Biology: previous-year questions

Answer: (A) Bats

Fruit bats of the genus Pteropus (flying foxes) are the natural reservoir of the Nipah virus, an RNA virus of the genus Henipavirus. It reaches humans through infected pigs, through fruit or raw date-palm sap contaminated by bats, or from person to person, and causes fever and encephalitis with high fatality; there is no licensed vaccine. It was first identified in Malaysia in 1998-99 (named after Sungai Nipah village). India's outbreaks include Siliguri (2001) and Nadia (2007) in West Bengal, repeated outbreaks in Kerala since 2018, and two cases in West Bengal in January 2026. Horses (D) are the intermediate host of the related Hendra virus in Australia, and migratory birds (C) are linked with avian influenza.

Answer: (D) it has nodes and inter-nodes.

Ginger is a rhizome, an underground stem, and the proof is that it bears nodes and internodes with scale leaves and buds, features that only stems have. The other options are true of ginger but are not what makes it a stem: roots such as sweet potato and carrot also grow in soil, store food and lack chlorophyll. Underground stem modifications to remember: rhizome (ginger, turmeric), tuber (potato, whose 'eyes' are nodes), corm (colocasia, gladiolus, saffron) and bulb (onion, garlic). Storage roots, by contrast, include sweet potato, carrot, radish, turnip and beet.

Answer: (B) Algae

Agar-agar is a jelly-like polysaccharide obtained from the cell walls of red algae (Rhodophyceae) such as Gelidium and Gracilaria. It is used as the solid culture medium for growing microbes in laboratories and as a vegetarian substitute for gelatin in food. Other algal products: carrageenan from red algae, algin and iodine from brown algae (kelps such as Laminaria), and protein-rich food supplements from Chlorella and Spirulina. The other groups give different products: bryophytes, peat (Sphagnum moss); fungi, antibiotics such as penicillin, and yeast; pteridophytes (ferns), ornamentals and biofertiliser (Azolla).

Answer: (B) The leaf contains green chlorophyll but it remains hidden under other pigments.

Leaves that look red, purple or yellow still contain chlorophyll; its green colour is simply masked by larger amounts of other pigments such as anthocyanins (red and purple, dissolved in the cell sap) and carotenoids (yellow and orange). Photosynthesis therefore goes on normally, as in Coleus, croton, red amaranth and red cabbage. Option A is wrong because heat cannot replace the light reactions of photosynthesis, and option C is fiction. Carotenoids act as accessory pigments that pass absorbed light energy on to chlorophyll a. Plants that truly lack chlorophyll, such as the parasite Cuscuta (dodder), cannot make food and draw it from a host through haustoria.

Answer: (B) Viruses neither have cell walls nor their own metabolic machinery that can be targeted by antibiotics.

Antibiotics act on structures and processes found only in bacteria: the peptidoglycan cell wall (penicillin), the 70S ribosome (streptomycin, tetracycline) or bacterial enzymes for copying DNA (ciprofloxacin). A virus is just nucleic acid inside a protein coat; it has no cell wall, ribosomes or metabolism of its own and multiplies by using the host cell's machinery, so an antibiotic has nothing to attack. Option C is a true statement about viruses but explains why vaccines and antiviral drugs lose effect, not why antibiotics fail; option A is false. Viral infections are handled with antivirals (acyclovir, oseltamivir) and prevented by vaccines. Needless antibiotic use promotes antimicrobial resistance (AMR). Penicillin was discovered by Alexander Fleming in 1928.

Answer: (A) Respiratory roots

Pneumatophores are 'breathing roots' that grow vertically upward out of the mud (negative geotropism) in mangrove plants such as Avicennia, Sonneratia and Heritiera (sundari). Waterlogged saline mud has almost no oxygen, so these roots take in air through pores called lenticels or pneumathodes. Mangroves of the Sundarbans also show stilt roots (Rhizophora) and vivipary, where seeds germinate while still on the tree. The other options describe different modifications: photosynthetic (assimilatory) roots occur in Tinospora and Trapa; food-storing roots in carrot, radish and sweet potato; and the spongy velamen of epiphytic orchid roots absorbs moisture from the air.

Answer: (D) Rubisco

RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase) is the enzyme that fixes atmospheric carbon dioxide in the first step of the Calvin ($C_3$) cycle in the chloroplast stroma. Because it works slowly, green plants and algae make it in huge quantities, and it is regarded as the most abundant enzyme, indeed the most abundant protein, on Earth. It can also bind oxygen, which causes wasteful photorespiration; $C_4$ plants such as maize and sugarcane avoid this by first fixing carbon dioxide with PEP carboxylase. The distractors: lipase digests fats, DNase breaks down DNA, and zymase is the yeast enzyme complex that ferments sugar to alcohol. The most abundant protein in animals is collagen.

Answer: (C) Zygotene

Synapsis, the pairing of homologous chromosomes to form bivalents, takes place in zygotene, the second stage of prophase I of meiosis, with the help of the synaptonemal complex. The five stages in order: leptotene (chromosomes condense into thin threads), zygotene (synapsis), pachytene (crossing over between non-sister chromatids), diplotene (the synaptonemal complex dissolves and chiasmata become visible) and diakinesis (terminalisation of chiasmata, nuclear membrane disappears). Memory aid: 'Lazy Zebra Pulls Down Dates'. The commonest confusion is with pachytene (D), which is the stage of crossing over, not of pairing.

Answer: (B) Pulmonary veins

Arteries normally carry oxygenated blood and veins deoxygenated blood, and the pulmonary vessels are the exception. The four pulmonary veins bring oxygenated blood from the lungs to the left atrium, while the pulmonary artery (D) carries deoxygenated blood from the right ventricle to the lungs. The hepatic portal vein (A) carries nutrient-rich but deoxygenated blood from the intestine to the liver, and the renal veins (C) carry deoxygenated blood, low in urea, from the kidneys. In the foetus the umbilical vein also carries oxygenated blood. Humans have double circulation (pulmonary and systemic); the circulation of blood was discovered by William Harvey.

Answer: (D) Abscisic acid

Abscisic acid (ABA) is the main growth-inhibiting plant hormone. It induces dormancy of seeds and buds, closes stomata during water shortage (hence 'stress hormone') and promotes ageing and shedding of leaves, acting against gibberellins. The other three are growth promoters: auxin (IAA) causes cell elongation, apical dominance and phototropism, and is used for rooting cuttings; gibberellin causes stem elongation and bolting and breaks dormancy (first found in the fungus Gibberella, which causes the 'foolish seedling' disease of rice); cytokinin promotes cell division and delays ageing. The fifth major hormone, ethylene, is a gas that ripens fruits.

Answer: (B) Capillary water

Soil water is classed as gravitational, capillary and hygroscopic. Capillary water, held in the fine pores between soil particles by surface tension, is the water that roots can absorb, so it is the 'available water' (roughly the water between field capacity and the permanent wilting point). Gravitational water drains down quickly to the water table; hygroscopic water forms a thin film held too tightly on soil particles to be absorbed; and chemically combined water is part of the mineral crystals. 'Mineral water' is not a category of soil water. Loam holds the most available water; sand drains too fast and clay holds much of its water too tightly.

Answer: (A) Chimpanzee and Bonobo

Chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) are the closest living relatives of humans, sharing nearly 99% of our DNA; the human line separated from theirs about 6 to 7 million years ago. Next in closeness come the gorilla, then the orangutan, and last the gibbons, which are 'lesser apes'. Humans and the great apes together form the family Hominidae. A related fact: humans have 46 chromosomes while the great apes have 48, because two ancestral chromosomes fused to form human chromosome 2. India's only ape is the hoolock gibbon of the North-East.

Answer: (D) Lactose

Lactose (milk sugar) is a disaccharide made of one glucose and one galactose unit. Glucose and fructose are hexose monosaccharides and ribose is a pentose monosaccharide, so A, B and C are all monosaccharides.

Answer: (B) Cyanobacteria

Cyanobacteria (blue-green algae) reproduce only asexually, by fission, fragmentation or spores, and no sexual process is known in them. Fungi and algae have clear sexual cycles, and bacteria show genetic recombination by conjugation, often described as a primitive sexual process, so A, C and D are not the intended answer.

Answer: (D) live microbial food supplement

Probiotics are live beneficial micro-organisms, such as Lactobacillus in curd, taken as food or supplements to improve gut health. They are not antibiotics (which kill bacteria), nor allergens or cancer-causing microbes.

Answer: (B) Bacteria

Denitrification, the conversion of soil nitrates back to nitrogen gas, is carried out by bacteria such as Pseudomonas and Thiobacillus denitrificans. Fungi, algae and protozoa do not perform this step of the nitrogen cycle.

Answer: (A) Abdominal region

The five lumbar vertebrae form the lower back, behind the abdomen, between the thoracic vertebrae above and the sacrum below. Thoracic vertebrae are in the chest, cervical vertebrae in the neck, and the sacrum and coccyx in the hip region.

Answer: (A) Lung

SARS-CoV-2 enters cells through ACE2 receptors that are abundant in the respiratory tract, so COVID-19 primarily attacks the lungs, causing pneumonia in severe cases. Liver, kidney and spleen can be affected secondarily but are not the primary target.

Answer: (A) 2000 nm.

Escherichia coli is a rod about 2 µm long and 0.5 µm wide, and 2 µm is 2000 nm. 200 nm and 20 nm are the size range of viruses, and 2 nm is about the width of DNA.

No option is correct as printed — this question is disputed. WBPSC awarded the mark to all candidates: two correct options — Mg (C) is part of chlorophyll and Fe (B) is required for its synthesis.

Magnesium is the central atom of the chlorophyll molecule, but iron is also essential for chlorophyll synthesis (its deficiency causes chlorosis even though Fe is not part of the molecule). With both (B) and (C) defensible, the Commission awarded the mark to all candidates.