NEB Class 11 Science Biology Question Paper 2078 Nepal

In DNA, adenine pairs with thymine (A=T) via two hydrogen bonds. Answer: (d) A = T.

The Miller–Urey experiment used a mixture of ammonia, methane, hydrogen and water vapour. Answer: (b).

Xerophytes are adapted to dry conditions (scarcity of water). The best answer is a habitat with dry condition. Answer: (a).

Since red rust of tea is caused by an alga (Cephaleuros), the relevant discipline is phycology (the study of algae). Answer: (d) Phycology.

In-situ conservation protects species in their natural habitat — e.g. a wildlife sanctuary. (Botanical garden, zoo and seed bank are ex-situ.) Answer: (b) Wildlife sanctuary.

Prop roots (e.g. in banyan) grow down from branches to the ground and give mechanical support to the heavy plant. Answer: (a) It has the function of support.

These wings perform the same function (flight) but have different structural origins/designs, so they are analogous organs (showing convergent evolution). Answer: (c) Analogous.

The nearest (most recent) ancestor among the options is Homo sapiens neanderthalensis. Answer: (a).

X is the vocal sac of a male frog. The croaking sound produced is the mating call used to attract females, indicating sex appeal. Answer: (d) Sex appeal.

Trichocysts are used by Paramecium for defence, offence (capturing prey) and anchorage/adhesion. Answer: (a) Defense, offence, and adhesion.

Given the high risk and population decline described, the one-horned rhino is classified as Endangered. Answer: (c) Endangered. (IUCN lists the greater one-horned rhino as Vulnerable globally, but per the description of high risk the intended answer here is Endangered/critically endangered.)

Characteristics of Chlorophyceae (green algae): (1) they contain chlorophyll a and b, giving a grass-green colour; (2) the cell wall is made of an inner cellulose and outer pectose layer; (3) the reserve food is starch (stored in pyrenoids); (4) they are mostly aquatic (fresh water), some terrestrial; (5) the body is unicellular to filamentous/colonial (e.g. Chlamydomonas, Spirogyra, Volvox, Ulothrix); (6) motile cells/gametes bear two equal whiplash flagella; reproduction is vegetative, asexual (zoospores) and sexual.

Gymnosperm advanced over Pteridophyta (any five): (1) Gymnosperms are seed-bearing (seed habit) whereas pteridophytes reproduce by spores; (2) gymnosperms show heterospory leading to seed formation; (3) pollination by wind — no water needed for fertilisation, unlike pteridophytes which need water for swimming sperms; (4) development of pollen tube (siphonogamy); (5) well-developed vascular tissue and secondary growth; (6) reduced, dependent gametophyte; (7) ovule and naked seed present.

OR — Simple vs multiple fruit (two points): A simple fruit develops from a single ovary of a single flower (e.g. mango); a multiple fruit develops from the whole inflorescence (many flowers fusing together, e.g. jackfruit, pineapple). Fruit aids continuity of generation by protecting the developing seeds and aiding their dispersal (by wind, water, animals), ensuring the species spreads and reproduces.

A pond is a self-sufficient freshwater ecosystem. Structural aspects: abiotic factors (light, temperature, dissolved O₂/CO₂, pH, nutrients, water) and biotic components — producers (phytoplankton, algae, aquatic plants), consumers (zooplankton, insects, fish — primary/secondary/tertiary) and decomposers (bacteria, fungi). Functional aspects: food chains/webs, energy flow (sunlight → producers → consumers), nutrient (biogeochemical) cycling, and productivity. Influence of ecological factors: e.g. light intensity controls photosynthesis (productivity); temperature affects metabolic rates and dissolved oxygen; nutrient levels affect algal growth (eutrophication if excessive); dissolved oxygen limits animal survival. Thus abiotic factors regulate the structure and functioning of the pond.

Roles of microorganisms in medicine (any five): (1) production of antibiotics (e.g. penicillin from Penicillium, streptomycin from Streptomyces) to treat bacterial infections; (2) production of vaccines (using attenuated/killed microbes) to immunise against diseases; (3) production of vitamins (e.g. B12, riboflavin) and other supplements; (4) production of enzymes and hormones (e.g. insulin via genetically engineered bacteria); (5) production of serums/antitoxins; (6) use in medical research and diagnosis; (7) production of organic acids and probiotics. (Any five.)

Three consequences of pesticide overuse: (1) soil and water pollution (contamination of drinking water and rivers); (2) harm to non-target organisms — bees, birds, fish, beneficial insects — and loss of biodiversity; (3) bioaccumulation/biomagnification in the food chain causing health hazards (cancer, poisoning) in humans; pesticide residues in food; development of pest resistance.

Awareness campaign (two points): (1) organise community education programmes/posters/street drama on safe use and alternatives (integrated pest management, organic/bio-pesticides); (2) train farmers on correct dosage, protective measures and promotion of organic farming, and lobby for regulation of harmful pesticides.

Transverse binary fission: An asexual reproduction in which the parent body divides into two equal daughter individuals by a division plane that runs transverse (at right angles) to the longitudinal axis.

Process in Paramecium: (1) The micronucleus divides first by mitosis (amitosis of macronucleus); the macronucleus elongates and divides amitotically. (2) A transverse constriction (furrow) appears at the middle of the body, deepening across the transverse axis. (3) The two contractile vacuoles and other organelles are distributed; new oral grooves form. (4) The body finally divides transversely into two daughter Paramecia, each with a micronucleus and macronucleus, which then grow to full size. It occurs in favourable conditions.

Pancreas in frog digestion: The pancreas is a mixed gland; its exocrine part secretes pancreatic juice into the duodenum (via the bile-pancreatic duct). Pancreatic juice contains enzymes — trypsin (proteins → peptides/amino acids), amylase (starch → maltose), and lipase (fats → fatty acids + glycerol) — and it is alkaline, neutralising the acidic chyme. Thus in intestinal digestion the pancreas provides the major digestive enzymes that complete the breakdown of carbohydrates, proteins and fats in the small intestine. (The endocrine part secretes insulin/glucagon for blood-sugar regulation.)

OR — Earthworm copulation & cocoon formation: Earthworms are hermaphrodite but cross-fertilise. During copulation two worms come together with their ventral surfaces in opposite directions, held by the clitellum and genital setae; they exchange sperm, which is stored in each other's spermathecae. After separation, the clitellum secretes a mucous cocoon (a girdle) which slides forward over the body; as it passes, it receives ova from the female pores and stored sperm from the spermathecae, where fertilisation occurs. The cocoon then slips off the anterior end, its ends seal, and it is deposited in moist soil where the young develop.

Darwinism (natural selection) explains organic evolution through: (1) overproduction — organisms produce far more offspring than can survive; (2) struggle for existence — limited resources cause competition (intraspecific, interspecific, environmental); (3) variation — individuals show heritable variations; (4) survival of the fittest / natural selection — individuals with favourable variations are better adapted, survive and reproduce, passing on those traits; (5) inheritance and origin of new species — over many generations, accumulation of favourable variations leads to new, better-adapted forms and ultimately new species. Thus nature 'selects' the fittest, driving evolution.

Importance of sexual reproduction in fungi: it introduces genetic variation (recombination) which aids adaptation and evolution; it restores the diploid phase and produces resistant resting structures (spores) for survival in unfavourable conditions and dispersal.

Sexual reproduction in Yeast: Under unfavourable conditions, two haploid yeast cells (or a cell with a bud) act as gametes; they fuse — plasmogamy (fusion of cytoplasm) followed by karyogamy (fusion of nuclei) — forming a diploid zygote. The diploid nucleus then undergoes meiosis to form four (or eight) haploid ascospores enclosed within the parent cell wall, which now acts as an ascus. On release in favourable conditions, the ascospores germinate into new haploid yeast cells.

OR — Solanaceae: Diagnostic floral characters: flowers bisexual, actinomorphic, hypogynous; calyx 5 sepals (gamosepalous, persistent); corolla 5 petals (gamopetalous); androecium 5 stamens epipetalous; gynoecium bicarpellary, syncarpous, superior ovary with axile placentation, obliquely placed. Floral formula: $\oplus\ \male\female\ K_{(5)}\ C_{(5)}\ A_5\ \underline{G}_{(2)}$. Examples: Solanum tuberosum (potato — edible), Atropa belladonna / Withania somnifera (medicinal); Solanum melongena (brinjal — edible).

Excretion in earthworm: carried out by segmentally arranged coiled tubular structures called nephridia (three types: septal, integumentary, pharyngeal). They remove nitrogenous wastes (urea, ammonia) and excess water.

Septal nephridia (structure): located on both sides of the inter-segmental septa from segment 15 backward; each consists of (1) the nephrostome (a ciliated funnel opening into the coelom that draws in coelomic fluid), (2) a short neck, (3) the body of the nephridium — a long twisted tube differentiated into a straight lobe and a twisted loop with ciliated and glandular regions, (4) the terminal duct leading to a common septal excretory canal, which opens through supra-intestinal excretory ducts into the gut (enteronephric type).

Function: coelomic fluid and wastes enter via the nephrostome; as the fluid passes along the tubule, water and useful materials are reabsorbed and nitrogenous wastes are concentrated; the waste is finally discharged into the alimentary canal and expelled with faeces (enteronephric excretion conserves water — an adaptation to terrestrial life).

(Labelled diagram of septal nephridium: nephrostome, neck, body, terminal duct — to be drawn.)

(a) Fig-1 shows Metaphase I — homologous chromosomes (bivalents/tetrads) are arranged in pairs at the equatorial plate, spindle fibres attached. Fig-2 shows Anaphase I — the homologous chromosomes of each bivalent disjoin (separate) and move to opposite poles (whole chromosomes, each still with two chromatids, move apart). The change is the disjunction/segregation of homologous chromosomes, reducing the chromosome number to half (reduction division) and distributing one of each pair to each pole, giving genetic variation.

(b) If disjunction did not occur (non-disjunction), the homologous chromosomes would not separate properly, the gametes would have an abnormal chromosome number (aneuploidy) — chromosomal instability — leading to disorders (e.g. Down's syndrome) and loss of chromosomal stability in offspring.

(c) The next step is Telophase I (followed by cytokinesis): two distinguishing features — (1) chromosomes reach the two opposite poles and a nuclear membrane reforms around each haploid set; (2) the cell divides (cytokinesis) into two haploid daughter cells. (Diagram to be drawn.)