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🧬 Class XI Biology — Short Questions & Notes
224 real short questions (Section B, including all OR choices) extracted from FBISE Class XI Biology past papers — 1A 2025 (both papers + Old Curriculum), 2A 2025 (+ Old Curriculum), Annual 2023 and Annual 2024 (both papers) — each with a concise model answer.
📚 How to use: Read each question, attempt it yourself first, then check against the short model answer. These are real Section B short questions exactly as they appeared in the papers — including the OR alternatives, so nothing is skipped.
🗂️ Annual 2023
Q1. Why are lysosomes called autophagosomes?
Ans: When a lysosome fuses with and digests worn-out organelles of its own cell (self-eating/autophagy), the resulting digestive vesicle is called an autophagosome, so lysosomes performing this role are given that name.
Q2. Differentiate microtubules and intermediate filaments in terms of diameter, protein type and function in the cell.
Ans: Microtubules are about 25 nm wide, made of tubulin, and function in intracellular transport, spindle formation and forming cilia/flagella. Intermediate filaments are about 10 nm wide, made of proteins like keratin, and provide mechanical strength and structural support to the cell.
Q3. Stereoisomers are isomers that differ in their H and OH group orientation. (a) What are enantiomers? (b) Draw the structure of D and L-glucose.
Ans: (a) Enantiomers are stereoisomers that are non-superimposable mirror images of each other, differing in the spatial arrangement around one chiral carbon (e.g., D and L forms). (b) In D-glucose the OH on C5 points to the right (same side as CH2OH drawn up); in L-glucose it points to the left, its exact mirror image.
Q4. Explain the given graph (rate of reaction vs substrate concentration).
Ans: The graph shows enzyme reaction rate rising sharply with substrate concentration at first, then levelling off into a plateau once all enzyme active sites are saturated with substrate — after this point adding more substrate no longer increases the rate.
Q5. Give any three carbohydrate digesting enzymes with their substrate and product.
Ans: Salivary amylase converts starch into maltose; sucrase converts sucrose into glucose and fructose; lactase converts lactose into glucose and galactose.
Q6. Write down any three differences between cyclic and non-cyclic photophosphorylation.
Ans: Non-cyclic photophosphorylation involves both PSI and PSII, produces ATP, NADPH and O2, and electrons flow one-way from water to NADP+. Cyclic photophosphorylation involves only PSI, produces only ATP (no NADPH or O2), and electrons return to PSI in a circular path.
Q7. Classify viruses on the basis of their genome.
Ans: Viruses are classified as DNA viruses (single-stranded or double-stranded DNA) and RNA viruses (single-stranded or double-stranded RNA); retroviruses are single-stranded RNA viruses that use reverse transcriptase.
Q8. How do slime layer and capsule increase the disease-causing ability of bacteria?
Ans: The slime layer/capsule protects bacteria from desiccation and from being engulfed by host phagocytic cells, and helps bacteria adhere to host tissues, all of which increase virulence and resistance to the immune system.
Q9. Normal bacterial flora in the human body is a blessing. Explain briefly.
Ans: Normal flora compete with and prevent colonization by pathogenic microbes, synthesize vitamins such as vitamin K and B-complex vitamins, and aid digestion, so they benefit the host rather than causing harm.
Q10. Why are plants and green algae considered monophyletic?
Ans: Plants and green algae are considered monophyletic because they share a single common ancestor and both possess chlorophyll a and b, similar cell wall composition (cellulose), and store food as starch, evidence of common descent.
Q11. Summarize the evolution of many-veined leaves in plants.
Ans: Early land plants had simple leaves with a single vein; through evolution, branching and fusion of veins produced leaves with net-like (reticulate) venation, which improved water/nutrient transport and support in larger leaves.
Q12. Give any three differences between cartilaginous and bony fishes.
Ans: Cartilaginous fishes (e.g., sharks) have a cartilage skeleton, no swim bladder, and exposed placoid scales; bony fishes have a bony skeleton, a swim bladder for buoyancy, and overlapping cycloid/ctenoid scales covered by mucus.
Q13. State the adaptations in plants to cope with high temperature stress.
Ans: Plants cope with heat stress by producing heat-shock proteins that protect other proteins from denaturation, increasing transpiration for evaporative cooling, and orienting or rolling leaves to reduce sunlight exposure.
Q14. (a) What is ascent of sap? (b) Enlist the forces responsible for ascent of sap in plants.
Ans: (a) Ascent of sap is the upward movement of water and dissolved minerals from roots to the leaves through the xylem. (b) The main forces are transpiration pull, cohesion between water molecules, adhesion of water to xylem walls, and root pressure.
Q15. Basidiomycota is a class in Fungi. (a) Why is Basidiomycota called club fungi? (b) Write any two differences between Basidiomycota and Ascomycota.
Ans: (a) They are called club fungi because their sexual spores (basidiospores) are produced on a club-shaped structure called a basidium. (b) Basidiomycota produce basidiospores externally on a basidium and typically form large fruiting bodies (mushrooms), while Ascomycota produce ascospores internally within a sac-like ascus and mostly form smaller/cup-shaped fruiting bodies.
Q16. Demonstrate the swallowing reflex in humans.
Ans: The swallowing reflex begins when the tongue pushes the food bolus to the pharynx; this triggers the soft palate to close the nasal passage and the epiglottis to cover the trachea, so peristaltic contractions push the bolus down the oesophagus into the stomach.
Q17. What major functions are performed by the large intestine?
Ans: The large intestine absorbs water and some remaining electrolytes from indigestible food, houses beneficial bacteria that synthesize certain vitamins, and compacts and stores waste as faeces before elimination.
Q18. Draw the pulmonary circuit of circulation.
Ans: In the pulmonary circuit, deoxygenated blood leaves the right ventricle through the pulmonary artery to the lungs, is oxygenated at the alveoli, and returns as oxygenated blood via the pulmonary veins to the left atrium of the heart.
Q19. Outline the events of the inflammatory response in case a pin has broken the skin.
Ans: Damaged cells and mast cells release histamine, causing blood vessels to dilate and become more permeable; this increases blood flow (redness, heat) and lets fluid and phagocytes (e.g., neutrophils) leak into the tissue to engulf pathogens and begin repair, producing swelling and pain.
Q20. State the natural and artificial active immunity.
Ans: Natural active immunity develops when the body is naturally exposed to a pathogen and produces its own antibodies (e.g., after recovering from an infection). Artificial active immunity is induced by vaccination, where a weakened/inactivated antigen stimulates the body to produce its own antibodies and memory cells.
🗂️ 1A 2025 — Paper I
Q21. How does the Golgi complex play a role in cell secretion and cell division?
Ans: The Golgi complex packages, modifies and sorts proteins/lipids into vesicles that are secreted from the cell (secretion), and during cell division it fragments and is distributed between daughter cells, later reassembling; it also forms the vesicles that build the new cell plate/membrane.
Q22. Compare the characteristics of cofactors of enzymes with one example of each.
Ans: Cofactors can be inorganic ions (e.g., Zn2+ needed by carbonic anhydrase), coenzymes (organic, e.g., NAD+ derived from vitamins), or prosthetic groups (tightly/permanently bound, e.g., heme in catalase).
Q23. Write any three main features of Bryophytes.
Ans: Bryophytes lack true roots, stems and leaves (have rhizoids instead), need water for fertilization since sperm are flagellated, and show a dominant gametophyte generation in their life cycle.
Q24. Enlist any three physical methods used to control bacteria with their mechanisms.
Ans: Heat (moist heat/autoclaving) denatures bacterial proteins and enzymes; radiation (UV light) damages bacterial DNA; filtration physically removes bacteria from liquids or air without using heat or chemicals.
Q25. Justify the statement 'Water is a universal solvent'.
Ans: Water's polar nature and ability to form hydrogen bonds let it surround and dissolve a very wide range of ionic and polar substances, which is why it is called a universal solvent.
Q26. Endospore formation is a mechanism in bacteria to survive under unfavorable conditions. Justify.
Ans: Endospores have a thick, resistant coat and dehydrated, metabolically inactive core, allowing bacteria to survive extreme heat, desiccation, radiation and lack of nutrients, then germinate back into active cells once conditions improve.
Q27. List the accessory glands associated with the male reproductive system with their respective locations.
Ans: The seminal vesicles lie behind the bladder, the prostate gland surrounds the urethra below the bladder, and the bulbourethral (Cowper's) glands lie below the prostate near the base of the penis.
Q28. What is incomplete dominance? Explain with a suitable example.
Ans: Incomplete dominance is when neither allele is completely dominant, so the heterozygote shows an intermediate phenotype; e.g., crossing red (RR) and white (WW) snapdragons gives pink (RW) offspring.
Q29. Why and how does cyclic photophosphorylation occur in light reactions?
Ans: It occurs when only Photosystem I is active and NADP+ is limited; electrons excited from PSI pass through an electron transport chain and return to PSI instead of going to NADP+, generating extra ATP without producing NADPH or O2.
Q30. Write the steps of the lytic cycle in bacteriophage.
Ans: Attachment of phage to the host bacterium, injection of viral DNA into the cell, replication of phage DNA and synthesis of viral proteins using host machinery, assembly of new phage particles, and lysis of the host cell releasing new phages.
Q31. What is mycorrhiza? Differentiate between its types.
Ans: Mycorrhiza is a mutualistic association between fungi and plant roots. In ectomycorrhiza the fungal hyphae surround the root surface without entering cells; in endomycorrhiza the hyphae penetrate into the root cortical cells.
Q32. What is point mutation? How does it cause sickle cell anaemia?
Ans: A point mutation is a change in a single nucleotide base in DNA. In sickle cell anaemia, a single base change alters one codon so that valine replaces glutamic acid at position 6 of the beta-globin chain, causing haemoglobin to distort red blood cells into a sickle shape.
Q33. Compare apoplast and symplast pathways involved in the uptake of water by roots.
Ans: In the apoplast pathway, water moves through cell walls and intercellular spaces without crossing membranes; in the symplast pathway, water moves through the cytoplasm of cells connected by plasmodesmata, crossing the plasma membrane.
Q34. Molecular biology provides evidence of evolution. Justify with reference to any three examples.
Ans: Similarities in DNA/protein sequences (e.g., cytochrome-c) across species indicate common ancestry, the universal genetic code shared by nearly all organisms points to a common origin, and comparing amino acid sequence differences can show evolutionary relatedness/divergence time between species.
Q35. Write about de-nitrification and assimilation processes in the nitrogen cycle.
Ans: Denitrification is the conversion of nitrates back into nitrogen gas by denitrifying bacteria, returning nitrogen to the atmosphere. Nitrogen assimilation is the uptake of nitrates/ammonium by plants and their conversion into amino acids and other organic nitrogen compounds.
Q36. Write about the structure and functions of the ovary and oviduct in the female reproductive system.
Ans: The ovary produces eggs (ova) and secretes the hormones oestrogen and progesterone. The oviduct (fallopian tube) receives the released egg, is the usual site of fertilization, and has ciliated walls that move the egg/embryo toward the uterus.
Q37. What is Y-linked inheritance? How are these traits transmitted? Give two examples.
Ans: Y-linked (holandric) inheritance involves genes located only on the Y chromosome, so traits are passed exclusively from father to all his sons and never to daughters. Examples include hypertrichosis (hairy ears) and some forms of male infertility genes.
Q38. Name and briefly explain the growth phases of bacteria (any three).
Ans: Lag phase — bacteria adjust to the medium with little division; log (exponential) phase — rapid, steady doubling of the population; stationary phase — growth rate equals death rate as nutrients become limited, so population size levels off.
Q39. Complete the enzyme table (enzyme, mechanism of action, example).
Ans: Hydrolases break large molecules using water (e.g., lipase); lyases remove groups without hydrolysis, often forming a double bond or ring (e.g., decarboxylase); oxidoreductases catalyse oxidation-reduction reactions, with cytochrome oxidase as an example.
Q40. In a cross between a black (BbEe) and a chocolate (bbEe) Labrador retriever showing epistasis: (a) show the genotypes/phenotypes of offspring, (b) calculate the probability of black and yellow coat colour.
Ans: (a) The cross BbEe x bbEe gives offspring in the ratio black:chocolate:yellow of 3:3:2 (yellow arises whenever the dog is ee regardless of B/b, due to epistasis). (b) Probability of black coat = 3/8; probability of yellow coat = 2/8 = 1/4.
Q41. Illustrate the conversion of pyruvic acid to acetyl-CoA as the link reaction between glycolysis and the Krebs cycle.
Ans: Pyruvate is oxidatively decarboxylated in the mitochondrial matrix: it loses a CO2, is oxidised, and the remaining 2-carbon acetyl group combines with coenzyme A to form acetyl-CoA, while NAD+ is reduced to NADH; acetyl-CoA then enters the Krebs cycle.
Q42. Interpret the results of the Meselson and Stahl experiment when bacteria grown in N15 were transferred to N14 medium for 2, 3 and 4 generations.
Ans: After one generation in N14, DNA was all hybrid (N15/N14); after the second generation, DNA appeared as half hybrid and half light (N14/N14); with further generations the proportion of light DNA kept increasing, confirming semi-conservative replication.
Q43. Write the difference in flower, stem and seed of dicots and monocots.
Ans: Dicot flowers usually have parts in fours/fives with reticulate-veined petals, dicot stems have vascular bundles arranged in a ring, and dicot seeds have two cotyledons. Monocot flowers have parts in threes, monocot stems have scattered vascular bundles, and monocot seeds have a single cotyledon.
Q44. Name any three enzymes with their functions involved in DNA replication.
Ans: DNA helicase unwinds the double helix; DNA polymerase synthesizes the new complementary strand by adding nucleotides; DNA ligase joins Okazaki fragments together on the lagging strand.
Q45. Elaborate the endosymbiont theory regarding the evolution of chloroplast and mitochondria.
Ans: The endosymbiont theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by an ancestral eukaryotic cell; the prokaryotes survived and formed a mutualistic relationship, retaining their own DNA, ribosomes and double membranes as evidence of this origin.
Q46. How is maltose different from sucrose in glycosidic linkage, monosaccharide units and specific role?
Ans: Maltose is formed of two glucose units joined by an alpha-1,4 glycosidic bond and is a product of starch digestion, while sucrose is formed of glucose and fructose joined by an alpha-1,2-beta glycosidic bond and functions as the main transport sugar in plants.
Q47. What are the limitations of Lamarckism?
Ans: Lamarckism's idea of inheritance of acquired characteristics is not supported because changes to body cells (somatic changes) cannot alter the DNA of gametes, and it cannot explain the genetic basis of variation or heredity established by later genetics.
Q48. Write the characteristics of angiosperms with reference to vascular tissue and alternation of generation.
Ans: Angiosperms possess well-developed vascular tissue (xylem with vessels and phloem with companion cells) for efficient conduction, and show a life cycle with alternation of a dominant sporophyte generation and a highly reduced gametophyte generation (pollen grain and embryo sac).
🗂️ 1A 2025 — Paper II
Q49. The diagram shows a section through the testis: (a) identify parts A & B, (b) write functions of C & D, (c) what is the function of inhibin hormone in males?
Ans: (a) A and B are typically the seminiferous tubule and epididymis. (b) The seminiferous tubules produce sperm, and the vas deferens/epididymis stores and transports maturing sperm. (c) Inhibin is secreted by Sertoli cells and inhibits FSH secretion, providing negative feedback to regulate sperm production.
Q50. Write the major points of the theory of natural selection (any three).
Ans: Individuals in a population show heritable variation; more offspring are produced than the environment can support, creating a struggle for existence; individuals with favourable traits survive and reproduce more (survival of the fittest), passing those traits to offspring over generations.
Q51. Compare any three adaptations for osmotic adjustment in hydrophytes and mesophytes.
Ans: Hydrophytes have reduced or absent root systems, thin cuticles, and abundant air spaces (aerenchyma) since water/gas exchange is easy; mesophytes have well-developed roots, a thicker cuticle to reduce water loss, and moderately developed stomata for balanced gas exchange and water conservation.
Q52. Why do post-transcriptional modifications occur in eukaryotic mRNA? Summarize the changes during mRNA processing.
Ans: They occur to protect, stabilize and prepare mRNA for translation. A 5' cap (modified guanine) is added to protect the mRNA and aid ribosome binding, a poly-A tail is added at the 3' end for stability, and introns are removed while exons are spliced together to form mature mRNA.
Q53. Write about the structure of prions and enlist two diseases caused by them.
Ans: Prions are infectious, misfolded protein particles with no nucleic acid that convert normal proteins into the misfolded form. They cause Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy (mad cow disease) in cattle.
Q54. Write any three distinguishing characteristics of vascular plants.
Ans: Vascular plants possess xylem and phloem for conduction of water/food, have true roots, stems and leaves, and show a dominant sporophyte generation in their life cycle.
Q55. What are homologous organs? Write an example. Which type of evolution is caused by them?
Ans: Homologous organs have the same basic structure and origin but may perform different functions in different species, e.g., the forelimbs of humans, whales and bats. They are evidence of divergent evolution.
Q56. Write any three chemical methods with their mechanisms used to control bacteria.
Ans: Alcohols denature bacterial proteins and dissolve lipids in the membrane; halogens (like chlorine/iodine) oxidize cell components and denature proteins; phenolics disrupt the cell membrane and denature proteins, all killing or inhibiting bacterial growth.
Q57. Complete the table with reference to plant groups (group, character, example).
Ans: Bryophytes are non-vascular with rhizoids, e.g., Marchantia/moss; Pteridophytes are vascular, seedless, reproduce by spores, e.g., fern; Gymnosperms are vascular, produce naked seeds in cones, e.g., pine.
Q58. Name the type of pattern of sex determination in humans. Also write the mechanism of sex determination in man.
Ans: Humans show the XY type of sex determination. Females are XX and males are XY; since males produce two types of sperm (X-bearing and Y-bearing) while all eggs carry X, the sperm that fertilizes the egg determines the sex of the offspring — XY (Y-bearing sperm) gives a male, XX (X-bearing sperm) gives a female.
Q59. Write the difference between leaf, seed and root of monocots and dicots.
Ans: Monocot leaves have parallel venation, one cotyledon in the seed, and a fibrous root system; dicot leaves have reticulate (net-like) venation, two cotyledons in the seed, and a taproot system.
Q60. Write down any three roles of smooth endoplasmic reticulum in a cell.
Ans: Smooth ER synthesizes lipids and steroid hormones, detoxifies drugs and harmful chemicals in liver cells, and stores/releases calcium ions, important for muscle contraction.
Q61. Peroxisomes are more abundant in liver cells. Justify.
Ans: Liver cells carry out heavy detoxification of alcohol and other harmful substances and break down excess fatty acids, both processes performed by peroxisomal enzymes, so liver cells contain many peroxisomes.
Q62. Outline the process of non-cyclic photophosphorylation with the help of a labelled flow sheet.
Ans: Light strikes PSII, exciting electrons that pass through an electron transport chain to PSI (generating ATP), water is split at PSII releasing O2 and replacing lost electrons, and electrons finally reduce NADP+ to NADPH at PSI — producing ATP, NADPH and O2 overall.
Q63. The diagram shows a molecule of lecithin: (a) identify parts A, B, C & D, (b) mention the polarity of E & F.
Ans: (a) A, B, C, D typically represent the phosphate head group, glycerol backbone, and two fatty acid tails of the phospholipid. (b) The phosphate head (E) is polar/hydrophilic, while the fatty acid tails (F) are non-polar/hydrophobic.
Q64. Following is a diagram of HIV: (a) identify parts A, B & C, (b) write the functions of protein gp40, gp120 and enzyme integrase.
Ans: (a) A, B, C typically represent the viral envelope, capsid and RNA genome/core. (b) gp120 binds to the CD4 receptor on host T-cells for attachment, gp41 (gp40) mediates fusion of the viral envelope with the host membrane, and integrase inserts the viral DNA into the host cell's genome.
Q65. Differentiate between the absorption spectrum of chlorophyll a, b and carotenoids.
Ans: Chlorophyll a absorbs strongly in the violet-blue and red regions with peaks around 430 nm and 662 nm; chlorophyll b absorbs slightly different wavelengths (peaks near 453 nm and 642 nm); carotenoids absorb mainly in the blue-violet region and pass the extra light energy to chlorophyll.
Q66. How can a virus survive under unfavorable conditions outside the host?
Ans: Viruses can remain as inert, non-metabolizing particles (virions) with a protective protein coat, allowing them to withstand harsh conditions outside a host cell until they encounter and infect a new host.
Q67. Justify that O-negative persons are universal donors and AB-positive are universal recipients of blood.
Ans: O-negative blood has no A, B or Rh antigens on its red cells, so it does not trigger antibody reactions when given to any recipient, making O-negative a universal donor. AB-positive blood has both A and B antigens and Rh antigen but no anti-A/anti-B antibodies in the plasma, so an AB-positive person can receive blood from any group, making them universal recipients.
Q68. How is photosynthesis in cyanobacteria different from bacteria (any three differences)?
Ans: Cyanobacteria perform oxygenic photosynthesis using chlorophyll a and water as an electron donor, releasing oxygen, similar to plants; most photosynthetic bacteria perform anoxygenic photosynthesis using bacteriochlorophyll and electron donors like H2S, and do not release oxygen.
Q69. How are starch and glycogen different from each other although both are polysaccharides?
Ans: Starch is the storage polysaccharide in plants, made of amylose (unbranched) and amylopectin (branched) glucose chains; glycogen is the storage polysaccharide in animals/fungi, more highly branched than starch, allowing faster glucose release.
Q70. Why are lysosomes also called suicidal bags?
Ans: Lysosomes contain powerful hydrolytic enzymes that, if released into the cytoplasm (e.g., when the cell is damaged or programmed to die), can digest and destroy the cell's own components, causing cell death — hence 'suicidal bags'.
Q71. Enlist any three fungal diseases in humans with one symptom each.
Ans: Athlete's foot (Tinea pedis) causes itching and cracking skin between the toes; ringworm (Tinea corporis) causes circular, scaly, itchy skin patches; candidiasis causes white patches and itching on mucous membranes.
Q72. Name the types of chromosomes on the basis of position of centromere, mentioning its location in each type (any three).
Ans: Metacentric chromosomes have the centromere in the middle, producing arms of equal length; submetacentric chromosomes have the centromere slightly off-centre giving unequal arms; acrocentric chromosomes have the centromere near one end, giving one very short and one long arm.
Q73. Write about the structure and function of each of the three layers of the uterus.
Ans: The perimetrium is the outer serous covering that protects the uterus; the myometrium is the thick middle layer of smooth muscle responsible for contractions during labour; the endometrium is the inner mucosal lining that thickens each cycle and supports implantation of the embryo.
Q74. How can the evolution of the giraffe's neck be explained on the basis of Lamarckism?
Ans: Lamarckism proposes that ancestral giraffes stretched their necks to reach higher leaves during their lifetime (use and disuse), and this acquired elongation was passed on to offspring, gradually producing longer-necked giraffes over generations — though this mechanism of inheritance is now known to be incorrect.
Q75. Write down the sources of mutagens with their examples (any three).
Ans: Physical mutagens include UV and ionizing radiation; chemical mutagens include substances like nitrous acid, mustard gas and certain pesticides; biological mutagens include transposons and certain viruses that insert into host DNA.
Q76. Name any three main greenhouse gases and their sources of emission.
Ans: Carbon dioxide is released by burning fossil fuels and deforestation; methane comes from livestock digestion, rice paddies and landfills; nitrous oxide comes mainly from agricultural fertilizers and industrial processes.
🗂️ 1A 2025 — Old Curriculum
Q77. Which organelle removes old and damaged mitochondria from the cell? Give its name and briefly explain the process.
Ans: The lysosome removes damaged mitochondria through a process called autophagy — it engulfs the worn-out organelle in a membrane (autophagosome), fuses with it, and its hydrolytic enzymes digest the contents for recycling.
Q78. Separate the following monosaccharides as aldo and keto sugars: Ribulose, Glucose, Fructose, Ribose, Dihydroxyacetone, Glyceraldehyde.
Ans: Aldo sugars (contain an aldehyde group): Glucose, Ribose, Glyceraldehyde. Keto sugars (contain a ketone group): Ribulose, Fructose, Dihydroxyacetone.
Q79. Name the three types of cytoskeletal proteins, their diameter and one function in the cell.
Ans: Microfilaments (~7 nm) made of actin help in cell movement and shape; intermediate filaments (~10 nm) provide mechanical strength; microtubules (~25 nm) made of tubulin help in intracellular transport and spindle formation.
Q80. Give the distinctive features of the apicomplexan group. Write one example.
Ans: Apicomplexans are non-motile, parasitic protists with a unique apical complex of organelles used to penetrate host cells, and most have complex life cycles involving multiple hosts; an example is Plasmodium, which causes malaria.
Q81. Where is the glyoxysome organelle found? State its role in the cell where it is found.
Ans: Glyoxysomes are found in the cells of germinating fatty seeds (e.g., castor bean). They convert stored fats into carbohydrates via the glyoxylate cycle to provide energy for the growing seedling before it can photosynthesize.
Q82. Name the nucleosides and nucleotides found in DNA.
Ans: The nucleosides in DNA are deoxyadenosine, deoxyguanosine, deoxycytidine and deoxythymidine (base + deoxyribose sugar); the corresponding nucleotides are these nucleosides plus a phosphate group, e.g., deoxyadenosine monophosphate (dAMP).
Q83. Why is pepsinogen secreted by stomach cells in an inactive form? How is it activated?
Ans: Pepsinogen is secreted inactive to prevent it from digesting the stomach's own proteins/cells. It is activated into pepsin by the acidic environment (HCl) of the stomach and by existing pepsin molecules acting on it (autocatalysis).
Q84. Depict the reaction of alcoholic fermentation done by yeast cells.
Ans: In alcoholic fermentation, yeast converts pyruvate (from glycolysis) into acetaldehyde by releasing CO2, and then acetaldehyde is reduced to ethanol using NADH, which regenerates NAD+ for glycolysis to continue: Pyruvate → Acetaldehyde + CO2 → Ethanol.
Q85. How is chemiosmosis carried out in the chloroplast?
Ans: Light reactions pump H+ ions from the stroma into the thylakoid lumen, creating a proton gradient; these H+ ions flow back into the stroma through ATP synthase enzyme complexes, and the energy released is used to synthesize ATP from ADP and phosphate.
Q86. Why do opportunistic diseases often attack AIDS patients? Give any two examples.
Ans: AIDS destroys helper T-cells, severely weakening the immune system so it cannot fight off organisms that a healthy immune system normally controls easily. Examples include Pneumocystis pneumonia and candidiasis (thrush).
Q87. Is a virus living or non-living? Justify your answer.
Ans: A virus shows both living and non-living traits: outside a host it is inert and shows no metabolism (non-living), but inside a host cell it can replicate and undergo mutation/evolution using host machinery (living-like), so it is considered to be at the border between living and non-living.
Q88. Differentiate between bacteria and cyanobacteria in terms of their photosynthetic properties.
Ans: Cyanobacteria carry out oxygenic photosynthesis using chlorophyll a and water as an electron donor, producing oxygen, similar to plants; most other photosynthetic bacteria are anoxygenic, using bacteriochlorophyll and donors other than water (e.g., H2S), and do not produce oxygen.
Q89. Suggest the appropriate name for bacteria according to their flagella distribution (given diagrams).
Ans: Bacteria with a single flagellum at one end are monotrichous; those with a tuft of flagella at one or both ends are lophotrichous/amphitrichous; those with flagella distributed all over the surface are peritrichous.
Q90. Complete the table related to inhibitors (name of inhibitor, competitive/non-competitive, enzyme inhibited).
Ans: Malonate is a competitive inhibitor of succinate dehydrogenase; cyanide is a non-competitive inhibitor of cytochrome oxidase.
Q91. Justify the names of slime molds and water molds. Give one example of each.
Ans: Slime molds are named for their slimy, amoeboid plasmodial stage that creeps like slime over surfaces, e.g., Physarum. Water molds are named because they thrive in aquatic or moist environments and have fungus-like, filamentous growth, e.g., Saprolegnia.
Q92. Give any three differences between naked seed plants and enclosed seed plants.
Ans: Naked-seed plants (gymnosperms) bear seeds exposed on cone scales, lack flowers and fruits, and are mostly wind-pollinated; enclosed-seed plants (angiosperms) bear seeds enclosed within an ovary/fruit, produce flowers, and use varied pollination methods including insects.
Q93. The three amino acids given (glycine, alanine, serine) — display the peptide bonds to make a tripeptide molecule.
Ans: A peptide bond forms between the carboxyl (-COOH) group of one amino acid and the amino (-NH2) group of the next, releasing water; linking glycine-alanine-serine in order forms a tripeptide with two peptide bonds joining the three residues.
Q94. Name the parts A and B of the given plant, and write one difference between the two parts.
Ans: Without the specific figure, in a typical fern/moss diagram A and B usually represent the sporophyte and gametophyte generations; the sporophyte is diploid and produces spores, while the gametophyte is haploid and produces gametes.
Q95. What are acoelomate, pseudocoelomate and coelomate animals? Mention one animal from each group.
Ans: Acoelomates have no body cavity between the gut and body wall (e.g., flatworm); pseudocoelomates have a body cavity not fully lined by mesoderm (e.g., roundworm/Ascaris); coelomates have a true body cavity completely lined by mesoderm (e.g., earthworm).
Q96. How do plants living in saline soil osmoregulate?
Ans: Halophytic plants accumulate salts in vacuoles or excrete excess salt through specialized salt glands, and increase their internal solute concentration so they can still absorb water osmotically from the saline soil.
Q97. What advancements are shown by reptiles over amphibians?
Ans: Reptiles have dry, scaly, waterproof skin that prevents water loss, lay shelled amniotic eggs that can be laid on land, and rely on internal fertilization, freeing them from dependence on water for reproduction unlike amphibians.
Q98. Classify plants on the basis of photoperiod and give one example of each class.
Ans: Short-day plants flower when day length is less than a critical value (e.g., chrysanthemum); long-day plants flower when day length exceeds a critical value (e.g., spinach); day-neutral plants flower regardless of day length (e.g., tomato).
Q99. Enlist human teeth in their correct order with their function.
Ans: Incisors (cutting food), canines (tearing food), premolars (crushing and grinding), and molars (grinding food finely) — arranged from front to back in each jaw quadrant.
Q100. Outline the inflammatory response of the body when the skin is pricked.
Ans: Damaged tissue releases histamine causing vasodilation and increased capillary permeability; this brings more blood and phagocytic white blood cells to the area, producing redness, heat, swelling and pain while pathogens are engulfed and the area begins healing.
Q101. Mention the different parts of the conduction system of the heart in their accurate order.
Ans: SA node (pacemaker) → AV node → Bundle of His → Purkinje fibers, in that order, coordinating the sequential contraction of the atria and then the ventricles.
Q102. Which blood cell is called the antigen-presenting cell? Why is it important in immune response?
Ans: Macrophages (and dendritic cells) act as antigen-presenting cells. They engulf pathogens, display fragments of the pathogen's antigens on their surface, and this presentation activates helper T-cells to coordinate the specific immune response.
Q103. Differentiate cell-mediated immunity from humoral immunity.
Ans: Cell-mediated immunity is carried out directly by T-cells that attack infected or abnormal cells; humoral immunity is carried out by B-cells that produce antibodies which circulate in blood/lymph to neutralize pathogens and toxins.
Q104. Make a flow chart of the pulmonary circuit.
Ans: Right ventricle → pulmonary artery → lungs (gas exchange: CO2 released, O2 picked up) → pulmonary veins → left atrium, completing the pulmonary circuit.
🗂️ 2A 2025 — New Curriculum
Q105. How did mitochondria develop according to endosymbiont theory? Briefly explain.
Ans: Mitochondria are believed to have originated when an ancestral eukaryotic cell engulfed a free-living aerobic bacterium; instead of being digested, it survived and formed a mutualistic relationship, retaining its own DNA and double membrane as mitochondria today.
Q106. Draw the ultrastructure of an animal cell and label three organelles present only in animal cells.
Ans: Centrioles (organize the spindle during cell division), lysosomes (though present in some plant cells too, they are prominent/typical in animal cells), and the true absence of a cell wall/chloroplast/large central vacuole distinguishes an animal cell structurally.
Q107. Compare amylose starch with amylopectin on the basis of structure, linkage and solubility.
Ans: Amylose is an unbranched chain of glucose joined by alpha-1,4 linkages and is less soluble in water; amylopectin is a branched chain with alpha-1,4 linkages in chains and alpha-1,6 linkages at branch points, and is more soluble due to its branched, compact structure.
Q108. Briefly describe the composition and role of the capsule in a bacterial cell.
Ans: The capsule is a sticky, gel-like layer usually made of polysaccharide surrounding the cell wall. It protects bacteria from desiccation and phagocytosis, helps them adhere to surfaces/host tissues, and increases virulence.
Q109. Distinguish between saturated and unsaturated fatty acids with examples.
Ans: Saturated fatty acids have no double bonds between carbon atoms, are solid at room temperature, e.g., palmitic acid; unsaturated fatty acids have one or more double bonds, are liquid at room temperature, e.g., oleic acid.
Q110. What are the three types of cofactors? Give one example of each.
Ans: Inorganic cofactors are metal ions, e.g., Zn2+ for carbonic anhydrase; coenzymes are loosely bound organic molecules, e.g., NAD+; prosthetic groups are tightly/permanently bound organic molecules, e.g., heme in catalase.
Q111. Identify the role of the active site of an enzyme. Also mention its component parts.
Ans: The active site is the region of the enzyme where the substrate binds and the reaction is catalysed; it consists of the binding site (recognizes and holds the substrate) and the catalytic site (carries out the chemical reaction).
Q112. Briefly explain the hormonal control of sperm production in the negative feedback system.
Ans: High testosterone/inhibin levels signal the hypothalamus and pituitary to reduce GnRH, LH and FSH secretion, which in turn reduces stimulation of the testes; when hormone levels fall, secretion increases again, keeping sperm production regulated within a set range.
Q113. Briefly explain the importance of competitive inhibitors.
Ans: Competitive inhibitors resemble the substrate and bind to the enzyme's active site, blocking the substrate from binding and thus regulating/reducing enzyme activity; they are important in metabolic regulation and are the basis of many drugs (e.g., some antibiotics).
Q114. Complete the table (specimen, sub-division, one special characteristic) for Rhynia, Lycopodium, Equisetum.
Ans: Rhynia belongs to Psilopsida and lacks true roots/leaves; Lycopodium belongs to Lycopsida and has small, scale-like leaves (microphylls); Equisetum belongs to Sphenopsida and has a jointed, ribbed, hollow stem.
Q115. Briefly explain the three steps of oxidation of pyruvate into acetyl-CoA.
Ans: Pyruvate is decarboxylated (loses CO2); the remaining 2-carbon fragment is oxidised, reducing NAD+ to NADH; the oxidised fragment combines with coenzyme A to form acetyl-CoA.
Q116. Why are the chances of a husband and wife having either a boy or a girl 50:50?
Ans: Since the mother produces only X-bearing eggs while the father produces X-bearing and Y-bearing sperm in equal numbers, there is an equal (50:50) chance that the fertilizing sperm carries an X (girl) or a Y (boy) chromosome.
Q117. Complete the table for the electron transport chain (site 1, 2, 3 — proton pumping site, enzyme involved).
Ans: Site 1 (Complex I, NADH dehydrogenase) pumps protons and passes electrons to Coenzyme Q; Site 2 (Complex III, cytochrome bc1) pumps protons and passes electrons to cytochrome c; Site 3 (Complex IV, cytochrome oxidase) pumps protons and passes electrons to oxygen, forming water.
Q118. Draw the labelled life cycle of HIV.
Ans: HIV attaches to CD4 receptors on a helper T-cell via gp120, fuses and releases its RNA; reverse transcriptase converts RNA to DNA, which integrates into the host genome via integrase; the host cell then transcribes/translates viral genes, and new virions assemble and bud off from the cell.
Q119. Name three types of enzymes for HIV replication and write the role of each.
Ans: Reverse transcriptase converts viral RNA into DNA; integrase inserts the viral DNA into the host cell's chromosome; protease cleaves large viral protein precursors into functional individual proteins during virion assembly.
Q120. What is heat of vaporization? What is its importance for animals?
Ans: Heat of vaporization is the amount of heat energy required to convert a liquid into vapour. In animals, evaporation of sweat/water requires large amounts of heat drawn from the body surface, which provides an effective cooling mechanism.
Q121. Briefly explain any three prokaryotic features of cyanobacteria.
Ans: Cyanobacteria lack a membrane-bound nucleus, have circular DNA not enclosed in a nuclear envelope, and lack membrane-bound organelles such as mitochondria and chloroplasts, all typical prokaryotic features despite performing photosynthesis.
Q122. Differentiate between incomplete dominance and codominance.
Ans: In incomplete dominance, the heterozygote shows a blended, intermediate phenotype (e.g., pink from red x white flowers); in codominance, both alleles are fully and separately expressed in the heterozygote (e.g., AB blood group showing both A and B antigens).
Q123. Illustrate with diagrams three types of rod-shaped bacteria.
Ans: Bacillus refers to a single rod-shaped bacterium; diplobacillus refers to rod-shaped bacteria occurring in pairs; streptobacillus refers to rod-shaped bacteria occurring in chains.
Q124. Draw a labelled Calvin cycle.
Ans: The Calvin cycle has three phases: carbon fixation (CO2 combines with RuBP via RuBisCO to form 3-PGA), reduction (3-PGA is reduced to G3P using ATP and NADPH), and regeneration (some G3P is used to regenerate RuBP using ATP, while the rest forms glucose).
Q125. Enlist any six stages of endospore formation in bacteria as a mechanism of survival.
Ans: The stages include: DNA replication and elongation of the cell, formation of a septum, engulfment of the forespore, formation of the cortex, deposition of the spore coat, and maturation followed by release of the free endospore.
Q126. Classify animals on the basis of body cavity with one example of each.
Ans: Acoelomates have no body cavity, e.g., flatworm; pseudocoelomates have a body cavity not fully lined by mesoderm, e.g., roundworm; coelomates have a true, mesoderm-lined body cavity, e.g., earthworm.
Q127. Describe the genomic organization of bacteria with respect to circular DNA and plasmids.
Ans: Bacteria have a single circular chromosome located in the nucleoid region, not enclosed by a membrane, and may additionally carry small circular DNA molecules called plasmids that often carry genes for antibiotic resistance and can be transferred between bacteria.
Q128. Why is post-transcriptional modification of mRNA necessary for eukaryotes (any three reasons)?
Ans: It protects mRNA from degradation by nucleases (5' cap and poly-A tail), allows removal of non-coding introns so only coding exons are translated, and helps mRNA to be recognized and exported from the nucleus and bind to ribosomes efficiently.
Q129. Write the names and causative agents of any three fungal diseases in man.
Ans: Athlete's foot is caused by Trichophyton; ringworm is caused by Microsporum/Trichophyton species; candidiasis is caused by Candida albicans.
Q130. Briefly describe population, community and ecosystem.
Ans: A population is a group of organisms of the same species living in a given area; a community is all the different populations of various species living and interacting in that area; an ecosystem is the community together with its non-living (abiotic) physical environment.
Q131. Describe any three general features of vascular plants.
Ans: Vascular plants have well-developed xylem and phloem tissues, true roots/stems/leaves, and a dominant sporophyte generation in their life cycle.
Q132. Briefly state three sources of mutation with examples.
Ans: Physical agents like UV/ionizing radiation, chemical mutagens like nitrous acid or mustard gas, and spontaneous errors during DNA replication can all introduce mutations.
🗂️ 2A 2025 — Old Curriculum
Q133. Differentiate between resolution and magnification with examples.
Ans: Resolution is the ability of an instrument to distinguish between two close points as separate (e.g., an electron microscope resolves down to nanometers); magnification is how much larger an image appears compared to the actual object (e.g., a light microscope may magnify 1000x).
Q134. Briefly explain condensation and hydrolysis with examples.
Ans: Condensation is the joining of two molecules with the removal of a water molecule, e.g., two glucose units forming maltose; hydrolysis is the breaking of a bond by adding a water molecule, e.g., maltose breaking down into two glucose units.
Q135. Compare the 'lock and key' model and 'induced fit' model of enzyme action.
Ans: In the lock and key model, the enzyme's active site has a fixed shape that exactly fits the substrate like a key in a lock; in the induced fit model, the active site is flexible and changes shape slightly to mould around the substrate as it binds.
Q136. Compare three differences in the cell wall of gram-positive and gram-negative bacteria.
Ans: Gram-positive bacteria have a thick peptidoglycan layer and no outer membrane, retaining crystal violet stain (appear purple); gram-negative bacteria have a thin peptidoglycan layer and an additional outer membrane containing lipopolysaccharide, losing the stain and appearing pink/red with safranin.
Q137. Why can artificial sweeteners not be metabolized by enzymes of the human body? Justify.
Ans: Artificial sweeteners have a molecular shape that does not fit the active sites of the enzymes that normally break down natural sugars, so the body's enzymes cannot bind and act on them, and they pass through largely unmetabolized.
Q138. Briefly explain any three evolutionary adaptations in class Aves.
Ans: Birds have hollow, pneumatic bones to reduce weight for flight, feathers that provide lift and insulation, and a highly efficient respiratory system with air sacs that allows continuous unidirectional airflow through the lungs.
Q139. Why is green algae considered the ancestor of plants?
Ans: Green algae share key features with land plants including chlorophyll a and b, cellulose cell walls, and starch as a storage product, and molecular/biochemical evidence shows they share a common ancestor, making them the most likely evolutionary precursors of land plants.
Q140. Classify proteins into globular and fibrous with examples.
Ans: Globular proteins are compact, roughly spherical, and usually soluble, functioning as enzymes or transport proteins, e.g., haemoglobin; fibrous proteins are long, thread-like, and structural, e.g., collagen and keratin.
Q141. State the three key features that distinguish fungi from other groups.
Ans: Fungi have cell walls made of chitin (not cellulose), are heterotrophic and absorb nutrients externally (absorptive nutrition) rather than ingesting food, and typically reproduce by spores.
Q142. Briefly explain the effect of temperature on enzyme action (no graph required).
Ans: As temperature rises, enzyme activity increases up to an optimum temperature where the reaction rate is highest; beyond this optimum, the enzyme's protein structure begins to denature, and activity rapidly declines as the enzyme loses its functional shape.
Q143. Draw a labelled life cycle of ferns.
Ans: The dominant diploid sporophyte (the visible fern plant) produces haploid spores by meiosis in sporangia; spores germinate into a small, independent haploid gametophyte (prothallus), which produces gametes that fuse to form a zygote, growing into a new sporophyte.
Q144. Draw and label the phases in a bacterial growth curve.
Ans: The growth curve has a lag phase (adjustment, minimal growth), a log/exponential phase (rapid multiplication), a stationary phase (growth rate equals death rate due to limited nutrients), and a death phase (nutrients exhausted, toxic waste accumulates, cells die faster than they multiply).
Q145. Differentiate between diploblastic and triploblastic animals with examples.
Ans: Diploblastic animals develop from two germ layers (ectoderm and endoderm), e.g., Hydra (Cnidaria); triploblastic animals develop from three germ layers (ectoderm, mesoderm and endoderm), e.g., earthworm.
Q146. Briefly describe the types of heterotrophic nutrition in bacteria.
Ans: Saprophytic bacteria obtain nutrients by decomposing dead organic matter; parasitic bacteria obtain nutrients by living on or in a host, often causing disease; symbiotic bacteria live in mutually beneficial association with another organism.
Q147. Briefly explain the TACT mechanism for movement of water in xylem.
Ans: TACT stands for Transpiration pull, Adhesion of water to xylem walls, Cohesion between water molecules, and Tension created in the water column — together these forces pull water continuously upward through the xylem from roots to leaves.
Q148. Briefly explain the structure of bacterial flagellum (no diagram required).
Ans: The bacterial flagellum consists of a long, hollow filament made of the protein flagellin, connected by a hook to a basal body embedded in the cell wall and membrane, which acts as a rotary motor to spin the flagellum and propel the cell.
Q149. Describe briefly any two structural features that increase the surface area of the small intestine.
Ans: Villi (finger-like projections) and microvilli (brush border on villi cells) both greatly increase the internal surface area of the small intestine, maximizing absorption of digested nutrients.
Q150. Describe bacterial endospore formation as an adaptation.
Ans: When conditions become unfavourable, some bacteria form a thick-walled, dehydrated, metabolically dormant endospore around their DNA, which can withstand heat, desiccation and chemicals, allowing survival until conditions improve and the spore germinates.
Q151. Briefly explain the coronary circulation.
Ans: Coronary arteries branch from the aorta just above the heart and supply oxygenated blood to the heart muscle itself; after passing through the cardiac tissue, deoxygenated blood is collected by cardiac veins that drain into the coronary sinus and then the right atrium.
Q152. Briefly state three economic gains of fungus yeast.
Ans: Yeast is used in baking (fermentation produces CO2 that leavens bread), in brewing/alcohol production (ferments sugars into ethanol), and in the production of vitamins and single-cell protein for nutritional supplements.
Q153. Distinguish between natural active immunity and artificial active immunity.
Ans: Natural active immunity develops when the body is naturally infected and produces its own antibodies; artificial active immunity is induced by vaccination with a weakened/inactivated antigen, prompting the body to produce its own antibodies without suffering the actual disease.
Q154. Briefly explain any three adaptations of bryophytes for terrestrial life.
Ans: Bryophytes have a waxy cuticle on aerial parts to reduce water loss, rhizoids to anchor to soil and absorb some water/minerals, and produce spores with resistant walls that can survive dry conditions for dispersal.
Q155. Differentiate among acoelomates and coelomates with examples.
Ans: Acoelomates have no body cavity between the gut and body wall, with the space filled by mesodermal tissue, e.g., flatworms; coelomates have a true, fluid-filled body cavity completely lined by mesoderm, e.g., earthworms, which allows organs to move and develop more freely.
Q156. Describe the role of lateral meristems in plant growth.
Ans: Lateral meristems (vascular cambium and cork cambium) are responsible for secondary growth, increasing the girth/diameter of stems and roots by producing new xylem and phloem, and protective cork tissue respectively.
Q157. Briefly explain obesity in terms of its causes and prevention.
Ans: Obesity is caused mainly by excessive calorie intake relative to energy expenditure, sedentary lifestyle, genetic predisposition and hormonal imbalances; it can be prevented through a balanced diet, regular physical activity, and monitoring calorie intake.
Q158. Briefly explain the role of the spleen as a lymphoid organ.
Ans: The spleen filters blood, removing old or damaged red blood cells and pathogens, stores platelets and white blood cells, and helps initiate immune responses against blood-borne antigens.
Q159. What are autoimmune diseases? Give two examples.
Ans: Autoimmune diseases occur when the immune system mistakenly attacks the body's own healthy cells/tissues as if they were foreign. Examples include rheumatoid arthritis and type 1 diabetes mellitus.
Q160. How do lysosomes act as suicidal bags?
Ans: Lysosomes contain powerful hydrolytic enzymes; if their membrane ruptures or they are triggered during programmed cell death, these enzymes are released into the cytoplasm and digest the cell's own components, destroying the cell — hence the term 'suicidal bags'.
🗂️ Annual 2024 — Paper I
Q161. Elaborate the role of peroxisomes in plant and animal cells.
Ans: Peroxisomes break down fatty acids by beta-oxidation and detoxify harmful substances (like alcohol and H2O2) using catalase in animal cells; in plant cells (as glyoxysomes) they also convert stored fats into carbohydrates in germinating seeds via the glyoxylate cycle.
Q162. Relate the variety of proteins of the plasma membrane with their respective roles (any three).
Ans: Transport proteins move substances across the membrane; receptor proteins bind signalling molecules (hormones) for cell communication; recognition/glycoproteins on the surface help cells identify each other, e.g., in immune responses.
Q163. Illustrate the formation of glycosidic bonds in (a) sucrose, (b) maltose.
Ans: In sucrose, glucose and fructose join via an alpha-1,2-beta glycosidic bond with loss of water; in maltose, two glucose units join via an alpha-1,4 glycosidic bond, also with loss of a water molecule (condensation reaction).
Q164. Illustrate the process of non-cyclic photophosphorylation with labelling.
Ans: Light excites electrons in PSII, which pass through an electron transport chain to PSI, generating ATP along the way; water is split at PSII to replace lost electrons and release O2; electrons from PSI ultimately reduce NADP+ to NADPH — net products: ATP, NADPH and O2.
Q165. Justify the significance of amino acid sequence by explaining the example of sickle cell haemoglobin.
Ans: A single amino acid substitution (valine for glutamic acid) at position 6 of the beta-globin chain changes haemoglobin's shape and behaviour, causing red blood cells to become sickle-shaped under low oxygen, showing how a tiny sequence change can have major functional consequences.
Q166. Outline the process of C4 photosynthesis in a flow chart.
Ans: CO2 is first fixed by PEP carboxylase into a 4-carbon compound (oxaloacetate) in mesophyll cells; this is converted to malate and transported to bundle sheath cells, where CO2 is released and enters the Calvin cycle, concentrating CO2 and minimizing photorespiration.
Q167. How do oxidoreductases, hydrolases and lyases work? Give one example of each group.
Ans: Oxidoreductases catalyse oxidation-reduction reactions, e.g., dehydrogenase; hydrolases catalyse breakdown of bonds using water, e.g., lipase; lyases catalyse removal of groups without hydrolysis, often forming double bonds, e.g., decarboxylase.
Q168. How does endospore formation occur in bacteria? How does it help bacteria withstand unfavorable conditions?
Ans: The bacterial DNA replicates and is enclosed within a tough, multi-layered, dehydrated spore coat, becoming metabolically dormant; this resistant structure can survive extreme heat, desiccation, radiation and lack of nutrients until conditions improve and it germinates back into an active cell.
Q169. Describe the steps of the lytic cycle of bacteriophage.
Ans: Attachment to the host bacterium, injection of phage DNA, replication of phage DNA and synthesis of viral proteins using host machinery, assembly of new phage particles, and lysis of the host cell releasing progeny phages.
Q170. Explain the pathogenic role of fungi in humans with their symptoms (any three).
Ans: Trichophyton causes ringworm/athlete's foot with itchy, scaly skin patches; Candida albicans causes candidiasis with white patches and itching on mucous membranes; Aspergillus can cause aspergillosis affecting the respiratory system with coughing and breathing difficulty.
Q171. A person bitten by a snake was injected with antivenom. (a) How does antivenom work? (b) Why was passive immunity preferred?
Ans: (a) Antivenom contains ready-made antibodies that directly bind and neutralize snake venom toxins in the bloodstream. (b) Passive immunity is preferred because it provides immediate protection, which is essential in an emergency, since there is no time to wait for the body to actively produce its own antibodies.
Q172. Name the structures P and R in the bacterium diagram; write chemical composition of Q; name the process by which the bacterium reproduces asexually.
Ans: Typical labelled parts of a bacterium include the cell wall/capsule and flagellum, with the cell wall/capsule (Q) composed mainly of peptidoglycan (murein). Bacteria reproduce asexually mainly by binary fission.
Q173. T-cells are involved in specific defence. Name any three types of T-cells with their specific roles.
Ans: Helper T-cells coordinate the immune response by activating B-cells and other T-cells; cytotoxic T-cells directly kill infected or cancerous cells; memory T-cells persist after infection to provide a faster response upon re-exposure to the same antigen.
Q174. Write about evolutionary adaptations in phylum Arthropoda regarding respiration, excretion and nervous system.
Ans: Arthropods show varied respiratory adaptations such as gills (aquatic forms), tracheae or book lungs (terrestrial forms); excretion via Malpighian tubules conserves water; and a ventral nerve cord with ganglia allows rapid, coordinated responses — all adaptations suited to diverse habitats.
Q175. Briefly explain the flow of blood through the heart as regulated by valves.
Ans: Blood flows from the atria through the atrioventricular valves (tricuspid/mitral) into the ventricles, then through the semilunar valves (pulmonary/aortic) into the pulmonary artery and aorta; the valves ensure blood flows in one direction only, preventing backflow.
Q176. Write down the steps of the swallowing action of a bolus in the oral cavity.
Ans: The tongue rolls food into a bolus and pushes it to the back of the mouth toward the pharynx, triggering the involuntary swallowing reflex that moves the bolus into the oesophagus.
Q177. (a) How does temperature affect the rate of an enzyme-catalysed reaction? (b) Compare the optimum temperature of enzymes in humans and thermophilic bacteria.
Ans: (a) Reaction rate increases with temperature up to an optimum, beyond which the enzyme denatures and activity falls sharply. (b) Human enzymes have an optimum around 37°C, while thermophilic bacterial enzymes have a much higher optimum, often above 70-80°C, reflecting their hot-environment adaptation.
Q178. Name any three groups of Protista with one salient feature and example of each group.
Ans: Protozoa (animal-like, heterotrophic, motile, e.g., Amoeba); Algae (plant-like, photosynthetic, e.g., Chlamydomonas); Slime molds/fungus-like protists (saprophytic, form plasmodium, e.g., Physarum).
Q179. In the TACT theory of ascent of sap, how do transpiration and adhesion help the process?
Ans: Transpiration from leaves creates a pulling force (tension) that draws water up through the xylem; adhesion is the attraction of water molecules to the polar walls of the xylem vessels, which helps maintain the continuous water column against gravity as it is pulled upward.
Q180. Name and explain two hypotheses for the evolution of single-veined leaves.
Ans: The enation theory suggests leaves arose as small outgrowths (enations) of the stem that later developed a vein connecting to the stem's vascular tissue; the microphyll theory suggests leaves evolved directly from a single, unbranched vascular strand (sterile sporangium branch) that flattened out.
Q181. Complete the table for phylum, distinguishing character and example (Cnidarian, metameric segmentation, snail).
Ans: Cnidarians show radial symmetry and possess stinging cells (cnidocytes), e.g., Hydra; metameric segmentation is a distinguishing feature of Annelida, e.g., earthworm; the mollusc with a coiled shell/muscular foot such as a snail belongs to phylum Mollusca.
Q182. The life cycle of a fern is shown: (a) name generations P and Q, (b) which processes are shown as X and Y, (c) which generation is dominant in ferns?
Ans: (a) P and Q typically represent the sporophyte and gametophyte generations. (b) X and Y usually represent meiosis (producing spores) and fertilization (producing the zygote). (c) The sporophyte generation is dominant in ferns.
Q183. Write the causative agent of cotton leaf curl disease. Also write symptoms and treatment.
Ans: Cotton leaf curl disease is caused by the Cotton Leaf Curl Virus, transmitted by whiteflies. Symptoms include upward/downward curling of leaves, thickened veins and stunted growth. Treatment/control involves using resistant varieties and controlling the whitefly vector with insecticides.
Q184. Write down the mechanism of translocation in plants.
Ans: According to the pressure-flow (mass-flow) hypothesis, sugars are actively loaded into phloem sieve tubes at the source, lowering water potential and drawing water in by osmosis, which raises turgor pressure; this pressure gradient pushes the phloem sap toward the sink, where sugars are unloaded and water potential rises again.
Q185. What are xerophytes? Enlist any four adaptations for their habitat. Also give an example.
Ans: Xerophytes are plants adapted to survive in dry, arid habitats. Adaptations include thick cuticles to reduce water loss, reduced leaves or spines to minimize transpiration, sunken stomata, and extensive root systems to reach deep water; example: cactus.
Q186. How is the dermis of skin involved in the first line of defence?
Ans: The dermis contains sweat glands (secreting acidic sweat that inhibits microbial growth), sebaceous glands (secreting sebum with antimicrobial properties), and dense connective tissue that provides a physical barrier against pathogen entry.
Q187. Name two growth promoters and one growth inhibitor in plants with one major effect of each.
Ans: Auxins promote cell elongation and apical dominance; gibberellins promote stem elongation and seed germination; abscisic acid (a growth inhibitor) promotes stomatal closure and seed/bud dormancy.
Q188. Write the mechanism of irreversible non-competitive enzyme inhibition with an example.
Ans: An irreversible non-competitive inhibitor binds permanently (often covalently) to a site other than the active site, changing the enzyme's shape so it can no longer function even in high substrate concentration; an example is cyanide binding to cytochrome oxidase, permanently blocking cellular respiration.
🗂️ Annual 2024 — Paper II
Q189. The diagram represents a neutral lipid: (a) name components X and Y, (b) name the type of bond between X and Y, (c) why is this molecule called a neutral lipid?
Ans: (a) X and Y typically represent glycerol and a fatty acid. (b) They are joined by an ester bond. (c) It is called neutral because it carries no charge/polar groups, being made only of glycerol and fatty acids without ionizable groups.
Q190. Complete the table with reference to bacterial cell wall (character, gram-negative, gram-positive) for peptidoglycan and thickness.
Ans: Gram-positive bacteria have a thick peptidoglycan layer (multiple layers, no outer membrane); gram-negative bacteria have a thin peptidoglycan layer plus an additional outer membrane containing lipopolysaccharide.
Q191. What are mesophytes? How do they adapt to their environment (any four adaptations)?
Ans: Mesophytes are plants adapted to environments with a moderate, adequate water supply. Adaptations include a moderately thick cuticle, well-developed but not excessive root systems, stomata on both leaf surfaces for balanced gas exchange, and broad leaves for efficient photosynthesis.
Q192. What are the evolutionary adaptations in echinoderms regarding digestion, respiration and nervous system?
Ans: Echinoderms have a simple digestive system with a mouth and often an anus adapted for their sedentary/slow-moving lifestyle, use dermal gills/tube feet for gas exchange and locomotion, and have a decentralized nerve ring with radial nerves rather than a brain, suited to their radial symmetry.
Q193. Differentiate between primary and secondary growth (any three differences).
Ans: Primary growth occurs at the apical meristems (root and shoot tips), increasing the length of the plant; secondary growth occurs at lateral meristems (vascular and cork cambium), increasing girth/diameter; primary growth occurs in all plants while secondary growth mainly occurs in gymnosperms and dicots.
Q194. Differentiate between mitochondria and chloroplasts (any three differences).
Ans: Mitochondria carry out aerobic respiration, releasing energy from food and are found in all eukaryotic cells; chloroplasts carry out photosynthesis, trapping light energy, and are found only in plant/algal cells; mitochondria contain cristae while chloroplasts contain thylakoids/grana.
Q195. How does a natural killer cell kill cells infected by bacteria and cancerous cells?
Ans: Natural killer cells recognize infected or cancerous cells (which often show altered or reduced surface markers) and release perforins and granzymes that create pores in the target cell membrane and trigger programmed cell death (apoptosis).
Q196. Laboratory-manufactured sugars are not metabolized by enzymes in the body. Justify.
Ans: Artificial sweeteners have a molecular structure that does not match the active site shape of the body's digestive/metabolic enzymes, so the enzymes cannot bind and break them down, meaning they largely pass through the body unmetabolized.
Q197. How does pH affect the rate of an enzyme action? Compare the optimum pH for trypsin and papain.
Ans: Enzyme activity is highest at an optimum pH and decreases sharply above or below it as the enzyme's shape is altered. Trypsin (a digestive enzyme in the small intestine) has an optimum pH around 8 (alkaline), while papain has a broader optimum around pH 6-7.
Q198. How does temperature affect the activities of RuBisCO?
Ans: RuBisCO activity increases with temperature up to an optimum, enhancing the rate of CO2 fixation in the Calvin cycle; beyond the optimum, its efficiency drops both because of denaturation and because it increasingly binds O2 instead of CO2, increasing photorespiration.
Q199. Complete the table (character, animal, plant, fungus) for mode of nutrition, cell wall composition, presence of centriole.
Ans: Mode of nutrition: animals are heterotrophic (ingestion), plants are autotrophic (photosynthesis), fungi are heterotrophic (absorption). Cell wall: animals have none, plants have cellulose, fungi have chitin. Centrioles: present in animal cells, generally absent in plant and fungal cells.
Q200. A student accidentally got a small cut on a finger in the lab. What series of events would occur as an inflammatory response?
Ans: Damaged cells release histamine causing vasodilation and increased capillary permeability, resulting in redness and swelling; phagocytic white blood cells migrate to the wound to engulf bacteria/debris, and clotting factors help seal the wound, followed by tissue repair.
Q201. Draw the structure of an antibody. Label its parts and write their specific roles.
Ans: An antibody has a Y-shape made of two heavy chains and two light chains; the variable regions at the tips of the arms form the antigen-binding sites that specifically recognize antigens, while the constant region (stem) determines the antibody class and can bind to immune cells to trigger further responses.
Q202. Draw an outline of the Calvin cycle mentioning substrates and products of each step.
Ans: Carbon fixation: CO2 + RuBP → two molecules of 3-PGA (via RuBisCO). Reduction: 3-PGA + ATP + NADPH → G3P. Regeneration: some G3P molecules are used with ATP to regenerate RuBP, continuing the cycle, while surplus G3P is used to form glucose.
Q203. (a) Why is the Human Immunodeficiency Virus called so? (b) Name two opportunistic diseases caused by HIV. (c) Name any two enzymes present in the HIV core.
Ans: (a) It is called so because it specifically infects and destroys human immune cells (helper T-cells), causing immunodeficiency. (b) Pneumocystis pneumonia and candidiasis are common opportunistic infections. (c) Reverse transcriptase and integrase (and protease) are enzymes present in its core.
Q204. (a) Why are bryophytes called amphibious plants? (b) Write any two distinguishing features of bryophytes.
Ans: (a) They are called amphibious because although they live on land, they require external water for their flagellated sperm to swim to and fertilize the egg, tying them to moist habitats like amphibians. (b) They lack true vascular tissue and true roots (having rhizoids instead).
Q205. How does the development of protostomes and deuterostomes differ in cleavage, coelom formation and blastopore fate?
Ans: Protostomes show spiral, determinate cleavage, coelom forms by splitting of mesoderm (schizocoely), and the blastopore becomes the mouth; deuterostomes show radial, indeterminate cleavage, coelom forms by outpocketing of the gut (enterocoely), and the blastopore becomes the anus while the mouth forms separately.
Q206. Draw and label the steps of the lysogenic cycle of bacteriophage.
Ans: The phage injects its DNA into the host, but instead of immediately replicating, the viral DNA integrates into the host chromosome as a prophage; it is replicated passively along with host DNA through many generations until, triggered by a stimulus, it excises and enters the lytic cycle.
Q207. How does blood circulation occur between the heart and kidneys?
Ans: Oxygenated blood leaves the left ventricle via the aorta and reaches the kidneys through the renal arteries, where filtration occurs; the filtered (deoxygenated, cleaned) blood returns via the renal veins to the inferior vena cava and back to the right atrium of the heart.
Q208. What is feedback inhibition in enzymes? Elaborate with a proper example.
Ans: Feedback inhibition occurs when the end product of a metabolic pathway accumulates and binds to (usually) the first enzyme in the pathway, slowing or stopping further production; for example, high levels of isoleucine inhibit the enzyme threonine deaminase, halting its own synthesis pathway.
Q209. Enlist the parts of the large intestine. Also write the specific roles of the large intestine.
Ans: Parts include the caecum, colon (ascending, transverse, descending, sigmoid) and rectum. Its roles are to absorb water and electrolytes from indigestible food residue, allow bacterial synthesis of some vitamins, and store/compact faeces before elimination.
Q210. What is photoperiodism? How does it affect short-day and long-day plants? Give one example of each.
Ans: Photoperiodism is a plant's physiological response to the relative length of day and night, especially in triggering flowering. Short-day plants flower when day length is below a critical value, e.g., chrysanthemum; long-day plants flower when day length exceeds a critical value, e.g., spinach.
Q211. Briefly explain racemose and cymose types of inflorescence.
Ans: In racemose inflorescence, the main axis continues to grow and produces flowers laterally, with the youngest flowers near the tip (indefinite growth), e.g., mustard; in cymose inflorescence, the main axis terminates in a flower, so growth continues through lateral branches, with the oldest flower at the top/centre (definite growth).
Q212. What is mycorrhizae? Name and differentiate between its two types.
Ans: Mycorrhizae are mutualistic associations between fungi and plant roots. Ectomycorrhizae form a sheath around the root surface without penetrating cells; endomycorrhizae penetrate into the root cortical cells, forming structures like arbuscules within them.
Q213. Name the parts of a bacterial flagellum. Also write their structures.
Ans: The flagellum consists of the filament (a long helical structure made of flagellin protein), the hook (a curved connector), and the basal body (a series of rings anchored in the cell envelope that acts as the rotary motor).
Q214. Briefly describe any three benefits of bacterial flora of humans.
Ans: Normal bacterial flora aid digestion of certain food components, synthesize vitamins such as vitamin K and B-complex vitamins, and prevent colonization by harmful pathogens through competitive exclusion.
Q215. Complete the table (group, character, example) for Whisk fern, jointed stem, Adiantum.
Ans: Whisk fern (Psilotum) lacks true roots and leaves; jointed, ribbed stem is characteristic of Equisetum (horsetail); Adiantum (maidenhair fern) is an example of a true fern with well-developed, divided fronds.
Q216. The gametophyte of ferns is shown in the diagram: (a) what is another name for this structure? (b) label parts A and B. (c) how does the gametophyte of ferns differ from other plants' gametophytes?
Ans: (a) It is called the prothallus. (b) A and B typically represent the antheridium (produces sperm) and archegonium (produces egg). (c) Unlike the highly reduced, dependent gametophytes of seed plants, the fern gametophyte is a small, free-living, independent, photosynthetic structure.
