Basic Sciences: Physics, Chemistry, Biology

Aflatoxins are secondary metabolites of filamentous fungi, principally Aspergillus flavus and Aspergillus parasiticus, which colonise groundnut, maize, cottonseed, chillies and tree nuts under warm and humid storage conditions, above roughly 25 degrees Celsius and 85 per cent relative humidity. Moulds are fungi in their filamentous growth form, so option (c) is correct. Aflatoxin B1 is a potent hepatotoxin and a Group 1 human carcinogen, acting synergistically with hepatitis B in liver cancer, and the detail in the stem about survival of cooking is accurate, the molecule being thermostable to about 250 degrees Celsius, which is why prevention at drying and storage rather than treatment at the kitchen is the whole of the control strategy.

• Option (a) is incorrect; bacteria produce their own important food toxins, the botulinum toxin of Clostridium botulinum and the enterotoxins of Staphylococcus aureus, but not aflatoxins, and the whole class name mycotoxin points to fungi.
• Option (b) is incorrect, protozoa being agents of infection rather than producers of food storage toxins.
• Option (d) is incorrect, viruses being obligate intracellular parasites with no independent metabolism, so they cannot synthesise a secondary metabolite in stored grain at all. The elimination route runs through the last of these: a candidate who holds the principle that viruses have no metabolism of their own can discard option (d) on structure alone, and the term mycotoxin then carries the rest.

• Item 1, calcium, is central and is the item no candidate should miss. Depolarisation of the sarcolemma releases calcium from the sarcoplasmic reticulum; calcium binds troponin C, which shifts tropomyosin off the myosin binding sites on actin, and cross bridge cycling follows. Without calcium there is no contraction, and the return of calcium to the reticulum is what produces relaxation.
• Item 3, sodium, is required for the event that precedes this. The action potential that travels along the motor neuron and then along the muscle fibre membrane and down the T tubules is generated by the inward sodium current through voltage gated sodium channels, so excitation contraction coupling cannot begin without sodium.
• Item 2, iron, is accepted by the official key. Its role is oxidative rather than mechanical: iron is the metal centre of haemoglobin, which delivers oxygen to the muscle, and of myoglobin, which stores it within the fibre, and it is also present in the cytochromes of the electron transport chain that regenerate the ATP consumed by the myosin head. Sustained contraction is impossible without it, which is why iron deficiency presents as muscular fatigue and weakness. Taking all three, the official answer is (d).

A harvested fruit remains metabolically alive, and its deterioration is driven by continuing respiration, which oxidises stored sugars and organic acids, releases heat and carbon dioxide, and progressively depletes the reserves that maintain tissue integrity, and by the ripening cascade that ethylene sets off. Both processes are enzyme catalysed, and the rate of an enzyme catalysed reaction falls sharply with temperature, the rule of thumb being a halving for every ten degrees Celsius of cooling. Refrigeration therefore slows respiration, delays ripening and senescence and also suppresses the growth of the spoilage fungi and bacteria that would otherwise colonise the tissue, and option (c) is correct.

• Option (a) is incorrect: darkness is incidental to cold storage and has no bearing on shelf life, harvested fruit not being photosynthetically dependent, and fruit stored in the dark at room temperature spoils on the ordinary timetable.
• Option (b) is incorrect as an explanation of what a cold chamber does. Raising carbon dioxide and lowering oxygen genuinely does extend storage life and is the basis of controlled atmosphere and modified atmosphere storage, but that is a different technology applied in addition to refrigeration, and it is not what cooling by itself achieves; the option is included precisely to catch a candidate who knows about controlled atmosphere storage and over reads the question.
• Option (d) is incorrect: high humidity is maintained in cold stores to prevent moisture loss and shrivelling, so it is a supporting condition rather than the reason, and excessive humidity encourages fungal growth. The governing principle is temperature dependence of metabolic rate.

The purpose of a ball bearing is to replace sliding friction between the shaft and its housing with rolling friction, and rolling friction is smaller by one to two orders of magnitude. The mechanism is that the hardened spheres interpose themselves between the two surfaces so that contact is reduced to a set of very small, nearly point contacts which roll rather than slide, and the relative motion is taken up by rotation of the balls instead of by shearing of the interface.

• Option (c) states this and is correct.
• Options (a) and (b) are incorrect because each asserts an increase in contact area, which would raise friction rather than reduce it and would defeat the purpose of the device.
• Option (d) fails once (c) is established. The physical qualification worth stating to a class is that classical friction between dry surfaces is independent of the apparent area of contact, which is what makes the wording of this question loose; what actually matters is the real area of contact at the asperity level and, more importantly, the change in the mode of relative motion from sliding to rolling. The examiner's phrase effective area of contact should be read as pointing at the near point contact of the rolling elements. The elimination route is direction alone: two options assert an increase and one a decrease, and friction must be reduced for the device to be useful.

The key groups together three phenomena in which what the observer sees is not what is physically there, and separates them from two in which the object is genuinely where and as it appears.

• Item 1 qualifies: the sun at the horizon subtends the same angle as at the zenith, as photography confirms, yet appears markedly larger, and the enlargement is a perceptual effect arising from the presence of intervening terrain and from the way the visual system judges distance against the apparent dome of the sky. It is a true illusion in the strict sense.
• Item 2 qualifies on the key's reading: the sun is not in fact red, and its apparent colour at dawn arises because the long slant path through the atmosphere scatters the shorter wavelengths out of the direct beam by Rayleigh scattering, leaving the transmitted light reddened.
• Item 4 qualifies on the same reading: the star does not vary in brightness, and the twinkling is scintillation produced by refraction through turbulent cells of differing density in the atmosphere, which is why planets, subtending a measurable disc, twinkle far less than point like stars.
• Item 3 does not qualify: the moon really is above the horizon at dawn whenever its phase and position place it there, and there is nothing false in the perception.
• Item 5 does not qualify: Polaris really is visible in the northern sky, standing almost on the extension of the earth's axis, which is why it appears nearly stationary while the other stars circle it. The official answer (c) follows. The elimination route is exceptionally clean and does not require the illusion category to be defined at all: items 3 and 5 are plainly real, and no option contains either of them together with the correct remainder except (c), so rejecting the two real phenomena settles the question.

Root hairs are unicellular tubular extensions of the epidermal cells of the young root, extremely fine and extremely numerous, and they are responsible for very nearly the whole of the plant's uptake of water and dissolved minerals because they multiply the absorptive surface area many times over. They are also delicate and short lived, and they are torn away when a seedling is lifted from the soil, since they are intimately bound to soil particles. The seedling therefore continues to transpire from its leaves while its capacity to absorb water has collapsed, and it wilts and often dies before new root hairs are regenerated behind the growing tip. This is exactly why transplanting is done in the evening or in overcast weather, why the seedling is watered immediately and shaded, and why nurseries raise seedlings in pots or plugs so that the root ball and its hairs are lifted intact.

• Option (c) is correct.
• Option (a) is incorrect as a general explanation, since transplanting shock occurs even into rich and well prepared soil and the failure is immediate rather than nutritional.
• Option (b) is incorrect and is close to meaningless; root hairs adhering firmly to soil is the normal and desirable condition, being how the absorptive contact is made.
• Option (d) is incorrect: leaves may be damaged incidentally, but leaf damage reduces transpiration and photosynthesis rather than causing the acute water stress that kills the seedling. The elimination route is to ask which structure is both indispensable and fragile.

• Statement 1 is correct and states the defining property of a virus. A virion consists of nucleic acid, either DNA or RNA, enclosed in a protein capsid and sometimes a lipid envelope, and it carries no ribosomes, no machinery of the citric acid cycle and no electron transport chain, so it can neither generate ATP nor synthesise protein on its own. It is metabolically inert outside a host and is an obligate intracellular parasite, which is the reason viruses sit on the border of the living and the non living.
• Statement 2 is incorrect and follows directly from statement 1. Because a virus cannot replicate without host machinery, it cannot be grown on any synthetic or cell free medium however enriched; cultivation requires living cells, in practice tissue culture, embryonated eggs or laboratory animals, and this is precisely what distinguishes viral from bacterial culture.
• Statement 3 is incorrect on the word only. Biological vectors are one route, and an important one for the arboviruses, dengue, chikungunya and Japanese encephalitis among them, but viruses are also transmitted directly by respiratory droplets and aerosols, by contact, by the faecal oral route, by blood and by vertical transmission from mother to child, as Q46 of this same paper illustrates with hepatitis B and HIV transmitted through tattooing. The official answer (a) follows. The elimination route is that statements 1 and 2 are logically inconsistent with one another, so a candidate who sees that a particle with no energy generating enzymes cannot be cultured on a lifeless medium has settled both statements from a single idea, which removes options (b) and (d) at once.

• Statement (a) is not correct and is therefore the answer. Gravity is by an enormous margin the weakest of the four. Taking the strong interaction as unity, the electromagnetic interaction is of the order of ten to the minus two, the weak interaction of the order of ten to the minus six or smaller, and gravity of the order of ten to the minus thirty nine. The comparison that fixes it is that the electrostatic repulsion between two protons exceeds their gravitational attraction by about thirty six orders of magnitude. Gravity dominates at astronomical scale for two reasons that have nothing to do with its strength: it has infinite range and, uniquely, it has only one sign of charge, so it never cancels, whereas the far stronger electromagnetic force is neutralised in bulk matter because positive and negative charges balance.
• Statement (c) is correct: the weak interaction mediates beta decay, converting a neutron into a proton with the emission of an electron and an antineutrino, and it is responsible for radioactive decays that the strong and electromagnetic interactions cannot produce, as well as for the first step of hydrogen fusion in the sun.
• Statement (d) is correct: the strong interaction binds quarks into nucleons and, in its residual form, binds nucleons to one another against the electrostatic repulsion of the protons, which is what makes the nucleus stable at all. The official answer (a) follows.

• Statement 1 is correct and is the whole significance of the particle. The Standard Model requires the carriers of the weak interaction, the W and Z bosons, to be massive, but a mass term inserted by hand destroys the gauge symmetry on which the theory rests. The Brout Englert Higgs mechanism resolves this by postulating a scalar field with a non zero vacuum expectation value which breaks the electroweak symmetry spontaneously; the gauge bosons acquire mass by interacting with this field, and the fermions acquire mass through Yukawa coupling to it, the strength of the coupling determining the mass. The Higgs boson is the quantum excitation of that field, so detecting it is the experimental confirmation that the field exists and that the mechanism is the right account of the origin of mass.
• Statement 2 is incorrect and is science fiction: teleportation of matter is not a prediction of the Standard Model or of any part of it, and quantum teleportation, which is a real laboratory technique, transfers a quantum state and not matter, requires a classical channel and moves nothing physically.
• Statement 3 is incorrect. Nuclear fission is governed by the strong and weak interactions acting on nuclei at energies of the order of millions of electron volts, and it depends on the properties of fissile nuclides, whereas the Higgs is a heavy unstable particle produced at energies of the order of hundreds of billions of electron volts with a lifetime of the order of ten to the minus twenty two seconds. There is no conceivable engineering pathway from one to the other. The official answer (a) follows. The elimination route is that statements 2 and 3 both promise a near term technology from a fundamental discovery, which is the signature of a planted falsehood in this paper's construction.

All three are correct, so the official answer is (c). Hibernation is a state of controlled metabolic depression in which body temperature, heart rate and oxygen consumption fall far below normal, allowing an animal to survive a season when food is scarce and thermoregulation would be prohibitively expensive.

• Statement 1 is correct: temperate insectivorous bats are among the deepest hibernators known, with body temperature dropping to within a degree or two of ambient and heart rate falling from several hundred beats a minute to a handful.
• Statement 3 is correct: rodents furnish the classical laboratory models of hibernation, including the ground squirrels, marmots, hamsters and the dormouse, whose name records the habit.
• Statement 2 is correct as conventionally taught: bears den through the winter, cease feeding, urinating and defecating for months, and depress metabolic rate substantially. The elimination route is decisive and requires only one fact. Rodents are indisputable hibernators, and (c) is the only option that contains statement 3, so the item is settled without any need to adjudicate the harder question of bears.

• Option (b) is correct on the reading the examiner intends. Photosynthesis takes radiant energy, which is available and unstored, and converts it into chemical energy locked in the bonds of carbohydrate, where it remains stored until released by respiration. The light reactions in the thylakoid membrane use photon energy to drive water oxidation and electron transport, generating ATP and NADPH, and the carbon fixation reactions in the stroma use those to reduce carbon dioxide to triose phosphate. The direction of energy flow is from an available form to a stored form, which is what the option asserts.
• Option (a) is incorrect because it reverses that direction and describes the release of stored energy, which is respiration.
• Option (c) is incorrect and describes aerobic respiration precisely, the oxidation of food to carbon dioxide and water.
• Option (d) is incorrect and describes the gas exchange of respiration, whereas photosynthesis takes in carbon dioxide and releases oxygen; a candidate who fixes the gas exchange of the two processes disposes of (c) and (d) together. The item then reduces to (a) against (b) and is settled by the direction of energy conversion.

All three are correct.

• Statement 1: viruses infecting bacteria are bacteriophages, among the best studied entities in molecular biology; T4, lambda and M13 are standard laboratory organisms, and phage biology gave us restriction enzymes and, more recently, the CRISPR system, which is a bacterial defence against phage.
• Statement 2: viruses infecting fungi are mycoviruses, mostly double-stranded RNA viruses transmitted by hyphal anastomosis and sporulation rather than by an extracellular route; the best known is the hypovirus of Cryphonectria parasitica, which attenuates the virulence of the chestnut blight fungus and has been used as a biocontrol agent.
• Statement 3: plant viruses are numerous and agriculturally serious, tobacco mosaic virus being the first virus ever characterised, and the group includes cassava mosaic, banana bunchy top and the geminiviruses. Hence (d).
• Options (a), (b) and (c) each exclude a genuine host group. The governing principle, and the elimination route for a candidate who has never met a mycovirus, is that viruses are obligate intracellular parasites of cells as such, and there is no reason in principle why any cellular life form should be exempt; archaea have their own viruses too. Fungi are the only statement likely to give pause, so the item is moderate rather than easy.

• Option (a) is correct, and the item is solved by recognising that all four terms belong to fundamental physics and cosmology. The event horizon is the boundary surrounding a black hole beyond which no signal can escape to a distant observer, its radius for a non rotating body being the Schwarzschild radius. A singularity is the point predicted by general relativity at which curvature and density diverge, arising both at the centre of a black hole and at the initial state of the Universe. String theory is the programme that replaces point particles with one dimensional extended objects in an attempt to reconcile general relativity with quantum mechanics. The Standard Model is the quantum field theoretic account of the electromagnetic, weak and strong interactions and of the known fundamental particles.
• Options (b) and (c) are wrong because eclipses and satellite injection are problems in classical celestial mechanics and orbital dynamics, requiring nothing beyond Newtonian gravitation and involving none of these four concepts.
• Option (d) is wrong because the origin and evolution of life belongs to biology and biochemistry, and shares no vocabulary with any of the four. Elimination route: any single one of the four terms, correctly located, disposes of all three wrong options at once, so no candidate needs all four. Governing principle: identify the discipline to which a term cluster belongs before attempting to define the terms.

• Statement 3 is the central content of the theory and is correct. General relativity replaces gravity as a force with the geometry of spacetime: the Einstein field equations relate the curvature of spacetime to the distribution of energy and momentum, so matter tells spacetime how to curve and curved spacetime tells matter how to move.
• Statement 1 is correct and is a direct consequence of statement 3. Since light follows null geodesics of the curved geometry, its path bends near a massive body, and it also loses frequency climbing out of a potential well. The deflection of starlight by the Sun measured at the 1919 eclipse was the observation that made the theory famous, and gravitational lensing is now a working tool of astronomy. Note that the Newtonian corpuscular calculation also predicts a deflection but only half the relativistic value, so the effect discriminates between the two theories rather than merely confirming that light responds to gravity.
• Statement 2 is the contested one and the key accepts it. The justification is that the field equations admit no stable static solution: Friedmann in 1922 and Lemaitre in 1927 showed that the generic cosmological solutions of general relativity are dynamic, either expanding or contracting, and Einstein had introduced the cosmological constant in 1917 precisely to force a static universe, later regarding that step as a mistake once Hubble's observations established recession. On that reading a non static universe is indeed a prediction of the theory and option (d) follows.

Hence (d).

Denisovans are an extinct group of archaic humans, a sister population to the Neanderthals, first identified in 2010 not from skeletal morphology but from DNA extracted from a finger bone and teeth found in Denisova Cave in the Altai mountains of Siberia, so (b) is correct. They are significant because the species was defined by genomics before it was described from anatomy, and because their genetic legacy survives in living people: Melanesians, Papuans and Aboriginal Australians carry several per cent of Denisovan ancestry, and the high altitude adaptation allele EPAS1 in Tibetans is of Denisovan origin, evidence of interbreeding with anatomically modern humans.

• Option (a) is wrong; the group is hominin and belongs to the late Pleistocene, tens of thousands rather than tens of millions of years ago.
• Option (c) is wrong, and is the deliberate trap for an Indian candidate, since Denisova Cave is in Siberia and the North East Indian cave associations are with the Meghalaya limestone systems, one of which gives its name to the Meghalayan Age.
• Option (d) is wrong for the same reason; the geological stage named from India is the Meghalayan, ratified in 2018, not anything Denisovan. The name itself is the guide, being a place name in the Altai, and the discipline is to resist the pull of the plausible Indian option.

• Statement 1 is correct. The cellulose wall external to the plasma membrane is the classic plant feature, absent in animal cells, which have only the membrane and an extracellular matrix.
• Statement 2 is incorrect and is decisive. Every living cell, plant, animal, fungal or bacterial, possesses a plasma membrane, since without it there is no cell; the plant cell has a wall in addition to the membrane, not instead of it.
• Statement 3 is correct. A mature plant cell is dominated by a single large central vacuole that maintains turgor and stores solutes, whereas animal cells contain small and transient vacuoles and vesicles. The answer is 1 and 3, giving (c).
• Options (a), (b) and (d) all admit the plasma membrane error, so recognising that single point settles the question.

The cooking temperature is the boiling point of water at the pressure maintained inside the vessel, and that pressure is set by the condition for the weight to lift. The steam must exert a force exceeding the weight of the regulator, so the gauge pressure at which venting begins is the weight divided by the area of the vent hole.

• Item 3 is therefore correct, since a heavier weight demands higher pressure.
• Item 1 is correct, since for a given weight a smaller hole area means a higher pressure is needed to produce the required lifting force, and the two together fix the operating pressure.
• Item 2 is incorrect and is the key exclusion: a hotter flame raises the rate of steam generation and so shortens the time to reach operating pressure, but it cannot raise the pressure beyond the point at which the weight lifts and vents the excess. The flame governs how fast, not how hot. Hence 1 and 3, giving (c). This is a clean application of the Clausius Clapeyron relation between pressure and boiling point, and is a good classroom example of a regulated equilibrium.

• Statement 1 is correct: a sodium lamp is a gas discharge tube radiating in all directions, so reflectors are needed to direct the output downward and much is wasted as glare and sky glow, whereas an LED is a semiconductor junction emitting into a hemisphere, which is why LED luminaires deliver more usable light per watt onto the road surface.
• Statement 3 is correct: the sodium D lines at about 589 nanometres make low pressure sodium light almost monochromatic yellow, under which colours cannot be distinguished, while LEDs provide broad spectrum white light with far better colour rendering, an appreciable safety advantage.
• Statement 2 is incorrect and is the decisive statement: LEDs substantially outlast sodium lamps, typically fifty thousand hours against fifteen to twenty four thousand, and this longevity together with efficiency is the entire economic case for the replacement programme. Hence 1 and 3, giving (c). Note that if statement 2 were true the national LED retrofit would make little sense, which is a useful plausibility check.

• Statement 1 is correct. Probiotic preparations draw on both bacteria, chiefly lactic acid bacteria of the genera Lactobacillus and Bifidobacterium, and yeasts, of which Saccharomyces boulardii is the standard example used against antibiotic associated diarrhoea.
• Statement 2 is incorrect, and it is the discriminating statement. Probiotic organisms are not foreign to the human gut; lactobacilli and bifidobacteria are ordinary residents of the healthy intestinal flora, and the rationale of probiotic administration is to restore or reinforce a population that already belongs there rather than to introduce an alien one.
• Statement 3 is correct. Lactic acid bacteria produce beta galactosidase, which hydrolyses lactose, so probiotic and fermented dairy products improve the tolerance of milk sugar in individuals with low intestinal lactase, which is the observed benefit in lactose maldigestion.
• Statements 1 and 3 hold and 2 fails, giving (c).
• Options (b) and (d) admit the false claim that these organisms do not occur naturally in the gut, and (a) drops the well supported statement 3.

B cells and T cells are the lymphocytes of the adaptive immune system, and their defining role is defence against pathogens. B cells mature in the bone marrow and, on activation, differentiate into plasma cells secreting antibody against a specific antigen, giving humoral immunity, while some persist as memory cells. T cells mature in the thymus, the cytotoxic subset killing cells that display foreign antigen, as virus infected cells do, and the helper subset coordinating the response by activating B cells and macrophages, which is cell mediated immunity.

• Option (d) captures this and is correct.
• Option (a) is wrong as a description of their role: B cells do produce the IgE that mediates allergic reactions, but that is the immune system misdirected against a harmless substance rather than protection from allergens.
• Option (b) is wrong because the relief of pain and inflammation is the action of pharmacological agents, and lymphocyte activity in fact drives inflammation rather than resolving it.
• Option (c) is wrong and inverts the function; regulatory T cells do restrain the response to prevent autoimmunity, but that is one specialised subset and not the role of B and T cells generally.

All three describe recognised classes of extremophiles.

• Statement 1 is correct. Hyperthermophilic archaea at deep sea hydrothermal vents, such as Pyrolobus fumarii and Methanopyrus, grow above 100 degrees Celsius because the high hydrostatic pressure at those depths raises the boiling point of water and their proteins and membranes are thermostable.
• Statement 2 is correct. Psychrophiles grow in polar brines and sea ice at sub zero temperatures, aided by antifreeze proteins and by the depressed freezing point of saline water.
• Statement 3 is correct. Acidophiles such as Acidithiobacillus and Picrophilus thrive below pH 3 in acid mine drainage and volcanic springs, maintaining near neutral cytoplasm through proton pumping.

Hence (c), and (a), (b) and (d) each require one of these well documented groups to be denied.

Nitric oxide is synthesised in the vascular endothelium from L arginine by nitric oxide synthase. It diffuses into adjacent smooth muscle, activates guanylate cyclase, raises cyclic GMP and produces relaxation, that is vasodilation and increased blood flow. Its identification as the endothelium derived relaxing factor won the 1998 Nobel Prize in Physiology or Medicine, and it is the mechanism through which nitroglycerine relieves angina.

• Option (b) is wrong because nitrous oxide is an inhaled anaesthetic and laughing gas, administered externally and not synthesised as a signalling molecule in the body.
• Option (c) is wrong because nitrogen dioxide is a reddish brown atmospheric pollutant from combustion, an irritant rather than a physiological messenger.
• Option (d) is wrong because nitrogen pentoxide is an unstable laboratory oxidising agent with no biological role.

All three statements are false, giving (d), and each is phrased as a universal negative that a single counterexample destroys.

• Statement I is incorrect: seawater is the most virus-rich environment known, holding on the order of ten million virus particles per millilitre, and marine viruses are a major ecological force, lysing bacterial and phytoplankton populations and thereby driving the viral shunt that recycles organic carbon and nutrients in surface waters.
• Statement II is incorrect: bacteriophages are viruses that specifically infect bacteria, they were central to the foundational experiments of molecular biology including the Hershey and Chase demonstration that DNA is the genetic material, and phage therapy is under renewed investigation as antibiotic resistance spreads.
• Statement III is incorrect: altering host transcription is fundamental to how viruses replicate, since they must divert host machinery to make viral products, and some go further by integrating into the host genome, as retroviruses do, or by driving oncogenesis through sustained transcriptional dysregulation, as human papillomavirus does. None holds, giving (d).
• Options (a), (b) and (c) each require at least one absolute negation to survive, and none can.