C4.2 Transfers of energy and matter

1. It is an open system: energy enters and leaves it and so does matter, such as water, gases and organisms. — Ecosystems are open systems in which both energy and matter can enter and exit. Light enters and heat leaves, and matter such as rainwater, carbon dioxide and migrating animals crosses the boundary in both directions.
2. It is a closed system: light energy enters and heat leaves, but all of its matter is recycled within it. — A student who pictures an ecosystem as a self-contained unit recycling its own matter picks this. That describes a closed system; real ecosystems exchange water, gases, ions and organisms with their surroundings, so they are open.
3. It is a closed system: neither energy nor matter can cross its boundary once the community is established. — A student who takes 'closed' to mean sealed against everything picks this. Even in a closed system energy passes in and out; only matter is confined. An ecosystem is not closed at all.
4. It is an open system for matter only, because the energy in it is recycled from decomposers back to producers. — A student who believes energy is recycled like nutrients picks this. Energy flows through an ecosystem and leaves as heat; it must enter continuously, so an ecosystem is open to energy as well as matter.

Syllabus statement C4.2.1 · Read this in Learn

1. In the carbon compounds of the rabbit's tissues, which the fox digests, absorbs and later oxidizes in cell respiration. — Chemical energy passes to a consumer as it feeds on an organism that is the previous stage in the food chain. The energy is held in the rabbit's carbohydrates, lipids and proteins, and is released only when the fox's cells oxidize the products of digestion.
2. In the ATP stored in the rabbit's cells, which the fox absorbs intact and uses directly as its energy currency. — A student who thinks of ATP as the molecule that carries energy from one organism to another picks this. ATP is made and used inside each organism's own cells; the energy passed on in feeding is in carbon compounds.
3. In the heat of the rabbit's body, which is transferred to the fox and then recycled back to the grass when the fox dies. — A student who believes energy cycles round the ecosystem picks this. Heat cannot be used by organisms and is lost to the environment; the fox gains chemical energy, and no energy returns to the grass.
4. In the oxygen the fox breathes in while it eats, since it is oxygen rather than food that supplies the energy for its cells. — A student who equates respiration with breathing and thinks energy comes from oxygen picks this. Oxygen is needed for aerobic respiration, but the energy released comes from oxidizing the carbon compounds obtained in food.

Syllabus statement C4.2.3 · Read this in Learn

1. Simple inorganic substances in the soil, from which decomposers synthesize their own carbon compounds using energy from sunlight. — A student who classes decomposers as producers picks this. Decomposers do not fix carbon; they obtain both carbon and energy from the organic matter of dead organisms.
2. Dead whole organisms and dead parts of organisms only, because faeces are waste from which all of the energy has been removed. — A student who thinks 'waste' means energy-free picks this. Faeces contain undigested carbon compounds and are an important energy source for decomposers and detritus feeders.
3. The bodies of tertiary consumers only, because decomposers are the final link in a food chain and feed on the top level. — A student who places decomposers as the last trophic level picks this. Decomposers receive dead matter and faeces from every trophic level, producers included, and are not usually shown as part of a food chain.
4. Carbon compounds in organic matter from dead organisms, including faeces, dead parts of organisms and dead whole organisms. — Decomposers are heterotrophs whose energy supply is the carbon compounds in dead organic matter. Faeces, shed leaves and other dead parts, and whole dead organisms all contain carbon compounds that the decomposers digest externally and then oxidize in cell respiration.

Syllabus statement C4.2.5 · Read this in Learn

1. From photosynthesis, using pigments that can capture the very low levels of light reaching the mine drainage water. — A student who assumes all autotrophs are photosynthetic picks this. These bacteria are chemoautotrophs; their energy source is an oxidation reaction, not light.
2. From the oxidation of iron(II) ions to iron(III) ions, which releases energy that is used to fix carbon dioxide. — Oxidation reactions release energy, so they are useful in living organisms. Iron-oxidizing bacteria are chemoautotrophs: the energy from oxidizing iron(II) ions to iron(III) ions is the external energy source for their carbon fixation.
3. From cell respiration of carbon compounds that they absorb from the organic matter dissolved in the water. — A student who thinks the only energy-yielding oxidation is respiration of organic matter picks this. Respiring absorbed carbon compounds would make the bacteria heterotrophs; iron-oxidizing bacteria oxidize an inorganic ion and fix their own carbon.
4. They need no energy source, because autotrophs generate energy themselves while synthesizing carbon compounds. — A student who believes autotrophs make energy picks this. Carbon fixation requires an input of energy from an external source; in these bacteria it is the oxidation of iron(II) ions.

Syllabus statement C4.2.7 · Read this in Learn

1. Heterotrophs at all times, but autotrophs only at night, when they cannot obtain energy from photosynthesis. — A student who sees photosynthesis and respiration as alternatives picks this. Autotrophs respire in the light as well; in daylight the net gas exchange is dominated by photosynthesis, but respiration has not stopped.
2. All autotrophs and all heterotrophs, in every living cell, throughout the day and the night. — Energy is released in both autotrophs and heterotrophs by oxidation of carbon compounds in cell respiration. Plants, bacteria, fungi and animals all respire continuously; photosynthesis in autotrophs is an additional process, not a replacement.
3. Only organisms with lungs or gills, because respiration depends on breathing to take in oxygen from the air or water. — A student who confuses cell respiration with breathing picks this. Cell respiration is a chemical process in cells; plants, fungi and bacteria respire without any breathing organs.
4. Heterotrophs and chemoautotrophs only, because photoautotrophs use light rather than oxidation as their energy source. — A student who thinks oxidation and light are alternative energy sources that exclude each other picks this. Light powers carbon fixation in photoautotrophs, but they still oxidize carbon compounds in cell respiration to release energy for their cells.

Syllabus statement C4.2.9 · Read this in Learn

1. Not all primary consumers are eaten, not all of what is eaten is absorbed, and most absorbed energy is released as heat in respiration. — Large energy losses occur between trophic levels: uneaten organisms and their dead parts go to decomposers, undigested material is egested as faeces, and most of the energy that is absorbed is released by cell respiration and lost as heat. Only energy in new biomass can pass on.
2. Primary consumers use up most of their energy in moving and feeding, so that energy is destroyed and no longer exists. — A student who thinks energy is consumed and disappears picks this. Energy is never destroyed; the energy used for movement is converted to heat by cell respiration and leaves the ecosystem as heat.
3. Decomposers, which form the final trophic level, take the energy from primary consumers and return it to the producers. — A student who places decomposers at the end of the chain and believes they recycle energy picks this. Decomposers release the energy in dead matter as heat; no energy is returned to producers.
4. Respiration in primary consumers captures all the released energy in ATP, and this ATP cannot be passed on to a predator. — A student who thinks respiration is 100% efficient picks this. Energy is lost as heat both when ATP is made and when it is used, and the losses also include uneaten material and faeces; ATP is not what passes between levels in any case.

Syllabus statement C4.2.12 · Read this in Learn

1. The number of trophic levels is set by the number of species in the community, and few communities have more than five. — A student who counts species rather than energy picks this. A rainforest with thousands of species still has food chains of only four or five levels, because energy, not species number, is the limit.
2. The tissues of organisms higher in the chain contain less energy per gram, so a predator cannot gain enough energy from each meal. — A student who thinks biomass becomes less energy-rich up the chain picks this. Energy content per unit mass is not reduced at higher levels; there is simply less biomass in total.
3. Decomposers occupy the last trophic level of every chain and take the remaining energy, so no further consumers can be added. — A student who places decomposers as a top trophic level picks this. Decomposers are not part of food chains and receive dead matter from every level; the limit comes from cumulative energy losses at each transfer.
4. So much energy is lost at each transfer that after a few levels too little remains to support a viable consumer population. — Restrictions on the number of trophic levels are due to energy losses. Most of the energy at each level is lost as heat or passes to decomposers, so at each successive stage there are fewer or smaller organisms and less biomass, and eventually there is not enough energy to sustain another level.

Syllabus statement C4.2.14 · Read this in Learn

1. Everything a heterotroph absorbs becomes biomass, so secondary production is lower only because heterotrophs eat relatively little. — A student who thinks assimilation is complete picks this. Much of the absorbed carbon is respired to carbon dioxide and water and never becomes biomass; that respiratory loss, not a small appetite, is the reason.
2. The biomass of heterotrophs contains less energy per gram than plant biomass, so less carbon is stored in each gram. — A student who thinks animal tissue is poorer in energy picks this. Energy content per unit mass is not reduced at higher trophic levels; the difference is in the quantity of biomass accumulated.
3. Autotrophs do not lose biomass by cell respiration, whereas heterotrophs do, so only heterotrophs suffer this loss. — A student who thinks plants do not respire picks this. Autotrophs also respire and lose carbon as carbon dioxide; secondary production is lower because heterotrophs start from the smaller pool of carbon that reaches them and then respire much of it.
4. Heterotrophs convert much of the carbon they consume to carbon dioxide and water in cell respiration, so less becomes biomass. — Secondary production is the accumulation of carbon compounds in biomass by heterotrophs. Due to loss of biomass when carbon compounds are converted to carbon dioxide and water in cell respiration, secondary production is lower than primary production in an ecosystem.

Syllabus statement C4.2.16 · Read this in Learn

1. A carbon sink, with a net uptake of 550 g m⁻², because only heterotroph respiration returns carbon to the atmosphere. — A student who leaves out plant respiration picks this: 1200 − 650 = 550. Plants respire too, releasing 600 g m⁻² here, so the true balance is 1200 − 1250 = −50 g m⁻².
2. A carbon source, with a net release of 50 g m⁻², because total respiration (1250) exceeds photosynthesis (1200). — If respiration exceeds photosynthesis there is a net release of carbon dioxide. Respiration here includes all organisms: 600 + 650 = 1250 g m⁻², which is 50 g m⁻² more than the 1200 g m⁻² fixed, so the forest was a source that year.
3. A carbon sink, because a forest full of photosynthesizing trees removes carbon dioxide no matter what its respiration is. — A student who assumes vegetation always makes an ecosystem a sink picks this. Sink or source depends on the balance between photosynthesis and total respiration, and in this year respiration was greater.
4. Neither, because an ecosystem recycles its carbon internally and has no net exchange with the atmosphere. — A student who treats the ecosystem as a closed system picks this. Ecosystems are open: carbon dioxide crosses the boundary in both directions, and here the outflow exceeded the inflow by 50 g m⁻².

Syllabus statement C4.2.18 · Read this in Learn

1. In the Northern Hemisphere growing season photosynthesis exceeds respiration, drawing carbon dioxide down; in autumn and winter respiration exceeds photosynthesis. — The annual fluctuation is analysed in terms of photosynthesis and respiration. Most land, and so most vegetation, is in the Northern Hemisphere; from May to September net uptake by photosynthesis lowers atmospheric carbon dioxide, and from autumn to spring respiration and decomposition return it.
2. People burn more fuel for heating in the Northern Hemisphere winter, so combustion raises carbon dioxide until spring and it falls again in summer. — A student who attributes every feature of the curve to fuel burning picks this. Seasonal changes in combustion are small compared with the seasonal swing in photosynthesis; combustion explains the long-term trend, not the annual cycle.
3. Plants respire only in winter, when they cannot photosynthesize, so carbon dioxide is released only in the cold months and absorbed in the warm months. — A student who sees photosynthesis and respiration as alternatives picks this. Plants respire all year round; in summer photosynthesis simply exceeds respiration, and in winter the balance reverses.
4. Biological processes are too small to change the whole atmosphere, so the regular swing must be an artefact of the measuring instruments at the observatory. — A student with no sense of the scale of global fluxes picks this. The yearly exchange of carbon dioxide between organisms and the atmosphere is huge, easily large enough to produce the measured swing, which is confirmed at many stations worldwide.

Syllabus statement C4.2.20 · Read this in Learn

1. Isolated, because nothing at all crosses the glass, so the plants survive on the energy that was sealed in with them. — A student who thinks a sealed container stops energy as well as matter picks this. Glass transmits light and conducts heat; without an energy input the plants would have died within months, since every energy transfer loses heat.
2. Closed, because light energy enters through the glass and heat leaves, but no matter passes in or out. — In closed systems only energy is able to pass in and out. The sealed bottle admits light for photosynthesis and loses heat, while every atom of water, carbon and oxygen inside is recycled between the plants and the soil organisms.
3. Closed, because energy is recycled inside the bottle between plants and decomposers, so nothing needs to enter. — A student who believes energy cycles like matter picks this. Chemical energy is released as heat at every transfer and cannot be reused, so the system depends on light entering continuously; only matter is recycled.
4. Closed, because light enters but no energy can leave, since plants do not release heat to their surroundings. — A student who believes only warm-blooded animals produce heat picks this. Plants and soil microorganisms respire and convert chemical energy to heat, which leaves through the glass; if it did not, the bottle would heat up continuously.

Syllabus statement C4.2.1 · Read this in Learn

1. Dead organic matter produced by photosynthesis at the surface, sinking down to the vents from the sunlit water above. — A student convinced that every ecosystem must trace back to sunlight picks this. A small amount of surface material does sink to the deep sea floor, but it is far too little to feed the dense vent communities, which are sustained by chemoautotrophs.
2. Bacteria that photosynthesize using the faint light emitted by the hot, mineral-rich water leaving the vents. — A student who assumes every autotroph must be photosynthetic picks this. The vent bacteria that support the community are chemoautotrophs; light is not their energy source.
3. Bacteria that fix carbon using energy released by oxidizing inorganic substances emerging from the vents. — Sunlight sustains most ecosystems, but ecosystems below the level of light penetration in oceans are exceptions. Chemoautotrophic bacteria oxidize inorganic substances such as hydrogen sulfide from the vent fluid and use the energy to synthesize carbon compounds, forming the base of the food chains.
4. Bacteria that respire dissolved carbon compounds carried up from below the sea floor in the hot vent water. — A student who thinks 'oxidation as an energy source' must mean respiring organic matter picks this. The vent bacteria oxidize inorganic substances and fix carbon dioxide; they do not depend on a supply of ready-made carbon compounds.

Syllabus statement C4.2.2 · Read this in Learn

1. It explains why energy is lost between trophic levels, because a law is a theory that has been proved by many observations. — A student who ranks laws above theories in certainty picks this. A law describes; it does not explain. The 10% rule says how much energy is passed on, not why the rest is lost, and laws are not proven theories.
2. It is disproved, because a law must hold without exception and the measured transfers are not all exactly 10%. — A student who reads 'law' as a rule that admits no exceptions picks this. A useful generalization holds in most cases and allows predictions; a rule of thumb is not falsified by the scatter of real values around it.
3. It shows that exactly 10% is transferred at every step, so the range of 5% to 20% must reflect measurement errors. — A student who treats the 10% figure as a fixed constant picks this. The rule is an approximation; the measured range reflects genuine differences between ecosystems and trophic levels, not errors.
4. It describes a pattern and can be used to predict the energy at a higher level, but it does not explain why so much energy is lost. — Laws in science are generalized principles, or rules of thumb, formulated to describe patterns observed in nature. Unlike theories they do not offer explanations, but like theories they can be used to make predictions. The explanation of the losses (respiration, uneaten material, faeces) is a separate matter.

Syllabus statement C4.2.2 · Read this in Learn

1. sparrowhawk → blue tit → caterpillar → hawthorn — A student who reads the arrow as 'eats' draws the chain from predator to prey. Arrows show the direction in which energy and biomass are transferred, so they must run from the hawthorn towards the sparrowhawk.
2. hawthorn leaves → caterpillar → blue tit → sparrowhawk — Each arrow points from the organism eaten to the organism that eats it, indicating the direction of transfer of energy and biomass: hawthorn leaves feed caterpillars, which feed blue tits, which feed sparrowhawks.
3. hawthorn → caterpillar → aphid → ladybird → sparrowhawk — A student who joins any two organisms that interact, including competitors, picks this. Caterpillars and aphids both eat hawthorn but neither eats the other, so no arrow can connect them; nor was any feeding link from ladybirds to sparrowhawks described.
4. soil fungi → hawthorn → caterpillar → blue tit → sparrowhawk — A student who thinks decomposers are producers that feed the plant picks this. The hawthorn does not eat fungi; it fixes its own carbon using light, so a food chain begins with the producer.

Syllabus statement C4.2.4 · Read this in Learn

1. The direction in which each consumer seeks its food, pointing from the feeder to the organism that it eats. — A student who reads the arrow as the verb 'eats' picks this. The convention is the reverse: the arrow follows the energy and biomass, from the eaten to the eater.
2. Every ecological interaction in the community, including competition between species that share a food source. — A student who treats a food web as a map of all relationships picks this. A food web shows feeding relationships only; competitors are not joined by arrows.
3. The direction in which energy and biomass are transferred, from each organism to the organism that feeds on it. — Arrows in food chains and food webs indicate the direction of transfer of energy and biomass, so they run from the food organism to the consumer.
4. The route by which energy is returned from decomposers to producers so that it can pass around the community again. — A student who believes energy is recycled picks this. Arrows show one-way transfer of energy and biomass by feeding; energy leaves the community as heat and is not returned to producers.

Syllabus statement C4.2.4 · Read this in Learn

1. Both carbon fixation and the anabolic reactions that build macromolecules need energy, supplied ultimately by light. — Autotrophs use an external energy source to synthesize carbon compounds from simple inorganic substances. Energy is required for carbon fixation and for the anabolic reactions that build macromolecules; in a photoautotroph that energy comes from light.
2. The autotroph generates its own energy as it synthesizes the carbon compounds, so no external energy source is needed. — A student who hears 'makes its own food' as 'makes its own energy' picks this. Energy cannot be created; the autotroph converts light energy into chemical energy, and without the light supply neither fixation nor anabolism could proceed.
3. Energy is needed for carbon fixation, but the anabolic reactions run on light directly because photoautotrophs do not respire. — A student who thinks photosynthesis replaces respiration in plants picks this. Anabolic reactions are driven by ATP, and photoautotrophs make ATP by cell respiration as well as in photosynthesis; they respire continuously.
4. No energy is needed for either process, because the carbon compounds absorbed through the roots already contain the energy. — A student who believes plants take in carbon compounds from the soil picks this. Roots absorb water and mineral ions; the carbon comes from carbon dioxide, and fixing it into carbon compounds requires an input of energy.

Syllabus statement C4.2.6 · Read this in Learn

1. The culture must have been exposed to some light, because synthesizing carbon compounds from carbon dioxide requires light. — A student who believes carbon fixation always depends on light picks this. Chemoautotrophs fix carbon in darkness using energy from oxidation reactions; the iron(III) that accumulated is the evidence of that oxidation.
2. The bacteria respired carbon compounds present as contaminants, and the iron(III) formed by ordinary chemical rusting. — A student who thinks bacteria can only gain energy by respiring organic matter picks this. The medium contained no carbon compounds, so the biomass gained must have been built from carbon dioxide, which needs an energy source: the iron oxidation.
3. The bacteria are chemoautotrophs that used energy from oxidizing iron(II) to synthesize carbon compounds from carbon dioxide. — Biomass increased with no organic carbon available, so the bacteria must have fixed carbon dioxide. There was no light, so the energy source was not photosynthesis; the accumulation of iron(III) shows that iron(II) was oxidized, and oxidation reactions release the energy needed for carbon fixation.
4. The bacteria generated the energy for growth themselves, so the oxidation of iron(II) was an unrelated side reaction. — A student who believes autotrophs create their own energy picks this. Energy cannot be generated from nothing; the growth was powered by the energy released when iron(II) was oxidized to iron(III).

Syllabus statement C4.2.7 · Read this in Learn

1. It absorbs the proteins intact through its gut wall and uses them directly, since a protein from food is already functional. — A student who thinks the protein eaten is the protein used picks this. Proteins are too large to be absorbed and would be the wrong proteins anyway; they are digested to amino acids and the heterotroph constructs its own.
2. It can use the proteins only if it has a gut, because digestion of complex carbon compounds cannot take place outside a cell. — A student who equates digestion with the gut picks this. Digestion can be external: fungi and many bacteria secrete enzymes onto food and absorb the products, then assimilate them just as an animal does.
3. It oxidizes all of the proteins in cell respiration to release energy, since heterotrophs cannot synthesize proteins themselves. — A student who thinks food is either burned or stored, never rebuilt, picks this. Heterotrophs synthesize their own proteins from absorbed amino acids; only some of the carbon compounds in food are respired.
4. It digests the proteins to amino acids, absorbs these and assimilates them by building the specific proteins that it needs. — Heterotrophs use carbon compounds obtained from other organisms to synthesize the carbon compounds that they require. Complex compounds such as proteins are digested, externally or internally, and the products are assimilated into the heterotroph's own molecules.

Syllabus statement C4.2.8 · Read this in Learn

1. It secretes enzymes onto the wood, absorbs the small molecules produced by this external digestion and assimilates them. — Complex carbon compounds are digested either externally or internally. Fungi digest externally: enzymes released into the wood break down its carbon compounds, the products are absorbed, and the fungus then constructs the carbon compounds that it requires.
2. It takes fragments of wood into its cells and digests them internally, since digestion can only happen inside an organism. — A student who thinks digestion is always internal picks this. Fungal cells cannot ingest solid particles; they digest the wood outside their cells and absorb the products.
3. It synthesizes its own carbon compounds from the mineral ions in the decaying wood, using the energy released by decay. — A student who classes decomposers as producers picks this. A fungus is a heterotroph: it cannot fix carbon and obtains both carbon and energy from the carbon compounds of the dead wood.
4. It absorbs the cellulose and other macromolecules of the wood intact through its cell walls and uses them unchanged. — A student who thinks food macromolecules can be used as they are picks this. Cellulose and lignin are far too large to cross membranes; they must be digested to small molecules first.

Syllabus statement C4.2.8 · Read this in Learn

1. A secondary consumer only, because each species has a single trophic level that is fixed by its usual diet. — A student who believes trophic level is a fixed property of a species picks this. The blackbird's level depends on which food chain is being considered; when it eats berries it is a primary consumer.
2. Secondary consumer in both chains, because a bird that eats animals is a carnivore and carnivores are secondary consumers. — A student who treats 'secondary consumer' as a synonym for 'carnivore' picks this. Trophic level is set by position in the chain, not by the type of animal; eating berries makes the blackbird a primary consumer in that chain.
3. A primary consumer in the chain where it eats berries and a secondary consumer in the chain where it eats snails. — Trophic level is a position in a particular food chain. In shrub → blackbird the bird feeds on the producer and is a primary consumer; in shrub → snail → blackbird it feeds on a primary consumer and is a secondary consumer. Many organisms have a varied diet and occupy different trophic levels in different food chains.
4. Secondary consumer when eating berries and tertiary consumer when eating snails, since the shrub is the first consumer. — A student who counts the producer as the first consumer picks this. Producers are not consumers; the herbivore feeding on the shrub is the primary consumer, so the numbering starts one level lower than this option assumes.

Syllabus statement C4.2.10 · Read this in Learn

1. About 1.8% — A student who compares every level with the producers picks this: 383 ÷ 20 810 × 100 = 1.8%. That is the fraction of the producer energy reaching secondary consumers, not the transfer from primary to secondary consumers.
2. Exactly 10% — A student who applies the 10% rule instead of calculating from the data picks this. The rule is only a rough generalization; the measured transfer at this step is 383 ÷ 3368, which is 11.4%.
3. Close to 89% — A student who subtracts and finds the proportion lost picks this: (3368 − 383) ÷ 3368 × 100 = 88.6%. The question asks for the percentage transferred, which is the remaining 11.4%.
4. About 11% — Transfer between adjacent levels is the energy at the higher level divided by the energy at the level below: 383 ÷ 3368 × 100 = 11.4%, about 11%. An energy pyramid built from these data would show this loss as a large narrowing between the second and third bars.

Syllabus statement C4.2.11 · Read this in Learn

1. It is returned to the plant through the soil, so that the plant can use it again for growth instead of needing sunlight. — A student who believes energy is recycled like matter picks this. Decomposers return the elements in faeces to the soil, but the chemical energy is released as heat; the plant obtains its energy only from light.
2. It passes to decomposers and detritus feeders, which release it as heat in respiration, not to the next trophic level. — Decomposers and detritus feeders are not usually considered part of the food chain, but they play a role in its energy transformations: the carbon compounds in faeces are their energy supply, and they oxidize them in cell respiration, releasing the energy as heat. Only the 15 kJ in biomass is available to a bird that eats the caterpillar.
3. There is no usable energy in the faeces, because the 50 kJ was already released as heat inside the caterpillar's gut. — A student who thinks faeces are energy-free waste picks this. Egested material was never absorbed, so its carbon compounds and their 50 kJ are intact; the only heat released by the caterpillar was the 35 kJ from respiration.
4. It has been used up by the caterpillar and ceases to exist, since energy that is not stored as biomass is destroyed. — A student who believes energy can be destroyed picks this. Energy is conserved; the 50 kJ remains in the carbon compounds of the faeces until decomposers and detritus feeders respire them.

Syllabus statement C4.2.12 · Read this in Learn

1. They do not: heat is produced only by warm-blooded animals; plants merely lose heat that they absorbed from sunlight. — A student who links heat production to being warm-blooded picks this. Plants respire continuously and convert chemical energy to heat; a compost heap or a germinating seed pile warms up for exactly this reason.
2. Plants respire only at night, so the heat they lose is produced in darkness and simply builds up until the next day. — A student who thinks plants alternate between photosynthesis and respiration picks this. Plants respire at all times, so heat is produced and lost continuously.
3. Cell respiration in every organism converts some chemical energy to heat, both when ATP is produced and when ATP is used. — Heat loss to the environment occurs in both autotrophs and heterotrophs because energy transfers are not 100% efficient: heat is produced when ATP is made in cell respiration and again when the ATP is used in cells.
4. Heat is released only when ATP is used in the cell, because the production of ATP in respiration transfers energy with no loss. — A student who assumes respiration captures all the energy in ATP picks this. Energy transfers are not 100% efficient at either stage; heat is produced during ATP production as well as when ATP is used.

Syllabus statement C4.2.13 · Read this in Learn

1. Energy per unit mass is similar at every level, so the fall in energy available up the chain is due to the fall in biomass. — At each successive stage there are fewer organisms or smaller organisms, so there is less biomass, but the energy content per unit mass is not reduced. The burning data confirm the second point, so the reduced energy at higher levels must come from the smaller quantity of biomass.
2. The biomass at higher levels must be poorer in energy per gram, since energy falls even faster than mass along the chain. — A student who expects tissue to become less energy-rich up the chain picks this. The burning results contradict it: each gram of biomass releases a similar amount of energy whatever its trophic level.
3. The data show that producers accumulate 800 g of biomass per square metre each year, which is their primary production. — A student who confuses a stock with a rate picks this. The figures are the biomass present at one time (g m⁻²); primary production is a rate of accumulation (g m⁻² yr⁻¹) and cannot be read from a snapshot.
4. The data confirm that exactly 10% of biomass is transferred between each pair of trophic levels in this ecosystem. — A student who reaches for the 10% rule without checking picks this. The ratios here are 12.5%, 15% and 13%, and in any case a biomass snapshot does not measure transfer of energy over time.

Syllabus statement C4.2.14 · Read this in Learn

1. Grams of carbon per square metre of land — A student who equates production with the biomass present picks this. Grams per square metre measure a stock of biomass at one time; production must include per unit time because it is a rate of accumulation.
2. Grams of carbon per square metre per year — Primary production is the accumulation of carbon compounds in biomass by autotrophs. It is a rate, so the units are mass of carbon per unit area per unit time, usually g m⁻² yr⁻¹.
3. Kilojoules per square metre per year — A student who carries over the units of an energy pyramid picks this. Energy flow is expressed in kJ m⁻² yr⁻¹, but primary production is defined by the mass of carbon accumulated, so its units are g m⁻² yr⁻¹.
4. Grams of carbon per individual per year — A student who thinks of production as the growth of an individual plant picks this. Production is a property of an area of ecosystem, measured per square metre, so that biomes can be compared.

Syllabus statement C4.2.15 · Read this in Learn

1. The desert value must be an error, because deserts receive the most sunlight and primary production depends on light alone. — A student who treats light as the only factor picks this. Light is necessary but not sufficient; without water, stomata close and photosynthesis stops, so desert production is genuinely very low.
2. Rainforest has the highest value simply because it contains the most standing biomass, not because it grows faster. — A student who confuses biomass with production picks this. The figures are rates of accumulation per year; a rainforest genuinely fixes and retains far more carbon per square metre each year than the other biomes.
3. Biomes differ greatly in capacity to accumulate biomass; deserts have high light but water limits their photosynthesis. — Biomes vary in their capacity to accumulate biomass. Photosynthesis and growth need water, warmth and mineral nutrients as well as light, so a hot desert, despite intense sunlight, has a production more than twenty times lower than a rainforest.
4. Rainforest soils are richer in carbon compounds, which the trees absorb through their roots to build biomass faster. — A student who believes plants take carbon from the soil picks this. The carbon in new biomass comes from atmospheric carbon dioxide fixed in photosynthesis; roots absorb water and mineral ions, not carbon compounds.

Syllabus statement C4.2.15 · Read this in Learn

1. Photosynthesis: CO₂ → producers; feeding: producers → consumers; respiration: producers, consumers, decomposers → CO₂. — Carbon is recycled in ecosystems by photosynthesis, feeding and respiration. Photosynthesis fixes carbon dioxide into the carbon compounds of producers, feeding passes carbon compounds along food chains, and cell respiration in all organisms, including the producers and the decomposers, returns carbon dioxide.
2. Photosynthesis: CO₂ → producers; feeding: producers → consumers; decomposition of dead organisms → CO₂, with no other return. — A student who thinks carbon leaves organisms only after death picks this. Living producers and consumers release carbon dioxide continuously by cell respiration; decay is only the route for carbon remaining in dead matter.
3. Absorption: soil carbon → producers; feeding: producers → consumers; respiration: all organisms → CO₂ in the air and water. — A student who believes plants take carbon from the soil picks this. Producers obtain carbon by fixing carbon dioxide in photosynthesis; the arrow into producers must start from carbon dioxide.
4. Photosynthesis: CO₂ → producers; feeding: producers → consumers; respiration: consumers and decomposers only → CO₂ in the air. — A student who thinks plants photosynthesize instead of respiring picks this. Producers respire as well, so a respiration arrow must also run from producers to carbon dioxide.

Syllabus statement C4.2.17 · Read this in Learn

1. All formed from organisms at about the same time in the distant past, so burning any of them has the same significance for the carbon cycle. — A student who treats all fuels of organic origin as one category picks this. Their dates of formation differ enormously, from living biomass to coal hundreds of millions of years old, and this affects how long the carbon had been out of circulation.
2. Burning biomass such as wood releases no carbon dioxide because it is renewable, whereas burning the fossil fuels does release it. — A student who reads 'carbon-neutral' as 'no carbon dioxide' picks this. Combustion of biomass releases carbon dioxide just as fossil fuels do; the difference is only that regrowth may fix an equivalent amount later.
3. All release carbon dioxide that was originally fixed by photosynthesis, at dates ranging from recent years to hundreds of millions of years ago. — Combustion of each of these releases carbon dioxide into the atmosphere. They vary in date of formation: biomass may be a few years old, peat accumulates over thousands of years, and coal, oil and natural gas formed over tens to hundreds of millions of years, but all began as carbon fixed by autotrophs.
4. Coal, oil and natural gas release carbon of mineral origin that did not come from living organisms, unlike biomass and peat. — A student who thinks 'fossil fuel' means a mineral picks this. Coal, oil and gas formed from the remains of organisms, so their carbon was fixed from the atmosphere by photosynthesis long ago.

Syllabus statement C4.2.19 · Read this in Learn

1. Only the coal releases carbon dioxide, because the grass and the stubble are renewable biomass and so are carbon-neutral fuels. — A student who equates renewable with carbon-free picks this. Burning grass and stubble releases carbon dioxide immediately; carbon-neutrality refers only to possible later regrowth.
2. The carbon in the coal and the carbon in the stubble were both fixed at about the same time, so the three releases are all equivalent. — A student who thinks all combustible carbon stores are the same age picks this. The stubble's carbon was fixed within the last year; the coal's was fixed hundreds of millions of years ago and had been locked out of the atmosphere ever since.
3. The coal releases carbon dioxide of geological origin that was not fixed by photosynthesis, unlike the grass and the crop stubble. — A student who regards coal as a mineral picks this. Coal formed from plant remains, so its carbon was fixed by photosynthesis in the distant past.
4. All three are combustion releasing carbon dioxide; the first is natural, but human activity has greatly increased combustion rates. — Combustion following lightning strikes sometimes happens naturally, but human activities such as burning crop residues and fossil fuels have greatly increased combustion rates. In each case carbon compounds are oxidized and carbon dioxide is released to the atmosphere.

Syllabus statement C4.2.19 · Read this in Learn

1. Breathing by the growing human population, together with its livestock, has added more carbon dioxide each year than plants can absorb. — A student who combines 'respiration releases carbon dioxide' with population growth picks this. The carbon exhaled by people and livestock was fixed from the atmosphere by crops within recent years, so it returns carbon already in circulation rather than adding to the total.
2. Combustion of fossil fuels and biomass releases carbon fixed long ago faster than photosynthesis and other processes remove it. — The long-term trend is analysed in terms of combustion. Burning coal, oil, gas and biomass adds carbon dioxide that had been locked out of the atmosphere, at a rate exceeding the net uptake by ecosystems and oceans, so the concentration climbs by an average of about 1.6 ppm per year over this period.
3. Increased volcanic activity has released carbon dioxide from deep within the Earth, and this geological source dominates the trend. — A student who thinks volcanoes are the main source of atmospheric carbon dioxide picks this. Volcanic emissions are a tiny fraction of those from combustion of fossil fuels, and there has been no sustained rise in volcanic activity since 1960.
4. Fossil fuels release mineral carbon that was not part of the carbon cycle, so it accumulates because organisms are unable to take it up. — A student who regards fossil carbon as non-biological picks this. Fossil-fuel carbon was fixed by photosynthesis and is chemically identical to any other carbon dioxide; it accumulates because it is being added faster than photosynthesis and ocean uptake remove it, not because it cannot be absorbed.

Syllabus statement C4.2.20 · Read this in Learn

1. Photosynthesis uses carbon dioxide that comes mainly from volcanoes and the burning of fuels, so it does not depend on respiration by other organisms. — A student who thinks of geological and industrial sources first picks this. Respiration by autotrophs and heterotrophs returns far more carbon dioxide to the atmosphere each year than volcanoes and combustion combined.
2. Only heterotrophs depend on atmospheric oxygen, because autotrophs obtain all the energy they need from light and do not carry out aerobic respiration. — A student who thinks plants do not respire picks this. Autotrophs carry out aerobic respiration in every cell, day and night, and so depend on atmospheric oxygen just as heterotrophs do.
3. Aerobic respiration depends on oxygen produced by photosynthesis, and photosynthesis depends on carbon dioxide produced by respiration; the yearly fluxes are huge. — Aerobic respiration depends on atmospheric oxygen produced by photosynthesis, and photosynthesis depends on atmospheric carbon dioxide produced by respiration. The fluxes involved per year are huge, so this is a major interaction between autotrophs and heterotrophs.
4. The exchange of gases between organisms and the atmosphere is small compared with the atmosphere itself, so it is a minor interaction on a global scale. — A student who pictures photosynthesis at the scale of a leaf picks this. The annual fluxes of oxygen and carbon dioxide between organisms and the atmosphere are enormous, which is exactly why this is a major interaction.

Syllabus statement C4.2.21 · Read this in Learn

1. Only carbon is recycled; the other elements are absorbed from the soil, used by organisms and then used up, so they must be replaced. — A student who has studied only the carbon cycle picks this. Elements are not used up; nitrogen, phosphorus and every other element required by organisms pass repeatedly between organisms and the environment.
2. Elements are released from dead organisms automatically as they die, so decomposers play only a minor part in recycling them. — A student who sees death and decay as the same event picks this. The elements in a dead organism are locked in complex carbon compounds until decomposers digest them; without decomposers, recycling would largely stop.
3. Both the chemical elements and the energy in dead organisms are recycled by decomposers and returned to producers for reuse. — A student who believes energy cycles like matter picks this. Decomposers return elements to the environment, but they release the energy in dead matter as heat, which producers cannot use.
4. All elements required by organisms are recycled; decomposers release them from dead matter in forms that autotrophs can take up. — All elements used by living organisms, not just carbon, are recycled in ecosystems, and decomposers play a key role by breaking down the carbon compounds of dead organic matter and returning the elements to the abiotic environment as simple inorganic substances.

Syllabus statement C4.2.22 · Read this in Learn

1. The elements built into crop biomass are removed from the field instead of being returned by decomposers, so the cycle is broken. — In a natural ecosystem the elements in plant biomass return to the soil when decomposers break down dead matter and faeces. Exporting the whole crop removes those elements from the ecosystem each year, so the recycling that would normally replenish the soil cannot occur.
2. Mineral elements are used up and destroyed by the growing plants, so any field loses them whether or not the crop is removed. — A student who thinks elements are consumed picks this. Elements are never destroyed; the ones in this field are lost only because they leave in the harvested biomass rather than being recycled.
3. The crop absorbed carbon compounds from the soil to build up its biomass, and it is this carbon that the soil is running short of. — A student who believes plants feed on soil carbon picks this. The crop's carbon came from atmospheric carbon dioxide; what the soil loses are the mineral elements, such as nitrogen and phosphorus, that were built into the exported biomass.
4. Decomposers in the soil cannot produce new mineral ions fast enough to keep pace with the amount that the crop plants absorb each year. — A student who thinks decomposers manufacture nutrients picks this. Decomposers only release elements already present in dead organic matter; if that matter is removed from the field, there is nothing for them to recycle.

Syllabus statement C4.2.22 · Read this in Learn

1. The bars represent the energy stored in the organisms present at each level when sampled, so the units should be kcal m⁻² with no time component. — A student who treats an energy pyramid as a snapshot like a pyramid of biomass picks this. Each bar is a rate of energy flow through the level, which is why the units include per year.
2. Four horizontal bars with producers at the base, each bar's width proportional to the energy flowing through that level per unit area per unit time. — An energy pyramid is constructed from research data for a specific ecosystem: one bar per trophic level, producers at the base, widths proportional to energy flow in kcal (or kJ) m⁻² yr⁻¹, so the narrowing between bars represents the energy losses between levels.
3. A fifth bar for decomposers should be drawn at the top, because they form the final trophic level and receive the remaining 21 kcal m⁻² yr⁻¹. — A student who places decomposers as the last trophic level picks this. Decomposers are not usually considered part of food chains; they receive dead matter and faeces from every level and are not a bar in the pyramid.
4. The bars narrow because the tissues at each higher level contain less energy per gram, so the widths show energy per unit mass of biomass. — A student who thinks biomass becomes poorer in energy up the chain picks this. Energy content per unit mass is not reduced; the bars narrow because less total energy flows through each successive level.

Syllabus statement C4.2.11 · Read this in Learn