) Explain why the badger cannot be given a single trophic level. [1]

10. Explain why heat is released in the respiration of plants and why this reduces the energy available to primary consumers. [5]

11. A grassland has 16 000 kJ m⁻² year⁻¹ in its producers. A model assumes that 9% of the energy at each level passes to the next.

(a) Calculate the energy at trophic levels two, three and four. [3] (b) A predator needs at least 5 kJ m⁻² year⁻¹ to support a viable population. Using the model, state the highest trophic level that can be supported. [1] (c) Explain why total energy falls at higher levels while the energy per gram of biomass does not. [2] (d) Explain why “9% passes on” is a generalization and not an exact law. [2]

12. A sealed glass sphere contains water, algae, small shrimps and bacteria. It has lived for several years on a sunny shelf. Explain how the sphere is maintained, with reference to energy and matter. [6]

Answers

1. An open system exchanges both energy and matter with its surroundings [1]; a closed system exchanges energy only [1]. [2] Examiner insight: A “distinguish” answer needs both sides stated. Defining only the open system usually leaves the contrast unclear.

2. Chemoautotrophic bacteria oxidize inorganic substances such as hydrogen sulfide [1], and use the energy released to fix carbon dioxide into carbon compounds [1]. Consumers obtain energy from these compounds by feeding on the bacteria or on organisms that eat them [1]. [3] Examiner insight: Saying “the bacteria photosynthesize” or “they use heat from the vent” does not answer the question. The energy source is oxidation reactions.

3. (a) Producer at the start, with two arrows from microalgae (to snail and to mud shrimp) [1]. Arrows run from food to feeder: snail → crab, mud shrimp → crab, mud shrimp → redshank, crab → redshank, crab → gull [1]. All seven arrows present and no extra arrows [1]. [3] (b) Microalgae → mud shrimp → redshank: secondary consumer [1]. Microalgae → snail → crab → redshank: tertiary consumer [1]. [2] Examiner insight: Arrows drawn from feeder to food are a common way to lose credit. Name the chain you use for each trophic level.

4. Faeces [1]; dead parts of organisms such as fallen leaves or shed bark [1]; dead whole organisms [1]. [3] Examiner insight: “Waste” alone is too vague. Name each type of material.

5. (a) A photoautotroph uses light as the external energy source to make carbon compounds [1]; a chemoautotroph uses energy released by oxidation reactions of inorganic substances [1]. [2] (b) Plant cells respire [1], oxidizing carbon compounds to release energy even when there is no light for photosynthesis, and carbon dioxide is produced [1]. [2] Examiner insight: Plants respire all the time. A claim that “plants only respire at night” is wrong and contradicts the idea that autotrophs also release energy by respiration.

6. The fungus secretes digestive enzymes onto the wood (external digestion) [1]. It absorbs the small products of digestion [1]. It assimilates them by building its own carbohydrates, proteins and nucleic acids [1]. [3] Examiner insight: Digestion and assimilation are separate steps. Credit comes from naming both.

7. (a) 1 665 ÷ 18 500 × 100 = 9.0% [1]; 233 ÷ 1 665 × 100 = 14.0% [1]; 24.5 ÷ 233 × 100 = 10.5% [1]. [3] (b) 18 500 − 1 665 = 16 835 kJ m⁻² year⁻¹ [1]. [1] (c) Animal tissue is easily digested, so less energy is lost in faeces [1]; plant material contains indigestible parts such as cellulose that pass out in faeces [1]. [2] Examiner insight: Show the division in a percentage question and give the unit in part (b). A suggestion must be biologically reasoned, not simply “crabs are more efficient”.

8. (a) 820 − 295 − 15 − 410 [1] = 100 kJ m⁻² year⁻¹ [1]. [2] (b) 410 ÷ 820 × 100 = 50.0% [1]. [1] (c) The energy stays in the ecosystem as carbon compounds in the faeces [1]; decomposers feed on it and release much of it as heat by respiration [1]. [2] (d) Heat cannot be converted into chemical energy in carbon compounds by organisms, so it is lost to the surroundings [1]. [1] Examiner insight: Faeces energy is not “lost from the ecosystem” at once. Say it passes to decomposers.

9. (a) Berries: primary consumer [1]. Voles: secondary consumer [1]. Shrews: tertiary consumer (leaves → beetles → shrews → badger) [1]. [3] (b) It feeds at different levels in different chains because its diet is varied [1]. [1] Examiner insight: Count the levels along each chain, starting from the producer, and name the chain you are using.

10. Cell respiration oxidizes carbon compounds such as glucose to release energy [1]. The conversion is not 100% efficient, so some energy is released as heat when ATP is produced [1]. More heat is released when ATP is used for processes such as active transport and synthesis [1]. The heat escapes to the surroundings and cannot be reused to make carbon compounds [1]. Less energy therefore remains stored in plant biomass for primary consumers [1]. [5] Examiner insight: Link each statement to the consequence for energy in biomass.

11. (a) Level two: 16 000 × 0.09 = 1 440 [1]; level three: 1 440 × 0.09 = 129.6 [1]; level four: 129.6 × 0.09 = 11.7 kJ m⁻² year⁻¹ (11.664) [1]. [3] (b) Level five would be about 1.05 kJ m⁻² year⁻¹, below 5, so the highest is level four [1]. [1] (c) Energy is lost at each transfer, so there are fewer or smaller organisms and less biomass [1]; each gram of biomass still contains a similar amount of energy [1]. [2] (d) A generalization describes a pattern seen in many ecosystems [1] but real transfer values vary and there are exceptions, so it is only approximate [1]. [2] Examiner insight: Give the working for each level.

12. Light is the only energy input [1]; algae photosynthesize and make carbon compounds [1]. Energy passes to shrimps when they feed [1], and is released as heat through the glass [1]. No matter crosses the glass, so it is a closed system [1], and carbon and nutrients are recycled by bacteria decomposing dead organisms and waste [1]. [6] Examiner insight: Separate the two ideas: energy flows through and leaves; matter stays and is recycled.

Where marks are usually lost

• Arrows drawn from the feeder to its food.
• Using “lost” for energy without saying it is released as heat.
• Writing that energy is recycled, or that matter flows one way.
• Omitting units such as kJ m⁻² year⁻¹.
• Not showing the division in a percentage transfer.
• Treating decomposers as a trophic level in a pyramid.
• Saying all faeces energy leaves the ecosystem.
• Forgetting that autotrophs respire too.

Next steps

• Review with the revision notes and the study guide.
• Course hub: IB DP Biology; checklist.
• Take the free 10-minute diagnostics to find weak topics.
• Book a free trial class.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Statements C4.2.1 to C4.2.14.