C4.2 Transfers of energy and matter

Transfers of energy and matter connect sunlight, feeding, trophic levels, decomposers, heat loss and carbon cycling to ecosystem structure, function and sustainability.

Learning objectives

C4.2.1Ecosystems as open systems• Ecosystems are open systems exchanging energy and matter with surroundings• Energy flows through ecosystems while matter can enter, leave, and recycle C4.2.2Sunlight as principal energy source• Sunlight is the principal energy source for most ecosystems• Exceptions include caves and deep-ocean vents supported by chemosynthesis C4.2.3Chemical energy flow• Chemical energy flows from producers to consumers through feeding• Energy enters as light or chemical energy and leaves ecosystems as heat C4.2.4Food chains and food webs• Food chains and food webs model feeding relationships in communities• Arrows point in the direction of energy and biomass transfer C4.2.5Supply to decomposers• Decomposers obtain energy from carbon compounds in detritus• Dead organisms, faeces, fallen leaves, and shed tissues supply organic matter C4.2.6Autotrophs as self-feeders• Autotrophs synthesize carbon compounds from simple inorganic substances• They use external energy sources to fix carbon and build biomass C4.2.7Energy sources• Photoautotrophs use light captured by photosynthetic pigments• Chemoautotrophs use oxidation of inorganic substances, such as iron or sulfur compounds C4.2.8Heterotrophs• Heterotrophs obtain carbon compounds from other organisms or organic matter• They use these compounds for respiration and synthesis of their own biomass C4.2.9Energy release• Both autotrophs and heterotrophs release energy by cell respiration• Oxidation of carbon compounds transfers energy to ATP and heat C4.2.10Trophic levels• Trophic levels classify organisms as producers, primary consumers, and higher consumers• Omnivores and decomposers may feed across more than one trophic level C4.2.11Energy pyramids• Energy pyramids represent energy flow per unit area per unit time• Each bar shows energy available to one trophic level C4.2.12Energy reductions• Energy availability decreases at each transfer between trophic levels• Losses occur through respiration, heat, egestion, excretion, and uneaten biomass C4.2.13Heat loss• Cell respiration converts some chemical energy to heat in all trophic levels• Heat dissipates to the environment, so energy cannot be recycled C4.2.14Restrictions on trophic levels• Large energy losses restrict food chains to a few trophic levels• Higher trophic levels usually support less biomass and fewer individuals C4.2.15Primary production• Primary production is accumulation of carbon compounds in autotroph biomass• Gross production minus respiration gives net primary production C4.2.16Secondary production• Secondary production is biomass accumulation by heterotrophs• It depends on food intake, assimilation, respiration losses, and biomass conversion C4.2.17Carbon cycle diagrams• Carbon cycle diagrams show stores as boxes and fluxes as labelled arrows• Include CO₂, photosynthesis, feeding, respiration, decomposition, fossil fuels, and combustion C4.2.18Carbon sinks and sources• Carbon sinks absorb more carbon than they release, such as forests, soils, and oceans• Carbon sources release more carbon than they absorb, such as fossil fuel combustion C4.2.19CO₂ release during combustion• Combustion of biomass, peat, coal, oil, and natural gas releases CO₂• Draining peat and forest fires increase carbon flux to the atmosphere C4.2.20Keeling Curve analysis• The Keeling Curve records atmospheric CO₂ at Mauna Loa since the late 1950s• Long-term rise reflects combustion; annual oscillation reflects Northern Hemisphere photosynthesis C4.2.21Dependence between respiration and photosynthesis• Photosynthesis supplies atmospheric O₂ used in aerobic respiration• Respiration supplies CO₂ used by photosynthesis, linking autotrophs and heterotrophs C4.2.22Recycling of all chemical elements• All chemical elements required by organisms are recycled in ecosystems• Decomposers convert detritus and waste into inorganic nutrients for producers

An ecosystem is a community interacting with its abiotic environment. It is an open system because both energy and matter can cross its chosen boundary.

Crossing the boundary	Example
energy enters	sunlight reaches producers
energy leaves	respiratory heat dissipates
matter enters	rain, mineral ions, migrating organisms
matter leaves	runoff, gases, harvested biomass

A closed system exchanges energy but not matter. Natural ecosystems are not closed, even when much of their matter is recycled internally.

An open system receives and releases both energy and matter across its boundary.

sunlight → photosynthetic pigments capture light → chemical energy is stored in carbon compounds → producer biomass supports consumers

Sunlight is the principal energy source for most ecosystems because photoautotrophs form the base of most food webs.

Low-light ecosystem	Initial energy source	Producer route
deep-ocean hydrothermal vent	oxidation of inorganic chemicals	chemoautotrophic bacteria or archaea
permanently dark cave	organic matter imported from outside, or local chemosynthesis	detrital food web or chemoautotrophs

The word most matters: sunlight is the dominant global input, but a food web can begin with chemical energy or with organic matter imported from another ecosystem.

Feature	Energy	Matter
common input	sunlight or chemical energy	CO₂, water and mineral ions
movement through biota	chemical energy in biomass and food	atoms in organic molecules
major exit / return	heat dissipates to surroundings	decomposition returns inorganic forms
recycled by ecosystem?	no	yes, although matter can also enter or leave

external energy → producer carbon compounds → feeding → ATP and biomass → heat

inorganic matter → producer biomass → feeding and waste → detritus → decomposers → inorganic matter

Energy is conserved, but after it disperses as low-temperature heat it is no longer available to organisms as the chemical-energy input for another trophic transfer.

A food-chain arrow shows the direction in which chemical energy and biomass are transferred: from the organism being eaten to the consumer.

grass → grasshopper → robin → hawk means grasshopper feeds on grass, robin feeds on grasshopper, and hawk feeds on robin.

Read each arrow as ‘provides energy to’, not ‘eats’. The arrow therefore points toward the feeder.

A food chain and food web use arrows from each food organism toward the organism that consumes it.

Model	What it shows	What it hides
food chain	one possible transfer sequence	alternative foods and indirect effects
food web	interconnected chains in a community	quantities and interaction strength unless data are added

To predict a change: locate the altered population → follow arrows to its consumers → follow arrows backward to its foods → check whether consumers have alternative pathways.

If one prey declines, its specialist predator loses an energy source. A generalist predator may switch prey, transferring stronger predation pressure to another population.

A food web is qualitative unless arrow widths or accompanying data quantify biomass, energy flow or feeding frequency.

Detritus includes dead organisms, shed tissue, fallen leaves and faeces. Its carbon compounds still contain chemical energy.

Organism	Action	Immediate result
detritivore	ingests and fragments detritus	smaller particles and faeces
saprotrophic bacterium or fungus	secretes enzymes, digests externally and absorbs products	carbon for respiration and biomass
decomposer respiration	oxidizes absorbed carbon compounds	ATP, CO₂, water and heat

Decomposition releases inorganic molecules and ions into soil, water or air. Producers absorb them, so matter returns even though the detrital chemical energy ultimately leaves as heat.

Dead material and waste pass to decomposers, which release inorganic nutrients that producers can absorb again.

An autotroph uses an external energy source to synthesize carbon compounds from simple inorganic substances. Carbon fixation brings inorganic carbon into organic molecules.

Input type	Example	Job
carbon source	CO₂	supplies carbon atoms
energy source	light or oxidation of inorganic substances	powers carbon fixation and synthesis
other matter	water and mineral ions	supplies atoms needed for biomass

‘Self-feeding’ does not mean creating matter or energy. Autotrophs transform external energy and inorganic matter into their own organic biomass.

Carbon dioxide, water, mineral ions and an external energy source enter an autotroph, which produces carbon compounds and biomass.

Producer	External energy source	Example	Shared outcome
photoautotroph	light captured by pigments	plant, alga, cyanobacterium	inorganic carbon fixed into organic molecules
chemoautotroph	energy released by oxidation of inorganic substances	nitrifying or iron-oxidizing bacterium	inorganic carbon fixed into organic molecules

An iron-oxidizing bacterium can obtain energy when Fe²⁺ is oxidized to Fe³⁺. The iron supplies energy through electron transfer; it is not the organism's organic-carbon source.

Both routes are autotrophic because the organism builds carbon compounds from inorganic carbon; only the external energy source differs.

A photoautotroph uses light, while an iron-oxidizing chemoautotroph uses energy from converting iron two-plus ions to iron three-plus ions; both build organic molecules.

A heterotroph obtains carbon compounds from other organisms or organic matter. It does not fix all the carbon it needs from inorganic carbon.

organic matter → digestion inside a gut or outside the body → small molecules absorbed into cells → assimilation into organism-specific proteins, lipids, polysaccharides and nucleic acids

Heterotroph	How organic matter is obtained
consumer	ingests living or recently killed organisms
detritivore	ingests dead material and waste
saprotroph	digests externally and absorbs soluble products

A heterotroph receives organic matter and directs it toward respiration and heat or toward synthesis of its own biomass.

Autotrophs and heterotrophs both oxidize carbon compounds in cell respiration. Energy is transferred to ATP for cellular work, while some becomes heat.

Organism	How it first gets carbon compounds	What respiration does
autotroph	synthesizes them from inorganic carbon	releases stored chemical energy
heterotroph	obtains them from other organisms	releases stored chemical energy

Plants respire as well as photosynthesize. Photosynthesis stores energy in carbon compounds; respiration releases usable energy from those compounds.

A producer, herbivore and carnivore each oxidize carbon compounds in cell respiration, transferring energy to ATP and heat.

Follow…	Start	Through organisms	End or return
energy	sunlight or inorganic oxidation	producer biomass → feeding → respiration	heat dissipates
carbon	CO₂ or organic matter crossing boundary	fixation → feeding → waste/death	CO₂ from respiration or decay; some enters longer-term stores
mineral element	inorganic ion	producer uptake → feeding	waste/death → decay → inorganic ion

For any arrow, ask: what is moving? In what chemical form? Which process moves it? Does it cross the ecosystem boundary, enter biomass, or return to an inorganic store?

Feeding can transfer energy and matter together, but their later fates differ: usable energy disperses as heat, while atoms can be rearranged and reused.

Trophic level	Feeding position	Example in one chain
1	producer	grass
2	primary consumer	grasshopper eating grass
3	secondary consumer	robin eating grasshopper
4	tertiary consumer	hawk eating robin

A trophic level is a feeding position in a particular food chain. It describes the transfer being traced, not an unchanging property of a species.

A human eating beans is a primary consumer; the same human eating a herbivorous cow is a secondary consumer. An omnivore can therefore occupy different trophic levels in different chains.

Decomposers receive material from organisms at every trophic level, so forcing them into one fixed level hides the detrital network.

A pyramid of energy shows the energy incorporated into biomass at each trophic level per unit area per unit time, for example kJ m⁻² yr⁻¹.

Use equal-height horizontal bars → centre every bar on the same vertical axis → make width proportional to the measured energy-flow value → place producers at the base.

Pyramid	Quantity represented	Can be inverted?
energy	rate of biomass-energy production	no; transfer losses make each higher rate smaller
biomass	standing biomass at one time	yes
numbers	count of organisms at one time	yes

A generalized energy pyramid has a wide producer base and progressively narrower primary-, secondary- and tertiary-consumer bars.

Energy route at one level	Available to next grazing level?	Destination
biomass never eaten	no	detritus and decomposers
eaten but not digested	no	faeces and decomposers
absorbed then excreted	no	waste pathway
oxidized in respiration	no	ATP transfer and heat
retained as new biomass	yes, if eaten	growth and reproduction

energy in existing biomass → fraction ingested → fraction assimilated → fraction left after respiration → new consumer biomass

Transfer efficiency varies with digestibility, metabolic rate and how much biomass is eaten. Carnivore diets are often more digestible than lignified plant tissue; ectotherms often spend less energy maintaining body temperature than endotherms.

‘Lost to the next consumer’ does not always mean lost from the ecosystem immediately. Uneaten biomass and faeces can still supply detritivores and decomposers before respiration releases heat.

$\eta=\frac{E_{higher}}{E_{lower}}\times100\%$

Trophic level	Energy flow
producers	18,000 kJ m⁻² yr⁻¹
primary consumers	2,160 kJ m⁻² yr⁻¹
efficiency	(2,160 ÷ 18,000) × 100 = 12%

Here η is transfer efficiency, E higher is the energy-flow value at the next trophic level and E lower is the value at the level supplying it. Both measurements must cover the same area and time interval.

About 10% is a useful rough model, not a fixed biological law. Use the supplied values whenever data are available, and draw bar widths to those values rather than forcing every transfer to exactly 10%.

carbon-compound oxidation → energy transferred to ATP plus heat → ATP powers cellular work and more heat is released → heat spreads into the surroundings

Energy is not destroyed. It becomes dispersed thermal energy that organisms cannot concentrate back into the chemical bonds required to restart the food chain.

Because usable energy is continually degraded to heat, ecosystems need a continuing external input—usually sunlight—even though atoms can be recycled.

Light energy enters a producer and chemical energy passes through consumers and detritivores, while respiration releases heat from every trophic level.

producer energy budget → incomplete transfer → smaller primary-consumer budget → another incomplete transfer → still smaller upper-level budget → an additional stable trophic level eventually becomes impossible

Toward the top of a food chain:

Energy flow decreases with every incomplete transfer.
Less total biomass can usually be supported.
Fewer large consumers can usually be supported.
A top predator often requires a larger feeding area.

Higher levels do not always contain fewer individual organisms. Body size matters; many small parasites can live on one host. The reliable restriction is the smaller energy budget.

Energy pyramids for tundra, temperate forest, temperate grassland and tropical rainforest show progressively narrower upper trophic levels.

$NPP=GPP-R$

Quantity	Meaning
GPP	total rate at which producers fix carbon / store energy by photosynthesis
R	producer respiratory loss
NPP	rate of new producer biomass accumulation available for growth, reproduction and consumers

If GPP = 2,400 g C m⁻² yr⁻¹ and producer respiration = 1,350 g C m⁻² yr⁻¹, then NPP = 1,050 g C m⁻² yr⁻¹.

Gross primary production is divided between producer respiration and new producer biomass called net primary production.

NPP falls when a resource or condition restricts new biomass production:

Low light: less energy is captured by photosynthesis.
Water shortage: stomata close, limiting CO₂ uptake, and growth slows.
Low temperature: enzyme-controlled reactions and nutrient cycling slow.
Excessive temperature: water loss rises and enzymes may lose function.
Short growing season: there are fewer days of positive carbon gain.

Biome	Typical NPP explanation
tropical rainforest	warm, wet and illuminated through most of the year
temperate forest or grassland	seasonal light and temperature shorten production
tundra	cold, short growing season and slow nutrient cycling

Higher NPP builds a larger producer energy budget, which can support more biomass and sometimes more trophic levels. Low NPP constrains the whole food web from its base.

NPP is a rate of new biomass production, not the amount of standing biomass visible on one date.

$GSP=I-F$

$NSP=GSP-R$

Symbol	Meaning
I	food energy or biomass ingested
F	faecal loss; not assimilated
GSP	energy or biomass absorbed by the consumer
R	consumer respiratory loss
NSP	new consumer biomass available to growth, reproduction and the next trophic level

A caterpillar ingests 900 kJ, loses 360 kJ in faeces and uses 420 kJ in respiration: GSP = 540 kJ and NSP = 120 kJ.

Food energy enters a consumer; some is not assimilated and leaves in faeces, while assimilated energy can support respiration or new biomass.

Stage	Calculation or decision	What becomes available next?
producer	NPP = GPP − producer R	new producer biomass
consumer assimilation	GSP = ingestion − faecal loss	absorbed energy
consumer production	NSP = GSP − consumer R	new consumer biomass
trophic transfer	higher-level energy ÷ lower-level energy × 100	transfer efficiency

Observation	First mechanism to test
low NPP	light, water, temperature or short season
much food but low GSP	low digestibility / large faecal loss
high GSP but low NSP	high respiration
short food chain	compounded transfer losses leave too little upper-level energy

An energy pyramid narrows because each level keeps only the fraction converted into new biomass. Every other route either feeds the detrital pathway or ultimately dissipates energy as heat.

Diagram feature	Meaning	Example
box	carbon store or reservoir	atmospheric CO₂; plant biomass
labelled arrow	carbon flux between stores	photosynthesis; feeding; respiration
arrowhead	direction of carbon transfer	atmosphere → producers
number on arrow	rate over a stated interval	Gt C yr⁻¹

Name the stores first → add biological fluxes → add long-term geological/fossil stores if required → check every arrow has a direction and process label → include atmospheric CO₂.

Draw stores as boxes and processes as arrows. A plant, animal or factory picture does not by itself state the carbon store, transfer direction or process.

A simplified carbon cycle links atmospheric carbon dioxide, green plants, animals, decomposers, soil organic carbon and fossil fuels with labelled process arrows.

Flux	From	To	Carbon form change
photosynthesis	atmospheric or dissolved CO₂	producer biomass	inorganic → organic
feeding	one organism's biomass	consumer biomass	organic → organic
respiration	organic carbon in cells	atmospheric or dissolved CO₂	organic → inorganic
death and waste	living biomass	detritus	organic → organic
decomposition	detritus	decomposer biomass and CO₂ / inorganic nutrients	organic redistributed and mineralized

CO₂ → photosynthesis → plant carbohydrate → feeding → animal biomass → respiration or death → CO₂ through respiration and decomposition

Producers, consumers and decomposers all respire. The carbon cycle therefore has several arrows returning CO₂, not one ‘animal-only’ respiration arrow.

A cycle diagram shows routes, not necessarily equal rates. Add values or arrow widths only when evidence supplies them.

Net balance over a stated interval	Classification	Store change
carbon in > carbon out	sink	increases
carbon out > carbon in	source	decreases
carbon in = carbon out	neither net sink nor net source	unchanged

$\Delta C=C_{in}-C_{out}$

Reservoir	Can act as a sink when…	Can act as a source when…
forest	photosynthesis and biomass growth exceed losses	respiration, fire, decay or harvest exceed uptake
soil / peat	organic inputs exceed decomposition	drainage and warming accelerate oxidation
ocean	uptake exceeds release	warming or circulation shifts favour release

A forest, soil or ocean is not permanently a sink. Classification depends on all relevant fluxes and on the time interval chosen.

carbon-rich fuel + O₂ → CO₂ + H₂O + released energy

Fuel	Approximate storage timescale before burning	Why combustion matters
fresh biomass	years to decades	rapidly returns recently fixed carbon; regrowth determines later uptake
peat	centuries to millennia	drainage exposes stored organic carbon to oxidation and fire
coal, oil, natural gas	millions of years	transfers geologically stored carbon into the active atmosphere-ocean cycle

Fire converts organic carbon in biomass or fuel to atmospheric CO₂ quickly. It can also weaken a sink by killing vegetation that would otherwise remove CO₂ through photosynthesis.

Combustion occurs naturally after lightning, but human fossil-fuel use and deliberate biomass burning have greatly increased the flux to the atmosphere.

The Keeling Curve is the long-running atmospheric CO₂ record begun at Mauna Loa in the late 1950s. Its rising baseline shows a long-term net gain of CO₂ in the atmosphere.

fossil-fuel combustion + land-use emissions + other sources > uptake by oceans and terrestrial photosynthesis → atmospheric CO₂ accumulates

Graph feature	Timescale	Main interpretation
rising baseline	decades	persistent net source to atmosphere
small repeating teeth	one year	seasonal biological fluxes

The curve records atmospheric concentration. It supports a net-balance conclusion; identifying individual sources requires additional measurements and carbon-cycle evidence.

Atmospheric carbon dioxide rises from about 315 parts per million in 1958 to above 400 parts per million, with a repeated seasonal oscillation around the rising baseline.

Northern Hemisphere season	Dominant change in terrestrial biota	Atmospheric CO₂ tendency
spring and summer	leaf area and photosynthesis increase; uptake exceeds biological release	falls
autumn and winter	photosynthesis falls while respiration and decomposition continue	rises

Northern Hemisphere land vegetation dominates the global seasonal signal because most of Earth's land and terrestrial plant biomass lie north of the equator.

Within each year CO₂ can fall for several months, while each year's average remains above the previous one. Seasonal uptake therefore coexists with the long-term rise.

Do not explain the annual fall by reduced combustion alone. The repeating biological drawdown is driven mainly by photosynthesis during Northern Hemisphere growth.

Process	Main gas input	Main gas output	Energy role
photosynthesis	CO₂	O₂	stores captured energy in carbon compounds
aerobic respiration	O₂	CO₂	releases energy from carbon compounds to ATP and heat

photosynthetic organisms replenish atmospheric O₂ used by aerobic respiration ↔ respiration by producers, consumers and decomposers returns CO₂ that photoautotrophs can fix

The processes are linked but are not simple reversals inside one organelle. They use different pathways, enzymes and energy conversions, and both occur in photosynthetic organisms.

Large opposing fluxes can occur at the same time. Atmospheric concentration changes only by the difference between total inputs and outputs.

inorganic molecule or ion → producer uptake and assimilation → organic molecules in biomass → feeding → waste or death → detritus and decomposers → inorganic form available again

Carbon is one example. Hydrogen, oxygen, nitrogen, phosphorus, sulfur, calcium, potassium and other required elements are also recycled between organisms and the atmosphere, hydrosphere or lithosphere.

Without decomposers, essential elements would remain locked in dead organic matter and waste. Recycling restores availability; it does not create a new supply of atoms.

Inorganic molecules or ions are absorbed by producers, move to consumers by feeding and digestion, enter decomposers after death, and return to the inorganic environment through decomposition.

Question	Energy answer	Matter / carbon answer
What enters?	usually sunlight	CO₂, water, ions, organisms and detritus can cross boundaries
What moves through food webs?	chemical energy in biomass	atoms in organic molecules
What returns?	no usable-energy cycle; heat dissipates	decomposition and respiration return inorganic forms
What can accumulate?	biomass energy temporarily	carbon in biomass, soil, peat, ocean or fossil stores
What reveals imbalance?	shrinking energy budgets up trophic levels	changing store size or atmospheric concentration

Use the correct evidence scale: energy-pyramid data for trophic transfer; GPP/NPP and GSP/NSP budgets for production; labelled store-and-flux diagrams for cycling; the Keeling trend for multi-decade atmospheric accumulation; seasonal teeth for annual biological fluxes.

Matter can be rearranged and reused because atoms remain available in biotic and abiotic stores. Usable energy must be replaced because each biological transfer disperses part of it as heat.

6 marks

Describe the movement of energy and nutrients in an ecosystem.

7 marks

Explain how the energy supply in an ecosystem is dependent on sunlight.

7 marks

Describe how populations in communities rely on each other for supplies of energy.

6 marks

Describe what is meant by a food chain and a food web.

2 marks

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.

1 mark

Define the term autotroph.

1 mark

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

5 marks

Compare and contrast how different types of heterotrophs obtain the energy that they need to produce ATP.

1 mark

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

4 marks

Explain, using an example of a food chain, how trophic levels can be deduced.

6 marks

Ecologists sometimes display data from an ecosystem using a diagram called a pyramid of energy. Describe what is shown in pyramids of energy.

3 marks

The efficiency of energy transfer along the various food chains in a food web varies. Suggest reasons for the differences.

1 mark

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?

1 mark

Outline why the number of trophic levels is limited in a food chain.

3 marks

In each forest, there are two or three trial plots per CO2 treatment. The bar chart shows the allocation of carbon from net primary production to different parts of the trees in these trial plots.

Evaluate the evidence from the bar chart that increases in carbon dioxide cause increases in carbon storage in young, developing forests.

1 mark

Calculate how much food would be required to produce 20 kg of boar meat.

kg

9 marks

Living organisms at every trophic level are part of the carbon cycle. Draw a labelled diagram of the carbon cycle to show the processes involved.

2 marks

The graph shows long-term fluctuations in atmospheric $\mathrm{CO}_{2}$ concentration of about 80 ppm . The decreases in $\mathrm{CO}_{2}$ concentration are probably due to oceans acting as a sink by holding large quantities of dissolved $\mathrm{CO}_{2}$ .

Identify two other examples of natural sinks that can remove carbon from the carbon cycle and reduce atmospheric $\mathrm{CO}_{2}$ concentration.

1 mark

The graph shows how the worldwide use of fossil fuels has increased from 1800 to 2019.

How has the increased combustion of fossil fuels contributed significantly to global warming?

1 mark

The graph shows data collected at Mauna Loa, USA, for monthly mean carbon dioxide concentration.

What causes the decreases in monthly mean carbon dioxide concentration each year?

7 marks

Explain how carbon is recycled in a terrestrial ecosystem.

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—IB BiologyRead

C4.2 Transfers of energy and matter

Learning objectives

An Ecosystem Boundary Passes Both Energy and Matter

Sunlight Powers Most Ecosystems—Not Every One

Energy Flows Through; Matter Can Return

A Food-Chain Arrow Points from Food to Feeder

A Food Web Reveals More Than One Dependence

Detritus Feeds Organisms and Returns Elements

Autotrophs Fix Inorganic Carbon into Biomass

Photoautotrophs and Chemoautotrophs Differ in Energy Source

Heterotrophs Rebuild Carbon Obtained from Other Organisms

Organisms at Every Trophic Level Respire Carbon Compounds

Trace One Energy Input and One Matter Pathway

Trophic Level Depends on the Food Chain Being Traced

An Energy Pyramid Measures a Rate, Not a Snapshot

Only New Biomass Can Feed the Next Trophic Level

Transfer Efficiency Compares Adjacent Energy Budgets

Respiratory Heat Is Conserved but Not Recycled

Repeated Losses Put a Ceiling on Food-Chain Length

Net Primary Production Is Producer Biomass Left after Respiration

Climate and Season Length Shape Net Primary Production

Consumer Production Has Two Loss Checkpoints

Follow the Energy Budget from Producer to Consumer Biomass

Carbon-Cycle Diagrams Separate Stores from Fluxes

Biological Fluxes Move Carbon Around the Short Cycle

Sink or Source Depends on the Net Balance

Combustion Moves Stored Carbon into Atmospheric CO₂

The Keeling Baseline Rises When Sources Exceed Sinks

Seasonal CO₂ Falls during Northern Summer Growth

Photosynthesis and Respiration Exchange Two Atmospheric Gases

Every Essential Element Returns through a Biogeochemical Cycle

Keep Energy, Matter, Stores and Timescales Separate

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Ecosystems as open systems

Sunlight as principal energy source

Chemical energy flow

Food chains and food webs

Supply to decomposers

Autotrophs as self-feeders

Energy sources

Heterotrophs exam focus

Energy release

Trophic levels

Energy pyramids

Energy reductions

Heat loss

Restrictions on trophic levels

Primary production

Secondary production

Carbon cycle diagrams

Carbon sinks and sources

CO₂ release during combustion

Keeling Curve analysis

Recycling of all chemical elements