AIPMT 1995 Biology Question Paper with Answer and Solution

(D) $Euglena$ is a unique organism that acts as a connecting link between plants and animals.
It possesses chloroplasts and performs photosynthesis like plants in the presence of sunlight.
In the absence of sunlight,it behaves like a heterotroph,feeding on organic matter like animals.
Therefore,it exhibits characteristics of both kingdoms.

(B) Bacteriophages are viruses that infect bacteria. They consist of a head and a tail structure. Despite having a tail,they do not possess any mechanism for independent locomotion. They are obligate parasites that rely on Brownian motion or random collisions to encounter their host cells. Therefore,tailed bacteriophages are non-motile.

(D) Bacteria were first observed and described by $A.V. Leeuwenhoek$ in $1676$. He referred to these microscopic organisms as 'animalcules'. Therefore,the correct answer is $A.V. Leeuwenhoek$.

(B) Sexual reproduction is a characteristic feature of Eukaryotic organisms.
$Nostoc$ is a Cyanobacterium (Prokaryote).
Prokaryotes do not undergo true sexual reproduction involving meiosis and gamete fusion.
$Ulothrix$ and $Volvox$ are green algae (Eukaryotes) that reproduce sexually.
$Aspergillus$ is a fungus (Eukaryote) that also exhibits sexual reproduction.

(C) The major function of the contractile vacuole is osmoregulation,which involves the removal of excess water from the cell. This process is essential for maintaining the internal water balance in organisms like $Amoeba$.

(C) Pyrenoids are specialized storage bodies found in the chloroplasts of many algae.
They consist of a central proteinaceous core,which is often surrounded by a sheath of starch.
Therefore,they are described as having a proteinaceous centre and a starchy sheath.

(A) In $Dryopteris$,the sporangium wall is differentiated into a specialized region called the $Annulus$.
This region consists of cells that develop a characteristic thickening along their radial and inner tangential walls.
Due to the differential drying of these cells,the $Annulus$ acts as a mechanical device that controls the dehiscence (opening) of the sporangium to release the spores.

(C) Metamerism is a condition in which the body is composed of a linear series of similar body segments called metameres or somites.
This phenomenon is a characteristic feature observed in the phyla $Annelida$,$Arthropoda$,and $Chordata$.
In $Chordata$,metamerism is primarily evident in the internal structures such as the vertebral column and the arrangement of muscles (myotomes).

(B) Organisms that are sessile or attached to a substratum, such as those in the phylum $Cnidaria$ (e.g., $Hydra$, $Sea$ $anemone$) or $Porifera$, often exhibit radial symmetry. Radial symmetry is an evolutionary adaptation for sessile organisms, as it allows them to interact with their environment and capture food from all directions equally, since they cannot move to hunt or escape.

(A) The functional kidney of a frog tadpole is the pronephros (also known as the head kidney).
It develops from the nephrostomes located in the anterior region of the embryo.
As the tadpole undergoes metamorphosis into an adult frog,the pronephros is replaced by the mesonephros.

(D) Desmosomes are specialized intercellular junctional complexes that provide strong adhesion between adjacent cells.
They act as 'spot welds' that anchor the intermediate filaments of the cytoskeleton to the plasma membrane,thereby maintaining the structural integrity of tissues,particularly in epithelial cells subjected to mechanical stress.
Therefore,the primary function of desmosomes is cell adherence.

(A) Prokaryotic flagella are composed of a protein called flagellin.
These flagellin molecules are arranged in a helical manner to form a filament.
Unlike eukaryotic flagella,prokaryotic flagella are not enclosed by a cell membrane and do not possess the $9+2$ microtubular arrangement.

(D) Water moves from a region of higher water potential to a region of lower water potential.
In the roots,water is absorbed from the soil and moves through the cortex cells towards the xylem.
This movement occurs because there is a continuous water potential gradient established between the soil,the cortical cells,and the xylem.
Therefore,the movement of water from one cell of the cortex to the adjacent one is driven by the water potential gradient.

(A) Molybdenum is an essential component of the enzyme nitrogenase,which is responsible for biological nitrogen fixation.
It is also a key component of nitrate reductase,an enzyme involved in nitrogen metabolism.
Therefore,$Molybdenum$ $(Mo)$ plays a crucial role in the process of biological nitrogen fixation.

(A) In $C_3$ plants,the primary carboxylation step of the Calvin cycle involves the enzyme RuBisCO,which catalyzes the reaction of $CO_2$ with Ribulose $1,5$-bisphosphate $(RuBP)$.
This reaction results in the formation of two molecules of $3$-phosphoglyceric acid ($3$-$PGA$).
$3$-$PGA$ is a $3$-carbon compound and is the first stable product formed during the dark reaction of photosynthesis in $C_3$ plants.

(C) In $C_3$ plants,the primary $CO_2$ acceptor is a $5$-carbon ketose sugar called Ribulose $1,5$-bisphosphate $(RuBP)$.
This reaction is catalyzed by the enzyme RuBisCO,which leads to the formation of two molecules of $3$-phosphoglyceric acid ($3$-$PGA$),which is the first stable product of the Calvin cycle.

(D) $Phytochrome$ is a proteinaceous pigment that exists in two interconvertible forms: $Pr$ (which absorbs red light at $660 \ nm$) and $Pfr$ (which absorbs far-red light at $730 \ nm$).
This interconversion between $Pr$ and $Pfr$ forms acts as a biological switch,allowing plants to sense light quality and regulate various developmental processes such as flowering and seed germination.

(B) The closure of the lid in a Pitcher Plant $(Nepenthes)$ is a type of nastic movement,specifically a thigmonastic movement.
These movements are caused by changes in the turgor pressure of the cells in the pulvinus or hinge region.
Since these movements are induced by external stimuli (like the touch of an insect) but are independent of the direction of the stimulus,they are classified as turgor movements.

(C) Glycogen is a complex carbohydrate known as a polysaccharide. It serves as the primary storage form of glucose in animals. It is synthesized from glucose molecules through a process called glycogenesis and is primarily stored in the liver and skeletal muscle cells to maintain blood glucose levels and provide energy during metabolic demand.

(B) Trypsinogen is an inactive enzyme (zymogen) secreted by the pancreas into the small intestine.
It is converted into its active form,trypsin,by an enzyme called enterokinase (also known as enteropeptidase).
Enterokinase is secreted by the intestinal mucosa.
Therefore,the correct option is $B$.

(D) Carbon dioxide $(CO_2)$ is transported in the blood in three forms:
$1$. As bicarbonates $(HCO_3^-)$: About $70\%$ of $CO_2$ is transported in this form.
$2$. As carbaminohaemoglobin: About $20-25\%$ of $CO_2$ is transported by binding with haemoglobin.
$3$. Dissolved in plasma: About $7\%$ of $CO_2$ is carried in a dissolved state through blood plasma.
Since the majority of $CO_2$ is transported as bicarbonates,the correct answer is $D$.

(D) The presence of a very large number of tiny alveoli significantly increases the surface area available for the exchange of gases ($O_2$ and $CO_2$).
This structural adaptation allows for a more efficient diffusion process.
While the lungs do contain residual air (the volume of air remaining in the lungs after a forced expiration),the branching structure ensures that the ventilation process remains highly efficient for gas exchange.

(D) During respiration,$CO_2$ is transported in the blood in three forms:
$1$. As dissolved gas in plasma (about $7\%$).
$2$. As bicarbonate ions $(HCO_3^-)$ in plasma,combined with sodium $(Na^+)$ and potassium $(K^+)$ ions to form $NaHCO_3$ and $KHCO_3$ (about $70\%$).
$3$. As carbaminohemoglobin (about $23\%$).
Therefore,the correct option is $(d)$ as it covers the dissolved state and the bicarbonate transport mechanism.

(D) The cardiac impulse originates at the $SA$ (Sinoatrial) node,which acts as the natural pacemaker of the heart.
From the $SA$ node,the impulse spreads through the atrial muscles to the $AV$ (Atrioventricular) node.
From the $AV$ node,the impulse travels through the Bundle of His (Atrioventricular bundle).
Finally,it is transmitted to the Purkinje fibres,which distribute the impulse throughout the ventricular musculature,causing the heart muscles to contract.
Therefore,the correct sequence is: $SA$ node $\to$ $AV$ node $\to$ Bundle of His $\to$ Purkinje fibres $\to$ Heart muscles.

(A) Valves are specialized structures that ensure the unidirectional flow of blood and lymph.
In the circulatory system,valves are found in the hearts of both vertebrates and invertebrates to prevent backflow during contraction.
Furthermore,they are present in the blood vessels (specifically veins) of many animals to assist in venous return against gravity.
Additionally,valves are present in the lymphatic vessels of vertebrates to maintain the unidirectional flow of lymph towards the venous system.
Therefore,option $A$ is the most comprehensive and accurate description.

(C) Lymph is a fluid connective tissue that acts as a middleman between the blood and the tissue cells.
It is formed from the interstitial fluid (tissue fluid) that leaks out of the blood capillaries into the intercellular spaces.
One of the primary functions of the lymphatic system is to collect this excess interstitial fluid and return it to the venous blood circulation,thereby maintaining fluid balance in the body.
Therefore,the correct function is to return the interstitial fluid to the blood.

(B) Antigens are substances that can induce an immune response. In the context of blood groups,antigens (such as $A$ and $B$ antigens) are present on the surface of red blood cells (erythrocytes). These antigens are recognized by the immune system,which produces specific antibodies that react with them.

(A) Blood contains various buffer systems,primarily the bicarbonate buffer system $(HCO_3^-/H_2CO_3)$ and hemoglobin,which help maintain the $pH$ of the blood within a narrow range $(7.35-7.45)$.
When $CO_2$ enters the blood,it reacts with water to form carbonic acid $(H_2CO_3)$,which dissociates into $H^+$ and $HCO_3^-$.
The buffer systems neutralize the excess $H^+$ ions,preventing a significant drop in $pH$ and keeping the blood from becoming acidic.

(D) The human rib cage consists of $12$ pairs of ribs.
Ribs $1$ to $7$ are called true ribs as they are attached directly to the sternum.
Ribs $8, 9,$ and $10$ are called false ribs (vertebrochondral ribs) as they join the $7^{th}$ rib instead of the sternum.
The last two pairs ($11^{th}$ and $12^{th}$) of ribs are not connected ventrally to the sternum at all and are therefore known as floating ribs.

(C) The sympathetic nervous system is a part of the autonomic nervous system.
In mammals,the sympathetic nerves arise from the thoraco-lumbar region of the spinal cord.
Specifically,the preganglionic neurons originate from the lateral horns of the gray matter in the thoracic and upper lumbar segments ($T_1$ to $L_2$ or $L_3$) of the spinal cord.
Therefore,the correct option is $(c)$.

(C) Hormones exert their physiological effects by binding to specific receptor molecules located either on the cell membrane or within the cytoplasm/nucleus of target cells.
These receptor-hormone complexes initiate a cascade of biochemical reactions that lead to the specific response.
If the receptor molecules are removed from the target organs,the hormone cannot bind to the cells.
Without this binding,the signal transduction pathway is not activated,and therefore,the target organ will not respond to the hormone.

(D) The $Thyroid$ gland is composed of tiny follicles,which are hollow spheres whose walls consist of a single layer of cuboidal epithelium.
In these follicles,thyroid hormones are stored in the extracellular space,bound to a protein called thyroglobulin,which is synthesized by the follicular cells.
To release the hormones into the blood,the epithelial cells take up thyroglobulin via pinocytosis and subsequently remove the active hormone from the thyroglobulin molecule.

(C) In the $S$ phase of the cell cycle,$DNA$ replication occurs,resulting in each chromosome consisting of two sister chromatids.
During metaphase,these chromosomes align at the equatorial plate.
At the onset of anaphase,the centromere of each chromosome splits,and the sister chromatids separate.
Once separated,each chromatid is considered an independent chromosome.
Therefore,at the anaphase stage,each chromosome consists of only one chromatid.

(B) Polytene chromosomes are giant chromosomes found in the salivary glands of Drosophila larvae.
In these chromosomes,the centromeric regions of all the chromosomes aggregate together to form a common central point called the $Chromocentre$.
This structure is characteristic of polytene chromosomes where multiple chromatids are held together at this central junction.

(A) Polytene chromosomes were first observed in the salivary glands of $Chironomus$ larvae by $E$.$G$. Balbiani in $1881$.
Because they are commonly found in the salivary glands of these dipteran insects,they are also known as salivary gland chromosomes.

(D) Leukemia is a type of cancer that affects the blood and bone marrow. It is characterized by the uncontrolled proliferation of abnormal white blood cells (leukocytes) in the body. Therefore,it is commonly referred to as 'blood cancer'.

(B) The correct answer is $(b)$.
$Molybdenum$ $(Mo)$ is a crucial micronutrient that plays an essential role in nitrogen fixation.
It is a key component of the enzyme $Nitrogenase$ and $Nitrate$ $reductase$.
$Nitrogenase$ is responsible for the reduction of atmospheric nitrogen to ammonia in leguminous plants, while $Nitrate$ $reductase$ is involved in the assimilation of nitrogen.

(A) Carbon monoxide $(CO)$ binds to haemoglobin to form carboxyhaemoglobin $(HbCO)$.
The affinity of haemoglobin for $CO$ is approximately $200$ to $250$ times greater than its affinity for oxygen $(O_2)$.
Due to this high affinity, $CO$ effectively displaces oxygen from haemoglobin, leading to hypoxia and tissue damage.

(C) Gibberellins are a group of plant growth regulators that are primarily responsible for stem elongation and internodal growth in plants.
They promote the elongation of internodes in genetically dwarf plants (rosette plants) by stimulating cell division and cell elongation.
Therefore,$Gibberellin$ is the correct answer for internodal elongation.

(C) : The growth movement in response to touch,or contact of a foreign body,in plants is called thigmotropism or thigmotropic movement.
The stems and tendrils of the climbers are positively thigmotropic in their response.
The coiling of garden pea tendrils around any support is an example of thigmotropism.
Thigmonastic (haptonastic) movements are induced by some external stimuli,but they are non-directional. For example,tentacles of $Drosera$ leaf curve and the lamina lobes of $Dionaea$ fold on coming in contact with an insect.

(D) Plasmids are small,circular,double-stranded $DNA$ molecules that are distinct from a cell's chromosomal $DNA$.
They are naturally found in bacterial cells and some eukaryotes like yeast.
Plasmids are considered extra-chromosomal genetic material because they replicate independently of the chromosomal $DNA$.
They often carry genes that provide advantages to the host,such as antibiotic resistance.

(C) Osmoregulation is the process of maintaining the water and salt balance within an organism. In freshwater protozoans like $Amoeba$ and $Paramecium$,the contractile vacuole is the specialized organelle responsible for osmoregulation. It collects excess water from the cytoplasm and expels it out of the cell to prevent the organism from bursting due to the influx of water by osmosis.

(C) In $Amoeba$,the excretion of metabolic wastes (primarily ammonia) occurs through the general body surface,which is the plasma membrane,via the process of diffusion. While the contractile vacuole is primarily involved in osmoregulation (water balance),the plasma membrane serves as the site for the elimination of nitrogenous waste products.

(A) The presence of two types of nuclei,a large macronucleus and a small micronucleus,is a characteristic feature of the phylum $Ciliophora$ (ciliates).
$Paramecium$,$Vorticella$,$Nyctotherus$,and $Balantidium$ are all examples of ciliates.
However,among the given options,$Paramecium$ and $Vorticella$ are the most classic examples of organisms exhibiting nuclear dimorphism (macronucleus and micronucleus).
Therefore,the correct option is $A$.

(C) The organism $Euglena$ is a unique member of the kingdom $Protista$.
It possesses characteristics of both plants and animals.
Like plants,$Euglena$ contains chlorophyll and performs photosynthesis in the presence of sunlight (autotrophic nutrition).
In the absence of sunlight,it behaves like a heterotroph by feeding on other organic matter (holozoic or saprophytic nutrition).
Additionally,it lacks a rigid cell wall,which is a characteristic feature of animal cells.

(D) White rust of crucifers is a common plant disease caused by the parasitic fungus $Albugo \ candida$ (also known as $Cystopus \ candidus$).
It belongs to the class Oomycetes within the kingdom Fungi.
This pathogen infects members of the Brassicaceae family,such as mustard and radish,leading to the formation of white,powdery pustules on the leaves and stems.

(D) In $Ulothrix$,the lowermost cell is modified into a specialized structure called a holdfast. This cell lacks chlorophyll because its primary function is to attach the filament to a substrate,not to perform photosynthesis. This specialization of a specific cell for a particular function (attachment) rather than the general function of the organism (photosynthesis) represents the beginning of the division of labor within a multicellular organism.

(A) Agar is a gelatinous substance that is commercially obtained from certain species of marine red algae (Rhodophyceae),specifically $Gelidium$ and $Gracilaria$. It is widely used in laboratories as a culture medium for growing microorganisms and in the food industry as a thickening agent.

(B) In bryophytes,including mosses like $Funaria$,the dominant phase of the life cycle is the gametophyte.
The plant body is haploid $(n)$ and represents the gametophytic generation.
The sporophyte $(2n)$ is dependent on the gametophyte for its nutrition and anchorage.
Therefore,the plant body is primarily gametophytic.

(C) The archegonium is the female sex organ in Bryophytes,Pteridophytes,and Gymnosperms.
In Bryophytes and Pteridophytes,the archegonium typically has a neck with a variable number of neck canal cells.
Specifically,in many Pteridophytes and Gymnosperms,the neck region of the archegonium is characterized by the presence of $4-6$ rows of neck canal cells.
Therefore,this feature is commonly observed in both Pteridophytes and Gymnosperms.

(B) $Pseudomonas$ species are well-known for their metabolic versatility. They possess a wide array of catabolic enzymes that allow them to break down a vast range of complex organic compounds,including hydrocarbons,pesticides,and various pollutants. This ability makes them highly significant in bioremediation processes.

(D) The correct answer is $D$. Yeast,specifically $Saccharomyces$ $cerevisiae$ (Brewer's yeast),is widely used in the fermentation industry.
During the process of alcoholic fermentation,yeast utilizes the enzyme complex $Zymase$ to convert glucose into ethyl alcohol and carbon dioxide.
The chemical reaction is as follows:
$C_6H_{12}O_6 \xrightarrow{Zymase} 2C_2H_5OH + 2CO_2$
In this reaction,$C_6H_{12}O_6$ is glucose,and $C_2H_5OH$ is ethyl alcohol.

(B) In flowering plants,one pollen mother cell $(PMC)$ undergoes meiosis to produce four haploid microspores (pollen grains).
Therefore,to produce $100$ pollen grains,the number of meiotic divisions required is calculated as:
Number of meiotic divisions = $\frac{\text{Total number of pollen grains}}{4} = \frac{100}{4} = 25$.
Thus,$25$ meiotic divisions are necessary.

(A) Polyembryony is defined as the occurrence of more than one embryo in a single seed.
In many species of $Citrus$,the nucellar cells surrounding the embryo sac start dividing,protrude into the embryo sac,and develop into embryos.
Thus,each ovule contains many embryos,making it a characteristic feature of $Citrus$.

(B) $Corpus$ $luteum$ literally means yellow body.
The luteal cells secrete small amounts of estradiol hormone and significant amounts of the progesterone hormone.
$Corpus$ $luteum$ also secretes the hormone relaxin.

(D) 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 $trans$ position (where the mutations are on different chromosomes) because there is no functional copy of the gene.
However,if the mutations are in different genes,they will show a wild-type (normal) phenotype in the $trans$ position because the functional copies of the genes complement each other.
If two genetic loci produce identical phenotypes in both $cis$ and $trans$ positions,it indicates that they are not functionally related in a way that suggests they are alleles of the same gene or interacting genes in a complementation sense; rather,they are considered to be different genes that do not exhibit functional interaction or complementation.

(C) Epistasis is a genetic interaction where the effect of one gene is dependent on the presence of one or more 'modifier' genes,i.e.,the genetic background.
In this phenomenon,an allele of one gene masks or suppresses the phenotypic expression of an allele of another gene.
For example,in dominant epistasis,the $F_2$ phenotypic ratio is typically modified to $12:3:1$ instead of the standard Mendelian $9:3:3:1$ ratio.

(D) Sickle-cell anaemia is a classic example of a point mutation. It is an autosomal recessive genetic disorder caused by a single nucleotide substitution ($GAG$ to $GUG$) in the $\beta$-globin gene of haemoglobin. This mutation results in the replacement of glutamic acid with valine at the sixth position of the $\beta$-globin chain,leading to the formation of abnormal haemoglobin $(HbS)$.

(D) The condition involving an extra $X$ chromosome in a human male,resulting in a genotype of $47, XXY$,is known as Klinefelter syndrome.
This chromosomal disorder occurs due to the presence of an additional copy of the $X$ chromosome in males.
Individuals with this syndrome exhibit overall masculine development,but they also show feminine characteristics such as the development of breast tissue (gynecomastia).
Such individuals are sterile.

(B) The Barr body is a condensed,inactive $X$ chromosome found in the somatic cells of female mammals. According to the Lyon hypothesis,one of the two $X$ chromosomes in female somatic cells undergoes heterochromatinization during early embryonic development. This inactive $X$ chromosome appears as a deeply stained,dense body near the nuclear envelope,known as the Barr body. Therefore,the correct answer is $B$.

(B) transgenic animal is defined as an animal that has had its $DNA$ manipulated to possess and express an extra (foreign) gene.
This is typically achieved by injecting foreign $DNA$ into the nucleus of a fertilized egg or zygote.
Since the $DNA$ is introduced at the zygote stage,it becomes integrated into the genome of the developing embryo.
Consequently,every cell in the resulting adult animal will contain the foreign $DNA$ sequence,allowing the trait to be expressed throughout the organism's body and passed on to its offspring.

(C) The original $DNA$ sequence is $ATG\ ATG\ ATG$.
When a cytosine $(C)$ base is inserted at the beginning,the sequence becomes $C\ ATG\ ATG\ ATG$.
This is an example of an insertion mutation.
Since the reading frame is shifted by one base,the new codons formed are $CAT$,$GAT$,$GAT$,and the remaining $G$ base.
Therefore,the resulting sequence is $CAT\ GAT\ GAT\ G$.

(D) The $lac$ operon is a cluster of genes that regulates the production of enzymes required to metabolize lactose in bacterial cells $(E. coli)$.
When glucose is present,the $lac$ operon is repressed.
When glucose is absent and lactose is provided as the sole carbon source,lactose acts as an inducer.
It binds to the repressor protein,preventing it from binding to the operator,thereby allowing transcription of the structural genes to occur.
Thus,the $lac$ operon is induced.

(D) An anticodon is a sequence of three nucleotides (a triplet) located on the $tRNA$ molecule.
It is complementary to the codon on the $mRNA$ strand.
These three bases are located on an exposed loop of the $tRNA$ molecule,known as the anticodon loop,which allows them to base-pair with the corresponding codon during the process of translation.

(B) In humans,reproduction occurs through sexual means involving the fusion of a haploid male gamete (sperm) and a haploid female gamete (ovum).
Each gamete contains $23$ chromosomes,which represent half of the total genetic material of the parent.
Upon fertilization,the zygote receives $23$ chromosomes from the father and $23$ chromosomes from the mother,totaling $46$ chromosomes ($23$ pairs).
Therefore,a child inherits exactly $50\%$ of their genetic material from their father and $50\%$ from their mother.

(C) In split genes,the coding sequences are known as $Exons$.
During the process of post-transcriptional modification,the non-coding sequences,known as $Introns$,are removed through splicing.
The remaining coding sequences,$Exons$,are then joined together to form a functional $mRNA$ molecule.

(C) Plague is an infectious disease caused by the bacterium $Yersinia$ $pestis$.
It is primarily transmitted to humans through the bite of infected fleas that live on small rodents.
$Salmonella$ $typhimurium$ causes typhoid fever, $Trichinella$ $spiralis$ causes trichinosis, and $Leishmania$ $donovani$ causes kala-azar (leishmaniasis).
Therefore, the correct option is $C$.

(A) $Yersinia pestis$ is a Gram-negative,non-motile,rod-shaped coccobacillus bacterium.
It is the causative agent of the disease known as Plague.
Plague is a zoonotic disease,typically transmitted to humans through the bite of infected fleas found on rodents.

(C) The correct answer is $C$.
Syphilis is a sexually transmitted disease $(STD)$ caused by the bacterium $Treponema \text{ } pallidum$.
$Trichuris \text{ } trichiura$ is a whipworm that causes trichuriasis, not syphilis.

(B) An allergy is an 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 allergens such as pollen grains,dust,or mold spores.
$Enteric \ fever$ (typhoid) is caused by the bacterium $Salmonella \ typhi$.
$Skin \ cancer$ is caused by uncontrolled cell division due to mutations,often linked to $UV$ radiation.
$Goitre$ is typically caused by an iodine deficiency.
Therefore,the correct option is $B$.

(B) Transgenic animals are animals that have had their $DNA$ manipulated to possess and express an extra (foreign) gene.
In the production of transgenic animals,the foreign $DNA$ is introduced into the germline cells or the early embryonic cells.
As a result,this foreign $DNA$ is incorporated into the genome of every cell in the developing organism.
Therefore,transgenic animals possess foreign $DNA$ in all of their cells.

(A) The silkworm $(Bombyx \text{ } mori)$ larva possesses a pair of specialized glands known as silk glands, which are modified salivary glands.
These glands secrete a viscous fluid containing fibroin protein, which hardens upon exposure to air to form the silk thread used by the larva to spin its cocoon.
Therefore, silk is the secretion of the salivary glands of the larva.

(B) Biological control is a method of controlling pests using other living organisms (predators or parasites).
One of the major challenges in this method is that the predator or parasite introduced to control a specific pest may not always survive or adapt well when transferred to a new environment,as it may be sensitive to the local climate,lack of alternative food sources,or other ecological factors.
Therefore,option $B$ is the most significant practical difficulty among the choices provided.

(C) Rotenone is a natural,powerful insecticide that is extracted from the roots of the plant $Derris$ $elliptica$.

(D) Rotenone is a naturally occurring compound found in the roots and stems of several tropical plants,such as those in the genus $Derris$. It acts as a broad-spectrum insecticide and piscicide. It works by inhibiting the mitochondrial electron transport chain,specifically targeting complex $I$,which prevents the production of $ATP$ in insects and fish,leading to their death. Therefore,it is classified as a natural insecticide.

(D) Biological control involves the use of natural predators,parasites,or pathogens to manage pest populations. Unlike chemical pesticides,which are often toxic,polluting,and require repeated application,biological control agents can establish themselves in the environment and reproduce,making them self-perpetuating. This method is sustainable and environmentally friendly.

(D) The brewing industry produces various types of alcoholic beverages depending on the fermenting agent and the raw material used.
$Saccharomyces$ $cerevisiae$,commonly known as brewer's yeast,is the primary organism used for alcohol fermentation.
Other species like $S. sake$,$S. ellipsoidens$ (wine yeast),and $S. pireformis$ (ginger yeast) are also used in specific fermentation processes.

(A) Citric acid is commercially produced by the aerobic fermentation of sucrose present in beet molasses using the fungus $Aspergillus$ $niger$.
$Aspergillus$ $niger$ is a filamentous fungus widely used in the biotechnology industry for the production of organic acids.

(C) The correct answer is $C$.
Desertification is the process of land degradation in arid,semi-arid,and dry sub-humid areas.
Overgrazing is a primary causative factor because it removes the protective vegetation cover from the soil.
When vegetation is removed,the topsoil becomes exposed to wind and water erosion,leading to the loss of soil fertility and the eventual transformation of the land into a desert.

(A) The change in the peppered moth population from the lighter variety to the darker variety $(carbonaria)$ is a classic example of natural selection.
In the $19^{th}$ century, during the Industrial Revolution in England, the environment became covered in soot and smoke.
The lighter-coloured moths were easily spotted by predators against the soot-covered tree trunks, while the darker-coloured moths were better camouflaged.
This provided a survival advantage to the darker variety, which then reproduced and passed on their genes to the next generation.
This phenomenon is driven by the mutation of a single Mendelian gene that confers a survival advantage in the smoke-laden industrial environment, leading to an increase in the frequency of the darker phenotype.

(A) Primary succession refers to the process of ecological succession that occurs in an area where no living organisms previously existed.
Examples of such areas include newly exposed sea floors,bare igneous rocks,sand dunes,lava sediments,or newly created ponds.
These environments are initially very hostile to life,making the establishment of a pioneer community a slow and gradual process.
In contrast,secondary succession occurs in areas where life previously existed but was destroyed (e.g.,cleared forests or harvested fields).

(D) The correct answer is $(d)$.
Approximately $2/3$ of the Earth's surface is covered by oceans.
In this ecosystem,various types of food chains interact to form complex food webs.
This ecosystem is considered the most stable due to the high buffering capacity of water,which maintains relatively constant environmental conditions.

(D) The correct answer is $D$.
Primary consumers are organisms that feed directly on producers (plants or algae).
Since producers are autotrophs,organisms that consume them are classified as herbivores.
Therefore,primary consumers are herbivores.

(A) Decomposers play a crucial role in the ecosystem by breaking down complex organic matter into simpler inorganic substances.
This process,known as mineralization,releases nutrients back into the soil,making them available for uptake by producers (plants).
If decomposers are removed,the recycling of nutrients stops,leading to the accumulation of dead organic matter and a deficiency of essential minerals for plant growth.
Therefore,the movement of minerals within the ecosystem will be blocked.

(C) . The $Gir$ National Park and Wildlife Sanctuary, located in $Gujarat$, is famous for being the only natural habitat of the Asiatic $Lion$ $(Panthera leo persica)$. Therefore, $Gir - Lion$ is the correct matching pair.

(C) The correct match is $Manas - Assam$.
$Corbett$ is located in $Uttarakhand$.
$Palamau$ is located in $Jharkhand$.
$Bandipur$ is located in $Karnataka$.

(D) The state of $Haryana$ has named all its tourist resorts after different birds. This initiative was taken by the $Haryana$ Tourism Corporation to promote wildlife awareness and tourism in the state. Examples include resorts named $Kingfisher$, $Magpie$, $Bulbul$, and $Parakeet$.

(A) Strontium-$90$ $(^{90}Sr)$ is considered one of the most dangerous radioactive pollutants for humans.
It is a byproduct of nuclear fission and is chemically similar to calcium.
When released into the environment,it enters the food chain and is absorbed by the body,where it accumulates in bones and teeth.
Because it mimics calcium,it replaces it in bone tissue,leading to long-term exposure to ionizing radiation,which increases the risk of bone cancer and leukemia.

(B) The Taj Mahal is primarily threatened by $SO_2$ (sulphur dioxide) emissions from nearby industries and refineries. $SO_2$ reacts with atmospheric water vapour to form sulphuric acid $(H_2SO_4)$,which leads to acid rain. This acid rain reacts with the calcium carbonate $(CaCO_3)$ present in the white marble of the Taj Mahal,causing it to corrode and turn yellow,a process often referred to as 'marble cancer'.

(A) The $BOD$ (Biochemical Oxygen Demand) is a measure of the amount of dissolved oxygen required by aerobic microorganisms to decompose the organic matter present in a given volume of water.
When a large amount of sewage is dumped into a river,it introduces a significant quantity of organic matter.
Microorganisms consume this organic matter,which leads to an increase in their metabolic activity and oxygen consumption.
Therefore,the $BOD$ of the water increases as the level of organic pollution rises.

(C) The correct answer is $C$.
Excessive concentration of $Fluorine$ in drinking water leads to a condition known as fluorosis.
The primary symptoms of dental fluorosis include the mottling of teeth,which appears as yellowish or brownish streaks or spots on the enamel.
In severe cases,it can also lead to skeletal fluorosis,causing abnormal bone structure and increased susceptibility to fractures.
This is a classic example of an endemic disease caused by environmental chemical contamination.

(B) Minamata disease is a neurological syndrome caused by severe mercury poisoning. It was first discovered in Minamata city,Japan,in $1956$. The disease was caused by the release of methylmercury in the industrial wastewater from the Chisso Corporation's chemical factory. This mercury bioaccumulated in shellfish and fish in Minamata Bay,which were then consumed by the local human population and animals. Cats,dogs,and pigs were observed to be affected by the disease due to the consumption of contaminated fish. Rabbits,however,were not part of the food chain involving the contaminated marine life in the bay and thus remained free from the disease.

(D) Antony van Leeuwenhoek was the first person to observe bacteria and protozoa in $1675$. He used his self-designed simple microscope to examine samples of rainwater,saliva,and plaque from his teeth. He referred to these microscopic organisms as 'animalcules'.

(A) The 'New World' refers to the Americas. Red pepper (Capsicum) originated in the Americas and was introduced to India by the Portuguese. It has since become an indispensable ingredient in Indian cooking,whereas spices like black pepper,cardamom,and ginger are native to the 'Old World' (specifically Asia/India).

(A) The term $New \ World$ refers to the Americas. Many crops,including $Red \ pepper$ (Capsicum),were introduced to India from the Americas by Portuguese traders. It has since become an integral part of Indian cuisine,whereas $Black \ pepper$,$Cardamom$,and $Ginger$ are native to the Old World (specifically India and Southeast Asia).

(A) Citric acid is commercially produced through the fermentation process using the fungus $Aspergillus$ $niger$.
This fungus is highly efficient at converting sugars into citric acid under controlled conditions.

(B) The number of individuals of a species per unit area or volume is known as population density. When this refers to the total number of individuals of a species in a specific habitat,it is called absolute density. However,in ecological studies,the term 'abundance' is often used to describe the population density of a species within its habitat. Among the given options,absolute density is the standard ecological term for the total count of individuals of a species in a given area.

(B) The correct answer is $B$. The age structure of a population is determined by birth rates and death rates. In India,there is a high birth rate,which leads to a larger proportion of young individuals in the population. Additionally,the average life span in India is relatively lower compared to developed nations,which contributes to a pyramid-shaped age structure where the base (younger age groups) is significantly wider than the top (older age groups).

(A) The concept that population tends to increase geometrically while food supply increases arithmetically was proposed by $Thomas \ Malthus$ in his essay on the principles of population.
This idea significantly influenced $Charles \ Darwin$, helping him realize that under intense competition for limited resources, natural selection operates as a mechanism for evolution.

(D) The correct answer is $D$. Sibling species (also known as cryptic species) are defined as two or more species that are morphologically indistinguishable or very similar but are reproductively isolated from each other. They occupy the same geographical area (sympatric) and share a very recent common ancestor.