IB Biology HL C4.2 Transfers of energy and matter Questions

Practise IB Biology HL C4.2 by tracing energy flow, nutrient cycling, biomass transfer, productivity and carbon fluxes across trophic levels.

212 questions in this syllabus scope, with question parts counted together.

Exam analysis

HL · 2010–2025

Papers with this topic67 / 117in the analysed sample

Latest appearanceNovember 2025

Tagged question parts94

Paper types5

Most tested objectives

C4.2.10—Trophic levels17 papers
C4.2.11—Energy pyramids9 papers
C4.2.3—Chemical energy flow8 papers
C4.2.17—Carbon cycle diagrams8 papers
C4.2.1—Ecosystems as open systems6 papers

Common question formats

Calculation
Diagram interpretation
Definition or recall
Comparison
Structured response
Data analysis
Evaluation
Process explanation
Extended response

Recent exam appearances

November 2025Paper 2 HL · TZ1Q4(b)[ 2 ]

November 2025Paper 1A HL · TZ3Q37[ 1 ]

May 2025Paper 2 HL · TZ3Q10(c)[ 5 ]

117 papers analysed · Tagging incomplete

Exam points

Define ecosystems as open systems exchanging energy and matter with their surroundings.
Trace chemical energy through producers, consumers and decomposers, and construct or interpret food chains and food webs.
Classify autotrophs and heterotrophs, including consumers, detritivores, decomposers and saprotrophs, by energy/carbon source and digestion.
Explain respiration as chemical-energy release producing ATP and CO2 with heat loss, and deduce trophic levels including omnivory or parasitism.
Construct, calculate and interpret energy pyramids and transfer efficiency using trophic labels, energy units, respiration, heat, uneaten biomass, egestion and excretion.
Explain why respiratory heat cannot be recycled and how inefficient energy transfer limits food-chain length and higher-level biomass.
Distinguish gross and net primary production and calculate or interpret productivity data with units.
Construct and interpret carbon-cycle and carbon-budget evidence, identifying stores, fluxes, sinks, sources and CO2 release from combustion.
Analyse Keeling-curve data to distinguish long-term atmospheric CO2 rise from seasonal photosynthesis and respiration oscillations.
Trace recycling of carbon and other elements through decomposers, inorganic nutrients and producer uptake, distinguishing matter recycling from one-way energy flow.

Covered syllabus points

C4.2.1—Ecosystems as open systems C4.2.2—Sunlight as principal energy source C4.2.3—Chemical energy flow C4.2.4—Food chains and food webs C4.2.5—Supply to decomposers C4.2.6—Autotrophs as self-feeders C4.2.7—Energy sources C4.2.8—Heterotrophs C4.2.9—Energy release C4.2.10—Trophic levels C4.2.11—Energy pyramids C4.2.12—Energy reductions C4.2.13—Heat loss C4.2.14—Restrictions on trophic levels C4.2.15—Primary production C4.2.16—Secondary production C4.2.17—Carbon cycle diagrams C4.2.18—Carbon sinks and sources C4.2.19—CO₂ release during combustion C4.2.20—Keeling Curve analysis C4.2.21—Dependence between respiration and photosynthesis C4.2.22—Recycling of all chemical elements

Exam points

Identify an ecosystem as an open system by stating that it exchanges energy and matter with its surroundings.
Distinguish the one-way flow of energy, which is lost and must be supplied, from matter and inorganic nutrients that can be transferred, enter or leave, and recycle through ecosystems.

[Maximum number: 2]

Carbon dioxide is released to the atmosphere when microorganisms decompose organic matter in thawing Arctic soil. The graph shows the rate of release of carbon dioxide at different soil temperatures during a moderate thaw.

Most of the surface of the Earth is covered with a wide diversity of ecosystems.
Outline two general characteristics of all ecosystems.

Exam points

Trace chemical energy from producers to consumers through feeding in food chains and food webs.
Describe energy entering as light converted to chemical energy, or as chemical energy in food, and leaving ecosystems as heat during respiration.

[Maximum number: 1]

Which of these energy conversions is/are possible in living organisms?

I. Light to chemical
II. Chemical to heat
III. Heat to light

I only

I and II only

II and III only

I, II and III

Exam points

Compare food chains and food webs as models of feeding relationships: a chain shows one pathway, while a web connects multiple feeding pathways.
Construct or interpret food-chain and food-web models from organisms, using arrows to show feeding links and the direction of energy transfer.
Evaluate food webs as models of ecological communities by describing what they represent and a limitation, such as complexity or omitted information.

[Maximum number: 2]

The table provides some information about organisms found in an Arctic environment.

Label the diagram to complete the food web for the organisms in the table above.

Exam points

Identify dead organic matter, including detritus from organisms, as a source of carbon compounds and energy for decomposers.
Use food-web evidence to predict how an increase in detritus can affect detritus feeders and organisms that feed on them.

[Maximum number: 2]

The map shows the widespread distribution of coral reef ecosystems (indicated by black dots) in the world's oceans. Death of coral reefs is related to increasing atmospheric carbon dioxide concentrations.

The image shows part of the food web for a coral reef. Lines show the relationships for energy between organisms in the web.

Detritus accumulates on coral reefs damaged by ocean acidification.
Suggest two possible impacts of an increase in detritus on the organisms in this food web.

Exam points

Classify an organism that synthesizes organic compounds from inorganic nutrients as an autotroph.
Identify dissolved carbon dioxide and hydrogen carbonate ions as carbon sources that aquatic autotrophs can use.

[Maximum number: 1]

What is the classification of an organism that is able to make organic compounds from inorganic nutrients?

Autotroph

Consumer

Detritivore

Saprotroph

Exam points

Identify chemoautotrophs as obtaining energy by oxidizing simple inorganic substances.

Exam points

Classify heterotrophs by their use of organic compounds from other organisms or organic matter.
Compare consumers, detritivores and saprotrophs by their organic sources and digestion, and explain how carbon compounds are used in respiration to produce ATP.

[Maximum number: 1]

The image shows a female Golden Orb-weaving spider (Nephila plumipes). They can grow as large as 4 cm and build webs strong enough to trap small birds for food.

Which of the following describe(s) this spider?

I. Primary consumer
II. Heterotroph
III. Arthropod

I only

I and II only

II and III only

I, II and III

[Maximum number: 1]

The mass of an individual organism can affect its physiology and feeding ecology. The diagram shows the relative mass of carbon (black) and total wet mass (grey) of a marine crustacean, Calanus hyperboreus and a jellyfish, Bathocyroe fosteri.

State one process that results in the loss of carbon dioxide from a marine organism such as a crustacean or a jellyfish.

Exam points

Deduce an organism’s trophic level from its feeding relationships, tracing what it eats back to producers and identifying its consumer level.
Explain that organisms feeding across trophic levels, including omnivores and decomposers, can occupy more than one trophic level.

[Maximum number: 2]

The tapeworm Schistocephalus solidus is a parasite that must grow in two intermediate hosts before it can reproduce in the gut of the final host, a fish-eating bird (Larus michahellis). For transmission, an infected crustacean (Macrocydops albidus) must be preyed upon by a stickleback fish (Gasterosteus aculeatus).

Discuss the trophic level of the tapeworm.

Exam points

Describe an energy pyramid as energy flow per unit area per unit time, with each bar showing energy available at one trophic level.
Interpret or construct stepped energy-pyramid bars to compare energy available at producer and consumer trophic levels.
Explain why an organism that feeds across trophic levels can be difficult to represent with a single energy-pyramid bar.

[Maximum number: 1]

The diagram shows a pyramid of energy. The unit is $\mathrm{kJm}^{-2}$ year $^{-1}$ .

What is a valid conclusion from the data displayed in the pyramid?

Producers lose almost $16000 \mathrm{~kJ} \mathrm{~m}^{-2}$ year $^{-1}$ as they reflect some sunlight.

$4006 \mathrm{~kJ} \mathrm{~m}^{-2}$ year $^{-1}$ is gained by primary consumers.

Tertiary consumers use $408 \mathrm{~kJ} \mathrm{~m}^{-2}$ year $^{-1}$ in life processes.

$21 \mathrm{~kJ} \mathrm{~m}^{-2}$ year $^{-1}$ is used by detritivores.

Exam points

Explain why less energy is available at each successive trophic level, including losses through respiration, heat, egestion, excretion and uneaten biomass.
Explain how limited energy transfer between trophic levels restricts food-chain length.
Calculate or compare energy transferred or lost between named trophic levels using supplied data, including conversion efficiency.

[Maximum number: 1]

Which statement applies to transfers in an ecosystem?

Green plants can transfer heat energy from the sun into chemical energy.

The greatest loss of energy occurs towards the end of a food chain.

Production of carbon dioxide by respiration results in loss of biomass in a food chain.

Both energy and nutrients are finite and must be recycled.

Exam points

Identify cellular respiration as a route by which energy that could otherwise be used by consumers is lost.

[Maximum number: 1]

Plankton are major producers in marine ecosystems. Only a small proportion of the energy harvested by plankton is passed to the primary consumers. Which process in phytoplankton results in the largest loss of energy that could otherwise be utilized by consumers?

Reproduction

Homeostasis

Excretion

Respiration

Exam points

Explain why fewer individuals are supported at higher trophic levels as energy is lost between levels.

[Maximum number: 1]

Animals in the highest trophic level of a food chain will often be the largest in body size but will be few in numbers. What accounts for the small numbers?

Food eaten by animals at the highest trophic level has a lower energy content per gram

Energy losses through the food chain

Conversion of heat energy into chemical energy

Biomass of producers is small

Exam points

Explain primary production as the accumulation of carbon compounds in autotroph biomass.
Use gross production, net production and respiration in the relationship gross production = net production + respiration.

[Maximum number: 1]

Annotate the equation to show the relationship between gross production, net production and respiration.

\underline{L}+\underline{ }=

Exam points

Explain secondary production as heterotroph biomass accumulation from food intake after respiration losses.
Use feed conversion ratios to compare animal feed intake with edible biomass output.

[Maximum number: 2]

The diagram shows a simplified marine food web in an area of the North Pacific Ocean.

Explain reasons for the feed conversion ratio in fish being lower than in cattle.

Exam points

Construct carbon-cycle diagrams with boxes for stores and labelled arrows for fluxes.
Label the carbon-cycle fluxes for photosynthesis, feeding, respiration, decomposition and combustion of fossil fuels.

[Maximum number: 1]

The diagram shows part of the carbon cycle involving methane.

Which conditions favour methane production in W, X and Y ?

Presence of eubacteria and organic matter

Presence of archaeans and waterlogged soil

Presence of eubacteria and waterlogged soil

Presence of archaeans and oxygen

Exam points

Explain that a carbon sink stores more carbon than it releases, while a carbon source releases more than it absorbs.
Classify forests, soils and oceans as sinks and fossil-fuel combustion as a source from the direction of carbon transfer.

[Maximum number: 1]

Studies show that adding iron to the ocean's surface triggers the growth of phytoplankton.

What could be a short-term consequence of stimulating phytoplankton growth in this area?

Rate of photosynthesis increases, so the ocean becomes the source of carbon.

Uptake of carbon dioxide into the ocean decreases, so it becomes a sink of carbon.

Rate of respiration increases, increasing the pH of the ocean.

Rate of photosynthesis increases, resulting in an increased uptake of carbon dioxide into the ocean.

Exam points

Interpret the Mauna Loa CO₂ record by distinguishing the long-term rise associated with combustion from annual decreases associated with Northern Hemisphere photosynthesis.

[Maximum number: 1]

The graph shows monthly mean atmospheric carbon dioxide concentrations measured at Mauna Loa Observatory, Hawaii in 2020 and 2021.

Which processes contribute to changes in monthly mean atmospheric carbon dioxide concentrations?

Combustion of coal, forest fires, ozone depletion

Photosynthesis, respiration, burning of fossil fuels

Emissions of carbon dioxide, methane and nitrogen oxides

Peat formation, drainage of wetlands, loss of coral reefs

Exam points

Explain how decomposers recycle nutrients by converting detritus or waste into inorganic forms that producers can use.
Identify sources of inorganic nutrients in soils, including ammonium from soil microorganisms, waste and dead organisms.
Explain how people and extreme weather can alter nitrogen or phosphorus cycling through nutrient inputs, removal and leaching.

[Maximum number: 6]

Suggest how nitrogen and phosphate cycles can be disrupted by humans or extreme weather events.

[Maximum number: 5]

The table provides some information about organisms found in an Arctic environment.

(a)

Label the diagram to complete the food web for the organisms in the table above.

[ 2 ]

(b)

Deduce the trophic level of Arctic cod.

[ 1 ]

(c)

Distinguish between the movement of energy and nutrients in an ecosystem.

[ 2 ]

All question bank results loaded

a	organisms/community plus the environment / biotic and abiotic «components»
b	interactions
c	ecosystems show sustainability
d	nutrients are recycled in ecosystems
e	energy flows through ecosystems
f	producers «are part of all ecosystems»
g	decomposers/saprotrophs «are part of all ecosystems»

a	excess rains/floods/irrigation can wash away nutrients/N/P by leaching;
b	excess rains/floods/irrigation can lead to waterlogged soils;
c	waterlogging leads to denitrification due to anaerobic conditions;
d	nutrients are added to the cycles by application of fertilizer;
e	nutrients are removed by the harvesting of agricultural crops;
Nitrogen:
f	lightning increases N in soil;
g	planting legumes increases N in soil;
Phosphate:
h	mining of P speeds up the P cycle/depletes P reserves;
i	phosphorus is added to waters in detergents;
j	phosphorus is mined/taken from rocks for making detergents/fertilizers;

Exam analysis

Most tested objectives

Common question formats

Recent exam appearances

Covered syllabus points

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