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

Practise IB Biology SL C4.2 by tracing energy flow, nutrient cycling, biomass transfer and ecological data across trophic levels.

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

Exam analysis

SL · 2010–2025

Papers with this topic105 / 141in the analysed sample

Latest appearanceNovember 2025

Tagged question parts156

Paper types5

Most tested objectives

C4.2.10—Trophic levels23 papers
C4.2.17—Carbon cycle diagrams17 papers
C4.2.11—Energy pyramids15 papers
C4.2.4—Food chains and food webs12 papers
C4.2.12—Energy reductions11 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 SL · TZ3Q5(a), 5(b), 5(c)[ 6 ]

November 2025Paper 1A SL · TZ3Q26, 28[ 2 ]

November 2025Paper 1A SL · TZ1Q27, 28[ 2 ]

141 papers analysed · Tagging incomplete

Exam points

Define ecosystems as open systems exchanging energy and matter, and explain sunlight as the initial source converted to chemical energy by autotrophs.
Trace chemical energy through producers, consumers and decomposers, and construct or interpret food chains and food webs with arrows showing transfer.
Classify autotrophs, heterotrophs, detritivores, decomposers and saprotrophs by carbon source, energy source and digestion, including photo- versus chemoautotrophy.
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 energy units, trophic labels, 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, calculate productivity with units, and evaluate secondary production, feed conversion and biomass gain.
Construct and interpret carbon-cycle and carbon-budget evidence, identifying stores, fluxes, sinks, sources and CO2 release from biomass or fossil-fuel 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

Describe how energy flows through ecosystems while inorganic nutrients cycle, distinguishing energy transfer and loss from nutrient recycling.

[Maximum number: 1]

In a stable natural ecosystem, how is the supply of nutrients and energy maintained?

Nutrients and energy are efficiently cycled within the ecosystem.

Energy from the Sun is cycled through the food chains, while nutrients are constantly lost.

Nutrients are efficiently cycled within the ecosystem, while there is a constant flow of energy.

Energy is cycled through photosynthesis and respiration, while food chains ensure that nutrients flow through the ecosystem.

Exam points

State that sunlight is the initial energy source for most ecosystem food chains.
Explain how autotrophs capture sunlight by photosynthesis, convert it to chemical energy and transfer energy through trophic levels with losses as heat.

[Maximum number: 1]

The diagrams below show different organisms (not drawn to scale).

State the initial energy source of a food chain.

Exam points

Explain how producers convert light energy into chemical energy stored in compounds that can be transferred through feeding.
Trace chemical energy from producers to consumers as organisms feed through a food chain or food web.
Explain how respiration releases some chemical energy as heat, which leaves the ecosystem.

[Maximum number: 4]

Living organisms depend on energy in order to carry out their essential functions.

Outline the flow of energy through an ecosystem.

Exam points

Describe a food chain as a sequence of feeding relationships from a producer to consumers, and distinguish it from a web that links multiple chains.
Construct or interpret a food chain or web from named organisms, using arrows to show feeding direction and the transfer of energy.

[Maximum number: 2]

Fish play a key role in the functioning of temperate shallow lakes. They affect nutrient cycles and interactions between trophic levels. Studies were done to compare fish community structure and dynamics between shallow lakes in Denmark (temperate) and in Uruguay (subtropical).

The following organisms were found in one of these lakes:
- macrophytes - large aquatic plants
- zooplankton - microscopic animals
- piscivorous fish (eat other fish)
- algae - aquatic plants
- omnivorous fish (eat plants and animals)
- planktivorous fish (eat plankton)
- phytoplankton - microscopic plants
- herbivorous fish (eat plants)

Draw a food chain of four trophic levels for the lake.

Exam points

Identify dead organic matter, including faeces, as material that decomposers break down.
Recognize saprotrophs as obtaining nutrients from dead organic matter, including by external digestion.

[Maximum number: 1]

Dry rot fungus (Serpula lacrymans) secretes digestive enzymes to externally digest dead wood.

What type of nutrition is used by dry rot fungus?

Holozoic

Saprotrophic

Mixotrophic

Autotrophic

Exam points

Distinguish autotrophs, which synthesize organic compounds from inorganic substances using light or chemical energy, from heterotrophs, which obtain organic compounds from other organisms.
Identify producers as the organisms that convert carbon into a form available to primary consumers.

[Maximum number: 2]

Distinguish between autotrophs and heterotrophs.

autotrophs make their own food/organic molecules/organic matter and heterotrophs feed on/obtain their food/organic molecules from other organisms;
autotrophs use/require inorganic molecules/ $\mathrm{CO}_{2}$ and heterotrophs require (complex) organic molecules;

Exam points

Identify photoautotrophs as organisms that capture light with photosynthetic pigments and convert that external energy into chemical energy during photosynthesis.
Distinguish photoautotroph light capture from chemoautotroph oxidation of simple inorganic substances as the external energy source for carbon fixation.

[Maximum number: 1]

Which organisms use oxidation of simple inorganic substances as an energy source?

Photoautotrophs

Heterotrophs

Chemoautotrophs

Saprotrophs

Exam points

Identify heterotrophs as obtaining organic molecules from other organisms or organic matter, unlike autotrophs that synthesize organic compounds from inorganic substances.
Recognize saprotrophs and detritivores as heterotrophs that obtain organic matter from dead organisms, and distinguish them from consumers feeding on living organisms.

[Maximum number: 1]

The image shows a single Paramecium with food vacuoles that contain ingested cells of the unicellular green alga Chlorella.

What can be deduced about Paramecium?

It is an autotroph.

It cannot perform all of the functions of life.

It carries out heterotrophic nutrition.

It is a prokaryote.

[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 by tracing its feeding relationships from producers through the organisms it eats.
Explain how an omnivore can occupy more than one trophic level because it feeds on organisms from different levels.

[Maximum number: 1]

The image shows a food web from Chesapeake Bay, USA.

Which organisms are tertiary consumers in at least one food chain?

Small planktivorous fish and tundra swans

Herbivorous ducks and bald eagles

Wading birds and geese

Ospreys and benthic invertebrates

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.
Construct or interpret stepped energy-pyramid bars from data, label trophic levels in order and compare the energy available at each level.
Discuss limitations of energy pyramids, including seasonal variation, difficulty assigning species to one trophic level and energy transfers to decomposers.

[Maximum number: 2]

An example of energy flow $\left(\mathrm{kJ} \mathrm{m}^{-2}\right.$ year $\left.{ }^{-1}\right)$ in the North Sea is shown.

Sketch a pyramid of energy for the trophic levels represented by A, B, C and D.

Exam points

Explain why energy availability decreases between trophic levels through losses in respiration, heat, egestion, excretion and uneaten biomass.
Calculate or compare energy-transfer efficiency using supplied trophic-flow data.

[Maximum number: 1]

The diagram shows the flow of energy through an ecosystem in $\mathrm{kJ} \mathrm{m}^{-2} \mathrm{y}^{-1}$ .

What percentage of the energy passed from primary producers to primary consumers is lost to cellular respiration by tertiary consumers?

0.001 %

0.08 %

0.2 %

0.4 %

Exam points

Identify heat as an energy loss between trophic levels that cannot be recycled.

[Maximum number: 1]

What is lost between trophic levels in ecosystems and cannot be recycled?

Heat

Nitrogen

Carbon compounds

Biomass

Exam points

Explain how large energy losses between trophic levels limit food chains to a few trophic levels.
Explain why higher trophic levels usually support fewer individuals and less biomass as less energy is transferred between levels.

[Maximum number: 1]

What restricts the length of a food chain?

Energy losses between the trophic levels

A greater biomass at the higher trophic levels

The number of species in the food web

The consumption of waste by detritivores

Exam points

Distinguish gross from net production by accounting for respiration: gross production = net production + respiration.
Interpret net primary production as carbon accumulation in autotroph biomass using supplied ecosystem data.

[Maximum number: 1]

Calculate the gross production of an aquatic ecosystem that has a net primary production of $96 \mathrm{~kJ} \mathrm{~m}^{-2} \mathrm{y}^{-1}$ and a respiration rate of $45 \mathrm{~kJ} \mathrm{~m}^{-2} \mathrm{y}^{-1}$ , giving the units.

$\quad(=96+45)=141 \mathrm{~kJ} \mathrm{~m}^{-2} \mathrm{yr}^{-1}$

Exam points

Calculate heterotroph biomass gain or required feed from a supplied food-to-biomass conversion ratio.
Compare feed conversion efficiency for edible biomass, accounting for assimilation and respiration losses.

[Maximum number: 4]

(a)

Not all of the energy ingested by animals in their food is ultimately converted to energy in the biomass of the animal's body.

[ 4 ]

(i)

State one factor that affects the percentage of ingested energy converted to biomass.

[ 1 ]

(ii)

The diagram shows the mass of edible parts produced by four animals when fed 100 kg of feed.

Figure for Question (ii) — IB Biology SL

Discuss the differences in the conversion ratios of the cow and fish.

[ 2 ]

(iii)

Crickets are said to be the food of the future. Using the data, deduce an advantage for crickets as a food source for humans.

[ 1 ]

Exam points

Draw a carbon-cycle diagram with carbon stores in boxes and directed arrows for fluxes.
Label flux arrows for photosynthesis, feeding, respiration, decomposition and fossil-fuel combustion.

[Maximum number: 1]

The diagram shows a simplified carbon cycle.

Which processes are taking place at W and Z ?

respiration

combustion

photosynthesis

decomposition

respiration

decomposition

photosynthesis

combustion

Exam points

Identify a carbon sink by net carbon storage and distinguish it from a source that releases more carbon than it absorbs.
Use forests and oceans as sink examples and fossil-fuel combustion as a source based on net carbon transfer.

[Maximum number: 3]

The table shows the global carbon budget over two decades; the years 1990 to 1999 and 2000 to 2009.

Using the table, explain causes of the changes in carbon flux over the two decades.

a	increased $\mathrm{CO}_{2}$ flux to the atmosphere due to increased burning of fossil fuels by industry/transportation/cement production;
b	land use change leading to decreased rate of forest burning OR better fire suppression leading to decrease in $\mathrm{CO}_{2}$ release OR example of land use changes that uses less fossil fuel OR increase in land covered by forests/plants / forests recovering from historical forestry OR any other reasonable explanation of land use change that would lead to decreased rate of carbon flow to atmosphere;
c	carbon storage in land decreased as less photosynthesis due to fewer forests/more construction OR release of methane due to drying of wetlands/sealing of land with concrete/buildings/roads;
d	carbon storage in ocean increased due to more photosynthesis/algae/greater concentration of $\mathrm{CO}_{2}$ in the atmosphere OR increased diffusion/rate of dissolving of $\mathrm{CO}_{2}$ into ocean from the atmosphere OR limestone/carbonate accumulation (more snails);

Exam points

Interpret reported CO₂ emissions from forest fires by comparing estimates and judging their scale as carbon flux to the atmosphere.

[Maximum number: 4]

There were widespread and intense wildfires in Southeast Australia during the summer season from November 2019 to January 2020. Estimates have been made of carbon dioxide emissions from these fires based on satellite monitoring data. The bar chart shows estimates made by five different research programmes: GFED, GFAS, QFED, FEER and FINN.

The table shows other statistics for comparison.

2019-2020 constrained by satellite. Nature (597), pp. 366-369. https://doi.org/10.1038/s41586-021-03712-y.]

(a)

The highest estimate of $\mathrm{CO}_{2}$ emitted from wildfires in Southeast Australia between November 2019 and January 2020 is 820 Tg , from the FEER research programme. State the lowest estimate.

Programme:
Estimate:

[ 1 ]

(b)

Using the data in the table, comment on the magnitude of the wildfires in Southeast Australia from November 2019 to January 2020.

In addition to carbon dioxide, carbon monoxide and other gases, the wildfires emitted smoke. Smoke is an aerosol of solid particles and liquid droplets suspended in air. The complex mixture of chemicals in this aerosol includes black carbon (soot) and compounds of nitrogen, phosphorus and iron. This aerosol absorbs light, allowing its optical density and movement to be assessed by satellite monitoring.

Much of the smoke from the wildfires in Southeast Australia in 2019-2020 was carried eastwards across the South Pacific Ocean. The study area shown in the map was affected by the smoke.

Australian wildfires. Nature (597), pp. 370-375. https://doi.org/10.1038/s41586-021-03805-8.]

The graph shows the optical density of the air and the chlorophyll concentrations of the water in this study area. The concentration of chlorophyll is proportional to the density of phytoplankton (photosynthetic bacteria and algae) in the water.

Daily measures of aerosol optical density (AOD)
8-day mean chlorophyll concentration in 2019-2020

Key:

[ 3 ]

a. larger than annual ( $\mathrm{CO}_{2}$ ) emissions for the whole of Australia;
b. larger than all fossil fuel emissions for the whole of Australia in a typical year;
c. extreme/intense/OWTTE;
d. (much) larger than the typical emissions of 9 Tg (for Nov to Jan) in SE Australia;

Exam points

Interpret Mauna Loa CO₂ records by identifying the long-term rise associated with combustion and annual fluctuations associated with Northern Hemisphere photosynthesis.

[Maximum number: 1]

The graph shows annual fluctuations of atmospheric carbon dioxide in the Northern Hemisphere. There is a relative increase in carbon dioxide concentration during winters.

Which reason might explain this increase during winters?

Increase in atmosphere-ocean diffusion

Decrease in photosynthesis of plants

Increase in respiration of all organisms

Decrease in combustion from wildfires

Exam points

Interpret plant CO₂-exchange data across light intensities to identify when respiration exceeds photosynthesis.

[Maximum number: 1]

Plants produce carbon dioxide in respiration and use carbon dioxide in photosynthesis. The graph shows the volume of carbon dioxide exchanged in a plant at different light intensities.

What is shown by the graph?

There is no photosynthesis between W and X .

There is no photosynthesis between Y and Z .

There is more respiration than photosynthesis between Y and Z .

There is more respiration than photosynthesis between W and X .

Exam points

Explain how decomposers and detritivores convert dead organic matter, detritus and waste into inorganic nutrients that producers can reuse.
Describe the recycling of elements such as carbon and nitrogen, and distinguish it from one-way energy flow.

[Maximum number: 7]

Cycles play an important role both within and outside the body.

Explain how carbon is recycled in a terrestrial ecosystem.

a. plants/autotrophs use $\mathrm{CO}_{2}$ in photosynthesis;

b. plants/autotrophs release $\mathrm{CO}_{2}$ during respiration;

c. ( $\mathrm{CO}_{2}$ ) converted to organic compounds/glucose/starch/cellulose/fat/protein;

d. (transferred) to consumers/animals by feeding;

e. consumers/animals/detritus feeders/heterotrophs release $\mathrm{CO}_{2}$ during respiration;

f. plants/animals die forming (dead) organic matter;

g. saprotrophs/decomposers digest dead plants/animals;

h. saprotrophs/decomposers release $\mathrm{CO}_{2}$ by respiration;

i. fossilization/incomplete decomposition produces fossil fuels/coal/oil/peat/gas OR anaerobic respiration releases methane/ruminant (cows) release methane;

j. combustion of fossil fuels/plants/biomass releases $\mathrm{CO}_{2}$ ;

Accept annotated diagrams that clearly show the relevant marking points.

[Maximum number: 3]

The diagrams below show different organisms (not drawn to scale).

(a)

Construct a possible food chain using three of the organisms shown opposite, stating the trophic level to which they belong.

[ 2 ]

(b)

State the initial energy source of a food chain.

[ 1 ]

All question bank results loaded

a	they have a low/a 1.25 conversion ratio;
b	produce higher quantity of edible mass with same mass of food (as other animals);
c	most parts are edible/ little is wasted;
d	they reproduce very quickly/large amounts easily grown;

a	(aerobic/cellular) respiration;
b	gas exchange / diffusion;

Exam analysis

Most tested objectives

Common question formats

Recent exam appearances

Covered syllabus points

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