AIIMS 1998 Biology Question Paper with Answer and Solution

(C) In $Nostoc$,the cells are arranged in a chain-like structure. When this filament of cells is present without its surrounding mucilaginous sheath,it is specifically referred to as a $Trichome$. Hyphae and Mycelium are terms associated with fungi,while a Colony refers to the entire aggregate of filaments.

(C) The correct answer is $C$. In mosses,the stomata are considered primitive because they lack functional guard cells that regulate opening and closing. These structures are specifically found in the $Apophysis$ region of the moss capsule,which is the sterile basal part of the sporophyte.

(D) $Selaginella$ is a heterosporous pteridophyte,meaning it produces two types of spores: microspores and megaspores.
Reduction division,also known as meiosis,occurs in the spore mother cells located within the sporangia to produce these haploid spores.
Since both microspores and megaspores are produced via meiosis from their respective mother cells,reduction division occurs during the formation of both.
Therefore,the correct answer is $(d)$.

(D) The correct answer is $(d)$.
At the base of the ligule,there is a sheath of elongated cells known as the glossopodium,which is secretory in nature.
The ligule serves both secretory and protective functions.
It secretes water or mucilage to keep the growing point of the stem and young leaves moist.
Additionally,it protects the young leaves during their development.

(C) Arbor vitae,also known as the 'tree of life',is a branching white matter structure located within the cerebellum of the brain.
It consists of myelinated nerve fibers that carry sensory and motor information to and from the cerebellar cortex.
Because it is composed of myelinated axons,it appears white in color,distinguishing it from the surrounding grey matter of the cerebellar cortex.

(A) The term $chromatophore$ was coined by the German botanist $F. \text{Schmitz}$ in $1882$.
$Chromatophores$ are pigment-containing and light-reflecting cells, or groups of cells, found in a wide range of animals including amphibians, fish, crustaceans, cephalopods, and reptiles.
In plants, the term was historically used to describe plastids containing pigments.

(C) The nucleolus is a non-membrane-bound,spherical,and conspicuous structure present within the nucleoplasm of the nucleus.
It is the site for active ribosomal $RNA$ $(rRNA)$ synthesis.
It is often found attached to a specific region of a chromosome known as the nucleolar organizer region $(NOR)$.

(C) Guttation is the process of exudation of water droplets from the margins or tips of leaves of uninjured plants.
This occurs through specialized pores called hydathodes.
It usually happens during the night or early morning when the rate of transpiration is low and root pressure is high.

(A) The primary function of the $phloem$ tissue in plants is the translocation of organic nutrients,primarily sucrose,from the source (leaves) to the sink (roots,fruits,or developing tissues).
Therefore,the chief function of $phloem$ is the conduction of food.

(A) The kidney is divided into two main regions: the outer $Cortex$ and the inner $Medulla$.
Bowman's capsules,along with the glomeruli and the convoluted tubules ($PCT$ and $DCT$),are located in the renal $Cortex$.
The $Medulla$ primarily contains the loops of Henle and the collecting ducts.
Therefore,all Bowman's capsules are found in the $Cortex$.

(C) The formation of urine involves three main processes: glomerular filtration,reabsorption,and secretion.
Uric acid is primarily removed from the blood and added to the filtrate in the renal tubules through the process of tubular secretion.
Tubular secretion is an active process where cells of the nephron (specifically in the proximal convoluted tubule and distal convoluted tubule) transport substances like $H^+$,$K^+$,and uric acid from the peritubular capillaries into the filtrate.

(D) The posterior pituitary (neurohypophysis) does not synthesize hormones itself.
It stores and releases two hormones,$Oxytocin$ and $Vasopressin$ $(ADH)$,which are synthesized by the neurosecretory cells of the hypothalamus.
These hormones are transported along the axons of these neurons to the posterior pituitary,where they are stored and released into the blood under the control of neurosecretory nerve axons.

(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 $(c)$.

(A) Sensory receptors are classified based on the source of stimuli.
$1$. $Interoceptors$ (or visceroceptors) are receptors located within the internal organs of the body,such as the stomach,intestines,and blood vessels. They detect internal physiological changes like pain,pressure,or chemical composition.
$2$. $Exteroceptors$ detect stimuli from the external environment (e.g.,skin receptors for touch,temperature).
$3$. $Proprioceptors$ provide information about body position and movement (e.g.,muscle spindles).
$4$. $Teloreceptors$ detect stimuli from a distance (e.g.,eyes for vision,ears for hearing).
Therefore,the sensation of stomach pain is detected by $Interoceptors$.

(C) The eye consists of three layers. The outermost layer is the sclera (sclerotic),which is composed of dense connective tissue. The middle layer is the choroid,which contains many blood vessels and looks bluish in color. The choroid layer is thin over the posterior two-thirds of the eyeball,but it becomes thick in the anterior part to form the ciliary body. This layer contains numerous pigment cells (melanocytes) that give it a dark/black appearance. The primary function of this black pigment is to absorb light rays and prevent internal reflection within the eyeball,thereby ensuring a clear image.

(A) Addison's disease is primarily caused by the hyposecretion (under-secretion) of hormones from the adrenal cortex,specifically glucocorticoids like cortisol and mineralocorticoids like aldosterone. This deficiency leads to symptoms such as fatigue,muscle weakness,weight loss,and low blood pressure. Therefore,the hyposecretion of these adrenal hormones is the underlying cause.

(C) In $Centrolecithal$ eggs,the yolk is concentrated in the centre of the ovum,while the cytoplasm is restricted to the peripheral layer.
This type of egg is commonly observed in insects (Arthropoda).

(A) Viruses are considered to be at the borderline between living and non-living. They are not considered true organisms because they lack a cellular structure,independent metabolic machinery,and the ability to replicate outside a host cell.
They consist of genetic material ($DNA$ or $RNA$) enclosed in a protein coat (capsid),making them nucleoproteins.
Since they lack cell organelles and cannot perform life processes independently,the Reason correctly explains why they are not classified as organisms.

(A) Sponges are multicellular organisms that belong to the phylum $Porifera$.
They possess a unique water transport or canal system,which is a defining characteristic of this phylum.
Water enters through minute pores called $ostia$ in the body wall into a central cavity,the $spongocoel$,and goes out through the $osculum$.
This pathway of water transport is helpful in food gathering,respiratory exchange,and removal of waste.
Since the canal system is a diagnostic feature that defines the phylum $Porifera$,the Reason correctly explains the Assertion.

(A) The Assertion is correct because the vascular cambium is a lateral meristem responsible for secondary growth,which increases the girth (width) of the stem.
The Reason is also correct because the vascular cambium consists of two types of cells: fusiform initials,which give rise to the axial system (tracheary elements,fibers,and axial parenchyma),and ray initials,which give rise to the radial system (vascular rays).
Since the presence of these specific initials is the structural basis that allows the cambium to function as a lateral meristem and produce secondary tissues,the Reason correctly explains the Assertion.

(C) The Assertion is correct because meiosis is a reductional division that reduces the chromosome number by half,resulting in the formation of haploid cells.
The Reason is incorrect because synapsis,which is the pairing of homologous chromosomes,occurs during the $zygotene$ stage of prophase-$I$,not during the $leptotene$ stage. $Leptotene$ is characterized by the compaction of chromosomes.

(C) The Assertion is correct because the $C_4$ pathway is an evolutionary adaptation found in many tropical plants to minimize photorespiration.
The Reason is incorrect because,in the $C_4$ pathway,the primary $CO_2$ acceptor is a $3$-carbon compound called Phosphoenolpyruvate $(PEP)$,but the $CO_2$ is fixed into a $4$-carbon compound called Oxaloacetic acid $(OAA)$. The statement 'fixed by $3C$ compound' is misleading as it implies the product is $3C$,whereas the fixation process involves the $3C$ acceptor to form a $4C$ stable product.

(A) Muscle fatigue is defined as the inability of a muscle to contract or relax effectively after prolonged or repeated stimulation.
This condition is primarily caused by the accumulation of lactic acid in the muscle tissue,which occurs due to anaerobic respiration during intense or repeated muscle contractions.
Therefore,both the Assertion and the Reason are correct,and the Reason provides a valid explanation for the phenomenon of muscle fatigue.

(C) The correct option is $C$.
Covered smut of barley is caused by the fungus $Ustilago \ hordei$,not $Ustilago \ nuda$.
$Ustilago \ nuda$ is responsible for the loose smut of barley.
Therefore,the pair in option $C$ is incorrectly matched.

(C) Apospory is the development of a gametophyte directly from the vegetative cells of the sporophyte without the involvement of meiosis or the formation of spores. This process bypasses the typical alternation of generations where spores are required to produce the gametophyte.

(B) The most primitive living mammals are the monotremes (egg-laying mammals),such as the platypus and echidna and echidna. These animals are primarily found in Australia and New Guinea. Their presence in these isolated regions provides significant evidence for organic evolution and continental drift,as they represent an ancient lineage that has survived due to geographic isolation.

(B) The correct answer is $B$.
Plant tissue culture media require specific nutrients for the growth of explants.
Coconut milk, derived from the endosperm of $Cocos nucifera$ (coconut), is widely used as a natural source of growth regulators (specifically cytokinins), vitamins, and amino acids in plant tissue culture media to promote cell division and growth.

(B) The correct answer is $B$.
Pollination by snails and slugs is known as $Malacophily$.
$Ornithophilous$ refers to pollination by birds.
$Anemophilous$ refers to pollination by wind.
$Chiropterophilous$ refers to pollination by bats.
An example of a plant pollinated by snails is $Arisaema$ (cobra plant).

(A) In humans,the placenta is classified as $Haemo-chorialis$.
This means that the chorionic villi are directly bathed in the maternal blood because the maternal endothelium,connective tissue,and uterine epithelium are eroded.
This type of placentation allows for efficient exchange of nutrients,gases,and waste products between the maternal and fetal bloodstreams.

(D) During the metamorphosis of a frog's tadpole, significant anatomical and physiological changes occur to transition from an aquatic, herbivorous larva to a terrestrial, carnivorous adult.
$1$. The $Intestine$ undergoes a drastic reduction in length as the diet shifts from herbivorous (algae) to carnivorous (insects).
$2$. The $Lung$ develops as the animal transitions from gill-based respiration to pulmonary respiration.
$3$. The $Heart$ undergoes structural remodeling to accommodate the shift from a single-circuit circulatory system to a double-circuit system.
$4$. The $Nervous \text{ } system$ remains fundamentally the same in its basic organization, although it undergoes maturation and refinement to support the complex behaviors of the adult frog.

(B) The distance between two linked genes on a chromosome is measured in map units or centimorgans $(cM)$.
This distance is directly proportional to the frequency of crossing over between them.
The cross-over value (or recombination frequency) represents the percentage of offspring that show recombinant phenotypes.
Therefore,the distance is measured as the cross-over value.

(A) The $tRNA$ (transfer $RNA$) acts as an adapter molecule.
It has many varieties,where each variety is specific for a particular amino acid.
During protein synthesis,$tRNA$ picks up its specific amino acid from the amino acid pool in the cytoplasm and transports it to the $mRNA$ template on the ribosomes.
This process ensures that the amino acids are added in the correct sequence as specified by the genetic code.

(C) $mRNA$ (messenger $RNA$) is a non-genetic $RNA$ molecule that carries the genetic code from $DNA$ to the ribosomes. It contains the instructions for the sequence of specific amino acids that need to be linked together to form a polypeptide chain,making it essential for protein synthesis.

(C) The genes present on the differential (non-homologous) region of the $Y$ chromosome are exclusively inherited from father to son.
These genes are known as Holandric genes.
Since they are located only on the $Y$ chromosome,they do not have corresponding alleles on the $X$ chromosome and are expressed in all males who inherit the $Y$ chromosome.

(C) The most primitive living mammals are monotremes (egg-laying mammals),such as the platypus and echidna and echidna. These organisms are primarily found in Australia and New Guinea. Their unique geographical distribution and primitive characteristics provide significant evidence for organic evolution,demonstrating how isolated landmasses allowed for the survival of ancestral mammalian lineages.

(D) 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 like pollen grains.
Goitre is caused by iodine deficiency.
Enteric fever (typhoid) is a bacterial infection caused by $Salmonella$ $typhi$.
Skin cancer is caused by uncontrolled cell division often due to $UV$ radiation exposure.
Therefore,the correct option is $D$.

(B) Alloxan is a toxic glucose analogue that selectively destroys the $Beta-$ cells of the islets of Langerhans in the pancreas.
These $Beta-$ cells are responsible for the production and secretion of insulin.
By destroying these cells,Alloxan induces experimental diabetes mellitus in laboratory animals,making it a standard tool for studying the disease.

(B) The ability of a single plant cell to divide and differentiate into a complete, mature plant is known as $Totipotency$.
This biological phenomenon forms the fundamental basis for $Tissue \text{ } culture$ techniques, where explants are grown in a nutrient medium to regenerate whole plants.

(C) Bordeaux mixture is a fungicide used to control various fungal diseases in plants.
It is prepared by mixing copper sulphate $(CuSO_4)$ and calcium hydroxide ($Ca(OH)_2$,also known as slaked lime) in water.
Therefore,both $(a)$ and $(b)$ are required to make the mixture.

(B) $Phytotron$ is a specialized growth chamber or facility designed to grow plants under strictly controlled environmental conditions.
It allows researchers to manipulate variables such as temperature,light,humidity,and $CO_2$ concentration to study plant growth,development,and physiological responses in a reproducible manner.

(D) Ecological pyramids are graphical representations of the relationship between different trophic levels in an ecosystem.
These relationships can be represented by the pyramid of energy,the pyramid of biomass,and the pyramid of numbers.
Therefore,all of these options correctly depict the relationships in an ecosystem.

(B) The correct answer is $B$.
The biosphere is the global ecological system integrating all living beings and their relationships,including their interaction with the elements of the lithosphere (land),hydrosphere (water),and atmosphere (air).
It encompasses all living organisms (biotic components) and the physical environment (abiotic components) they inhabit.

(D) Deciduous forests are characterized by moderate rainfall,which is sufficient for the growth of trees that shed their leaves annually. These biomes typically experience distinct seasons,including a summer period with moderate precipitation that supports a dense canopy of broad-leaved trees.

(D) Tropical wet evergreen forests in India are primarily found in regions with high rainfall, such as the Western Ghats, the Andaman and Nicobar Islands, and parts of the North-Eastern states (including West Bengal).
$Madhya Pradesh$ is located in central India and is characterized by tropical dry deciduous forests rather than wet evergreen rain forests due to lower annual rainfall compared to the coastal and northeastern regions.

(A) The process of double fertilization involves the fusion of one male gamete $(n)$ with the secondary nucleus $(2n)$ to form the primary endosperm nucleus $(3n)$.
This process is known as triple fusion.
The primary endosperm nucleus develops into the endosperm,which is a triploid $(3n)$ nutritive tissue.
The endosperm provides essential nutrients to the developing embryo.

(C) The Assertion is correct because the origin of life occurred billions of years ago,and the geological processes,such as plate tectonics,erosion,and climatic changes,have destroyed most of the direct evidence (fossils or transitional forms) from that era.
The Reason is incorrect because the loss of evidence is a natural geological and environmental phenomenon,not a result of human negligence or lack of skill in preservation.

(C) The Assertion is correct because male $Anopheles$ mosquitoes feed on plant juices and nectar,not blood.
They lack the piercing and sucking mouthparts required to bite humans and transmit pathogens.
The Reason is incorrect because male $Anopheles$ mosquitoes do not carry $Plasmodium$ at all,regardless of the strain's virulence. $Plasmodium$ requires a female $Anopheles$ mosquito to complete its life cycle and be transmitted to humans.