AIIMS 2002 Biology Question Paper with Answer and Solution

(B) In the initial stage of cyst formation,two or more transparent,rod-like structures are present in the cytoplasm,which are called chromatid bodies.
These chromatid bodies are composed of ribonucleoprotein and are characteristic of the early cystic stage of $Entamoeba$.

(B) Lichens are symbiotic associations between algae and fungi.
$Cyphella$ are small,cup-like depressions found on the lower surface of the thallus in certain foliose lichens (e.g.,$Sticta$).
These structures facilitate gaseous exchange,thereby assisting in the respiration of the lichen thallus.

(B) The sporogonium of $Marchantia$ is differentiated into foot,seta,and capsule.
The capsule contains elaters $(2n)$.
Elaters are diploid,spindle-shaped,hygroscopic,elongated structures with $2$ spiral bands that aid in spore dispersal.

(D) Bath sponges,such as $Spongia$,belong to the phylum $Porifera$. Their body wall is supported by a skeleton composed of proteinaceous $Spongin$ fibres. When the sponge is dried and the living cells are removed,only the tough,flexible $Spongin$ fibre network remains.

(D) $Hydra$ possesses a primitive nervous system consisting of a nerve net composed of interconnected neurons.
Sensory cells are distributed throughout the epidermis to detect environmental stimuli.
$Nematocytes$ (or $Cnidocytes$) are specialized cells that contain stinging organelles and also function as sensory receptors to trigger the discharge of the nematocyst in response to external stimuli.
Therefore,all these structures contribute to the reception of impulses and stimuli in $Hydra$.

(B) $Wuchereria$ $bancrofti$ is a filarial nematode that causes filariasis (elephantiasis). The adult worms typically reside in the lymphatic vessels and lymph nodes of the human host. They cause chronic inflammation of the organs in which they live for many years,leading to the characteristic swelling of the lower limbs and other body parts.

(A) The excretory organs of the cockroach are $Malpighian$ tubules.
These are yellow-colored,thin,filamentous structures.
They are present at the junction of the midgut and the hindgut.
They help in the removal of excretory products from the hemolymph.

(B) In the class $Ophiuroidea$,the ambulacral grooves are either absent or completely covered by the ossicles. This is a characteristic feature that distinguishes them from other classes like $Asteroidea$,where open ambulacral grooves are present.

(B) The life cycle of $Ascaris$ $lumbricoides$ involves several stages of moulting:
$1$. The first moulting occurs in the soil within the egg.
$2$. The second moulting occurs in the lungs (specifically in the alveoli) after the larvae migrate through the circulatory system.
$3$. The third moulting also occurs in the lungs.
$4$. The fourth moulting occurs in the intestine,where the larvae develop into adults.
Therefore,the sites for the first,second,and third moulting are soil,lung,and lung respectively. However,based on the provided options,the most accurate sequence representing the progression is soil,lung,and lung. Since the question asks for the sites of the first,second,and third moulting,the correct sequence is soil,lung,lung.

(C) Passage cells are thin-walled cells found in the endodermis of roots,specifically located opposite to the protoxylem elements.
These cells allow the radial movement of water and dissolved minerals from the cortex into the xylem.
They are characteristic of monocot roots,where the endodermis remains thin-walled except for the Casparian strips,and these specific cells lack the suberin deposition.

(B) In an $Acrocentric$ chromosome,the $centromere$ is situated close to its end,forming one extremely short arm and one very long arm.
In a $Telocentric$ chromosome,the $centromere$ is at the terminal end.
In a $Submetacentric$ chromosome,the $centromere$ is slightly away from the middle,resulting in one shorter and one longer arm.
In a $Metacentric$ chromosome,the $centromere$ is in the middle,forming two equal arms.
Therefore,the correct answer is $Acrocentric$.

(D) Photorespiration is a complex metabolic pathway that involves the cooperation of three distinct cell organelles:
$1$. $Chloroplast$: Where the initial reaction occurs involving $RuBisCO$ and $O_2$.
$2$. $Peroxisome$: Where the glycolate pathway continues.
$3$. $Mitochondria$: Where glycine is converted to serine, releasing $CO_2$.
Therefore, all three organelles are required for the process to complete.

(A) Glycolysis is the process of the breakdown of glucose into two molecules of pyruvic acid.
It occurs in the cytoplasm of the cell and does not require oxygen.
The overall reaction is: $C_6H_{12}O_6 + 2NAD^+ + 2ADP + 2Pi \rightarrow 2 \text{Pyruvic acid} + 2NADH + 2H^+ + 2ATP$.
Therefore, the correct product formed is pyruvic acid.

(B) Glycolysis is the first stage of cellular respiration that occurs in the cytoplasm of the cell.
In this metabolic pathway,one molecule of glucose $(C_6H_{12}O_6)$,which is a $6$-carbon compound,undergoes a series of enzymatic reactions to produce two molecules of pyruvic acid $(CH_3COCOOH)$,which is a $3$-carbon compound.
This process is common to both aerobic and anaerobic respiration.

(A) $ABA$ (Abscisic acid) acts as a plant growth inhibitor.
It plays a crucial role in inducing bud dormancy,especially as winter approaches,to protect the plant from harsh environmental conditions.
Therefore,$ABA$ is responsible for the induction of dormancy in buds.

(B) The correct answer is $B$.
Lophodont teeth are characterized by the presence of transverse ridges on the occlusal surface,which are known as $lophos$.
These types of teeth are typically found in herbivores such as rabbits and elephants.
Among the given options,premolars and molars exhibit this structure to facilitate the grinding of plant material.

(C) The $pH$ values of various digestive juices are as follows:
$1$. Bile: $7.6 - 8.6$ (Alkaline)
$2$. Saliva: $6.2 - 7.6$ (Slightly acidic to slightly alkaline)
$3$. Gastric juice: $1.5 - 3.5$ (Highly acidic)
$4$. Pancreatic juice: $7.5 - 8.8$ (Alkaline)
Among the given options,gastric juice has the lowest $pH$ due to the presence of $HCl$.

(B) The enzyme $Rennin$ (also known as $Chymosin$) is a proteolytic enzyme found in the gastric juice of infants. It specifically acts on the milk protein $Caseinogen$ (a soluble protein) and converts it into $Paracasein$ (an insoluble protein),which then reacts with calcium ions to form calcium paracaseinate (curd). This process facilitates the digestion of milk protein by $Pepsin$.

(C) Nicotinamide (a form of Niacin or Vitamin $B_3$) is a water-soluble vitamin.
In the human body,it can be synthesized from the essential amino acid $Tryptophan$.
Approximately $60 \ mg$ of $Tryptophan$ is required to synthesize $1 \ mg$ of Niacin.
This metabolic pathway occurs primarily in the liver.

(B) The animals that excrete nitrogenous waste mainly in the form of uric acid are called uricotelic animals.
Uric acid is excreted in the form of a pellet or paste with a minimum loss of water.
Terrestrial reptiles (like lizards and snakes),birds,and most insects are uricotelic animals.
Therefore,among the given options,lizards and crows are uricotelic.

(D) The excretory organs of a cockroach are the Malpighian tubules.
These tubules are yellow-colored,thin,filamentous structures.
They are present at the junction of the midgut and the hindgut.
They help in the removal of excretory products from the hemolymph.

(D) In vertebrates,including frogs,the nervous system originates from the ectoderm. Specifically,the neural tube forms the central nervous system (brain and spinal cord). The neural crest cells are a specialized group of ectodermal cells that migrate away from the neural tube during development. These cells give rise to various structures,including the ganglia of the sympathetic nervous system,the adrenal medulla,and sensory ganglia of the peripheral nervous system. Therefore,the correct origin for these ganglia is the neural crest cells.

(D) Living organisms are considered open systems because they continuously exchange energy and matter with their external environment.
They require a constant input of energy (e.g.,sunlight or food) to maintain their metabolic processes and structural organization.
Since they exchange both energy and matter with the surroundings,they cannot be classified as closed systems.
Therefore,both the Assertion and the Reason are incorrect.

(B) Systematics is the branch of biology that deals with the diversity of organisms and their comparative and evolutionary relationships. It includes identification,nomenclature,and classification of organisms.
Classification is the process by which anything is grouped into convenient categories based on some easily observable characters.
Therefore,both the Assertion and the Reason are correct,but the Reason does not fully explain the definition of Systematics,as Systematics also includes identification and nomenclature.

(A) The lateral line system is a sensory organ found in fishes and aquatic larval amphibians that helps them detect movement and vibration in the surrounding water.
This system consists of specialized sensory cells called neuromasts,which are derived from the ectoderm.
Since the presence of these ectodermally derived sensory receptors is the reason for the functional existence of the lateral line system in these organisms,the Reason correctly explains the Assertion.

(D) Collenchyma consists of living cells with unevenly thickened cell walls.
The cell walls are composed of cellulose,hemicellulose,and pectin.
Since collenchyma cells are living,the assertion that they are 'dead tissue' is incorrect.
However,the reason stating that collenchymatous cells show thickenings of pectin is correct.
Therefore,the Assertion is incorrect,but the Reason is correct.

(A) The Assertion states that organisms are made up of cells,which is a fundamental principle of the Cell Theory.
The Reason states that cells are the structural units of living organisms and maintain a steady chemical composition within their boundaries (homeostasis).
Since the structural organization of an organism is derived from its cellular composition,the Reason correctly explains why organisms are made up of cells.

(A) Specialization of cells refers to the process where cells differentiate to perform specific functions.
By focusing on a single task,cells become highly efficient at that particular function.
This division of labor among specialized cells significantly increases the overall operational efficiency of a multicellular organism.
Therefore,both the Assertion and the Reason are correct,and the Reason is the correct explanation of the Assertion.

(D) Assertion is incorrect because the number of cells in a multicellular organism is generally directly proportional to the size of the body,not inversely proportional. Larger organisms typically have more cells.
Reason is incorrect because not all cells are alive; for example,xylem vessels and tracheids in plants,and cork cells,are dead at maturity.

(A) In purple bacteria,photosynthesis utilizes infrared light with wavelengths longer than $700\, nm$ (typically around $800-900\, nm$).
These bacteria contain bacteriochlorophyll as the primary pigment.
The reaction centre in these purple bacteria is known as $B-890$,which absorbs light at this specific wavelength.
Therefore,both the Assertion and the Reason are correct,and the Reason correctly explains why these bacteria can utilize wavelengths longer than $700\, nm$.

(B) The Assertion is correct because the Calvin cycle requires $6$ molecules of $CO_2$,$12$ molecules of $NADPH + H^+$,and $18$ molecules of $ATP$ to synthesize one molecule of hexose (glucose).
The Reason is also correct because the light-dependent reaction (photophosphorylation) produces $ATP$ and $NADPH + H^+$ (often written as $NADPH_2$) which are then utilized in the dark reaction (Calvin cycle).
However,the Reason does not directly explain why exactly $6$ $CO_2$,$12$ $NADPH$,and $18$ $ATP$ are required; it only states the source of these molecules. Therefore,the Reason is not the correct explanation of the Assertion.

(A) Glycolysis occurs in the cytoplasm because all the necessary enzymes for this metabolic pathway are present there.
This process is common to both aerobic and anaerobic respiration.
In this process,one molecule of glucose is partially oxidized to form $2$ molecules of pyruvic acid.

(A) $WBCs$ (leucocytes) are wandering cells that are capable of moving out of blood capillaries into the tissues through the capillary walls. This process of squeezing out of the blood vessels is known as diapedesis. Since the accumulation of $WBCs$ at the site of injury is directly facilitated by this process,the Reason correctly explains the Assertion.

(C) The resting membrane potential is generated due to the differential permeability of the axonal membrane to $Na^+$ and $K^+$ ions and the active transport of these ions by the $Na^+-K^+$ pump.
This pump requires metabolic energy in the form of $ATP$ (chemical energy),not electrical energy,to maintain the unequal distribution of ions against their concentration gradients.
Therefore,the Assertion is correct,but the Reason is incorrect because it mentions electrical energy instead of chemical energy $(ATP)$.

(D) The regulation of $RBC$ production (erythropoiesis) is accomplished by the hormone erythropoietin $(EPO)$.
Erythropoietin is a glycoprotein produced primarily by the kidneys in response to low oxygen levels in the blood.
It stimulates the red bone marrow to increase the rate of stem cell mitosis and accelerates the maturation of $RBCs$,thereby increasing the oxygen-carrying capacity of the blood.
Follicle-stimulating hormone $(FSH)$ is a gonadotropic hormone secreted by the anterior pituitary gland,which regulates gametogenesis and reproductive functions,not $RBC$ production.
Therefore,the Assertion is incorrect,and the Reason is correct.

(C) Plasmids are small,circular,double-stranded extra-chromosomal $DNA$ molecules found in prokaryotic cells,such as bacteria.
$1$. The Assertion is incorrect because plasmids are double-stranded,not single-stranded.
$2$. The Reason is incorrect because plasmids are typically found in prokaryotic cells,not eukaryotic cells.
Therefore,both the Assertion and the Reason are incorrect.

(A) In $Cycas$ (a gymnosperm),the megasporophyll is a leaf-like structure that bears ovules. In angiosperms,the female reproductive organ is the carpel (or pistil),which also bears ovules. Therefore,the megasporophyll of $Cycas$ is homologous to the carpel of angiosperms.

(D) $Wuchereria$ $bancrofti$ is a filarial nematode that causes filariasis in humans.
The adult worms typically reside in the lymphatic vessels and lymph glands of the human host.
It is a viviparous nematode,meaning it gives birth to live young ones called microfilariae.

(B) $Xenia$ is the direct effect of foreign pollen on the development and characteristics of the endosperm in seeds. Since the endosperm is formed by the fusion of one male gamete with two polar nuclei (double fertilization),it carries paternal genetic traits that can influence its phenotype,such as color or texture.

(D) $Xenia$ refers to the direct effect of pollen grains on the tissues of the seed,specifically the endosperm,outside the embryo.
$Metaxenia$ refers to the effect of pollen grains on the maternal tissues of the fruit,such as the $testa$ and fruit wall.
$Maize$ is a classic example of $Xenia$,while $Date$ $palm$ is a well-known example of $Metaxenia$.

(C) According to Mendel's law of dominance,when a homozygous dominant plant (red-flowered,$RR$) is crossed with a homozygous recessive plant (white-flowered,$rr$),the $F_1$ generation offspring will all be heterozygous $(Rr)$.
Since the allele for red color is dominant over the allele for white color,all $F_1$ offspring will exhibit the red-flowered phenotype.

(D) The Wobble hypothesis was proposed by $F. H. C. Crick$ in $1965$.
This hypothesis explains that the third base of a codon (at the $3'$ end) can form non-standard base pairs with the first base of an anticodon (at the $5'$ end),allowing a single $tRNA$ to recognize more than one codon.

(B) pure line breed refers to a population of individuals that are genetically identical and homozygous for the traits of interest.
This is achieved through continuous self-pollination over several generations.
As a result,the individuals exhibit homozygosity for all their alleles,ensuring that the offspring will consistently show the same traits as the parents.

(D) Genetic engineering is a branch of biotechnology that involves the direct manipulation of an organism's genes.
It specifically refers to the process of introducing foreign genes (transgenes) into the genome of an organism to improve its genotype and express desired traits.
This technique allows for the creation of genetically modified organisms (GMOs) with enhanced characteristics such as disease resistance,higher yield,or improved nutritional value.

(D) Pesticides are non-biodegradable synthetic chemicals that are not easily broken down by biological processes.
When ingested in small quantities over a long period,these chemicals undergo biomagnification.
They are lipophilic in nature,meaning they have an affinity for fats.
Consequently,they accumulate in the adipose tissues (body fat) of the organism,leading to potential long-term health hazards.

(D) Large green plants are planted in high noise pollution zones because they have the capacity to absorb sound waves. These green plants,used for checking noise pollution,are known as green mufflers.

(B) The Assertion is correct because cotyledons are indeed embryonic leaves that store food or perform photosynthesis in early development.
The Reason is also correct because the embryo consists of an embryonal axis,which includes the radicle (future root) and the plumule (future shoot).
However,the Reason does not explain why cotyledons are considered embryonic leaves; it merely describes the components of the embryo. Therefore,the Reason is not the correct explanation of the Assertion.

(D) The Assertion is incorrect because the megaspore mother cell $(MMC)$ undergoes meiosis,not mitosis,to produce four megaspores.
The Reason is correct because the $MMC$ is diploid $(2n)$ and the resulting megaspores are haploid $(n)$.
Therefore,the Assertion is false,but the Reason is true.

(A) Death is a natural and essential biological process that ensures the continuity of life by maintaining ecological balance.
Continuous reproduction by organisms leads to an increase in population size.
If organisms did not die,the earth would face severe over-crowding and depletion of resources.
Therefore,death acts as a regulatory mechanism to prevent over-crowding and maintain the stability of ecosystems.
Thus,both the Assertion and the Reason are correct,and the Reason is the correct explanation of the Assertion.

(D) The Assertion is incorrect because $mRNA$ attaches to the ribosome at the $5'$ end,specifically at the Shine-Dalgarno sequence in prokaryotes or the $5'$ cap in eukaryotes,not the $3'$ end.
The Reason is also incorrect as there is no such thing as an '$F-$ capsular nucleotide' in $mRNA$ structure.
Therefore,both the Assertion and the Reason are incorrect.

(A) Histamine is a chemical mediator released by mast cells during allergic and inflammatory reactions.
It acts as a potent vasodilator,meaning it causes the widening of blood vessels.
This vasodilation increases blood flow and the permeability of blood vessel walls,which contributes to the symptoms of inflammation and allergy,such as redness,swelling,and itching.
Therefore,the Reason correctly explains why Histamine is involved in these reactions.

(D) Clones are genetically identical individuals produced through asexual reproduction,not sexual reproduction. Therefore,the Assertion is incorrect.
Cloning involves the production of organisms from a single parent,resulting in offspring that are clones of the parent. The Reason provided is also scientifically inaccurate as it describes processes that do not result in clones. Thus,both the Assertion and the Reason are incorrect.