IB Biology C4.2: Transfers of energy and matter worked solutions

B. Energy enters, flows through and leaves as heat, while matter such as carbon and nitrogen is recycled by decomposers. A and C let energy be recycled or matter be lost, and D describes a closed system with no energy supply.

B. Photoautotrophs use light. Chemoautotrophs use energy released by oxidising inorganic substances such as sulfur compounds. A gives light to both, C swaps the pair and D gives oxidation to both.

A. The sparrowhawk eats a vole, a primary consumer, so it is a secondary consumer there, and a blue tit, a secondary consumer, so it is a tertiary consumer there. The shrew eats a caterpillar, so it is a secondary consumer; the vole is a primary consumer and the blue tit is a secondary consumer, not a producer.

C. That is secondary production. Primary production is the same gain made by autotrophs, and neither respiration nor the light absorbed by leaves is production.

B. 600\div5000\times100=12\%. A is the share that does not pass on, C is 70\div600 and D is 70\div5000.

C. At each transfer much of the energy is lost as heat from respiration and left in uneaten or indigestible parts, so too little remains to support another level. Energy is not recycled and biomass does not increase up a chain.

D. Decomposers respire the carbon compounds in dead leaves and release carbon dioxide. Photosynthesis takes carbon dioxide in, combustion needs burning and fossilization stores carbon.

B. Burning or clearing releases stored carbon and removes the producers that take it in. Planting seedlings and increased photosynthesis increase the sink, and slow growth leaves it small but still positive.

B. In the northern spring and summer photosynthesis in the large northern land vegetation exceeds respiration, so there is a net uptake of carbon dioxide. Combustion does not fall in summer, and the other options would raise the level.

A. Aerobic respiration depends on the oxygen released by photosynthesis, and photosynthesis depends on the carbon dioxide released by respiration. B reverses the gases, and C and D describe only one process.

(a) Each arrow runs from the food to the feeder: kelp to urchin, kelp to snail, urchin to crab, snail to crab, urchin to otter, crab to otter. All six correct (3); four or five correct (2); two or three correct (1). (b) The otter eats urchins, which are primary consumers, so it is a secondary consumer (1). It also eats crabs, which are secondary consumers, so it is a tertiary consumer (1). (c) Without otters the urchins lose a predator and increase in number (1). More urchins graze more kelp, so the kelp decreases (1). Credit a reasoned answer that notes the crabs, which also eat urchins, may increase and partly offset this.

(a) 2400-600-1560=240\,\mathrm{kJ} goes into growth (1). (b) 240\div2400\times100=10\% of the ingested energy (1). (c) The energy absorbed is 2400-600=1800\,\mathrm{kJ}, so 240\div1800\times100=13.3\% (1). (d) A fish that swims more respires more, releasing more energy as heat, so a smaller share of the food becomes biomass (1).

(a) A scale stated and used, for example 1\,\mathrm{cm}=500\,\mathrm{kJ\,m^{-2}\,yr^{-1}}, giving bars of 9.6 cm, 1.04 cm, 0.12 cm and 0.01 cm (1); bars of equal height, centred, producers at the base (1); each bar labelled with its trophic level and the units (1). (b) 61\div520\times100=11.7\% (1). (c) The tertiary step is 5\div61\times100=8.2\%, similar to the others, so the top bar is tiny because of the repeated losses below it, not because that step is unusually inefficient (1).

Any six of: sunlight is the energy source (1); producers absorb light and convert it to chemical energy in carbon compounds by photosynthesis (1); chemical energy passes to consumers by feeding (1); each trophic level uses energy in cell respiration (1); respiration releases energy for ATP and heat (1); energy is lost as heat, which leaves the ecosystem (1); dead organisms and waste pass chemical energy to decomposers (1); energy is not recycled, so a continuous supply of light is needed (1).

Energy is lost at each trophic level as heat from cell respiration (1). It is also lost in indigestible and uneaten parts (1). Less energy is therefore available to each higher level, so fewer or smaller organisms can be supported (1). Because a gram of tissue holds about the same energy at every level, less total energy means less biomass (1).

Photosynthesis moves carbon dioxide from the air into carbon compounds in the trees (1). Feeding passes carbon compounds to consumers (1). Dead organisms and waste pass carbon to decomposers (1). Respiration by plants, animals and decomposers returns carbon dioxide (1); combustion, for example a forest fire, also returns it (1). Long-term storage: peat formed in waterlogged, anaerobic, acid soil, or fossilization to fossil fuels (1).

(a) Gain =27.6-18.0=9.6 tonnes of carbon (1); 9.6\times10^6\,\mathrm{g}\div8000\,\mathrm{m^2}=1200\,\mathrm{g\,m^{-2}} (1); 1200\div3=400\,\mathrm{g\,C\,m^{-2}\,yr^{-1}} (1). (b) 12\times10^6\div15\,000\div3=267\,\mathrm{g\,C\,m^{-2}\,yr^{-1}} (1). (c) Plot X has the higher production, because production is a rate per square metre per year; Y gained more in total only because it is larger (1).

(a) Release =240+190+12+18=460 (1); net =520-460=+60\,\mathrm{g\,C\,m^{-2}\,yr^{-1}} (1). (b) Positive, so the peatland is a net carbon sink (1). (c) Decomposition rises to 330, so release is 240+330+12+18=600 and the net is 520-600=-80, a net source (1). The drought lowers the water level, air enters the peat and decomposers respire the stored carbon (1).

(a) Rainforest: biomass (430); boreal forest: soil (220) (1). (b) Litter is decomposed quickly in the warm, moist conditions (1) and the released nutrients are taken up quickly by plants, so little accumulates in the litter (1). (c) Clearing and removing the trees takes away most of the nutrients, which are held in the biomass (1); the soil store is small, so soil fertility falls quickly (1).

(a) The dark jar measures respiration alone, because there is no photosynthesis without light, so the light jar's net gain understates photosynthesis: net change in light = photosynthesis - respiration, so photosynthesis =3.6+1.5=5.1\,\mathrm{mg\,dm^{-3}\,h^{-1}} (2). (b) Aerobic respiration oxidises carbon compounds using oxygen, so oxygen falls (1), and it releases carbon dioxide, so that rises (1). (c) Photosynthesis releases oxygen, which aerobic respiration uses (1); respiration releases carbon dioxide, which photosynthesis uses (1).

(a) The bacteria make carbon compounds from carbon dioxide, so they are autotrophs (1), and they use oxidation of an inorganic substance as the energy source, so they are chemoautotrophs (1). (b) Oxidation of hydrogen sulfide (1). (c) Energy enters the community as chemical energy in the carbon compounds made by the bacteria (1); it passes to the shrimps, which are primary consumers, and then to the fish, a secondary consumer, by feeding (1); at each level respiration releases heat, which leaves the pool, and none of it is recycled (1).

(a) The yearly mean rises each year from 408.0 to 415.5 ppm (1), an increase of 7.5 ppm in three years or 2.5 ppm a year (1). (b) Swing in 2018: 411.4-404.6=6.8\,\mathrm{ppm} (1). The high is in late spring and the low in early autumn; in spring and summer northern plants take up more carbon dioxide in photosynthesis than organisms release in respiration, so the level falls, which makes the northern vegetation a net carbon sink between May and October, and in autumn and winter respiration exceeds photosynthesis (1). (c) Combustion of coal, oil, gas, peat and wood releases carbon dioxide (1) faster than photosynthesis and the oceans remove it (1).

(a) Primary production is the carbon compounds autotrophs add to their stored biomass (1); secondary production is the carbon compounds heterotrophs add to theirs (1). Phytoplankton is the primary production, and the other three are secondary production. (b) Zooplankton to small fish =100\div840\times100=11.9\% (1); small fish to pike =9\div100\times100=9\% (1). (c) The first step is 840\div6000=14.0\%, so the percentage hardly changes between steps (1); the pike get little energy because losses add up through the earlier levels, not because that step is unusually inefficient (1); the claim is not supported (1).

(a) Photosynthesis exceeds respiration in the growing trees (1), so carbon dioxide is taken up and carbon accumulates in the biomass, making the plantation a net sink (1). (b) Raw materials: carbon dioxide and water; the product that stores energy is a carbohydrate such as glucose (1). (c) Burning the wood is combustion, which releases the stored carbon as carbon dioxide (1); carbon dioxide absorbs infrared radiation and re-emits it, which warms the atmosphere, the enhanced greenhouse effect (1); the carbon released returns the plantation to being a source (1).

(a) Earthworms are detritivores, which ingest dead organic matter and digest it inside the body (1). Fungi are saprotrophs, which secrete digestive enzymes onto the matter and absorb the products (1). (b) Decomposers break down dead matter to release the elements in it (1), which are taken up by plants and used again, so elements are recycled (1). (c) Less leaf litter means less food (1), so the carrying capacity for the earthworm population falls and the population declines (1).

(a) Respiration is catalysed by enzymes (1); a higher temperature gives more collisions and faster reaction rates, so more carbon dioxide is released (1). (b) A warmer climate raises the release of carbon dioxide from soil (1); the added carbon dioxide strengthens the greenhouse effect and warms the climate further, which is a positive feedback (1). (c) In waterlogged peat there is little oxygen (1), the soil is acid, and decomposers cannot fully decompose the plants, so carbon stays stored (1).