AIPMT 1995 Biology Question Paper with Answer and Solution

(A) Organisms that are sessile (attached to a substratum) often exhibit radial symmetry. This type of symmetry allows the organism to detect food or threats from any direction in their environment,which is advantageous for a stationary lifestyle. Examples include members of the phylum Cnidaria (like sea anemones) and Echinodermata (adults). Therefore,radial symmetry is a common feature associated with a sessile mode of life.

(B) Metamerism (or true segmentation) is a characteristic feature where the body is externally and internally divided into segments with a serial repetition of at least some organs.
In the animal kingdom,true metamerism is primarily observed in three phyla: $Annelida$,$Arthropoda$,and $Chordata$.
While $Annelida$ and $Arthropoda$ show prominent external and internal segmentation,$Chordata$ (specifically in vertebrates) exhibit metamerism in their internal structures such as muscles,nerves,and vertebral column.

(D) An annual plant completes its life cycle (germination,growth,flowering,and seed production) within a single growing season and then dies. $A$ biennial plant completes its life cycle in two years; it grows vegetatively in the first year and produces flowers,fruits,and seeds in the second year before dying. Therefore,the fundamental difference is that an annual plant completes its life cycle and dies within one year,whereas a biennial plant does not die after the first year's growth.

(C) Tetradynamous condition refers to the presence of $6$ stamens,where $4$ are long (inner) and $2$ are short (outer).
This specific arrangement of stamens is a characteristic feature of the family $Brassicaceae$ (formerly known as $Cruciferae$).

(C) Buttress roots are large,wide-spreading roots on all sides of a shallowly rooted tree. These roots typically grow above the ground and provide extra stability to large trees in tropical rainforests where the soil is often thin and nutrient-poor. Among the given options,$Terminalia$ is a well-known example of a plant that exhibits buttress roots to support its massive trunk.

(C) Desmosomes are specialized structures found in animal cells that function as intercellular junctions.
They act as 'spot welds' or anchoring junctions that provide mechanical strength to tissues by firmly attaching adjacent cells to one another.
Therefore,they are primarily associated with cell adhesion.

(D) The prokaryotic flagellum is a thin,hair-like appendage that extends from the cell surface.
It is composed of a protein called flagellin.
Unlike eukaryotic flagella,which have a $9 + 2$ microtubule arrangement,the prokaryotic flagellum is a helical structure made of protein subunits arranged in a spiral or helical fashion.
Therefore,the correct description is that the protein molecules are arranged in a helical manner.

(C) The smooth endoplasmic reticulum $(SER)$ is primarily responsible for the synthesis of lipids and lipid-like steroidal hormones. While the rough endoplasmic reticulum $(RER)$ is involved in protein synthesis due to the presence of ribosomes,the $SER$ lacks ribosomes and focuses on lipid metabolism and detoxification. Among the given options,the synthesis of lipids is the most comprehensive and standard biological function attributed to the $SER$ in $NCERT$ textbooks.

(A) In the human rib cage, there are $12$ pairs of ribs.
These are classified into three types based on their attachment to the sternum:
$1$. True ribs ($1^{st}$ to $7^{th}$ pairs): These are directly attached to the sternum via hyaline cartilage.
$2$. False ribs ($8^{th}$, $9^{th}$, and $10^{th}$ pairs): These do not attach directly to the sternum but join the $7^{th}$ rib.
$3$. Floating ribs ($11^{th}$ and $12^{th}$ pairs): These are not connected to the sternum at all, hence they are called floating ribs.
Therefore, there are $2$ pairs of floating ribs.

(C) The autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems.
$1$. The sympathetic nervous system originates from the thoraco-lumbar region of the spinal cord,specifically from the $T_1$ to $L_2$ segments.
$2$. The parasympathetic nervous system originates from the cranial nerves $(III, VII, IX, X)$ and the sacral region $(S_2-S_4)$ of the spinal cord.
Therefore,the sympathetic nerves originate from the thoraco-lumbar region.

(D) The $Thyroid$ gland is unique among endocrine glands because it stores its hormones in an inactive form within the extracellular space of the thyroid follicles, known as the colloid.
These hormones, $Thyroxine$ $(T_4)$ and $Triiodothyronine$ $(T_3)$, are synthesized and stored as part of the protein $Thyroglobulin$ in the follicular lumen.
When the body requires these hormones, they are reabsorbed from the colloid into the follicular cells, processed, and then released into the bloodstream.

(A) The thyroid gland specifically absorbs iodine to synthesize thyroid hormones ($T_3$ and $T_4$).
Radioactive Iodine-$131$ $(^{131}I)$ is used in medical diagnostics and therapy because the thyroid gland concentrates it,allowing for the detection of cancerous cells or the destruction of overactive thyroid tissue.
Other isotopes like Carbon-$14$ are used for radiocarbon dating,Uranium-$238$ for geological dating,and Phosphorus-$32$ for molecular biology research.

(A) Hormones exert their physiological effects by binding to specific proteins called hormone receptors located in the target tissues.
These receptors are either present on the cell membrane (for peptide hormones) or inside the cell (for steroid hormones).
The hormone-receptor complex formation is essential for triggering the biochemical changes within the target cell that lead to the physiological response.
If the receptor molecule is removed or absent,the hormone cannot bind to the target cell,and consequently,the target organ will not be able to recognize or respond to the hormone.

(A) When a person encounters a sudden threat (like a snake),the body initiates the '$fight-or-flight$' response.
This response is mediated by the sympathetic nervous system.
The sympathetic nerves stimulate the adrenal medulla to secrete catecholamines,specifically epinephrine (adrenaline) and norepinephrine (noradrenaline).
These hormones prepare the body for immediate action by increasing heart rate,blood pressure,and alertness.
Therefore,option $A$ is correct because the adrenal medulla is the site of secretion for these hormones,not the adrenal cortex.

(B) Polytene chromosomes are giant chromosomes that are common to many dipteran flies. They were first discovered by $E.G. Balbiani$ in $1881$ in the salivary glands of $Chironomus$ larvae. These chromosomes are formed by repeated rounds of $DNA$ replication without cell division,a process known as endoreduplication.

(D) In a nucleic acid chain,individual nucleotides are linked together to form a polynucleotide chain.
This linkage occurs between the $3'$-carbon atom of the sugar moiety of one nucleotide and the $5'$-carbon atom of the sugar moiety of the adjacent nucleotide.
The bond formed by this linkage is known as a $3'-5'$ phosphodiester bond.
Therefore,the correct option is $D$.

(D) Glycogen is a multi-branched polysaccharide of glucose that serves as a form of energy storage in animals,fungi,and bacteria. In humans,it is primarily synthesized and stored in the liver and muscle cells. When the body requires energy,the liver breaks down glycogen into glucose and releases it into the bloodstream. Lactose,galactose,and arabinose are not polysaccharides stored in the liver.

(A) Pyrenoids are storage bodies found in the chloroplasts of algae.
They typically consist of a central core of protein,which is the site of the enzyme RuBisCO,surrounded by a sheath of starch.
Therefore,they are composed of a central protein core and a starch sheath.

(D) Polytene chromosomes are giant chromosomes found in the salivary glands of Drosophila larvae.
These chromosomes consist of many chromatids that are held together at a common central point.
This central point where all the chromatids of the polytene chromosome converge is known as the $Chromocenter$.

(D) Atmospheric nitrogen fixation is the process of converting atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$,which can be utilized by plants.
$Anabaena$ is a genus of filamentous cyanobacteria that exists as plankton.
These organisms possess specialized cells called heterocysts,which provide an anaerobic environment for the enzyme nitrogenase to function.
Nitrogenase is essential for the biological fixation of atmospheric nitrogen.
$Albugo$,$Cystopus$,and $Aprolegnia$ are fungi or fungus-like organisms that do not perform nitrogen fixation.

(A) Kranz anatomy is a specialized structure found in the leaves of $C_4$ plants (such as maize,sugarcane,and sorghum).
In this anatomy,the mesophyll cells are arranged in a ring-like manner around the bundle sheath cells.
This structural arrangement facilitates the $C_4$ photosynthetic pathway,which helps in minimizing photorespiration and increasing the efficiency of carbon fixation in high-temperature and high-light conditions.

(B) In the Calvin cycle,which occurs in the stroma of chloroplasts,the primary $CO_2$ acceptor is a $5$-carbon ketose sugar called Ribulose $1,5$-bisphosphate $(RuBP)$.
This reaction is catalyzed by the enzyme Ribulose bisphosphate carboxylase-oxygenase $(RuBisCO)$.
$PEP$ is the primary $CO_2$ acceptor in $C_4$ plants,while $PGA$ is the first stable product of the Calvin cycle.

(B) The Respiratory Quotient $(RQ)$ is defined as the ratio of the volume of $CO_2$ evolved to the volume of $O_2$ consumed during respiration.
For fatty acids, the oxidation process requires more oxygen relative to the amount of carbon dioxide produced.
For example, in the oxidation of tripalmitin (a fatty acid), the reaction is: $2(C_{51}H_{98}O_6) + 145O_2 \rightarrow 102CO_2 + 98H_2O + \text{Energy}$.
Calculating the $RQ$: $RQ = \frac{\text{Volume of } CO_2}{\text{Volume of } O_2} = \frac{102}{145} \approx 0.7$.
Since $0.7 < 1$, the $RQ$ for fatty acids is always less than $1$.

(D) The conversion of pyruvic acid into acetyl $CoA$ is catalyzed by the pyruvate dehydrogenase complex. This reaction is an oxidative decarboxylation process. The essential cofactors and coenzymes required for this reaction include:
$1$. $TPP$ (Thiamine pyrophosphate)
$2$. $NAD^+$ (Nicotinamide adenine dinucleotide)
$3$. $CoA$ (Coenzyme $A$)
$4$. $LAA$ (Lipoic acid)
$5$. $Mg^{2+}$ ions.
Since all the listed substances are essential components of this enzymatic complex,the correct answer is $D$.

(C) Florigen is a hypothetical hormone or signaling molecule that is believed to be responsible for inducing flowering in plants. It is synthesized in the leaves in response to photoperiodic stimuli and then transported to the shoot apical meristem to initiate the transition from vegetative growth to reproductive growth (flowering).

(C) Ethylene is a gaseous plant hormone known primarily for its role in fruit ripening. It promotes the ripening process by stimulating the breakdown of cell walls and the conversion of starches into sugars. It also accelerates senescence and abscission of plant organs like leaves and flowers.

(B) Phytochrome is a proteinaceous pigment found in plants that acts as a photoreceptor. It exists in two interconvertible forms: $P_r$ (which absorbs red light at $660 \ nm$) and $P_{fr}$ (which absorbs far-red light at $730 \ nm$). This pigment plays a crucial role in photomorphogenesis,including seed germination,stem elongation,and flowering.

(D) Haemorrhagic disease of the newborn $(HDN)$, also known as Vitamin-$K$ deficiency bleeding, occurs because newborns have low levels of Vitamin-$K$ at birth.
Vitamin-$K$ is essential for the synthesis of blood clotting factors $(II, VII, IX, \text{and } X)$ in the liver.
A deficiency in Vitamin-$K$ prevents proper blood coagulation, leading to an increased risk of bleeding.
Therefore, the correct option is $D$.

(B) The enzyme $Enterokinase$ (also known as $Enteropeptidase$) is secreted by the intestinal mucosa.
It acts on the inactive enzyme $Trypsinogen$,which is secreted by the pancreas into the duodenum.
$Enterokinase$ converts $Trypsinogen$ into its active form,$Trypsin$.
$Trypsin$ then activates other pancreatic enzymes like $Chymotrypsinogen$ and $Procarboxypeptidase$ to facilitate protein digestion.

(C) Vitamin $- C$,also known as ascorbic acid,is a water-soluble vitamin essential for the synthesis of collagen,a protein that helps in the formation of connective tissues,skin,and blood vessels.
Deficiency of Vitamin $- C$ leads to a condition known as scurvy,which is characterized by symptoms such as bleeding gums,weakness,fatigue,and poor wound healing.
Rickets is caused by a deficiency of Vitamin $- D$.
Pellagra is caused by a deficiency of Vitamin $B_3$ (niacin).
Therefore,Vitamin $- C$ prevents scurvy.

(B) The blood maintains a relatively constant $pH$ (approximately $7.4$) despite the transport of large amounts of $CO_2$. This is primarily due to the presence of blood buffers. The bicarbonate buffer system,involving plasma proteins and hemoglobin,acts to neutralize the hydrogen ions $(H^+)$ produced when $CO_2$ reacts with water to form carbonic acid $(H_2CO_3)$. Hemoglobin,in particular,acts as a very effective buffer by binding to $H^+$ ions,thereby preventing the blood from becoming acidic.

(D) Carbon dioxide $(CO_2)$ is transported in the blood from tissues to the lungs in three main forms:
$1$. As dissolved gas in blood plasma (about $7\%$).
$2$. As carbaminohemoglobin,where $CO_2$ binds to the amino groups of hemoglobin (about $20-25\%$).
$3$. As bicarbonate ions $(HCO_3^-)$,which is the primary method of transport (about $70\%$).
While the provided option $D$ mentions carbaminohemoglobin,it incorrectly lists carbonic acid instead of bicarbonate ions. However,in the context of standard multiple-choice questions,option $D$ is the most appropriate choice as it covers the major transport mechanisms involving hemoglobin and chemical conversion.

(C) At high altitudes,the atmospheric pressure is low,which leads to a lower partial pressure of oxygen $(pO_2)$.
This results in hypoxia (low oxygen availability in tissues).
To compensate for this reduced oxygen supply,the body stimulates the production of erythropoietin from the kidneys.
Erythropoietin stimulates the bone marrow to produce more red blood cells $(R.B.C.s)$,thereby increasing the oxygen-carrying capacity of the blood.
Therefore,the $R.B.C.$ count increases in humans living at high altitudes.

(D) The heartbeat is initiated by the $SA$ (Sinoatrial) node,which acts as the natural pacemaker of the heart.
From the $SA$ node,the impulse travels to the $AV$ (Atrioventricular) node.
From the $AV$ node,it is conducted through the Bundle of His.
Finally,the impulse spreads through the Purkinje fibers to the ventricular heart muscles,causing contraction.
Therefore,the correct sequence is: $SA$ node $\rightarrow$ $AV$ node $\rightarrow$ Bundle of His $\rightarrow$ Purkinje fibers $\rightarrow$ Heart muscles.

(A) Blood cancer is medically known as $Leukemia$.
It is a type of cancer that affects the blood and bone marrow,characterized by the abnormal and uncontrolled production of white blood cells $(leukocytes)$.
$Thrombosis$ refers to the formation of a blood clot within a blood vessel.
$Hemolysis$ is the destruction of red blood cells.
$Hemophilia$ is a genetic disorder that impairs the body's ability to make blood clots.

(C) Lymph is a clear,colorless fluid that acts as a middleman between the blood and the tissues.
Its primary function is to collect interstitial fluid (tissue fluid) from the intercellular spaces and return it to the blood circulation via the lymphatic vessels.
It also plays a crucial role in the immune system by transporting lymphocytes and antibodies,and in the absorption of fats from the digestive tract.

(B) Antigens are specific proteins or carbohydrates present on the surface of red blood cells $(RBCs)$.
These antigens determine the blood group of an individual.
For example, in the $ABO$ blood grouping system, the presence of antigen $A$ or $B$ on the surface of $RBCs$ defines the blood type.
Therefore, antigens are found on the surface of the cell.

(D) Stomata are small pores present on the epidermis of leaves and other aerial parts of plants,which facilitate gas exchange and transpiration.
In submerged hydrophytes (plants that grow completely underwater),stomata are absent because gas exchange occurs directly through the general body surface via diffusion in the water.
Therefore,the correct option is $D$.

(C) Sunken stomata are stomata that are located in small pits or depressions on the leaf surface.
This adaptation helps in reducing the rate of transpiration by creating a humid microclimate around the stomatal pore.
This is a critical survival mechanism for plants living in dry or arid environments where water conservation is essential.
Therefore,sunken stomata are a characteristic feature of $Xerophytes$.

(D) $1$. Uricotelic organisms (like birds and insects) excrete uric acid to conserve water,which is an adaptation for terrestrial,not aquatic,habitats.
$2$. Parasitism is an interspecific relationship (between two different species),not an intraspecific one.
$3$. High transpiration is a disadvantage in desert environments; desert plants have adaptations to reduce transpiration (e.g.,sunken stomata).
$4$. $A$ boat-shaped (streamlined) body reduces water resistance,which is a classic adaptation for aquatic animals to swim efficiently.

(B) Desert plants,also known as $Xerophytes$,have evolved specific adaptations to survive in water-scarce environments.
One of the most common adaptations is the presence of $Succulent$ tissues,which store water in their stems,leaves,or roots.
This allows them to survive long periods of drought.
Therefore,the correct option is $B$.

(A) : Cleavage is a series of rapid mitotic cell divisions by which a single fertilized egg cell (zygote) is transformed into a multicellular structure called the blastula.
During cleavage,there is no significant growth in the size of the embryo,and the shape remains largely unchanged except for the formation of a central cavity known as the blastocoel.
The ratio of nuclear material $(DNA)$ to cytoplasm increases with each division.
The pattern or mode of cleavage is primarily determined by the amount of yolk present in the egg and its distribution (pattern of yolk deposition).
Based on the yolk content,cleavage is classified as holoblastic (total) or meroblastic (partial).

(C) Phytoalexins are antimicrobial substances synthesized de novo by plants that accumulate rapidly at areas of pathogen infection. They act as a chemical defense mechanism against various pathogens,including fungi,bacteria,and viruses. Therefore,the correct answer is Phytoalexins.

(C) The correct answer is $C$.
$1$. Root knot disease is caused by the nematode $Meloidogyne$ $javanica$.
$2$. Smut of pearl millet is caused by $Tolyposporium$ $penicillariae$.
$3$. Covered smut of sorghum is caused by $Sphacelotheca$ $sorghi$,whereas $Ustilago$ $nuda$ causes loose smut of wheat. Therefore,this pair is incorrectly matched.
$4$. Late blight of potato is caused by $Phytophthora$ $infestans$.

(B) The hormone $Oxytocin$ is responsible for the ejection of milk from the mammary glands in females. When a baby suckles,sensory signals are sent to the hypothalamus,which triggers the posterior pituitary to release $Oxytocin$. This hormone acts on the smooth muscles of the mammary glands,causing them to contract and eject milk. $Prolactin$,on the other hand,is responsible for the production of milk.

(D) The $Leydig$ cells,also known as interstitial cells,are located in the connective tissue spaces between the seminiferous tubules of the testis.
These cells are responsible for the synthesis and secretion of a group of hormones called androgens,primarily $Testosterone$.
$Testosterone$ is the main male sex hormone responsible for the development of secondary sexual characteristics in males.

(A) In angiosperms,each microspore mother cell $(MMC)$ undergoes meiosis to produce $4$ microspores (pollen grains).
Therefore,the number of $MMC$ required to produce $n$ pollen grains is calculated as $n / 4$.
For $100$ pollen grains,the number of $MMC$ required = $100 / 4 = 25$.
Thus,$25$ microspore mother cells are needed.

(B) The $Corpus$ $Luteum$ is a temporary endocrine structure formed from the ruptured follicle after ovulation.
Its primary function is to secrete large amounts of $Progesterone$,which is essential for maintaining the endometrium for potential implantation of the embryo.
While it also secretes some $Estrogen$ and $Relaxin$,$Progesterone$ is its most significant and primary hormonal product.

(D) population that is heavily weighted towards the younger age group is known as an expanding population.
In such a population,the number of individuals in the pre-reproductive and reproductive age groups is very high.
This demographic structure typically leads to a high birth rate because a large proportion of the population is entering or is currently in the reproductive phase.
Furthermore,improvements in healthcare and nutrition have contributed to a longer average lifespan for individuals in India compared to previous decades.
Therefore,the combination of a large youth population and better survival rates results in a high birth rate and a longer lifespan for many individuals.

(A) Epistasis is a genetic interaction where the expression of one gene is modified or masked by the presence of one or more other genes.
In this phenomenon,the gene that suppresses the effect of another gene is called the epistatic gene,and the gene whose effect is suppressed is called the hypostatic gene.
Unlike dominance,which involves the interaction between alleles of the same gene,epistasis involves the interaction between different gene loci.

(A) The $cis-trans$ test (or complementation test) is used to determine if two mutations are in the same gene or different genes.
If two mutations are in the same gene,they will show a mutant phenotype in the $trans$ configuration (complementation does not occur).
If two mutations are in different genes,they will show a wild-type phenotype in the $trans$ configuration (complementation occurs).
However,if two genetic loci produce identical phenotypes in both $cis$ and $trans$ configurations,it indicates that the mutations are not affecting the same functional unit in a way that prevents complementation,or they are effectively acting as independent units. In the context of classical genetics,when mutations at two closely linked loci show identical phenotypes and do not exhibit the expected $cis-trans$ effect,they are often referred to as Pseudoalleles.

(D) Point mutation is a change in a single base pair of $DNA$.
Sickle cell anemia is a classic example of a point mutation where a single base pair substitution occurs in the $\beta$-globin gene.
Specifically,the codon $GAG$ is replaced by $GUG$,which leads to the substitution of glutamic acid with valine at the sixth position of the $\beta$-globin chain of hemoglobin.
This change alters the structure of the hemoglobin molecule,causing red blood cells to become sickle-shaped under low oxygen conditions.

(C) In $E. coli$,the $lac$ operon is responsible for the metabolism of lactose.
When glucose is present,the $lac$ operon is repressed because glucose is the preferred energy source.
When glucose is removed and only lactose is provided,lactose acts as an inducer.
It binds to the repressor protein,preventing it from binding to the operator region.
This allows $RNA$ polymerase to transcribe the structural genes,thereby inducing the $lac$ operon.

(C) The anticodon is a sequence of three nucleotides forming a unit of genetic code in a transfer $RNA$ $(tRNA)$ molecule,corresponding to a complementary codon in messenger $RNA$ $(mRNA)$.
During protein synthesis,the anticodon of $tRNA$ base-pairs with the codon of $mRNA$ to ensure the correct amino acid is added to the polypeptide chain.
Therefore,the anticodon is present on $tRNA$.

(A) During the process of transcription,$DNA$ acts as a template to synthesize $mRNA$.
According to the principle of complementarity,$Adenine$ $(A)$ pairs with $Uracil$ $(U)$ in $RNA$,$Thymine$ $(T)$ pairs with $Adenine$ $(A)$,$Cytosine$ $(C)$ pairs with $Guanine$ $(G)$,and $Guanine$ $(G)$ pairs with $Cytosine$ $(C)$.
Given $DNA$ template sequence: $A-T-T-C-G-A-T-G$.
Complementary $mRNA$ sequence: $U-A-A-G-C-U-A-C$.
Therefore,the correct sequence is $UAAGCUAC$.

(C) In eukaryotic organisms,genes are often 'split' or 'interrupted'.
These genes contain coding sequences known as $Exons$ and non-coding sequences known as $Introns$.
$Exons$ are the sequences that appear in mature or processed $RNA$ and code for proteins.
$Introns$ are intervening sequences that do not code for proteins and are removed during $RNA$ splicing.
Therefore,the coding sequences are called $Exons$.

(C) The condition described is known as dental fluorosis.
It occurs due to the excessive intake of fluoride (a form of fluorine) through drinking water.
High concentrations of fluoride in water lead to the mottling (discoloration or pitting) of tooth enamel.
Therefore,the correct element is fluorine.

(B) Homologous organs are defined as organs that share a common evolutionary origin and structural plan,even if they perform different functions in different organisms. $1.$ They arise from the same embryonic primordia. $2.$ They indicate divergent evolution. $3.$ Examples include the forelimbs of whales,bats,cheetahs,and humans,which share the same anatomical structure (humerus,radius,ulna,carpals,metacarpals,and phalanges) despite being used for swimming,flying,running,or grasping. Therefore,the correct answer is $B$ (Origin).

(B) The presence of gill slits in the embryos of all vertebrates is a classic example of embryological evidence for evolution.
This observation supports the $Recapitulation$ $theory$ (also known as $von$ $Baer's$ $law$ or $Haeckel's$ $Biogenetic$ $law$),which states that 'ontogeny repeats phylogeny'.
This means that an individual organism's biological development,or ontogeny,parallels and summarizes its species' evolutionary development,or phylogeny.

(D) The alkaloid $Ajmalicine$ (also known as $Raubasine$) is a pharmaceutical compound used primarily for the treatment of circulatory disorders.
It is extracted from the roots of the plant $Rauwolfia$ $serpentina$,commonly known as $Sarpagandha$ in India.
$Rauwolfia$ $serpentina$ is well-known for producing various alkaloids,including $Reserpine$ and $Ajmalicine$,which have significant medicinal properties.

(B) Nicotine is a chemical compound found in tobacco that acts as a stimulant in mammals. It functions by binding to and activating nicotinic acetylcholine receptors $(nAChRs)$ in the central and peripheral nervous systems. Because its structure allows it to bind to these receptors,it mimics the action of the neurotransmitter $Acetylcholine$,leading to increased alertness,heart rate,and blood pressure. Therefore,the correct option is $B$.

(B) Allergy is the exaggerated response of the immune system to certain antigens present in the environment.
$Hay$ $fever$ (also known as allergic rhinitis) is a classic example of an allergic reaction caused by exposure to allergens like pollen grains,dust,or spores.
Typhoid is a bacterial infection caused by $Salmonella$ $typhi$.
Goitre is typically caused by iodine deficiency.
Skin cancer is caused by uncontrolled growth of abnormal skin cells,often due to $UV$ radiation exposure.
Therefore,the correct option is $B$.

(C) The silk produced by the silkworm $(Bombyx \ mori)$ is a proteinaceous secretion.
This secretion is produced by the specialized salivary glands,also known as silk glands,present in the larval stage of the silkworm.
As the larva prepares to pupate,it secretes this liquid protein through a spinneret,which solidifies upon contact with air to form the silk fiber.
Therefore,the correct answer is the salivary glands of the larva.

(C) Biological control refers to the use of biological methods for controlling plant diseases and pests. The main difficulty in this method is that the introduced predator or parasite may not always survive or adapt well when introduced into a new environment,which limits its effectiveness in controlling the target pest population.

(A) Restriction endonucleases are enzymes that recognize specific palindromic nucleotide sequences in $DNA$ and cut the $DNA$ duplex at specific positions,usually within or near the recognition site. These are essential tools in recombinant $DNA$ technology,often referred to as 'molecular scissors'.

(B) Transgenic animals are defined as animals that have had their $DNA$ manipulated to possess and express an extra (foreign) gene.
In the process of creating a transgenic animal,the foreign gene is introduced into the germline cells (such as the zygote or early embryonic cells).
As a result,the foreign $DNA$ becomes integrated into the genome of the organism.
Since this integration occurs at the embryonic stage,the foreign $DNA$ is replicated and passed on to every cell of the developing organism.
Therefore,all cells of a transgenic animal contain the foreign $DNA$.

(D) The concept that food supply increases in an arithmetic progression $(1, 2, 3, 4, ...)$ while human population tends to increase in a geometric progression $(1, 2, 4, 8, ...)$ was proposed by the economist Thomas Malthus in his work 'An Essay on the Principle of Population'. This theory highlights the potential for a population crisis when the growth of the population outpaces the growth of resources.

(C) Decomposers (saprotrophs) play a crucial role in the ecosystem by breaking down complex organic matter into simpler inorganic substances. This process is known as mineralization. If decomposers are removed,the dead organic matter will accumulate,and the essential mineral nutrients locked within this matter will not be released back into the soil or environment. Consequently,the cycling of nutrients will cease,leading to a shortage of nutrients for producers (plants),which will eventually disrupt the entire ecosystem.

(B) Biogeochemical cycles are broadly classified into two types: Gaseous cycles and Sedimentary cycles.
$1$. Gaseous cycles: The reservoir for these elements is the atmosphere or hydrosphere (e.g.,Nitrogen,Oxygen,Carbon).
$2$. Sedimentary cycles: The reservoir for these elements is the Earth's crust (lithosphere). Phosphorus and Sulfur are classic examples of sedimentary cycles because their primary reservoir is rock/soil.

(D) Primary succession is the process of ecological succession that occurs in an area where no community previously existed.
This includes environments such as newly cooled lava,bare rock,or sand dunes where soil formation has not yet occurred.
In contrast,secondary succession occurs in areas where a pre-existing community has been destroyed (e.g.,after a fire or flood),as the soil is already present.

(D) In an ecosystem,the flow of energy starts from producers (plants).
Primary consumers are organisms that feed directly on producers.
Since producers are plants,organisms that eat plants are known as herbivores.
Therefore,primary consumers are herbivores.

(C) Desertification is the process by which fertile land becomes desert,typically as a result of drought,deforestation,or inappropriate agriculture.
$1$. Overgrazing: Excessive grazing by livestock removes vegetation cover,leading to soil erosion and loss of soil fertility.
$2$. Deforestation: Removal of trees exposes the soil to wind and water erosion.
$3$. Inappropriate agricultural practices: Intensive irrigation in arid regions can lead to soil salinization,making the land unsuitable for plant growth.
Therefore,all the listed factors contribute to desertification.

(B, D) The correct answer is $B$ and $D$.
$1$. Gir Forest is famous for the Asiatic Lion.
$2$. Sariska Tiger Reserve is primarily known for Tigers,not Elephants.
$3$. The Rann of Kutch (Wild Ass Sanctuary) is famous for the Indian Wild Ass $(Equus hemionus khur)$.
$4$. Kaziranga National Park is famous for the One-horned Rhinoceros,not the Musk Deer.
Since both $B$ and $D$ are incorrectly matched,the question contains multiple incorrect pairs.

(A) Biochemical Oxygen Demand $(BOD)$ refers to the amount of dissolved oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria.
When a large amount of sewage (organic waste) is discharged into a river,the population of decomposer bacteria increases to break down this organic matter.
These bacteria consume a large amount of dissolved oxygen from the water for the process of decomposition.
Consequently,the $BOD$ of the river water increases significantly as the demand for oxygen by microorganisms rises.

(D) The Taj Mahal is primarily threatened by acid rain,which is caused by the high concentration of sulfur dioxide $(SO_2)$ in the atmosphere.
Sulfur dioxide is released from industrial activities,particularly from the Mathura oil refinery.
When $SO_2$ reacts with moisture in the air,it forms sulfuric acid,which leads to the corrosion of the marble,a process often referred to as 'marble cancer'.

(B) Minamata disease is a neurological syndrome caused by severe mercury poisoning. It was first discovered in Minamata City,Kumamoto Prefecture,Japan,in $1956$. The disease was caused by the release of methylmercury in the industrial wastewater from the Chisso Corporation's chemical factory. This highly toxic chemical bioaccumulated in shellfish and fish in Minamata Bay,which were then eaten by the local human population and animals. Cats in the area were severely affected,exhibiting symptoms like convulsions and erratic behavior,which led to the term 'dancing cat fever'. Pigs and dogs were also affected. However,rabbits were not reported to be affected by the disease in this specific ecological context.