AIIMS 2003 Biology Question Paper with Answer and Solution

(C) The quiescent centre is a region in the root apical meristem where the rate of cell division is very low. It acts as a reservoir of active initials that can replace damaged cells of the meristem,ensuring the continuous growth of the root.

(D) The $Patella$,commonly known as the 'knee cap',is a flat,triangular bone.
It is embedded within the tendon of the quadriceps muscle at the knee joint in birds and mammals.
$A$ bone that develops within a tendon due to ossification is specifically classified as a $Sesamoid$ bone.
Therefore,the correct option is $(d)$.

(B) Plasmodesmata are microscopic channels that traverse the cell walls of plant cells and some algal cells.
These channels enable transport and communication between individual cells.
They act as cytoplasmic bridges,allowing the cytoplasm of adjacent cells to remain continuous,facilitating the movement of molecules,ions,and nutrients.

(A) Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required for the reaction to proceed.
Activation energy is the minimum amount of energy required for the reactant molecules to undergo a chemical transformation.
By providing an alternative reaction pathway with a lower energy barrier,enzymes allow a greater number of substrate molecules to reach the transition state at a given temperature,thereby increasing the rate of the reaction.

(A) Succinic dehydrogenase catalyzes the oxidation of succinate to fumarate. Malonic acid is structurally similar to succinate. Because of this structural similarity,malonic acid competes with the substrate (succinate) for the active site of the enzyme succinic dehydrogenase. This type of inhibition,where the inhibitor resembles the substrate and competes for the active site,is known as competitive inhibition.

(D) The Hill reaction,discovered by Robert Hill in $1937$,refers to the light-dependent reaction of photosynthesis where water is photolyzed to release oxygen in the presence of an artificial electron acceptor.
This process occurs in the light and requires the presence of an electron acceptor (like ferricyanide or $DCPIP$) to accept electrons released from the photolysis of water.
Therefore,it is a light-dependent process that demonstrates the evolution of oxygen from water during photosynthesis.

(B) The correct option is $B$.
$1$. Normal blood sugar (fasting) in a healthy human adult is typically in the range of $70-110 \text{ mg}/100 \text{ ml}$ (often cited as $80-100 \text{ mg}/100 \text{ ml}$),making option $B$ the correct match.
$2$. Normal blood urea levels are typically $15-40 \text{ mg}/100 \text{ ml}$ of blood,so option $A$ is incorrect.
$3$. Total blood volume in an average adult is approximately $5-6 \text{ litres}$,not $3-4 \text{ litres}$,so option $C$ is incorrect.
$4$. The normal $ESR$ (Erythrocyte Sedimentation Rate) by the Wintrobe method is $0-9 \text{ mm}$ per hour for males and $0-20 \text{ mm}$ per hour for females,making option $D$ incorrect.

(B) The vagus nerve ($CN$ $X$) plays a crucial role in the parasympathetic control of the digestive system. It stimulates the secretion of gastric juices,including hydrochloric acid $(HCl)$ and pepsin,in the stomach. Excessive stimulation of the vagus nerve leads to hypersecretion of $HCl$. This excess acid can erode the protective mucosal lining of the stomach and duodenum,resulting in the formation of peptic ulcers.

(A) $Lubb$ is the first heart sound,which is low-pitched and produced due to the closure of the bicuspid and tricuspid $(AV)$ valves at the beginning of ventricular systole.
$Dupp$ is the second heart sound,which is high-pitched and produced due to the closure of the semilunar valves at the beginning of ventricular diastole.
Therefore,option $A$ is the only correctly matched pair.

(C) Myopia,also known as near-sightedness,is a refractive defect of the eye.
In this condition,the eyeball becomes too long or the curvature of the cornea is too steep.
As a result,the light rays entering the eye converge at a point in front of the retina rather than directly on it.
This causes distant objects to appear blurred,while near objects remain clear.

(B) $Vasopressin$ (also known as $ADH$) reduces the excretion of water in urine by promoting its reabsorption from the $Distal$ $Convoluted$ $Tubule$ $(DCT)$ and collecting ducts of the nephron.
When $vasopressin$ is released in lesser amounts,the reabsorption of water decreases,leading to excessive loss of water through urine,a condition known as diabetes insipidus.
This results in increased thirst (polydipsia) and frequent urination,while the blood glucose level remains normal because insulin secretion is unaffected.

(B) Somatostatin is secreted by the delta cells of the Islets of Langerhans in the pancreas.
Insulin is secreted by the beta cells and glucagon is secreted by the alpha cells of the same Islets of Langerhans in the pancreas.
Therefore,the source of somatostatin is the same as that of insulin and glucagon.

(B) Both the Assertion and the Reason are correct statements.
Bats and whales are classified as mammals because they possess mammary glands,have hair (at some stage of development),and are warm-blooded (homeothermic).
They also possess a $4$-chambered heart,which is a characteristic feature of mammals.
However,the presence of a $4$-chambered heart is not the defining characteristic that classifies an animal as a mammal; rather,it is the presence of mammary glands and other mammalian traits.
Therefore,the Reason is a correct statement but does not explain why they are classified as mammals.

(D) The Assertion is incorrect because birds generally build nests primarily for laying eggs and rearing their young,not specifically for resting or sleeping at night. Many birds roost on branches or in cavities without building nests for that purpose.
The Reason is also incorrect because the Koel (Eudynamys scolopaceus) is a brood parasite that typically lays its eggs in the nest of a crow,not a tailor bird.
Therefore,both the Assertion and the Reason are incorrect.

(A) The fluid mosaic model,proposed by $Singer$ and $Nicolson$ in $1972$,describes the cell membrane as a fluid structure.
The fluidity of the membrane is due to the lateral movement of lipids and proteins within the bilayer.
The membrane is described as a mosaic because it is composed of a diverse arrangement of proteins embedded in a phospholipid bilayer.
Since the fluid nature of the membrane allows for the movement of these embedded proteins and lipids,the mosaic composition is directly related to its fluid behavior.
Therefore,both the Assertion and Reason are correct,and the Reason is the correct explanation of the Assertion.

(A) The movement of water through the root occurs via two pathways: the apoplast and the symplast. The apoplast is the system of adjacent cell walls that is continuous throughout the plant,except at the casparian strips of the endodermis in the roots. Water and minerals move through the apoplast until they reach the endodermis. The endodermis is impervious to water because of a band of suberized matrix called the casparian strip. Because the apoplastic pathway is blocked at the endodermis by these suberized strips,water is forced to enter the symplast pathway to cross the endodermis. Therefore,both the assertion and the reason are correct,and the reason correctly explains why the apoplastic pathway is interrupted at the endodermis.

(C) The Assertion is correct because $Rhoeo$ leaves possess anthocyanin pigments in their epidermal cells,which provide a purple or reddish coloration to the leaves.
However,the Reason is incorrect. Anthocyanins are water-soluble vacuolar pigments that provide red,purple,or blue colors to plant parts like petals,fruits,and leaves. They are not involved in photosynthesis and do not act as accessory photosynthetic pigments. Accessory photosynthetic pigments include chlorophyll $b$,xanthophylls,and carotenoids,which capture light energy and transfer it to chlorophyll $a$.

(A) The first heart sound, $Lubb$, is produced primarily due to the closure of the atrioventricular $(AV)$ valves (bicuspid and tricuspid valves) at the beginning of ventricular systole.
$Dup$ is the second heart sound, caused by the closure of the semilunar valves at the beginning of ventricular diastole.
$Purkinje$ fibres are responsible for the conduction of the cardiac impulse throughout the ventricles, not for the initiation of the heartbeat (which is the function of the $SA$ node).

(B) The normal fasting blood glucose level in a healthy adult is approximately $70-110\, mg/100\, ml$ (or $mg/dl$).
Option $B$ provides the range $70-100\, mg/100\, ml$, which is clinically accepted as the normal fasting blood sugar range.
Total blood volume in a normal adult human is typically $5-6\, litres$.
Blood is a fluid connective tissue consisting of formed elements (corpuscles) suspended in plasma.
It is mesodermal in origin and slightly alkaline with a $pH$ of $7.3-7.4$.

(C) Both $corpus \ luteum$ and $macula \ lutea$ are characterized by a yellow colour.
$1$. The $corpus \ luteum$ is a temporary endocrine structure in the ovary of mammals,formed after ovulation,which secretes progesterone to maintain pregnancy.
$2$. The $macula \ lutea$ is an oval-shaped yellowish pigmented area near the center of the retina in the human eye,responsible for sharp,detailed central vision.

(A) Old age (senescence) is a natural biological process characterized by the progressive decline in the structural and functional integrity of cells,tissues,and organs. It is not a disease but a phase of life.
The cessation of mitosis (cell division) in many somatic cells is a genetically programmed event (senescence) that contributes to the aging process,as the body's ability to repair and replace damaged cells diminishes over time. Thus,both statements are correct,and the reason explains why the capacity for adaptation decreases.

(C) $Spirulina$ is a cyanobacterium (blue-green alga) that is considered a superfood.
It is exceptionally rich in high-quality proteins,vitamins,and minerals.
During a long illness,the body undergoes significant tissue damage and muscle wasting.
Since protein is the primary building block for tissue repair and regeneration,consuming $Spirulina$ helps in the rapid recovery of the body by providing essential amino acids required for rebuilding damaged tissues.

(B) . $Rhizobium$ is a symbiotic nitrogen-fixing bacterium,not a parasite.
$B$. $Mycorrhizae$ are symbiotic associations between fungi and roots of higher plants,which help in the absorption of minerals like phosphorus from the soil. This is the correct match.
$C$. $Yeast$ is used in the production of ethanol and bread,not biogas.
$D$. $Myxomycetes$ are slime molds,while ringworm is caused by fungi like $Microsporum$ or $Trichophyton$.

(B) . $Spirulina$ is a filamentous cyanobacterium (often referred to as blue-green alga) that is widely used as a single-cell protein $(SCP)$ source.
It is highly valued in biotechnology because it contains approximately $60-70$% protein by dry weight,along with essential amino acids,vitamins,and minerals.
It is cultivated on a large scale for human consumption and animal feed due to its high nutritional profile.

(C) $Azolla$ is used as a biofertilizer because it contains a symbiotic association with nitrogen-fixing cyanobacteria, specifically $Anabaena \, azollae$, in its leaf cavities.
These cyanobacteria are capable of fixing atmospheric nitrogen, which enriches the soil with nitrogenous nutrients, making $Azolla$ highly beneficial for crops like rice.

(C) The outer layer of the pollen grain is called the exine.
It is composed of a highly resistant organic material known as sporopollenin.
Sporopollenin is one of the most resistant organic materials known; it can withstand high temperatures,strong acids,and alkali,and no enzyme that degrades sporopollenin is known so far.
It is derived from carotenoids through oxidative polymerization.

(B) During cleavage,the zygote undergoes rapid mitotic divisions without any significant growth in size. As a result,the total mass of the protoplasm does not increase; instead,the cytoplasm is partitioned into smaller cells called blastomeres.
$1$. Cleavage does not increase the mass of protoplasm; it decreases the size of individual blastomeres.
$2$. In the second cleavage,the division is asynchronous,meaning one blastomere divides slightly earlier than the other.
$3$. With each division,the blastomeres become progressively smaller.
$4$. The morula is a solid ball of cells (typically $8-16$ cells),not a hollow one; the hollow structure is known as the blastocyst.

(B) The menstrual cycle in humans is divided into four main phases: the menstrual phase,the follicular (or proliferative) phase,the ovulatory phase,and the luteal (or secretory) phase.
$1$. The menstrual phase usually lasts for $3-5$ days.
$2$. The follicular phase follows the menstrual phase and lasts for about $10-14$ days.
$3$. The ovulatory phase occurs around the $14^{th}$ day.
$4$. The luteal phase lasts for about $14$ days.
However,the question refers to the phase lasting $7-8$ days,which is often associated with the proliferative part of the follicular phase or specific clinical descriptions. Among the given options,the follicular phase is the most appropriate as it encompasses the growth and maturation of follicles over a significant duration.

(B) The correct answer is $(b)$. Red-green colour blindness is a well-known $X$-linked recessive disorder in humans.
Because the gene responsible for this condition is located on the $X$-chromosome,it is expressed more frequently in males (who have only one $X$-chromosome) than in females (who have two $X$-chromosomes).
Baldness is typically sex-influenced,while facial hair is a secondary sexual characteristic influenced by hormones,and night blindness is often related to Vitamin $A$ deficiency.

(D) $DNA$ replication is semi-conservative and proceeds in the $5' \to 3'$ direction on both strands.
Since the two strands of the $DNA$ double helix are antiparallel, one strand (leading strand) is synthesized continuously in the $5' \to 3'$ direction towards the replication fork.
The other strand (lagging strand) is synthesized discontinuously in the $5' \to 3'$ direction away from the replication fork, using $RNA$ primers.
Option $D$ correctly depicts the antiparallel nature of the template strands ($3' \to 5'$ and $5' \to 3'$) and the synthesis of new strands in the $5' \to 3'$ direction.

(C) The correct answer is $C$.
$1$. $tRNA$ binds to an amino acid at its $3'$ end (acceptor arm),not the $5'$ end.
$2$. The cloverleaf model represents the two-dimensional structure of $tRNA$,while the $L$-shaped model represents the three-dimensional structure.
$3$. $tRNA$ contains an anticodon that recognizes the codon on $mRNA$,not the other way around.
Therefore,statement $C$ is false because it incorrectly states that $tRNA$ has a codon that recognizes an anticodon on $mRNA$.

(A) The genetic code is degenerate,meaning that some amino acids are encoded by more than one codon.
Both $UGC$ and $UGU$ are codons that code for the amino acid $Cysteine$ $(Cys)$.
Therefore,$UGU$ codes for the same information as $UGC$.

(B) The possession of pharyngeal gill slits and gill pouches is one of the three diagnostic characters of chordates. These structures appear in the embryonic stages of all vertebrates,including humans,which serves as evidence for evolutionary relationships.

(A) Kala-azar,also known as visceral leishmaniasis,is a disease caused by the protozoan parasite $Leishmania$ $donovani$.
This parasite is transmitted to humans through the bite of an infected female sand fly,which belongs to the genus $Phlebotomus$.
Therefore,the causative agent is $Leishmania$ and the vector (transmitter) is $Phlebotomus$.

(D) The correct answer is $D$. Prolonged ingestion of pesticides,even at low concentrations,leads to bioaccumulation. In India,studies have shown that as a result of prolonged exposure,$13-31 \ ppm$ of $DDT$ (a pesticide) can be detected in the body fat of the population,which is among the highest levels in the world.

(B) The correct matching pair is $Algae$ and $fungi$ in $lichens$ - $Mutualism$. In this association, both organisms benefit: the $algae$ provide food through photosynthesis, while the $fungi$ provide protection and absorb water and minerals. Shark and sucker fish represent $Commensalism$, Orchids on trees represent $Commensalism$, and $Cuscuta$ on other plants represents $Parasitism$.

(C) In a food chain,the first trophic level is occupied by producers (plants).
The second trophic level is occupied by primary consumers (herbivores) that eat plants.
The third trophic level is occupied by secondary consumers (carnivores) that eat herbivores.
Since curd/yoghurt is a dairy product derived from milk (which comes from a herbivore like a cow),the cow occupies the second trophic level.
Therefore,Mr. $X$,by consuming the curd,acts as a secondary consumer,placing him at the third trophic level.

(B) Biological 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.
It serves as a standard measure of the degree of water pollution caused by organic matter.
The higher the $BOD$ value,the greater the amount of organic pollution in the water body.

(A) In Indian metropolitan cities like Delhi,the primary source of air pollution is the combustion of fossil fuels in vehicles and industries.
This process releases a high concentration of $SPM$ (Suspended Particulate Matter),which consists of fine dust,smoke,and soot particles.
According to the Central Pollution Control Board $(CPCB)$,$SPM$ of size $2.5$ micrometers or less in diameter $(PM_{2.5})$ is responsible for causing the greatest harm to human health.
Therefore,$SPM$ is considered the major atmospheric pollutant in these cities.

(A) Photochemical smog is formed by the action of sunlight on nitrogen oxides $(NO_x)$ and hydrocarbons released by automobiles and factories.
It is primarily composed of ozone $(O_3)$,peroxyacetyl nitrate $(PAN)$,and nitrogen oxides $(NO_x)$.
These components are secondary pollutants formed through complex photochemical reactions in the atmosphere.

(B) Ernst Ruska was a German physicist who designed the first electron microscope in $1931$.
Due to his pioneering work in electron optics and the design of the first electron microscope,he was awarded half of the Nobel Prize in Physics in $1986$.

(D) The World Food Prize is a prestigious international award recognizing the achievements of individuals who have advanced human development by improving the quality,quantity,or availability of food in the world.
$M.S. Swaminathan$ was the first recipient of the World Food Prize in $1987$ for his leadership in India's Green Revolution,which significantly increased agricultural productivity and food security in the country.

(B) Positron emission tomography $(PET)$ is a nuclear medicine functional imaging technique.
It is highly effective because it utilizes isotopes of basic body elements (such as $C^{11}$,$N^{13}$,$O^{15}$,and $F^{18}$) that have very short half-lives.
These isotopes are incorporated into biologically active molecules (like glucose),allowing for the visualization of metabolic processes in real-time.
The short half-life minimizes the radiation exposure to the patient,making it a safe and preferred diagnostic tool.

(A) Magnetic Resonance Imaging $(MRI)$ relies on the magnetic properties of hydrogen nuclei (protons).
In biological tissues,these protons are primarily found in water molecules $(H_2O)$ and in the hydrocarbon chains of lipids (fats).
Specifically,the protons in the long alkane chains of fatty acid moieties contribute significantly to the signal intensity in $MR$ images,especially in adipose (fatty) tissue.
Therefore,the correct answer is the long alkane chain protons of the fatty acid moieties.

(A) July $11$ is observed as World Population Day.
This date was chosen because the world population reached the $5$ billion mark on July $11, 1987$.

(B) $Spirulina$ is a type of blue-green alga (cyanobacteria) that is widely used as a dietary supplement.
It is considered a superfood because it is exceptionally rich in high-quality proteins,containing all essential amino acids.
In addition to protein,$Spirulina$ is packed with vitamins,minerals,carotenoids,and essential fatty acids.
Due to its high nutritional density,it is often recommended for individuals recovering from long illnesses to help restore strength and provide essential nutrients efficiently.

(C) The Assertion is correct because placental mammals exhibit holoblastic (total) cleavage,resulting in blastomeres of nearly equal size.
The Reason is incorrect because the eggs of most mammals,including humans,are microlecithal (isolecithal),meaning they contain a very small amount of yolk distributed evenly. Centrolecithal eggs,which have yolk concentrated in the center,are characteristic of insects.

(A) The Assertion is correct because many visitors experience allergic reactions in hilly regions due to high concentrations of airborne pollen from plants like conifers.
The Reason is also correct because conifer trees are wind-pollinated (anemophilous) and release massive amounts of pollen grains into the air,which act as potent allergens.
Therefore,the Reason correctly explains why visitors suffer from such allergies.

(B) The first patient of $SARS$ was reported in February $2003$ in China. Therefore,the Assertion is correct.
Although China is a highly populated country,the population density is not the direct biological cause for the origin of the $SARS$ virus. Therefore,the Reason is a true statement but not the correct explanation for the Assertion.

(B) $DNA$ replication is semi-conservative and proceeds in the $5' \to 3'$ direction.
Because the two strands of the $DNA$ double helix are antiparallel,the replication fork moves in one direction,but the synthesis of new strands occurs differently on the two templates.
The leading strand is synthesized continuously in the $5' \to 3'$ direction towards the replication fork.
The lagging strand is synthesized discontinuously in the $5' \to 3'$ direction away from the replication fork,forming Okazaki fragments.
Option $B$ correctly depicts the antiparallel nature of the template strands ($5' \to 3'$ and $3' \to 5'$) and the resulting continuous and discontinuous synthesis of the new strands in the $5' \to 3'$ direction.

(A) Mast cells contain granules rich in histamine,serotonin,and heparin.
When allergens enter the body,they bind to $IgE$ antibodies on the surface of mast cells in sensitive individuals.
This binding triggers the degranulation of mast cells,causing them to release large amounts of inflammatory chemicals like histamine into the bloodstream.
Histamine acts as an initiator of the inflammatory response,leading to symptoms such as capillary dilation,increased permeability,mucus secretion,and swelling,which are characteristic of allergic reactions.
Therefore,the Reason correctly explains why mast cells release these chemicals.

(B) The Assertion is correct because fertilizers provide essential nutrients like $N, P,$ and $K$ to the soil,which significantly boosts crop yield.
The Reason is also correct because irrigation ensures the timely and adequate supply of water,which is vital for plant growth and metabolic processes.
However,irrigation is a separate agricultural practice from the application of fertilizers. Therefore,the Reason does not explain why fertilizers enhance productivity. Both statements are independent facts regarding agricultural enhancement.

(A) The concept of totipotency states that any living plant cell has the inherent capacity to generate a whole new plant.
In plant tissue culture,somatic embryos can be induced from various plant cells because of this property of totipotency.
Since any viable plant cell possesses the potential to differentiate into somatic embryos under appropriate culture conditions,the Reason correctly explains the Assertion.
Therefore,both the Assertion and Reason are correct,and the Reason is the correct explanation of the Assertion.

(C) The Assertion is correct because $Saccharomyces \text{ } cerevisiae$ (brewer's yeast) is widely used in the baking industry to ferment sugars in the dough.
The Reason is incorrect because the rising of the bread dough is caused by the entrapment of $CO_2$ gas bubbles produced during fermentation, not by thermal expansion. Thermal expansion would only occur during the actual baking process in the oven, whereas the 'rising' of the dough occurs primarily due to the accumulation of gas.

(C) The Assertion is correct because leaf butterflies and stick insects exhibit mimicry and camouflage to protect themselves from predators.
The Reason is incorrect because the definition provided describes 'camouflage' (or crypsis),not 'mimicry'.
Mimicry is a phenomenon where an organism resembles another organism (the model) to gain protection or advantage,whereas camouflage is the ability to blend into the background environment to avoid detection.

(A) In a food chain, energy is transferred from one trophic level to the next. According to the $10\%$ law of energy transfer, only about $10\%$ of the energy available at one trophic level is transferred to the next higher level.
Due to this significant loss of energy, the amount of biomass and the number of organisms that can be supported decreases as we move to higher trophic levels.
Therefore, the Assertion is correct because higher trophic levels have fewer organisms.
The Reason is also correct because the population size at any trophic level is limited by the energy and food supply provided by the lower trophic level.
Since the limitation of food/energy at lower levels directly causes the reduction in the number of individuals at higher levels, the Reason is the correct explanation of the Assertion.

(C) The Assertion is correct because high-intensity noise pollution near airports causes various health issues like stress,sleep disturbance,and hearing impairment.
The Reason is incorrect because the sound level of jet aeroplanes typically ranges between $140 \, dB$ to $150 \, dB$,not exceeding $160 \, dB$. While $150 \, dB$ is extremely high and harmful,the specific value mentioned in the reason is factually inaccurate.

(C) The Assertion is correct because organochlorine pesticides are indeed organic compounds containing chlorine atoms.
The Reason is incorrect because $Fenitrothion$ is an organophosphate pesticide,not an organochlorine pesticide.
Organochlorine pesticides,such as $DDT$,are persistent organic pollutants that accumulate in the food chain (biomagnification).
Therefore,the correct choice is $C$.