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PSEB SS Biology study materials
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PSEB SS Biology practice questions

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Question 1Biology (Senior Secondary / Class 12 science subject)

In the human male reproductive system, which structure stores and matures sperm before ejaculation?

  1. Epididymis Correct answer.
  2. Seminal vesicle
  3. Prostate gland
  4. Vas deferens

Explanation

The epididymis is a coiled tube on the posterior surface of each testis where sperm complete maturation and are stored until ejaculation. The vas deferens only transports sperm, while the seminal vesicle and prostate are accessory glands that add secretions to semen.

Question 2Biology (Senior Secondary / Class 12 science subject)

Which hormone surge directly triggers ovulation in the human menstrual cycle?

  1. Follicle-stimulating hormone
  2. Progesterone
  3. Prolactin
  4. Luteinising hormone Correct answer.

Explanation

A sharp mid-cycle surge of luteinising hormone (LH) causes the mature Graafian follicle to rupture and release the oocyte. FSH supports follicle growth earlier in the cycle, while progesterone rises after ovulation to maintain the endometrium.

Question 3Biology (Senior Secondary / Class 12 science subject)

During which stage of meiosis does crossing over between homologous chromosomes occur?

  1. Metaphase I
  2. Anaphase II
  3. Telophase II
  4. Pachytene of prophase I Correct answer.

Explanation

Crossing over takes place at the pachytene stage of prophase I, when non-sister chromatids of homologous chromosomes exchange segments at chiasmata. This recombination is a major source of genetic variation in gametes.

Question 4Biology (Senior Secondary / Class 12 science subject)

A test cross is performed to determine the genotype of a tall pea plant. If half the offspring are tall and half are dwarf, what was the genotype of the tested plant?

  1. Heterozygous tall (Tt) Correct answer.
  2. Homozygous tall (TT)
  3. Cannot be inferred from the ratio
  4. Homozygous dwarf (tt)

Explanation

A test cross uses a homozygous recessive partner (tt). A 1:1 tall-to-dwarf ratio means the tested plant contributed a t allele half the time, so it must be heterozygous Tt. A TT parent would give all tall offspring.

Question 5Biology (Senior Secondary / Class 12 science subject)

Which of the following is an example of a sex-linked disorder inherited through the X chromosome?

  1. Haemophilia Correct answer.
  2. Thalassaemia
  3. Sickle-cell anaemia
  4. Phenylketonuria

Explanation

Haemophilia is caused by a recessive allele on the X chromosome, so it appears far more often in males who carry only one X. The other conditions listed are autosomal disorders, not sex-linked.

Question 6Biology (Senior Secondary / Class 12 science subject)

In the lac operon of E. coli, what is the role of the repressor protein produced by the i gene?

  1. It cleaves lactose into glucose and galactose
  2. It codes for the permease enzyme
  3. It binds the operator and blocks transcription when lactose is absent Correct answer.
  4. It activates RNA polymerase at the promoter

Explanation

The i gene product is a repressor that binds the operator region in the absence of lactose, preventing RNA polymerase from transcribing the structural genes. When lactose is present, its inducer allolactose inactivates the repressor and transcription proceeds.

Question 7Biology (Senior Secondary / Class 12 science subject)

Which enzyme is responsible for joining Okazaki fragments on the lagging strand during DNA replication?

  1. DNA ligase Correct answer.
  2. DNA helicase
  3. Primase
  4. DNA polymerase III

Explanation

DNA ligase catalyses the formation of phosphodiester bonds that seal the nicks between adjacent Okazaki fragments. Helicase unwinds the double helix, polymerase extends chains, and primase lays down RNA primers.

Question 8Biology (Senior Secondary / Class 12 science subject)

A DNA segment has the sequence 5'-ATGCGT-3'. What is the sequence of its complementary strand written in the conventional 5' to 3' direction?

  1. 5'-ACGCAT-3' Correct answer.
  2. 5'-ATGCGT-3'
  3. 5'-UACGCA-3'
  4. 5'-TACGCA-3'

Explanation

The complementary strand is antiparallel, so it pairs A with T, T with A, G with C and C with G, and the strand direction reverses. Reading the complement in the 5' to 3' direction gives ACGCAT.

Question 9Biology (Senior Secondary / Class 12 science subject)

Which of the following best describes the process of transcription in eukaryotes?

  1. Synthesis of protein from mRNA on ribosomes
  2. Removal of introns from pre-mRNA
  3. Synthesis of mRNA from a DNA template catalysed by RNA polymerase Correct answer.
  4. Replication of DNA before cell division

Explanation

Transcription is the synthesis of an RNA molecule using one DNA strand as a template, carried out by RNA polymerase. Translation and splicing are separate later steps in gene expression.

Question 10Biology (Senior Secondary / Class 12 science subject)

In the genetic code, what property ensures that a single base substitution does not always change the amino acid specified?

  1. Degeneracy of the code Correct answer.
  2. Non-overlapping nature of the code
  3. Comma-less nature of the code
  4. Universality of the code

Explanation

Degeneracy means that most amino acids are coded by more than one codon, often differing in the third base. A substitution in that position may therefore specify the same amino acid, a phenomenon called silent mutation.

Question 11Biology (Senior Secondary / Class 12 science subject)

Which of the following is a correct description of the theory of natural selection as proposed by Darwin?

  1. Individuals with favourable heritable variations survive and reproduce more successfully Correct answer.
  2. All individuals in a population have equal reproductive success
  3. Variations arise only in response to a need felt by the organism
  4. Acquired characters during a lifetime are passed to offspring

Explanation

Darwin proposed that heritable variations exist within populations and that those conferring an advantage in the environment lead to greater survival and reproduction. Over generations, such favourable traits increase in frequency.

Question 12Biology (Senior Secondary / Class 12 science subject)

Which of the following is an example of an analogous structure?

  1. Thorns of Bougainvillea and tendrils of Cucurbita
  2. Wings of a butterfly and wings of a bird Correct answer.
  3. Vertebrate hearts and brains
  4. Forelimbs of a whale and forelimbs of a human

Explanation

Analogous structures perform similar functions but have different evolutionary origins, as with insect and bird wings used for flight. Homologous structures share a common ancestry but may differ in function.

Question 13Biology (Senior Secondary / Class 12 science subject)

Which of the following statements about the human immune system is correct?

  1. Antibodies are produced by plasma cells derived from B lymphocytes Correct answer.
  2. Interferons are antibodies that neutralise bacterial toxins
  3. T lymphocytes secrete antibodies into the blood plasma
  4. Innate immunity involves memory cells that give lifelong protection

Explanation

B lymphocytes, when activated, differentiate into plasma cells that synthesise and secrete antibodies specific to an antigen. T cells mediate cell-mediated immunity rather than secreting antibodies.

Question 14Biology (Senior Secondary / Class 12 science subject)

A person with blood group A receives blood from a donor of blood group B. What is the most likely consequence?

  1. No reaction occurs because both groups are compatible
  2. The recipient's anti-A antibodies attack the donor cells
  3. The donor's anti-A antibodies destroy the recipient's red blood cells
  4. The recipient's anti-B antibodies agglutinate the donor red blood cells Correct answer.

Explanation

Group A blood contains anti-B antibodies in the plasma. When group B red cells carrying B antigens enter, the recipient's anti-B antibodies cause agglutination and haemolysis, making this a mismatched transfusion.

Question 15Biology (Senior Secondary / Class 12 science subject)

Which part of the human brain regulates body temperature, hunger and the sleep-wake cycle?

  1. Cerebrum
  2. Cerebellum
  3. Medulla oblongata
  4. Hypothalamus Correct answer.

Explanation

The hypothalamus is the master control centre for homeostasis, containing centres for thermoregulation, hunger, thirst and circadian rhythms. The cerebellum coordinates movement, the medulla controls vital reflexes, and the cerebrum handles higher functions.

Question 16Biology (Senior Secondary / Class 12 science subject)

Which of the following correctly describes the mechanism of action of a steroid hormone such as cortisol?

  1. It binds a cell-surface receptor and generates cAMP
  2. It acts as a neurotransmitter at the synapse
  3. It opens ligand-gated ion channels in the plasma membrane
  4. It enters the cell, binds a receptor and regulates gene transcription Correct answer.

Explanation

Steroid hormones are lipid-soluble and pass through the plasma membrane to bind intracellular receptors. The hormone-receptor complex enters the nucleus and regulates transcription of specific genes, producing effects that develop slowly but last longer.

Question 17Biology (Senior Secondary / Class 12 science subject)

In a nephron, in which region is the filtrate most concentrated due to the counter-current mechanism?

  1. The Bowman's capsule
  2. The proximal convoluted tubule
  3. The loop of Henle in the renal medulla Correct answer.
  4. The distal convoluted tubule

Explanation

The long loop of Henle establishes an osmotic gradient in the renal medulla through the counter-current multiplier mechanism, allowing water reabsorption and concentration of urine. Filtration occurs at the Bowman's capsule, and most reabsorption in the proximal tubule is isosmotic.

Question 18Biology (Senior Secondary / Class 12 science subject)

Which of the following is the correct sequence of events during muscle contraction?

  1. ATP hydrolysis occurs before calcium release and prevents cross-bridge formation
  2. Calcium binds troponin, myosin heads attach to actin, and sarcomeres shorten Correct answer.
  3. Acetylcholine enters the muscle fibre and directly binds actin
  4. Myosin heads detach permanently from actin when calcium rises

Explanation

Nerve stimulation releases calcium from the sarcoplasmic reticulum; calcium binds troponin, moving tropomyosin to expose actin binding sites. Myosin heads then form cross-bridges and pull actin filaments toward the centre, shortening the sarcomere.

Question 19Biology (Senior Secondary / Class 12 science subject)

In C3 plants, which enzyme fixes CO2 during the Calvin cycle?

  1. Nitrogenase
  2. ATP synthase
  3. RuBisCO Correct answer.
  4. PEP carboxylase

Explanation

RuBisCO catalyses the carboxylation of ribulose-1,5-bisphosphate with CO2 to form two molecules of 3-phosphoglycerate in C3 plants. PEP carboxylase is the primary fixer in C4 and CAM plants.

Question 20Biology (Senior Secondary / Class 12 science subject)

Which of the following is a characteristic difference between C4 and C3 photosynthesis?

  1. C3 plants have Kranz anatomy with bundle sheath cells
  2. C4 plants lack the Calvin cycle entirely
  3. C4 plants fix CO2 only at night
  4. C4 plants show little photorespiration because CO2 is concentrated around RuBisCO Correct answer.

Explanation

In C4 plants, CO2 is first fixed by PEP carboxylase in mesophyll cells and released in bundle sheath cells, maintaining a high CO2 concentration around RuBisCO. This suppresses the oxygenase activity of RuBisCO and greatly reduces photorespiration.

Question 21Biology (Senior Secondary / Class 12 science subject)

In aerobic respiration, which statement correctly describes the role of oxygen?

  1. It acts as the final electron acceptor in the electron transport chain Correct answer.
  2. It donates electrons to NADH dehydrogenase
  3. It is produced during the Krebs cycle
  4. It directly phosphorylates ADP to ATP

Explanation

Oxygen accepts electrons and protons at the end of the electron transport chain, forming water and allowing the chain to keep running so ATP synthesis continues. Without oxygen, the chain and oxidative phosphorylation halt.

Question 22Biology (Senior Secondary / Class 12 science subject)

Which of the following is a correct statement about the human respiratory system?

  1. The trachea is the site of gas exchange
  2. The diaphragm relaxes during inspiration
  3. The partial pressure of oxygen is higher in alveolar air than in deoxygenated blood entering the lungs Correct answer.
  4. Carbon dioxide diffuses from the alveoli into the blood

Explanation

Alveolar PO2 is about 104 mm Hg while deoxygenated blood arriving at the lungs has a PO2 near 40 mm Hg, so oxygen diffuses into the blood down its gradient. Carbon dioxide moves in the opposite direction, from blood to alveoli.

Question 23Biology (Senior Secondary / Class 12 science subject)

Which of the following is an example of a biotechnology application using recombinant DNA technology?

  1. Brewing of beer using yeast
  2. Production of human insulin by bacteria carrying the human insulin gene Correct answer.
  3. Fermentation of milk to curd by Lactobacillus
  4. Production of biogas from cattle dung

Explanation

Recombinant DNA technology joins a human insulin gene to a vector, which is introduced into bacteria so they produce insulin. The other examples rely on traditional microbial fermentation rather than recombinant DNA.

Question 24Biology (Senior Secondary / Class 12 science subject)

A gene of interest is inserted into a plasmid vector. Which feature of the plasmid is essential for selecting transformed bacterial cells?

  1. A restriction enzyme recognition site
  2. The origin of replication
  3. A selectable marker gene conferring antibiotic resistance Correct answer.
  4. A promoter for the inserted gene

Explanation

A selectable marker such as an antibiotic resistance gene allows transformed cells to grow on a medium containing the antibiotic while non-transformed cells die. The origin of replication supports replication but does not by itself allow selection.

Question 25Biology (Senior Secondary / Class 12 science subject)

Which of the following is an example of an in-situ conservation strategy for biodiversity?

  1. Maintenance of plants in a botanical garden
  2. Cryopreservation of seeds in a gene bank
  3. Storage of gametes in liquid nitrogen
  4. Establishment of a biosphere reserve Correct answer.

Explanation

In-situ conservation protects species within their natural habitat, as in biosphere reserves, national parks and wildlife sanctuaries. Gene banks, cryopreservation and botanical gardens are ex-situ methods that conserve organisms outside their natural environment.

Question 26Biology (Senior Secondary / Class 12 science subject)

Which of the following components is found in a DNA nucleotide but not in an RNA nucleotide?

  1. Deoxyribose sugar Correct answer.
  2. Uracil base
  3. Ribose sugar
  4. Phosphate group

Explanation

A DNA nucleotide contains deoxyribose, a phosphate group and one of the bases adenine, thymine, guanine or cytosine. RNA nucleotides contain ribose instead of deoxyribose and uracil instead of thymine, so only deoxyribose distinguishes DNA from RNA at the sugar level.

Question 27Biology (Senior Secondary / Class 12 science subject)

In a dihybrid cross between two heterozygous pea plants (RrYy x RrYy), what is the expected phenotypic ratio in the offspring, assuming independent assortment?

  1. 3:1
  2. 1:1:1:1
  3. 9:3:3:1 Correct answer.
  4. 1:2:1

Explanation

Independent assortment of two gene pairs during meiosis produces four gamete types in equal frequency, and random fertilisation gives the classic 9:3:3:1 phenotypic ratio. This assumes the genes are on different chromosomes or far apart on the same chromosome.

Question 28Biology (Senior Secondary / Class 12 science subject)

Which of the following is the site of synthesis of the ribosomal subunits in a eukaryotic cell?

  1. Smooth endoplasmic reticulum
  2. Lysosome
  3. Nucleolus Correct answer.
  4. Golgi apparatus

Explanation

Ribosomal RNA is transcribed and assembled with ribosomal proteins in the nucleolus, a dense region within the nucleus. The assembled subunits then pass through nuclear pores into the cytoplasm to form functional ribosomes.

Question 29Biology (Senior Secondary / Class 12 science subject)

During which phase of mitosis do sister chromatids separate and move to opposite poles of the cell?

  1. Prophase
  2. Telophase
  3. Anaphase Correct answer.
  4. Metaphase

Explanation

In anaphase the centromeres split and the spindle fibres shorten, pulling sister chromatids apart toward opposite poles. By contrast, chromosomes align at the equator in metaphase and nuclear envelopes reform in telophase.

Question 30Biology (Senior Secondary / Class 12 science subject)

A woman who is a carrier of an X-linked recessive disorder marries an unaffected man. What proportion of their sons is expected to be affected?

  1. 50% Correct answer.
  2. 0%
  3. 100%
  4. 25%

Explanation

Sons inherit their single X chromosome from the mother, so each son has a one-in-two chance of receiving the mutant allele and being affected. Daughters receive a normal X from the father and can at worst be carriers.

Question 31Biology (Senior Secondary / Class 12 science subject)

Which of the following structures in a flowering plant develops into the fruit after fertilisation?

  1. Stigma
  2. Ovule
  3. Ovary Correct answer.
  4. Anther

Explanation

After fertilisation the ovary wall thickens to form the pericarp of the fruit, while the ovules inside develop into seeds. The other floral parts usually wither and fall away.

Question 32Biology (Senior Secondary / Class 12 science subject)

Which pigment absorbs light most strongly in the blue-violet and red regions and reflects green light?

  1. Carotene
  2. Xanthophyll
  3. Chlorophyll a Correct answer.
  4. Phycobilin

Explanation

Chlorophyll a has absorption peaks in the blue-violet and red parts of the spectrum and reflects green wavelengths, which is why leaves appear green. Accessory pigments absorb in other regions and transfer energy to chlorophyll a.

Question 33Biology (Senior Secondary / Class 12 science subject)

In the light reactions of photosynthesis, what is the immediate source of electrons that replaces those lost by photosystem II?

  1. Oxidation of NADPH
  2. Photolysis of water Correct answer.
  3. Reduction of plastoquinone
  4. Splitting of carbon dioxide

Explanation

Photosystem II loses electrons to the electron transport chain and replaces them by splitting water in a reaction catalysed by the oxygen-evolving complex. This photolysis releases electrons, protons and molecular oxygen.

Question 34Biology (Senior Secondary / Class 12 science subject)

Which of the following is the correct balanced equation for aerobic respiration of one molecule of glucose?

  1. C6H12O6 + 6O2 → 6CO2 + 6H2O + energy Correct answer.
  2. C6H12O6 → 2C2H5OH + 2CO2 + energy
  3. 6CO2 + 6H2O → C6H12O6 + 6O2
  4. C6H12O6 + 6CO2 → 6O2 + 6H2O + energy

Explanation

Aerobic respiration completely oxidises glucose using oxygen, releasing carbon dioxide, water and usable energy. The equation balances six carbons, twelve hydrogens and eighteen oxygens on both sides.

Question 35Biology (Senior Secondary / Class 12 science subject)

Which of the following statements about the light-dependent and light-independent reactions of photosynthesis is correct?

  1. The light reactions fix carbon dioxide directly
  2. Both reactions occur only in the dark
  3. The light reactions supply ATP and NADPH to the Calvin cycle Correct answer.
  4. The Calvin cycle produces ATP that is used in the light reactions

Explanation

The light reactions convert light energy into chemical energy as ATP and NADPH, which are then consumed by the Calvin cycle to reduce carbon dioxide. The two stages are linked by these energy carriers rather than by a direct exchange of gases.

Question 36Biology (Senior Secondary / Class 12 science subject)

Which of the following describes the role of the sinoatrial node in the human heart?

  1. It generates the electrical impulse that initiates each heartbeat Correct answer.
  2. It increases the heart rate during exercise
  3. It carries oxygenated blood from the lungs to the heart
  4. It prevents backflow of blood from the ventricle into the atrium

Explanation

The sinoatrial node is a patch of specialised cardiac muscle in the right atrium that spontaneously depolarises and sets the rhythm of the heart. Because it initiates impulses without external input, it is called the natural pacemaker.

Question 37Biology (Senior Secondary / Class 12 science subject)

Which of the following is the main function of the loop of Henle in the mammalian kidney?

  1. Reabsorbing glucose and amino acids from the filtrate
  2. Establishing a concentration gradient in the medulla for water reabsorption Correct answer.
  3. Filtering blood plasma under high pressure
  4. Secreting hydrogen ions to regulate blood pH

Explanation

The loop of Henle creates a hypertonic medullary interstitium through countercurrent multiplication, allowing water to be reabsorbed from the collecting duct. This mechanism is essential for producing concentrated urine.

Question 38Biology (Senior Secondary / Class 12 science subject)

Which of the following correctly describes the role of tRNA in translation?

  1. It removes introns from the pre-mRNA transcript
  2. It carries specific amino acids to the ribosome and pairs with mRNA codons Correct answer.
  3. It forms the structural core of the ribosome
  4. It copies the DNA template into messenger RNA

Explanation

Each tRNA has an anticodon loop that base-pairs with a complementary codon on the mRNA and an attachment site for a specific amino acid. This adaptor function ensures amino acids are added in the order encoded by the mRNA.

Question 39Biology (Senior Secondary / Class 12 science subject)

Which of the following is an example of a transgenic organism produced by recombinant DNA technology?

  1. A mule produced by crossing a horse and a donkey
  2. Bt cotton that carries a bacterial gene for insect resistance Correct answer.
  3. A seedless watermelon produced by hybridisation
  4. A micropropagated banana plant grown from tissue culture

Explanation

Bt cotton is transgenic because a gene from Bacillus thuringiensis has been inserted into its genome using recombinant DNA techniques so that the plant produces an insecticidal protein. The other examples involve hybridisation or clonal propagation without foreign gene transfer.

Question 40Biology (Senior Secondary / Class 12 science subject)

A restriction enzyme recognises the sequence GAATTC and cuts between G and A. Which feature makes this enzyme useful in cloning?

  1. It adds methyl groups to protect DNA from digestion
  2. It synthesises a complementary DNA strand from an RNA template
  3. It cuts DNA at a specific palindromic sequence, generating defined fragments Correct answer.
  4. It joins two DNA fragments by forming phosphodiester bonds

Explanation

Restriction endonucleases recognise specific palindromic sequences and cleave at defined positions, producing predictable fragments that can be ligated into vectors. The specificity allows reproducible construction of recombinant molecules.

Question 41Biology (Senior Secondary / Class 12 science subject)

Which of the following best describes the principle of the polymerase chain reaction?

  1. Proteins are separated according to their charge and size in a gel
  2. Repeated cycles of denaturation, primer annealing and extension amplify a target DNA sequence Correct answer.
  3. A DNA probe hybridises to a target sequence to detect its presence
  4. Foreign DNA is inserted into a plasmid and cloned in a bacterial host

Explanation

PCR uses thermal cycling to separate the DNA strands, anneal sequence-specific primers and extend them with a thermostable DNA polymerase, doubling the target each cycle. The result is exponential amplification of a chosen region.

Question 42Biology (Senior Secondary / Class 12 science subject)

Which of the following observations best supports the theory of evolution by common descent?

  1. The seasonal change in coat colour of an arctic fox
  2. The ability of bacteria to acquire antibiotic resistance
  3. Similar forelimb bone arrangements in humans, whales and bats Correct answer.
  4. Different beak shapes in finches on different islands of one archipelago

Explanation

Homologous structures such as the pentadactyl forelimb of humans, whales and bats share the same underlying bone pattern despite different functions, indicating inheritance from a common ancestor. This structural correspondence is direct anatomical evidence of common descent.

Question 43Biology (Senior Secondary / Class 12 science subject)

In a population in Hardy-Weinberg equilibrium, the frequency of a recessive allele is 0.2. What is the expected frequency of heterozygous individuals?

  1. 0.04
  2. 0.32 Correct answer.
  3. 0.64
  4. 0.16

Explanation

If q = 0.2, then p = 0.8, and the heterozygote frequency is 2pq = 2 x 0.8 x 0.2 = 0.32. The Hardy-Weinberg equation p² + 2pq + q² = 1 gives the genotype frequencies directly from the allele frequencies.

Question 44Biology (Senior Secondary / Class 12 science subject)

Which of the following is a characteristic feature of the phylum Arthropoda?

  1. Jointed appendages and a chitinous exoskeleton Correct answer.
  2. Radial symmetry in the adult stage
  3. A water vascular system used for locomotion
  4. A notochord present throughout life

Explanation

Arthropods are characterised by a segmented body with jointed appendages and a hard exoskeleton made largely of chitin, which they must moult to grow. These features are shared by insects, crustaceans and arachnids.

Question 45Biology (Senior Secondary / Class 12 science subject)

Which of the following describes the role of decomposers in an ecosystem?

  1. They fix atmospheric nitrogen into amino acids inside root nodules
  2. They transfer energy from herbivores to carnivores
  3. They break down dead organic matter and release inorganic nutrients into the soil Correct answer.
  4. They convert solar energy into chemical energy

Explanation

Decomposers such as bacteria and fungi break down dead organisms and wastes, releasing mineral nutrients that producers can absorb again. This recycling keeps nutrients available within the ecosystem.

Question 46Biology (Senior Secondary / Class 12 science subject)

In an ecosystem, energy is transferred between trophic levels with about ten percent efficiency. What is the main reason for this low efficiency?

  1. Energy is converted back into sunlight by consumers
  2. Energy is destroyed when an organism is eaten by a predator
  3. Most energy is lost as heat through respiration and in undigested material Correct answer.
  4. Energy is stored entirely in the bones of consumers

Explanation

At each trophic level much of the assimilated energy is dissipated as heat during respiration, and some remains in uneaten or undigested material. Only a small fraction is incorporated into new biomass available to the next level.

Question 47Biology (Senior Secondary / Class 12 science subject)

Which of the following is the correct sequence of stages in the nitrogen cycle, starting from atmospheric nitrogen?

  1. Ammonification → nitrification → denitrification → nitrogen fixation → assimilation
  2. Nitrogen fixation → nitrification → assimilation → ammonification → denitrification Correct answer.
  3. Denitrification → assimilation → nitrification → ammonification → nitrogen fixation
  4. Nitrification → nitrogen fixation → denitrification → assimilation → ammonification

Explanation

Atmospheric nitrogen is first fixed into ammonia, which is nitrified to nitrite and nitrate, then assimilated by plants. Organic nitrogen from dead matter is ammonified back to ammonia, and denitrification returns nitrogen gas to the atmosphere.

Question 48Biology (Senior Secondary / Class 12 science subject)

Which of the following statements about the human male reproductive system is correct?

  1. Sertoli cells nourish developing sperm while Leydig cells secrete testosterone Correct answer.
  2. Leydig cells nourish developing sperm while Sertoli cells secrete testosterone
  3. Both Sertoli and Leydig cells secrete testosterone
  4. Sperm mature fully inside the epididymis before entering the testis

Explanation

Within the seminiferous tubules, Sertoli cells support and nourish developing spermatids, while interstitial Leydig cells produce testosterone under stimulation by luteinising hormone. The two cell types have distinct supportive and endocrine roles.

Question 49Biology (Senior Secondary / Class 12 science subject)

A couple is unable to conceive and the woman is diagnosed with blocked fallopian tubes. Which assisted reproductive technique directly bypasses this blockage?

  1. Oral administration of gonadotropins
  2. Surgical reversal of a vasectomy
  3. Intrauterine insemination of the husband's semen
  4. In vitro fertilisation followed by embryo transfer Correct answer.

Explanation

In IVF, oocytes are collected and fertilised by sperm outside the body, and the resulting embryo is transferred directly into the uterus, so the fallopian tubes are not needed. This makes it suitable when tubal blockage prevents natural fertilisation.

Question 50Biology (Senior Secondary / Class 12 science subject)

Which of the following is the primary role of memory B cells formed during an immune response?

  1. They directly engulf and digest invading bacteria
  2. They release histamine during allergic reactions
  3. They produce complement proteins in the liver
  4. They persist after infection and give a rapid, stronger response on re-exposure to the same antigen Correct answer.

Explanation

Memory B cells are long-lived clones that survive after an infection and respond quickly by differentiating into antibody-secreting plasma cells when the same antigen reappears. This is the cellular basis of immunological memory and of vaccination.