NEB Class 12 · Past paper
The complete NEB Class 12 2070 exam paper for Biology, all 38 questions with solved model answers.
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What is sclerosis?
Sclerosis is the hardening of a plant cell wall by the deposition of lignin, which converts thin-walled living cells into thick-walled, dead mechanical cells (sclerenchyma).
Define ascent of sap.
Ascent of sap is the upward movement of water and dissolved minerals from the roots to the leaves through the xylem, mainly explained by the transpiration pull (cohesion-tension theory).
Mention two plant hormones.
Auxin (IAA) and Gibberellin (GA). (Others: cytokinin, ethylene, abscisic acid.)
What does it mean by homozygous?
An organism is homozygous when it carries two identical alleles of a gene for a trait, e.g. $TT$ (homozygous dominant) or $tt$ (homozygous recessive).
What do you mean by recessive?
A recessive allele is one whose effect is masked in the presence of its dominant allele; it is expressed phenotypically only in the homozygous ($tt$) condition.
Define sexual reproduction.
Sexual reproduction is the mode of reproduction involving the fusion of male and female gametes (fertilization) to form a zygote, producing offspring that show genetic variation.
Elaborate about female gametophyte.
The female gametophyte of an angiosperm is the embryo sac, a typically 7-celled, 8-nucleate structure containing the egg cell, two synergids, three antipodal cells and a central cell with two polar nuclei.
Point out about anemophily.
Anemophily is pollination by wind; such flowers are usually small, inconspicuous, produce large amounts of light, dry pollen (e.g. maize, grasses).
Significance of green manure.
Green manuring (ploughing green leguminous plants like Sesbania into the soil) enriches the soil with nitrogen and organic matter, improving fertility and soil structure.
Mention about genetic engineering.
Genetic engineering is the deliberate manipulation of an organism's genes using recombinant DNA technology (cutting, joining and transferring genes) to obtain desired characters.
Differentiate between spongy and palisade parenchyma.
Process of glycolysis.
Glycolysis is the anaerobic breakdown of one molecule of glucose into two molecules of pyruvate in the cytoplasm. $$C6H{12}O6 + 2NAD^+ + 2ADP + 2Pi \rightarrow 2CH3COCOOH + 2NADH + 2ATP$$ Glucose is first phosphorylated (using 2 ATP) to ...
Types of RNA and its functions.
Reasons for the selection of pea plant on the Mendel's experiment.
Mendel chose Pisum sativum because it has: (i) clear-cut contrasting characters (tall/dwarf, round/wrinkled seeds); (ii) it is naturally self-pollinating yet can be easily cross-pollinated; (iii) a short life cycle giving many offspring ...
Semi-conservation method of DNA replication.
In semi-conservative replication the two strands of the parent DNA separate, and each old strand acts as a template for a new complementary strand. Thus each daughter DNA molecule contains one parental (old) and one newly synthesised str...
Development of female gametophyte.
A megaspore mother cell in the nucellus undergoes meiosis to form four haploid megaspores; three degenerate and one functional megaspore enlarges. Its nucleus divides mitotically three times to give 8 nuclei, which organise into the 7-ce...
Fermentation technology and its significances.
Fermentation technology uses microorganisms (yeast, bacteria) in controlled bioreactors to convert substrates into useful products. Significance: production of alcohol and beverages, antibiotics (penicillin), organic acids, enzymes, vita...
What is transpiration? Discuss the experiment in detail to demonstrate the unequal transpiration with clean diagram. (Or) Describe the T.S. of monocot stem with well labelled diagram and point out its differences with that of dicot stem.
Transpiration is the loss of water in the form of vapour from the aerial parts (mainly leaves through stomata) of a plant. Experiment to show unequal transpiration (two surfaces of a leaf): A well-watered potted plant leaf is smeared wit...
Describe the sex linked inheritance with special reference to the eye colour of Drosophila melanogaster (fruit fly).
Sex-linked inheritance is the inheritance of genes located on the sex chromosomes (mainly the X chromosome), so the trait's transmission is linked to the sex of the individual.
In Drosophila, red eye (W) is dominant over white eye (w) and the gene lies on the X chromosome. Morgan crossed a red-eyed female ($X^W X^W$) with a white-eyed male ($X^w Y$):
In a reciprocal cross (white female $X^w X^w$ × red male $X^W Y$), F1 females are red-eyed and all F1 males are white-eyed, showing criss-cross inheritance (father's trait passes to daughters and mother's trait to sons).
What is ligament?
A ligament is a tough band of white fibrous / yellow elastic connective tissue that joins bone to bone at a joint and holds it in place.
Name the pigments found in bile.
Bilirubin (yellow) and biliverdin (green) are the bile pigments.
What is the location of Adam's apple?
The Adam's apple is the projection of the thyroid cartilage of the larynx in the front of the neck (more prominent in males).
Which hormone is useful for retaining salt in body?
Aldosterone, a mineralocorticoid secreted by the adrenal cortex, promotes reabsorption of sodium (salt) and water in the kidney.
Name two neurotransmitters.
Acetylcholine and Noradrenaline (norepinephrine). (Others: dopamine, serotonin, GABA.)
Which system of body is affected by Tuberculosis?
Pulmonary tuberculosis mainly affects the respiratory system (lungs), though Mycobacterium tuberculosis can also infect bones, lymph nodes and other organs.
What is sperm lysine?
Sperm lysine is an enzyme released from the acrosome of the sperm that dissolves the egg membranes (zona pellucida), enabling the sperm to penetrate the egg during fertilization.
Why ultra filtration is called so?
It is called ultrafiltration because blood is filtered under high hydrostatic (glomerular) pressure through the fine glomerular capillaries and Bowman's capsule, forcing out water and small solutes while retaining cells and proteins.
What is glycogenolysis?
Glycogenolysis is the breakdown of stored glycogen into glucose (glucose-1-phosphate) in the liver and muscles to raise blood glucose.
What do you mean by vaccination?
Vaccination is the introduction of a vaccine (weakened, killed or antigenic part of a pathogen) into the body to stimulate the immune system to produce antibodies and provide active acquired immunity against that disease.
Working mechanism of human eye.
Light rays from an object pass through the cornea, aqueous humour, pupil and lens, which refracts and focuses them to form a real, inverted image on the retina. Rod and cone cells (photoreceptors) convert the light into nerve impulses; t...
Ill effects of alcohol on health.
Chronic alcohol use causes liver damage (cirrhosis, fatty liver), gastritis and ulcers, high blood pressure and heart disease, damage to the nervous system (loss of memory, coordination), nutritional deficiency, and social/psychological ...
Differentiate between inspiration and expiration.
The importance of poultry farming in Nepal.
Poultry farming provides nutritious eggs and meat (protein), generates employment and income for farmers, needs relatively low investment and space, supplies manure, and supports allied industries, thereby contributing to the national ec...
Natural pace maker.
The sino-atrial (SA) node, located in the wall of the right atrium, is the natural pacemaker of the heart. It spontaneously generates rhythmic electrical impulses (about 70-75 per minute) that spread over the atria to the AV node and ini...
Advantages of organ transplantation.
Organ transplantation replaces a diseased or non-functional organ (kidney, heart, liver, cornea) with a healthy donor organ, restoring normal function and saving or prolonging the patient's life, improving quality of life, and reducing d...
Blastula formation in frog.
After fertilization the zygote undergoes repeated cleavage (holoblastic, unequal) forming a solid morula, and continued division creates a hollow, fluid-filled cavity called the blastocoel. The resulting single-layered, hollow spherical ...
Describe the respiration in human beings with suitable diagrams. (Or) Write, what you know about the AIDS?
Respiration in humans occurs through the respiratory system: nostrils, pharynx, larynx, trachea, bronchi, bronchioles and alveoli of the lungs. - External respiration (breathing): during inspiration the diaphragm and external intercostal...
Discuss the physiology of digestion in human beings.
Digestion is the breakdown of complex food into absorbable simple molecules along the alimentary canal.
An organism is homozygous when it carries two identical alleles of a gene for a trait, e.g. (homozygous dominant) or (homozygous recessive).
A recessive allele is one whose effect is masked in the presence of its dominant allele; it is expressed phenotypically only in the homozygous () condition.
Glycolysis is the anaerobic breakdown of one molecule of glucose into two molecules of pyruvate in the cytoplasm. Glucose is first phosphorylated (using 2 ATP) to ...
Sex-linked inheritance is the inheritance of genes located on the sex chromosomes (mainly the X chromosome), so the trait's transmission is linked to the sex of the individual.
In Drosophila, red eye (W) is dominant over white eye (w) and the gene lies on the X chromosome. Morgan crossed a red-eyed female () with a white-eyed male ():
In a reciprocal cross (white female × red male ), F1 females are red-eyed and all F1 males are white-eyed, showing criss-cross inheritance (father's trait passes to daughters and mother's trait to sons).