AIIMS 2006 Biology Question Paper with Answer and Solution

(D) In the classification system proposed by Bentham and Hooker, the "Ordines Anomali" (meaning anomalous orders) was a category created to include families that did not fit well into the established classification scheme.
These were groups of plants with uncertain affinities or characteristics that made them difficult to place within the existing taxonomic hierarchy of the time.
Therefore, it represents a few orders which could not be placed satisfactorily in the classification.

(B) Myxomycetes,commonly known as acellular slime molds,consist of a slimy mass of multinucleate protoplasm called a plasmodium. They exhibit amoeboid movement using pseudopodia to engulf food particles. Their reproduction occurs through the formation of spores or by fragmentation of the plasmodium.

(D) Rust,smut and mushroom belong to the class Basidiomycetes.
Members of this class are characterized by the production of reproductive bodies known as basidiocarps,which bear basidia and basidiospores.

(A) Gram-negative bacteria possess a thin peptidoglycan layer and an outer membrane rich in lipids.
When treated with alcohol,the lipid-rich outer membrane dissolves,allowing the crystal violet-iodine complex to wash out,which is why they do not retain the stain.
The outer membrane of Gram-negative bacteria indeed contains lipopolysaccharides $(LPS)$,which are integral components of the outer leaflet of the outer membrane.
Therefore,both the Assertion and the Reason are correct,and the presence of the lipid-rich outer membrane (containing $LPS$) is the reason why the stain is not retained.

(B) The correct match is $B$.
$1$. Millipedes belong to the class $Myriapoda$,not $Arachnida$.
$2$. The Duckbill platypus ($Ornithorhynchus$ $anatinus$) is a unique,semi-aquatic,egg-laying $(oviparous)$ mammal belonging to the class $Mammalia$.
$3$. Silverfish is an insect (phylum $Arthropoda$),not a chordate; it lacks pectoral and pelvic fins.
$4$. Sea anemones are members of the phylum $Cnidaria$ and are $diploblastic$,not $triploblastic$.

(D) Mammals are unique among vertebrates because they possess a muscular diaphragm,which aids in respiration,and mammary (milk-producing) glands,which are used to nourish their young. While other features like a four-chambered heart or heterodont teeth are common,they are not present in every single mammal without exception (e.g.,some monotremes have different characteristics,and not all mammals have $12$ pairs of cranial nerves).

(B) Grafting is a horticultural technique whereby tissues from one plant are inserted into those of another so that the sets of vascular tissues may join together.
Grafting is successful in dicots because their vascular bundles are arranged in a ring and possess cambium,which allows for secondary growth and the fusion of vascular tissues between the scion and the stock.

(D) $P-$ proteins (phloem proteins) are found in the sieve tube elements of angiosperms. Their primary function is to act as a sealing mechanism. When the sieve tube is injured or wounded,these proteins,along with callose,rapidly accumulate at the sieve plate pores to plug them,thereby preventing the loss of phloem sap.

(C) The Assertion is correct because the presence of vessels in the xylem of angiosperms provides a more efficient pathway for water transport compared to tracheids alone.
However,the Reason is incorrect. The conduction of water through xylem vessels is a passive process driven by transpiration pull (cohesion-tension theory),not an active process requiring energy from xylem parenchyma.

(C) Ciliated epithelium consists of cells that possess fine hair-like outgrowths called cilia on their free surfaces.
This type of epithelium lines the inner surface of hollow organs like the fallopian tubes (oviducts),bronchioles,and small bronchi.
The primary function of these cilia is to move particles,mucus,or cells (like the ovum) in a specific direction.
Squamous epithelium is found in the walls of blood vessels and air sacs of lungs.
Columnar epithelium lines the stomach and intestines.
Cuboidal epithelium is found in kidney tubules and glandular ducts.

(A) The metamorphosis of a tadpole into an adult frog is regulated by the hormone thyroxine,which is secreted by the thyroid gland.
If the production of thyroxine is inhibited,the tadpole fails to undergo metamorphosis and continues to grow in size,eventually becoming a giant-sized tadpole.
Antithyroid substances,such as thiourea,inhibit the synthesis of thyroxine.
Therefore,administering thiourea prevents metamorphosis,allowing the tadpole to continue growing without transforming into an adult frog.

(D) In prokaryotes,chromatophores are internal membrane systems that may become extensive and complex in photosynthetic bacteria. They contain pigments and are structurally and functionally similar to the thylakoids found in eukaryotic chloroplasts,facilitating photosynthesis.

(D) Plastids are organelles found in plant cells and algae. They are classified into three types based on the type of pigments they contain: chloroplasts,chromoplasts,and leucoplasts.
All these plastids originate from proplastids and possess the ability to multiply by a fission-like process.
Chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.
Chromoplasts contain fat-soluble carotenoid pigments like carotene,xanthophylls,and others,which give plants their yellow,orange,or red color.
Leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: amyloplasts store carbohydrates (starch),elaioplasts store oils and fats,and aleuroplasts store proteins.
Therefore,the common feature among all these plastids is their origin from proplastids and their ability to divide by fission.

(A) Eukaryotic cells possess a complex network of proteinaceous filaments in the cytoplasm collectively called the cytoskeleton.
This cytoskeleton is composed of three principal types of protein filaments: $1$. Microfilaments,$2$. Microtubules,and $3$. Intermediate filaments.
These structures provide mechanical support,maintain the characteristic shape of the cell,and facilitate intracellular transport and directed cell movements.
Therefore,the presence of the cytoskeleton is the direct reason why eukaryotic cells can adopt various shapes and perform directed movements.

(A) In the given graph,curve $a$ represents the normal enzyme reaction with the highest $V_{max}$.
Curve $b$ shows competitive inhibition,where the $K_m$ increases but the $V_{max}$ remains the same as the normal reaction because the inhibition can be overcome by increasing the substrate concentration.
Curve $c$ shows non-competitive inhibition,where the $V_{max}$ is decreased because the inhibitor binds to a site other than the active site,reducing the enzyme's efficiency regardless of substrate concentration.

(D) The phenomenon of loss of water in the form of droplets from the surface of plants is known as guttation. Guttation typically occurs through specialized pores called hydathodes,not lenticels. Lenticels are involved in gaseous exchange. Therefore,the Assertion is incorrect.
Root pressure is a positive pressure generated in the xylem due to the active absorption of minerals and subsequent osmotic influx of water. While root pressure is responsible for guttation,it does not regulate water loss from lenticels. Thus,the Reason is also incorrect.

(C) Photorespiration is a process that occurs when $CO_2$ levels are low and $O_2$ levels are high,causing the enzyme $RuBisCO$ to act as an oxygenase rather than a carboxylase.
Under high light intensity,if $CO_2$ is limited,the light energy absorbed by the photosynthetic apparatus cannot be fully utilized for the Calvin cycle.
This excess energy can lead to the formation of reactive oxygen species $(ROS)$,which cause photo-oxidative damage to the plant cells.
Photorespiration helps dissipate this excess energy by consuming $O_2$ and $ATP$,thereby protecting the photosynthetic machinery from damage.
Therefore,the Assertion is correct,but the Reason is incorrect because the excess energy,if not utilized,$DOES$ cause damage to the plants.

(D) $C_4$ plants are actually more efficient than $C_3$ plants in terms of photosynthesis,especially under conditions of high temperature,high light intensity,and limited water availability,because they lack photorespiration.
Therefore,the Assertion is incorrect.
The $C_4$ pathway involves both mesophyll cells and bundle-sheath cells,which work in coordination to concentrate $CO_2$ around the enzyme $RuBisCO$.
Therefore,the Reason is also incorrect.
Since both the Assertion and the Reason are incorrect,the correct option is $D$.

(A) The Avena curvature test is a classic bioassay developed by Frits Went in $1928$.
It is specifically used to detect and measure the biological activity of auxins,particularly Indole$-3-$Acetic Acid $(IAA)$.
In this experiment,agar blocks containing auxin are placed asymmetrically on the coleoptile tips of Avena sativa (oat) seedlings,causing the coleoptile to curve due to differential growth.

(A) Oxyntic cells,also known as Parietal cells,are found in the stomach epithelium and secrete hydrochloric acid $(HCl)$ and intrinsic factor. The secretion of $HCl$ makes the gastric juice acidic,typically maintaining a $pH$ between $2.0$ and $3.0$.
Alpha cells of the Islets of Langerhans secrete glucagon,which increases blood glucose levels,not decreases them (insulin decreases blood sugar).
Kupffer cells are specialized macrophages in the liver that perform phagocytosis to destroy bacteria and worn-out blood cells,not digestive enzymes.
Sebaceous glands are skin glands that secrete sebum (an oily substance) to lubricate the skin and hair,not digestive enzymes.

(B) In this figure,coronary angioplasty is being performed.
During this procedure,an atherosclerotic plaque,which builds up and clogs the coronary arteries,is compressed against the vessel wall.
This is achieved by expanding a balloon-like device inserted through a catheter that has been threaded through the artery.

(D) During fast running or chasing,the lower limbs are primarily involved in locomotion.
The bones of the lower limb include the $Femur$ (thigh bone),$Tibia$ (shin bone),$Tarsals$ (ankle bones),and $Metatarsals$ (foot bones).
These bones work together to provide support,balance,and propulsion during rapid movement.
$Malleus$ is an ear ossicle,$Sternum$ is a chest bone,and $Ulna$ is a forearm bone,which are not directly involved in the locomotion of the legs.

(A) The assertion is correct because inflammation of joints (arthritis) leads to stiffness and reduced mobility.
The reason is also correct as it describes specific causes of joint disorders: accumulation of uric acid crystals causes gout,and ossification of articular cartilage (ankylosis) leads to the fusion and immobilization of joints.
Therefore,the reason correctly explains why inflammation and associated pathological changes lead to the immobilization of joints.

(B) Based on the morphological structure,cell $A$ represents a cone cell,which is characterized by a conical outer segment. Cone cells are highly concentrated in the fovea centralis (yellow spot) of the retina. They are responsible for colour vision and high-resolution vision in bright light. Cell $B$ represents a rod cell,which is characterized by a cylindrical outer segment and is sensitive to low light intensities (scotopic vision) but does not provide colour vision.

(C) Glaucoma is an eye disease characterized by increased ocular pressure within the eyeball.
It is a group of diseases of the optic nerve involving the loss of retinal ganglion cells in a characteristic pattern of optic neuropathy.
Untreated glaucoma leads to permanent damage of the optic nerve and resultant visual field loss that can progress to blindness.
$A$ (Hepatitis) is an inflammation of the liver.
$B$ (Measles) is a highly infectious viral disease that usually spreads by droplet infection.
$D$ (Bronchitis) is the inflammation of the membrane lining of the bronchial tubes.

(B) The sympathetic nervous system prepares the body for 'fight or flight' responses,which includes dilating the pupils,increasing heart rate,and inhibiting intestinal motility.
In contrast,the parasympathetic nervous system promotes 'rest and digest' activities,which includes constricting the pupils,decreasing heart rate,and stimulating intestinal motility.
Comparing the given options:
Option $A$ is incorrect because the sympathetic system inhibits salivary secretion,while the parasympathetic system stimulates it.
Option $B$ is correct because the sympathetic nervous system causes pupil dilation,and the parasympathetic nervous system causes pupil constriction.
Option $C$ is incorrect because the sympathetic system increases heart rate,while the parasympathetic system decreases it.
Option $D$ is incorrect because the sympathetic system inhibits intestinal motility,while the parasympathetic system stimulates it.

(A) Adrenaline is an emergency hormone,also known as the fight-or-flight hormone,which is secreted by the adrenal medulla during stress conditions such as intense cold,fear,or anger.
Adrenaline increases the metabolic rate by stimulating glycogenolysis (breakdown of glycogen to glucose) and increasing oxygen consumption,which helps in thermogenesis (heat production) to maintain body temperature in cold conditions.
Therefore,the secretion of adrenaline in intense cold is a physiological response to increase the metabolic rate and generate heat.
Thus,both the Assertion and the Reason are correct,and the Reason is the correct explanation of the Assertion.

(A) Polytene chromosomes are giant chromosomes found in the salivary glands of Drosophila larvae.
They are formed through a process called endoreduplication or endomitosis,where the $DNA$ replicates repeatedly without the cell dividing.
Because of this repeated replication of $DNA$ strands,these chromosomes contain a very high amount of $DNA$ and appear as thick,banded structures.
Therefore,the Assertion is correct,and the Reason correctly explains why they have a high amount of $DNA$.

(D) $Chamaeleon$ is a specialized lizard known for its prehensile tail,which it uses to coil around twigs for stability while climbing trees.
It exhibits protective colouration (camouflage) to blend in with its surroundings,such as leaves and twigs,to avoid predators and hunt prey effectively.

(D) Leghaemoglobin acts as an oxygen scavenger in the root nodules of legumes.
It has a high affinity for oxygen and binds with it,thereby maintaining a low oxygen concentration in the nodule.
This is essential because the enzyme nitrogenase,which is responsible for biological nitrogen fixation,is highly sensitive to oxygen and functions only under anaerobic conditions.

(B) $Escherichia$ $coli$,$Shigella$ $sp.$,and $Salmonella$ $sp.$ are pathogenic bacteria that cause diarrhoeal diseases,typically through the ingestion of contaminated food or water.
Dehydration is a major clinical consequence of all diarrhoeal diseases due to the loss of water and essential salts from the body.
Therefore,both statements are correct,but the Reason does not explain why these specific bacteria cause the disease; it only describes a common clinical management aspect of the condition. Thus,the Reason is not the correct explanation of the Assertion.

(C) The correct match is Tobacco - Root-knot - $Meloidogyne \text{ } incognita$.
$1$. Citrus canker is caused by $Xanthomonas \text{ } citri$.
$2$. Late blight of potato is caused by $Phytophthora \text{ } infestans$.
$3$. Root-knot disease in tobacco and many other plants is caused by the nematode $Meloidogyne \text{ } incognita$.
$4$. Pigeon pea wilt is caused by $Fusarium \text{ } udum$.

(C) Pollination is the transfer of pollen grains from the anther to the stigma of a flower. Different plants use different agencies for pollination.
$1$. $Triticum$ (wheat) is pollinated by wind (anemophily),$Cocos$ (coconut) by wind or insects,and $Mangifera$ (mango) by insects.
$2$. $Ficus$ is pollinated by wasps,$Kigelia$ by bats,and $Casuarina$ by wind.
$3$. $Salvia$,$Morus$,and $Euphorbia$ are all pollinated by insects (entomophily).
$4$. $Bombax$,$Butea$,and $Bauhinia$ are primarily pollinated by birds (ornithophily).
Therefore,the correct group is $Salvia$,$Morus$,and $Euphorbia$.

(C) Hirudin is a polypeptide that acts as a potent thrombin inhibitor,preventing blood clotting. It is naturally found in the saliva of medicinal leeches ($Hirudo$ $medicinalis$). Through genetic engineering,the gene encoding hirudin has been introduced into transgenic $Brassica$ $napus$ (rapeseed) plants,allowing for the large-scale production of this protein for medical use.

(A) Borderline personality disorder $(BPD)$ is a mental health condition characterized by a pattern of ongoing instability in moods,behavior,self-image,and functioning.
Individuals with this disorder often exhibit impulsive actions,unpredictable mood swings,intense outbursts of emotion,and difficulty maintaining stable relationships,which frequently leads to quarrelsome behavior and conflicts with others.

(D) Hexaploid wheat ($Triticum$ $aestivum$) has a chromosome number of $2n = 6x = 42$,where $x = 7$ is the basic chromosome number.
$1$. Haploid number $(n)$: $n = 2n / 2 = 42 / 2 = 21$.
$2$. Monosomic $(2n - 1)$: $42 - 1 = 41$.
$3$. Nullisomic $(2n - 2)$: $42 - 2 = 40$.
$4$. Trisomic $(2n + 1)$: $42 + 1 = 43$.
Comparing these values with the given options,option $D$ is correct.

(C) The image shows the $X$ and $Y$ chromosomes. Genes $a$ and $b$ are located on the same chromosome (the $X$ chromosome) and are very close to each other,indicating that they are linked genes.
Linked genes are genes that are physically located on the same chromosome and tend to be inherited together.
Haemophilia and red-green colour blindness are both well-known $X$-linked recessive disorders in humans.
Because the genes responsible for these two conditions are located on the $X$ chromosome,they exhibit linkage and are often inherited together,which is a classic example of sex-linked inheritance.

(C) Genetic diversity refers to the variation of genes within a species. It allows a population to adapt to its environment and ensures survival. In the case of mangoes $(Mangifera \text{ } indica)$, the existence of over $1,000$ varieties in India, which differ in flavour, colour, fibre content, sugar content, and shelf-life, is a classic example of genetic diversity at the intraspecific level.

(A) Polygenic inheritance refers to traits that are controlled by two or more gene pairs,often resulting in a continuous range of phenotypes.
Human height and skin colour are classic examples of polygenic inheritance,as they are influenced by the additive effects of multiple genes.

(C) Protein synthesis is terminated when the ribosome encounters a stop codon on the $mRNA$ strand.
These stop codons are also known as nonsense codons because they do not code for any amino acid.
The three stop codons are $UAA$ (Ochre),$UAG$ (Amber),and $UGA$ (Opal).
Therefore,the correct group is $UAG, UGA, UAA$.

(C) The Assertion is correct because lethal mutations involve genes that are essential for survival,and their expression can lead to the death of the organism at the embryonic or zygotic stage.
The Reason is incorrect because not all mutations are lethal. Mutations can be classified as beneficial,neutral (silent),harmful,or lethal depending on their effect on the phenotype and survival of the organism. Many mutations have no significant effect on the organism's fitness.

(A) The earliest organisms that appeared on the earth were heterotrophic because of the reducing atmosphere,and the first autotrophs were chemoautotrophs.
An anaerobic organism does not require oxygen for growth and may even die in its presence. Chemoautotrophs were the first autotrophic organisms to appear on Earth,obtaining energy by the oxidation of inorganic molecules in their environment. These organisms did not release oxygen as a byproduct of their metabolic processes. Therefore,both the Assertion and the Reason are correct,and the Reason provides a valid explanation for the nature of the earliest organisms.

(A) Leptospirosis is a bacterial disease transmitted through contact with water contaminated by the urine of infected animals. Bilharzia (schistosomiasis) is caused by parasitic flatworms (flukes) whose larvae,known as cercariae,penetrate the human skin when a person wades or swims in contaminated freshwater. Both diseases are commonly associated with exposure to dirty or flood water.

(D) $LSD$ (Lysergic acid diethylamide) is a powerful hallucinogen,not an analgesic. Marijuana (cannabinoids) acts on cannabinoid receptors in the brain and is primarily used for its psychoactive effects,though it has some medicinal applications for pain management in specific chronic conditions,it is not classified as a standard clinical analgesic. The assertion is incorrect because they are not primarily used as analgesics. The reason is also incorrect as they are hallucinogens or psychoactive substances that alter brain function rather than simply suppressing it.

(C) The Assertion is correct because wounds can be a portal of entry for $Clostridium$ $tetani$ spores,which cause tetanus.
However,the Reason is incorrect. Anti-tetanus serum $(ATS)$ provides passive immunity by introducing pre-formed antibodies directly into the body,rather than stimulating the body to produce its own antibodies (which would be active immunity). Therefore,the correct choice is $C$.

(A) Cancer cells exhibit uncontrolled cell division and lack contact inhibition,making them virtually immortal as long as they receive nutrients from the host body. This uncontrolled proliferation is primarily caused by mutations in genes that regulate the cell cycle,such as proto-oncogenes and tumor suppressor genes. Therefore,the Reason correctly explains why cancer cells continue to divide indefinitely (immortality).

(B) In grafting,the scion is the upper part of the plant that is selected for its desirable traits,such as fruit quality,yield,or disease resistance. The stock is the lower part that provides the root system. Since the scion contains the genetic information that dictates the development of branches,leaves,flowers,and fruits,the quality of the fruits produced is determined by the genotype of the scion.

(D) Keystone species have a disproportionate effect on their environment relative to their biomass.
These species play a critical role in maintaining the structure of an ecological community.
They help to determine the types and numbers of various other species within the community,making their conservation essential for ecosystem stability.

(C) food web is a complex network of interconnected food chains within an ecosystem,representing the various pathways through which energy and nutrients flow.
In a food web,organisms occupy different trophic levels,such as producers,primary consumers,secondary consumers,and tertiary consumers.
Since a kite is a bird of prey (a secondary or tertiary consumer),it feeds on various organisms like small mammals,reptiles,or insects,making it an integral part of multiple food chains within a food web.
Therefore,the Assertion is correct,but the Reason is incorrect.

(C) The Montreal Protocol $(1987)$ was a landmark international agreement designed to protect the stratospheric ozone layer.
It achieved this by establishing regulations to limit the production and consumption of ozone-depleting substances,such as chlorofluorocarbons $(CFCs)$.

(B) Biosphere reserves are a special category of protected areas of land or coastal environments where people are an integral component of the system. Unlike National parks and Wildlife sanctuaries,which are strictly protected areas where human interference is minimal or prohibited,Biosphere reserves are designed to include human settlements and their sustainable use of resources as part of the ecosystem management.

(C) The Assertion is correct: In tropical rain forests,high temperature and heavy rainfall lead to rapid decomposition of organic matter and intense leaching of nutrients,resulting in shallow and nutrient-poor $O$ and $A$ horizons.
The Reason is incorrect: While micro-organisms are abundant,they do not deplete the organic content in a way that makes the soil nutrient-poor; rather,the rapid nutrient cycling and leaching are the primary causes of low soil fertility in these regions. The organic matter is decomposed and absorbed by plants almost immediately.

(C) Vermiculture is the process of using earthworms to decompose organic waste. These worms consume organic matter,such as animal excreta and kitchen waste,and excrete it as vermicompost,which is rich in nutrients and humus. This process is highly profitable as it converts waste into a valuable organic fertilizer,thereby minimizing pollution effectively.

(B) Blue baby disease,also known as methaemoglobinaemia,is caused by the high concentration of nitrates in drinking water,often resulting from the heavy use of nitrogen-rich fertilizers in agricultural areas.
When nitrates are ingested,they are converted into nitrites,which react with haemoglobin to form methaemoglobin.
Methaemoglobin cannot bind oxygen effectively,leading to tissue hypoxia and a characteristic bluish tint to the skin.

(B) Deforestation leads to a reduction in the number of trees that absorb $CO_2$ through photosynthesis,thereby increasing the concentration of $CO_2$ in the atmosphere,which contributes to global warming.
Greenhouse gases are gases that trap heat in the atmosphere,leading to the greenhouse effect and global warming.
$CO_2$,methane $(CH_4)$,$CFCs$,and nitrous oxide $(N_2O)$ are the primary greenhouse gases.
While both statements are scientifically true,the Reason does not directly explain why deforestation causes global warming; it merely lists other greenhouse gases. Therefore,the Reason is not the correct explanation for the Assertion.

(C) The Assertion is correct: In the stratosphere,$UV$ radiation causes the photodissociation of ozone $(O_3)$ into molecular oxygen $(O_2)$ and atomic oxygen $(O)$. This is a natural process,but the depletion of the ozone layer is primarily accelerated by chlorofluorocarbons $(CFCs)$.
The Reason is incorrect: The ozone hole refers to the thinning of the stratospheric ozone layer,which leads to increased $UV$ radiation reaching the Earth's surface,causing skin cancer,cataracts,and damage to $DNA$. While ozone depletion is a serious environmental issue,it is not the primary cause of global warming or climate change; global warming is primarily caused by the greenhouse effect due to the accumulation of greenhouse gases like $CO_2$,$CH_4$,and $N_2O$.

(A) The concentration of methane in the atmosphere has significantly increased due to human activities,more than doubling over the last $250$ years.
Wetlands,rice fields,and livestock are indeed the primary natural and anthropogenic sources of methane.
Since the increase in methane concentration is directly linked to the expansion of these sources (like rice cultivation and livestock farming),the Reason correctly explains the Assertion.