C4.2 Transfers of energy and matter

An ecosystem is an open system because both energy and matter can cross its boundary.

A forest receives sunlight, rainfall and atmospheric gases, exports heat and dissolved materials, and cycles carbon and nutrients among organisms, soil and air.

Matter can be recycled because atoms remain available in different compounds; useful energy is not recycled in the same way and ultimately dissipates as heat.

Sunlight is the principal energy source sustaining most ecosystems because photoautotrophs convert light into chemical energy stored in biomass.

Chemical energy then reaches consumers through feeding. This is a useful generalization: it describes a widespread pattern and supports predictions, but it is not an explanation of every ecosystem and must allow known exceptions.

Cave ecosystems can depend on organic matter imported from sunlit areas, while communities below ocean light penetration can depend on sinking detritus or chemoautotrophic production powered by oxidation reactions.

In a grassland, sunlight → producer biomass → herbivore → predator. In a deep-sea vent community, chemical oxidation can supply the primary energy input instead.

Sunlight supplies energy, not carbon atoms. ‘Principal source’ means the dominant global pattern, not a universal rule without exceptions.

Chemical energy moves through feeding.

Organic molecules contain chemical potential energy. Feeding transfers that energy between organisms, while respiration releases some for work and dissipates much as heat.

source molecule; consumer; respiration; useful work versus heat.

Carbon compounds pass from plant to caterpillar to bird; at every step some energy supports metabolism and some leaves as heat.

Energy flow is not identical to carbon flow: carbon atoms may remain while usable energy declines.

Food webs connect many feeding pathways.

A food chain shows one route of feeding; a food web joins overlapping routes. The web reveals alternative prey, predators and indirect effects when one population changes.

trace arrow direction; identify trophic position; follow direct and indirect paths.

Removing a predator can increase one prey species and indirectly reduce the plants eaten by that prey.

Arrows show transfer of food/energy from resource to consumer, not who is ‘stronger’.

Decomposers obtain energy from carbon compounds in organic matter derived from faeces, dead parts and whole dead organisms.

Fungi and bacteria secrete enzymes onto detritus, absorb soluble digestion products and oxidize some carbon compounds in respiration. This transfers chemical energy to decomposer metabolism.

Faeces/dead tissue/dead organism → extracellular digestion → soluble carbon compounds absorbed → decomposer respiration and biomass → inorganic nutrients released.

A fungus digests dead leaf tissue externally, absorbs sugars and other products, uses some for respiration and growth, and releases mineral nutrients into soil.

Decomposers transform both matter and energy, but matter can return to nutrient cycles while respiratory energy ultimately leaves as heat.

Autotrophs use an external energy source to synthesize carbon compounds from simple inorganic substances.

External energy is required both to fix inorganic carbon such as CO₂ into organic intermediates and to drive anabolic reactions that assemble carbohydrates, lipids, proteins and nucleic acids.

External light or chemical energy → carbon fixation → small organic compounds → energy-requiring anabolic synthesis → autotroph biomass.

A plant uses light energy to fix CO₂ into carbon compounds and then combines carbon skeletons with mineral nutrients to build macromolecules for new tissue.

Autotroph refers to carbon-compound synthesis, not independence from external energy, water or mineral elements.

Photoautotrophs use light as their external energy source; chemoautotrophs use energy released by oxidation reactions.

Oxidation transfers electrons and releases energy that chemoautotrophs can couple to ATP production and carbon fixation even where light is unavailable.

Chemosynthesis is not photosynthesis without light: the external energy source is an oxidation reaction. Both groups are autotrophs because they synthesize carbon compounds from inorganic carbon.

Heterotrophs obtain carbon compounds from other organisms and use them to synthesize the carbon compounds they require.

Complex molecules such as proteins and nucleic acids are too large to assimilate directly. They are digested internally in animals or externally by organisms such as fungi, producing smaller molecules that can be absorbed.

Other organism/detritus → internal or external digestion → small molecules absorbed → assimilation into new, organism-specific proteins, nucleic acids and other carbon compounds.

An animal digests dietary protein to amino acids and then assembles those amino acids into its own enzymes; a fungus performs digestion outside its body before absorption.

Assimilation is not simply absorption: it is incorporation and reconstruction into the organism's own compounds. Heterotrophs still respire and recycle nutrients.

Both autotrophs and heterotrophs release energy by oxidizing carbon compounds in cell respiration.

Electrons and hydrogen are transferred from respiratory substrates through enzyme-controlled pathways; part of the released chemical energy is captured in ATP and the rest becomes heat.

Autotroph: makes carbon compounds, then can respire them. Heterotroph: obtains carbon compounds from other organisms, then can respire them. Both use ATP for cellular work.

A plant leaf respires glucose that the plant synthesized, while an animal respires carbon compounds derived from food; in each, oxidation supports ATP production.

Autotrophs do not only photosynthesize—they also respire. Respiration is cellular oxidation, not gas movement in breathing; photoheterotroph details are not required.

A trophic level classifies an organism by its feeding position in a particular food chain.

An omnivorous bird may be a primary consumer when eating seeds and a secondary consumer when eating herbivorous insects, so trophic level depends on the food chain.

Trophic level is not body size, intelligence or a permanent label for a species with a varied diet.

An energy pyramid uses measured research data to show energy available at successive trophic levels per unit area per unit time.

Choose one specific ecosystem dataset; verify that all values use compatible energy, area and time units; order producer to higher consumers; draw bars with widths proportional to the reported values; label every value and unit.

For each transfer, calculate loss as energy at lower level minus energy at next level, and transfer efficiency as (next-level energy ÷ lower-level energy) × 100%. Use the dataset's real values rather than assuming a fixed percentage.

A correctly constructed pyramid lets the reader compare the measured producer energy with each consumer level and see both absolute loss and proportional transfer between adjacent levels.

An energy pyramid is not a pyramid of numbers or standing biomass. Do not invent missing values or mix units or measurement intervals from different ecosystems.

Energy availability decreases at successive trophic levels because only part of one level's chemical energy becomes biomass eaten and assimilated by the next.

Losses include uneaten material, indigestible material egested as faeces, carbon compounds used in respiration, and heat from metabolism. Movement, maintenance and other work reduce energy stored as new biomass.

Decomposers and detritus feeders are not usually drawn as one step in a food chain, but they receive uneaten, egested and dead organic matter and transform its chemical energy through feeding and respiration.

Plant roots left uneaten and herbivore faeces can supply detrital pathways, while only new herbivore biomass remains available to a predator.

Transfer efficiency is not a universal ten-percent rule. Species, tissues, temperature and ecological conditions alter the size of each loss.

Both autotrophs and heterotrophs release heat when chemical energy is converted during cell respiration and when ATP is used for cell work.

Energy transfers are not 100% efficient. Some substrate energy becomes heat while ATP is produced, and further heat is released when ATP hydrolysis drives synthesis, transport or movement.

Carbon-compound oxidation → ATP plus heat; ATP use → cell work plus heat; heat dissipates to the environment and is unavailable as chemical energy to the next trophic level.

A plant and a mammal both respire and lose heat during ATP production; both also release heat as ATP powers active transport or biosynthesis.

Heat loss is energy transfer, not loss of carbon mass. Ecosystems require continuing external energy input because dissipated heat is not recycled into chemical energy by organisms.

Repeated energy losses restrict the number of trophic levels that an ecosystem can support.

Each higher level receives less total chemical energy, so less total biomass can be maintained. Successive levels therefore tend to contain fewer organisms or organisms with smaller combined body size.

Lower-level energy → incomplete transfer → smaller upper-level energy budget → lower total biomass → fewer/smaller organisms → an additional trophic level eventually becomes unsustainable.

A food web with low primary production or large transfer losses may support herbivores but too little predator biomass to support another persistent consumer level.

Total biomass decreases upward, but the energy content per unit mass is not assumed to decrease. The limit comes from less total energy/biomass, not poorer-quality joules.

Primary production is the accumulation of carbon compounds in autotroph biomass through growth and reproduction.

Report it as mass of carbon per unit area per unit time, commonly g C m⁻² yr⁻¹. A rate requires a defined area and interval rather than only a standing biomass measurement.

Biomes differ in capacity to accumulate biomass because light, temperature, water, nutrients and growing-season length constrain autotroph growth.

Compare ecosystem primary-production values only after checking that carbon mass, area and time units match; the larger rate represents faster producer carbon accumulation.

Primary production is producer biomass accumulation, not total biomass already present. Biomass also accumulates when heterotrophs grow or reproduce, but that is secondary production.

Secondary production is the accumulation of carbon compounds in heterotroph biomass through growth and reproduction.

Heterotrophs obtain organic carbon by feeding, but not all intake becomes biomass. Some is not assimilated, and respiration converts absorbed carbon compounds to carbon dioxide and water while releasing energy.

Food carbon → ingestion/digestion → assimilation → respiration losses as CO₂ and water + retained carbon in growth/reproduction. Retained carbon is secondary production.

A growing fish retains part of assimilated food as new tissue, while respiring another part; only the retained new tissue contributes to secondary production.

Secondary production is lower than primary production at ecosystem scale because carbon is lost from biomass at each heterotrophic transfer, especially through respiration.

Carbon-cycle diagrams track stores and transfers.

A carbon cycle represents carbon stores such as atmosphere, biomass, soil and ocean, and transfers such as photosynthesis, respiration, decomposition and combustion.

label store versus flow; direction; rate or stock; time interval.

A forest store can gain carbon through growth while releasing carbon through respiration and decomposition at the same time.

Arrows are transfers, not extra carbon; a diagram should not be read as one-way flow.

An ecosystem is a carbon sink when photosynthesis removes more CO₂ than respiration releases, and a carbon source when respiration releases more CO₂ than photosynthesis removes.

Classification depends on the net balance across a stated ecosystem boundary and time interval, not on whether both fluxes occur—both photosynthesis and respiration normally continue.

Photosynthesis > respiration → net CO₂ uptake → sink. Respiration > photosynthesis → net CO₂ release → source. Equal fluxes → no net exchange from these two processes.

A growing forest can be a sink while carbon accumulation exceeds respiratory release; after disturbance, high respiration and decomposition can make the same area a source.

Sink/source status is not permanent and changes with season, disturbance, ecosystem age, boundary and measurement interval.

Combustion of biomass, peat, coal, oil and natural gas oxidizes stored carbon and releases carbon dioxide to the atmosphere.

These stores formed at different times: biomass stores recent carbon, peat accumulates over longer periods, and coal, oil and natural gas contain ancient geological carbon. Burning transfers that stored carbon rapidly to atmospheric CO₂.

Lightning can ignite biomass naturally, but human burning of biomass and fossil fuels has greatly increased combustion rates and the flux of stored carbon to the atmosphere.

Complete combustion of methane in natural gas forms CO₂ and water; burning coal, oil, peat or wood likewise transfers their carbon to atmospheric products.

Combustion moves existing carbon—it does not create carbon atoms. Incomplete combustion can also form carbon monoxide or soot, but CO₂ release is the required main flux.

The Keeling Curve shows both a long-term rise in atmospheric CO₂ and repeated annual fluctuations.

Seasonal photosynthesis and respiration, especially across Northern Hemisphere land ecosystems, create the annual oscillation: growing-season uptake lowers CO₂, while reduced photosynthesis and continuing respiration raise it later.

Over many years, CO₂ peaks and troughs both shift upward because combustion adds carbon faster than global sinks remove the additional amount.

When reading the curve, compare equivalent points in successive years to identify the long-term trend; do not mistake one seasonal decline for a reversal of the multi-year rise.

Photosynthesis, respiration and combustion explain different components of the pattern. Axis units and time scale must be checked before interpreting magnitude or rate.

Photosynthesis and aerobic respiration are interdependent through the atmosphere: photosynthesis supplies oxygen, while respiration supplies carbon dioxide.

Photosynthetic organisms release atmospheric O₂ when water is split and use atmospheric CO₂ to build carbon compounds. Aerobic organisms require O₂ as the terminal electron acceptor and release CO₂ when carbon compounds are oxidized.

Photosynthesis: CO₂ consumed, O₂ released. Aerobic respiration: O₂ consumed, CO₂ released. The annual fluxes are huge, making this a major interaction between autotrophs and heterotrophs.

A plant contributes O₂ used by animal aerobic respiration; the animal's respiratory CO₂ can later be fixed by photosynthetic organisms.

The two processes are complementary at ecosystem scale but are not exact reverse reaction pathways, and plants themselves also carry out aerobic respiration.

Every chemical element required by living organisms is recycled through ecosystems; carbon is only one example.

Atoms move between organisms and abiotic stores through uptake, feeding, excretion and death. Decomposers digest dead organic matter and wastes, returning elements in inorganic forms that producers can use again.

Organic matter → decomposer action → inorganic compounds in soil, water or air → producer uptake → food-web transfer → waste/death → decomposition.

Carbon, nitrogen, phosphorus and other required elements can all pass through biomass and return to abiotic stores; the detailed nitrogen or other nutrient cycles are not required here.

Recycling does not mean immediate or unlimited availability. Elements may be lost across ecosystem boundaries or remain in slow stores, while energy follows a one-way dissipative flow.