ESAT Biology: Ecosystems and plant physiology, set 2

• Answer all questions. No calculator.
• Each question has exactly one correct answer.
• Work only with the values given in each question; no calculator is allowed or needed.

1. 11 mark

   In parts of the UK, the introduced grey squirrel has largely replaced the native red squirrel in many woodlands, partly because grey squirrels are more efficient at digesting acorns and can build up larger fat reserves from the same woodland food supply, allowing them to survive in greater numbers than red squirrels in the same habitat. Which term best describes the relationship between the two squirrel species, and why?

   1. A Mutualism, because both squirrel species benefit from living in the same woodland and sharing access to its food supply.
   2. B Interspecific competition, because both species compete for the same limited woodland food supply and habitat space, and one outcompetes the other.
   3. C Predation, because the grey squirrel is directly killing and consuming red squirrels in order to reduce their numbers.
   4. D Parasitism, because the grey squirrel benefits from the presence of the red squirrel in the woodland, while the red squirrel is harmed by living alongside it.
2. 21 mark

   A tapeworm lives attached to the inner wall of a mammal's intestine, absorbing digested food from its host over a long period. The tapeworm gains all of its nutrients this way, while the host mammal loses nutrients it would otherwise absorb and can suffer damage to its gut lining. Which term correctly describes this relationship, and why?

   1. A Predation, because the tapeworm is effectively consuming part of its host's body over time, in the same way a predator consumes its prey.
   2. B Competition, because the tapeworm and the mammal are both trying to make use of the same digested food that passes through the intestine.
   3. C Parasitism, because the tapeworm benefits at the host's direct expense, over a prolonged, close association between the two species.
   4. D Mutualism, because both the tapeworm and the host mammal gain some benefit from living in such close association with one another.
3. 31 mark

   A population of a particular moth species in a woodland numbers approximately 4000 individuals in year 1. An unusually cold, wet winter leads to widespread failure of the moth's overwintering pupae, and the population falls to approximately 1000 individuals by year 2. Which statement correctly classifies the winter weather as a cause of this population change, and explains why?

   1. A Cold, wet weather is an abiotic factor: a non-living, physical condition that can reduce survival directly, distinct from a biotic factor such as a predator, competitor, pathogen or food shortage.
   2. B Cold, wet weather is a biotic factor, because it is produced by the interaction of living organisms with their environment over many years, such as through large-scale patterns of vegetation growth.
   3. C Since the moth population itself is a living thing, any factor that affects the moths, including the weather, must automatically be classed as a biotic factor.
   4. D The population change described here cannot be caused by an abiotic factor at all, since abiotic factors can only ever affect population size gradually, over many generations, rather than causing a sharp fall within a single year.
4. 41 mark

   In a lake ecosystem, algae are eaten by water fleas, which are eaten by small fish, which are in turn eaten by herons. Which statement correctly explains why the algae, rather than any of the animals in this food chain, are described as the primary producers, and what this means for biomass in the ecosystem?

   1. A Algae are called primary producers simply because they are eaten first in the food chain, before the water fleas, fish or herons feed, and this ordering has no connection to how algae obtain their energy.
   2. B Algae are primary producers because they have the greatest biomass of any single organism in the food chain, larger than a heron or a fish, regardless of how they obtain their energy.
   3. C Algae, water fleas, small fish and herons are all equally described as primary producers, because every organism in a food chain contributes some biomass to the ecosystem.
   4. D Algae are photosynthetic organisms that build up their own biomass directly from light energy, carbon dioxide and water, without needing to eat another organism to obtain it.
5. 51 mark

   An ecologist wants to estimate the number of woodlice living in a large compost heap using the mark-release-recapture method. She catches, marks and releases 50 woodlice back into the compost heap. A few days later, she catches a second sample of 40 woodlice, of which 8 are found to be already marked. Using the formula: estimated population = (number marked and released in the first sample x total number caught in the second sample) / number of marked individuals found in the second sample, what is the best estimate of the total woodlice population in the compost heap?

   1. A 16000
   2. B 250
   3. C 50
   4. D 400
6. 61 mark

   Salmon are farmed in floating cages in a sea loch, at a much higher density than salmon would occur naturally in the wild. Waste food and faeces from the farmed fish accumulate in the water and on the seabed below the cages. Which statement correctly describes a negative impact this fish farm can have on biodiversity in the loch?

   1. A Nutrients from the accumulated waste can cause excessive growth of algae and bacteria near the cages, and as this extra biomass decomposes it uses up dissolved oxygen in the water, harming other species.
   2. B The farmed salmon compete directly with wild seabirds for nesting space along the shoreline, and this competition for nesting sites is the main threat the farm poses to biodiversity.
   3. C Because the farmed salmon are kept in cages, they cannot have any effect at all on wild species living in the surrounding loch, since none of the water inside a cage interacts with the water outside it.
   4. D Farmed salmon that escape from damaged cages cannot interbreed with or compete against wild salmon populations, because the medicines used to treat them make escaped fish unable to survive outside the cage.
7. 71 mark

   Sulfur dioxide and nitrogen oxides released by burning fossil fuels dissolve in atmospheric water droplets and fall as acid rain, lowering the pH of some lakes and streams. Which statement correctly describes a negative impact of acid rain of this kind on biodiversity, and its underlying cause?

   1. A Acid rain increases biodiversity in affected lakes, because the extra hydrogen ions provide an additional food source that supports larger populations of every aquatic species living there.
   2. B Acid rain is caused mainly by carbon dioxide released during photosynthesis, which dissolves in rainwater and lowers its pH as it falls on lakes and forests.
   3. C Acid rain can leach toxic aluminium ions out of soil and into waterways, and the resulting low pH and aluminium can damage fish gills and reduce reproductive success.
   4. D Acid rain affects only the specific plant species it directly lands on, so it cannot have any indirect effect on animal species living in or around the same lake or forest.
8. 81 mark

   Respiration and decomposition both release carbon dioxide into the atmosphere as part of the ongoing carbon cycle, and this carbon dioxide can be taken up again by photosynthesis within the same cycle. Burning fossil fuels also releases carbon dioxide. Which statement best explains why burning fossil fuels, unlike respiration and decomposition, increases the overall concentration of carbon dioxide in the atmosphere?

   1. A Fossil fuels release a different gas from carbon dioxide when burned, and only this different gas builds up in the atmosphere over time, since true carbon dioxide is always fully reabsorbed by plants.
   2. B Respiration and decomposition do not actually release any carbon dioxide at all, so burning fossil fuels is the only process in the carbon cycle that adds carbon dioxide to the atmosphere.
   3. C Burning fossil fuels releases carbon dioxide more quickly than respiration does, and it is only this faster rate of release, not the total amount released, that raises the atmospheric concentration over time.
   4. D Fossil fuels contain carbon that photosynthesis removed from the atmosphere millions of years ago and locked underground; burning them adds this ancient carbon on top of the carbon already cycling.
9. 91 mark

   A large area of rainforest is cleared and replaced with bare, exposed soil. Which statement best describes the most likely direct effect of this change on the local water cycle, and why?

   1. A The water cycle is entirely unaffected, because water still evaporates from bare soil at exactly the same rate as it transpired from a forest of leaves.
   2. B Less water vapour is released into the local atmosphere, because far less transpiration now takes place without the trees' leaves and stomata, which can reduce local cloud formation and rainfall over time.
   3. C More water vapour enters the local atmosphere, because bare soil heats up more than a forest canopy in sunlight, so evaporation from the exposed soil surface increases enough to outweigh the lost transpiration.
   4. D Precipitation over the area increases, because clouds no longer have to travel around a tall forest canopy, so rain that used to fall elsewhere now falls directly on the cleared area.
10. 101 mark

    A pondweed's rate of photosynthesis is measured as temperature is raised from 10 degrees C to 45 degrees C, with light intensity and carbon dioxide concentration both kept high and non-limiting throughout. The rate of photosynthesis increases steadily up to about 35 degrees C, but then falls sharply as temperature rises further towards 45 degrees C. Which explanation best accounts for the sharp fall in rate above 35 degrees C?

    1. A Carbon dioxide becomes less soluble in the plant's cells at higher temperatures, so despite the concentration being kept high in the surrounding water, none of it can enter the plant at all.
    2. B Light intensity has become the limiting factor as temperature rises, since warmer water absorbs light less effectively, reducing the light available to the pondweed.
    3. C The rate continues to rise at the same steady pace above 35 degrees C, and only appears to fall because the measuring apparatus becomes less accurate at higher temperatures.
    4. D Enzymes involved in photosynthesis begin to denature at these higher temperatures, so their active sites lose their normal shape and can no longer catalyse the reactions as effectively.
11. 111 mark

    In a laboratory experiment, the rate of photosynthesis in pondweed is measured at increasing carbon dioxide concentrations, while light intensity is kept constant throughout and temperature is kept constant at the plant's optimum. The rate rises steadily as carbon dioxide concentration increases, then levels off at higher concentrations despite further increases in carbon dioxide. Which factor most likely explains this plateau?

    1. A Light intensity, which was kept constant throughout, has itself now become the limiting factor, so extra carbon dioxide can no longer increase the rate further.
    2. B Temperature has become limiting despite being kept constant, since enzymes work optimally only within a narrow temperature range that gradually narrows further during a long experiment.
    3. C The pondweed has absorbed the maximum possible amount of carbon dioxide it can ever use, so its cells have no further capacity to process the gas at all.
    4. D Carbon dioxide concentration continues to be the limiting factor throughout the experiment, so the plateau shows that even more carbon dioxide would eventually raise the rate again if given enough time.
12. 121 mark

    A student wants to show that light is needed for photosynthesis. She takes a plant that has been kept in the dark for 48 hours to remove existing starch from its leaves, then covers half of one leaf with black foil, leaving the other half exposed, and places the plant in bright light for several hours. She removes the leaf, decolourises it in boiling ethanol to remove its pigment, then tests it with iodine solution. Which result would support the conclusion that light is required for photosynthesis?

    1. A Both halves turn blue-black with iodine, showing starch is present throughout, because starch was already present in the leaf before the experiment began and 48 hours in the dark cannot remove it.
    2. B The exposed half stays orange-brown and the covered half turns blue-black, because chlorophyll is destroyed by exposure to bright light, preventing photosynthesis in the exposed region.
    3. C The exposed half of the leaf turns blue-black with iodine, showing starch is present, while the covered half stays orange-brown, because only the exposed part received light.
    4. D Neither half changes colour with iodine, because boiling in ethanol destroys all the starch present in the leaf before the iodine test can be carried out.
13. 131 mark

    Root hair cells are specialised epidermal cells in a plant's root, adapted for the absorption of water and mineral ions from the soil. Which feature of a root hair cell is a genuine adaptation for this function, and what is its effect?

    1. A The cell contains many chloroplasts, allowing it to photosynthesise and provide itself with the energy needed for the active transport of mineral ions.
    2. B The cell has a long, thin extension into the soil, greatly increasing the surface area of cell membrane in contact with soil water and dissolved mineral ions.
    3. C The cell is covered by a thick, waterproof cuticle over its extension, which prevents water loss from the cell and so increases the efficiency of water uptake from the soil.
    4. D The cell is heavily lignified, in the same way as xylem tissue, making it rigid enough to push its way between soil particles.
14. 141 mark

    Stomata are pores found mostly on the lower epidermis of a leaf, each controlled by a pair of guard cells. Which statement correctly describes how stomata affect the rate of transpiration from a leaf?

    1. A Stomata are permanently open holes with no cellular control over them, so the transpiration rate depends only on the temperature and humidity outside the leaf, not on the leaf itself.
    2. B Guard cells become flaccid and the stomatal pore closes in bright light, as the guard cells lose water by osmosis, so stomata close during the day when photosynthesis needs the most gas exchange.
    3. C Stomata allow only oxygen to leave the leaf, while carbon dioxide diffuses in through the waxy cuticle instead, so transpiration occurs mainly through the cuticle rather than through the stomata.
    4. D Guard cells become turgid in the light, as they take up water by osmosis, and this change in shape opens the stomatal pore, letting water vapour diffuse out of the leaf into the atmosphere.
15. 151 mark

    In a potometer experiment, a leafy shoot takes up 0.9 cm^3 of water in 6 minutes. What is the rate of transpiration, expressed in cm^3 of water per hour?

    1. A 9 cm^3 per hour
    2. B 54 cm^3 per hour
    3. C 0.0025 cm^3 per hour
    4. D 0.15 cm^3 per hour

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: B

   1. Both squirrel species rely on the same woodland food supply and habitat space, so they are in direct competition for a shared, limited resource; this is interspecific competition, competition between two different species.
   2. Grey squirrels digest acorns more efficiently and store more fat from the same food supply, so they survive and reproduce more successfully than red squirrels on that resource, and the red squirrel population falls as a result.
   3. This rules out mutualism (which needs both species to benefit), predation (no species is being caught and eaten) and parasitism (no host individual is being lived on and fed from directly), leaving interspecific competition as the only relationship that fits.
   4. Only B correctly identifies this as competition for a shared resource, so B is correct.
   • Why not A: Mutualism requires both species to gain a benefit. Here the red squirrel's population falls as the grey squirrel out-competes it for the shared food supply, so only one species gains, ruling this out.
   • Why not C: The scenario describes both species competing for the same food resource; a true predator-prey relationship requires one species to actually catch and consume the other, which is not described here.
   • Why not D: Parasitism requires one organism to live in or on a specific host individual and take nutrients directly from it. The grey squirrel gains nothing directly from red squirrels; it simply out-competes them for a shared, external food resource.

2. Question 2Answer: C

   1. The tapeworm gains all of its nutrients from the host's digested food, while the host is harmed by the loss of those nutrients and by damage to its gut lining, so only one of the two species benefits from the association.
   2. This is the defining feature of parasitism: the parasite (the tapeworm) benefits at the direct expense of a host it lives closely with, over a prolonged period, rather than the two species killing, competing with or mutually benefiting each other.
   3. This rules out predation (no organism is caught and killed), competition (this is not two organisms independently seeking the same external resource) and mutualism (the host does not benefit).
   4. Only C correctly names this one-sided, harmful-to-the-host relationship as parasitism, so C is correct.
   • Why not A: Predation involves one organism killing and eating another distinct organism relatively quickly. A parasite instead lives in or on a host over a prolonged period, typically without killing it outright, which is what distinguishes parasitism from predation here.
   • Why not B: Competition describes two organisms competing on comparable terms for the same limited resource. Here the tapeworm is intercepting nutrients its host has already digested from the host's own food, a one-directional benefit-and-harm relationship rather than a shared struggle for a resource.
   • Why not D: Mutualism requires both species to benefit. The host mammal is harmed here, losing nutrients and suffering gut damage, so only the tapeworm gains, which rules mutualism out.

3. Question 3Answer: A

   1. A biotic factor is one caused by another living organism, such as a predator, a competitor, a pathogen or a shortage of food produced by other organisms; an abiotic factor is a non-living, physical or chemical condition, such as temperature, rainfall, light or pH.
   2. Cold, wet winter weather is a physical condition of the environment, not the action of another organism, so it is classified as an abiotic factor even though its effect (failure of the overwintering pupae) falls on a living population.
   3. This rules out B and C, which both wrongly reclassify a physical condition as biotic just because it affects living organisms, and D, which wrongly claims abiotic factors can only act slowly, when in fact a single severe winter can cause a sharp, rapid fall like the one described.
   4. Only A correctly identifies the weather as an abiotic factor and explains its direct physical effect on the pupae, so A is correct.
   • Why not B: Biotic factors are the effects of other living organisms, such as predators, competitors, food availability or disease, not physical or climatic conditions. Weather is non-living, so it is abiotic regardless of any long-term links to living systems.
   • Why not C: It is the nature of the factor causing the change, not the fact that a living population is affected, that determines whether a factor is biotic or abiotic. A non-living factor such as temperature or rainfall stays abiotic even though it affects living organisms.
   • Why not D: Abiotic factors, including a single extreme weather event, can cause rapid, large population changes within one year or generation; there is no rule restricting abiotic effects to slow, gradual change only.

4. Question 4Answer: D

   1. A primary producer is an organism that generates new biomass directly, using light energy, carbon dioxide and water in photosynthesis, rather than by consuming another organism's biomass.
   2. Algae are photosynthetic, so they fit this definition, while water fleas, small fish and herons all obtain their biomass indirectly, by eating another organism that already contains it.
   3. Because every other organism in the chain ultimately depends on biomass that the algae first created from non-living raw materials, algae's biomass is the base that supports the biomass of everything feeding on them, directly or indirectly.
   4. This rules out A (which misreads "primary" as feeding order), B (which confuses producer status with an individual's size) and C (which wrongly extends producer status to consumers), leaving D as the correct explanation.
   • Why not A: This mistakes the term "primary" for referring to feeding order, when it actually refers to algae's role as the ultimate, non-consumer source of new biomass, produced by photosynthesis rather than by being eaten first.
   • Why not B: "Producer" status depends on how an organism obtains its energy (by photosynthesising, not by consuming others), not on the size or biomass of one individual; a single alga is tiny compared with a heron, so this reasoning would not even pick out the correct organism.
   • Why not C: Only photosynthetic organisms, which generate new biomass directly from light energy rather than by consuming other organisms, are described as primary producers; the water fleas, fish and herons in this food chain are all consumers, not producers.

5. Question 5Answer: B

   1. The mark-release-recapture formula is: estimated population = (number marked in first sample x total caught in second sample) / number of marked individuals found in second sample.
   2. Substituting the values given: (50 x 40) / 8 = 2000 / 8 = 250.
   3. The other options come from common slips in applying this formula: multiplying all three numbers instead of dividing (16000), dividing by the wrong count from the second sample (50), or multiplying the wrong pair of values together (400).
   4. Only 250 correctly applies the formula to the numbers given, so B is correct.
   • Why not A: This comes from multiplying all three given numbers together (50 x 40 x 8), treating the formula's division step as though it were another multiplication, rather than dividing by the number of marked individuals found in the second sample.
   • Why not C: This comes from dividing by the total number caught in the second sample (40) instead of by the number of marked individuals found within that sample (8), confusing the two different counts from the second sample.
   • Why not D: This comes from multiplying the first-sample count by the number of marked individuals recaptured (50 x 8) instead of by the total size of the second sample, using the wrong pair of values in the numerator.

6. Question 6Answer: A

   1. Waste food and faeces accumulating below densely stocked cages add a large amount of nutrients to the water and seabed in a small area.
   2. These extra nutrients encourage rapid growth of algae and bacteria, and as this extra biomass dies and decomposes, the decomposers' own respiration uses up dissolved oxygen from the surrounding water, in a process similar to eutrophication.
   3. Species that depend on well-oxygenated water, including many invertebrates and fish, can be harmed or displaced by this local drop in oxygen, reducing biodiversity in the loch near the farm.
   4. This rules out B (a nesting-space claim unconnected to underwater waste), C (cages are not sealed off from surrounding water) and D (escaped farmed fish are a genuine, documented risk to wild populations), leaving A as the correct mechanism.
   • Why not B: The impact described (waste accumulating below the cages) is a water-quality and nutrient issue, not a competition for shoreline nesting space; seabird nesting is not the mechanism affected by underwater fish waste.
   • Why not C: Fish farm cages are located in open water and are not sealed off from the surrounding loch; waste, nutrients, parasites and chemicals can pass between the water inside and outside a cage, so effects on wild species are possible.
   • Why not D: Escaped farmed salmon are a well-documented cause for concern precisely because they can interbreed with and compete against wild salmon, reducing the fitness of wild populations; treatments given to farmed fish do not make an escapee unable to survive in open water.

7. Question 7Answer: C

   1. Acid rain lowers the pH of soil and water it falls on, and this increased acidity can dissolve and release aluminium ions that were previously bound up in the soil.
   2. These aluminium ions, together with the low pH itself, can damage the gills of fish and disrupt enzymes involved in reproduction, reducing survival and reproductive success.
   3. This lowers the numbers and diversity of species able to survive in the affected water, which is the negative impact on biodiversity described in the question.
   4. This rules out A (acidity harms rather than feeds organisms), B (acid rain's cause is fossil fuel emissions, not photosynthesis) and D (effects spread through the food web, not just to directly rained-on organisms), leaving C as the correct mechanism.
   • Why not A: Hydrogen ions are not a food source for organisms, and lower pH generally harms rather than helps most aquatic species, so acidification reduces rather than increases biodiversity.
   • Why not B: Acid rain is caused mainly by sulfur dioxide and nitrogen oxides released by burning fossil fuels, not by carbon dioxide released during photosynthesis; photosynthesis removes carbon dioxide from the atmosphere rather than adding it.
   • Why not D: This understates the ecosystem-wide effects of acid rain: harming plants, soil chemistry or invertebrates affects the whole food web, so species that never directly touch the rain (such as fish or animals feeding on affected plants) can still be harmed indirectly.

8. Question 8Answer: D

   1. Respiration and decomposition release carbon dioxide that was only recently taken in by photosynthesis, so this carbon simply cycles between the atmosphere and living organisms without changing the total amount in the cycle.
   2. Fossil fuels formed from the remains of organisms that photosynthesised millions of years ago, locking that carbon away underground, outside the current, actively cycling carbon budget.
   3. Burning fossil fuels releases this ancient, previously locked-away carbon back into the atmosphere in addition to the carbon that is already being recycled by respiration and decomposition, so the overall amount of carbon dioxide in the atmosphere and cycle increases.
   4. This rules out A (fossil fuel combustion does release carbon dioxide), B (respiration and decomposition do release it) and C (the issue is additional carbon entering the cycle, not merely a faster release rate), leaving D as the correct explanation.
   • Why not A: Combustion of carbon-based fossil fuels such as coal and oil releases carbon dioxide, the same gas released by respiration and decomposition, not a different gas.
   • Why not B: This directly contradicts the question's own statement that both respiration and decomposition release carbon dioxide as part of the ongoing carbon cycle.
   • Why not C: The key issue is that fossil fuel carbon is additional carbon, previously locked away for millions of years, entering the cycle for the first time in that time, not simply a faster release of carbon that would otherwise cycle at the same total amount regardless of rate.

9. Question 9Answer: B

   1. A forest's leaves, with their many stomata, transpire large volumes of water vapour into the local atmosphere, in addition to any evaporation from the soil surface.
   2. Removing the forest and replacing it with bare soil removes this large transpiring surface, so the total amount of water vapour entering the local atmosphere falls sharply, even though some evaporation from the soil continues.
   3. Because water vapour in the local atmosphere contributes to local cloud formation and rainfall, a sustained fall in transpiration can reduce local rainfall over time.
   4. This rules out A (evaporation from soil does not match transpiration's contribution), C (soil warming does not outweigh the lost leaf surface area) and D (an invented mechanism), leaving B as the correct explanation.
   • Why not A: This overstates the equivalence between the two processes: transpiration through a large total leaf surface area, with many stomata, typically returns far more water vapour to the atmosphere than evaporation from bare soil alone.
   • Why not C: While exposed soil may warm and evaporate somewhat more than shaded soil, this increase does not outweigh the much larger surface area for water loss that a forest's leaves and stomata previously provided, so overall local water vapour input typically falls rather than rises.
   • Why not D: This invents a mechanism, clouds having to travel around a canopy, that is not how cloud formation or rainfall distribution works; the real link between deforestation and local rainfall is the loss of water vapour previously added by transpiration.

10. Question 10Answer: D

    1. The enzymes that catalyse the reactions of photosynthesis each have an optimum temperature at which their active site has the ideal shape for their substrate.
    2. As temperature rises well above this optimum, the bonds holding the enzymes' tertiary structure together begin to break, changing the shape of the active site; this is denaturation.
    3. A denatured enzyme's active site no longer fits its substrate as well, so fewer successful reactions occur per unit time, and the overall rate of photosynthesis falls sharply, exactly as described above 35 degrees C.
    4. This rules out A (an overstated solubility effect), B (light was held non-limiting) and C (a real fall in rate is described, not a measurement fault), leaving D as the correct explanation.
    • Why not A: Reduced gas solubility at higher temperature is a real but minor secondary effect; it does not stop carbon dioxide entering the plant altogether, and the dominant cause of a sharp fall in rate at these temperatures is enzyme denaturation, not blocked gas entry.
    • Why not B: This conflates two unrelated variables: light intensity was kept high and non-limiting by the question's own conditions, and raising temperature does not meaningfully change how much light reaches the plant.
    • Why not C: This contradicts the description of a genuine sharp fall in rate, and blames measurement error rather than identifying a real biological cause for the observed change.

11. Question 11Answer: A

    1. In this experiment, carbon dioxide concentration is deliberately varied while light intensity and temperature are both held constant.
    2. As long as carbon dioxide is scarce, increasing it raises the rate of photosynthesis, because carbon dioxide is the limiting factor at those low concentrations.
    3. Once carbon dioxide is abundant enough, one of the other two conditions that was held fixed, here light intensity, becomes the new limiting factor, so further increases in carbon dioxide can no longer raise the rate, producing the plateau.
    4. This rules out B (temperature was fixed at its optimum, with no mechanism to become newly limiting), C (an unsupported absolute cap) and D (contradicts the plateau itself), leaving A as the correct explanation.
    • Why not B: Temperature was held constant at the plant's optimum throughout; there is no mechanism by which an enzyme's optimum temperature range would narrow further simply because an experiment continues, so this does not explain a new limitation appearing.
    • Why not C: This overclaims a fixed, absolute cap on carbon dioxide use, rather than explaining the plateau through another factor, held constant, that has become limiting once carbon dioxide is abundant.
    • Why not D: This directly contradicts the observed plateau: if carbon dioxide were still the limiting factor, increasing its concentration further would still raise the rate, which is the opposite of what is described.

12. Question 12Answer: C

    1. The 48 hours in the dark ensures the leaf starts the experiment with no starch present, so any starch detected afterwards must have been made during the timed period of the experiment itself.
    2. Only the exposed half of the leaf receives light during the experiment, so only that half can carry out photosynthesis and convert the glucose it produces into starch for storage.
    3. Testing with iodine after removing the leaf's pigment in boiling ethanol shows a blue-black colour where starch is present and leaves an orange-brown colour where it is absent, so a blue-black exposed half and an orange-brown covered half directly shows that light was needed to produce starch.
    4. This rules out A (contradicts the point of destarching), B (reverses the expected pattern and misreads chlorophyll's role) and D (misreads the purpose of the ethanol step), leaving C as the result that supports the conclusion.
    • Why not A: This contradicts the purpose of the 48-hour dark period, which is to allow the plant to use up any starch already present in its leaves, so that any starch found afterwards must have been made newly during the experiment.
    • Why not B: This reverses the expected result and misunderstands chlorophyll's role: chlorophyll captures light energy for photosynthesis, it is not destroyed by exposure to it under these conditions.
    • Why not D: This misunderstands the purpose of the ethanol step, which removes the leaf's green pigment so that a colour change with iodine can be seen clearly; it does not destroy any starch present in the leaf.

13. Question 13Answer: B

    1. A root hair cell's long, thin extension projects out into the spaces between soil particles, greatly increasing the surface area of cell membrane exposed to soil water and dissolved mineral ions, compared with a cell with no such extension.
    2. This larger surface area allows more water to move in by osmosis, and more mineral ions to be taken up by diffusion or active transport, per unit time, increasing the overall rate of absorption.
    3. This rules out A (root hair cells lack chloroplasts, having no light underground), C (a waterproof cuticle would block rather than assist uptake) and D (lignification belongs to xylem, not to a living, absorbing root hair cell).
    4. Only B correctly identifies the increased surface area from the cell's shape as the relevant adaptation, so B is correct.
    • Why not A: Root hair cells are found underground, where there is no light available for photosynthesis, so they do not contain chloroplasts; they obtain energy for active transport from respiration instead.
    • Why not C: A thick, waterproof cuticle would block rather than assist the movement of water into the cell; root hair cell membranes need to be permeable to water, not waterproofed against it.
    • Why not D: Lignification is a feature of xylem tissue, which provides rigid, dead vessels for water transport; a root hair cell needs a living, flexible membrane across which absorption can occur, not a rigid, lignified wall.

14. Question 14Answer: D

    1. In the light, guard cells take up water by osmosis and become turgid, which changes their shape and opens the stomatal pore between them.
    2. An open stomatal pore provides a direct route for water vapour to diffuse out of the air spaces inside the leaf into the drier air outside, which is the main route by which transpiration occurs.
    3. The more widely open the stomata are, the more easily water vapour can diffuse out, so a greater number of open stomata increases the rate of transpiration, all other conditions being equal.
    4. This rules out A (guard cells do actively control the pore), B (which reverses the light response) and C (which wrongly routes water loss through the cuticle instead of the stomata), leaving D as the correct description.
    • Why not A: This ignores the guard cells' active control over the stomatal pore; guard cells open and close stomata in response to conditions such as light, so the leaf itself does influence its own transpiration rate, not only external temperature and humidity.
    • Why not B: This reverses the actual mechanism: guard cells become turgid, not flaccid, in bright light, which opens rather than closes the stomatal pore, and stomata typically open in daylight precisely because photosynthesis then needs the most gas exchange.
    • Why not C: This confuses gas exchange pathways; the waxy cuticle is largely waterproof and allows very little gas or water movement across it compared with the stomata, which are the main route for both carbon dioxide entry and water vapour loss.

15. Question 15Answer: A

    1. The rate of transpiration is the volume of water taken up divided by the time taken: 0.9 cm^3 / 6 minutes = 0.15 cm^3 per minute.
    2. There are 60 minutes in an hour, so converting this rate to cm^3 per hour means multiplying by 60: 0.15 x 60 = 9 cm^3 per hour.
    3. The other options come from common slips: multiplying the raw volume by 60 without first finding the per-minute rate (54), dividing instead of multiplying when converting units (0.0025), or forgetting to convert the per-minute rate into a per-hour rate at all (0.15).
    4. Only 9 cm^3 per hour correctly completes both the division and the unit conversion, so A is correct.
    • Why not B: This comes from multiplying the raw volume of water taken up (0.9 cm^3) by 60 directly, skipping the step of first finding the rate per minute before converting to a rate per hour.
    • Why not C: This comes from dividing the correct per-minute rate by 60 instead of multiplying by it, confusing the direction of the conversion from minutes to hours.
    • Why not D: This is the correct rate per minute (0.15 cm^3 per minute), but presented with the wrong unit, having stopped before converting it into a rate per hour.