AIIMS 2015 Biology Question Paper with Answer and Solution

(D) $(i)$ Growth is observed in non-living objects (like mountains and boulders) by accumulation of material on the surface,so it is not a defining property.
$(ii)$ Dead organisms do not show growth.
$(iii)$ Reproduction is not a defining characteristic because many organisms like mules,worker bees,and infertile human couples do not reproduce.
$(iv)$ Self-replication is a unique property of living systems.
$(v)$ Metabolism in a test tube (in vitro) is a chemical reaction,not a living thing,but it is a living reaction.
$(vi)$ Metabolism is the sum total of all chemical reactions occurring in a living body and is a defining feature of all living organisms.
Since all statements are scientifically correct,the answer is $(D)$.

(A) Slime moulds are saprophytic protists.
$(i)$ The body moves along decaying twigs and leaves engulfing organic material,which is correct.
$(ii)$ Under suitable conditions,they form an aggregation called Plasmodium which may grow and spread over several feet. When conditions become unfavourable,the Plasmodium differentiates and forms fruiting bodies bearing spores at their tips,which is correct.
$(iii)$ The spores possess true walls (cellulosic),so statement $(iii)$ is incorrect.
$(iv)$ The spores are dispersed by air currents,which is correct.
$(v)$ The spores are extremely resistant and survive for many years,even under adverse conditions,which is correct.
$(vi)$ Plasmodium can grow up to several feet,which is correct.
Therefore,statements $(i), (ii), (iv), (v)$ and $(vi)$ are correct.

(A) All the given statements $(i)$ through $(vii)$ regarding the characteristics and habitats of algae are scientifically correct.
- Green algae $(Chlorophyceae)$ are found in diverse habitats including fresh,brackish,and salt water.
- Brown algae $(Phaeophyceae)$ are mostly marine,with rare occurrences in fresh water.
- Red algae $(Rhodophyceae)$ are predominantly marine but also occur in fresh and brackish water.
- Red algae are mostly multicellular and exhibit a wide range of distribution from surface waters to great depths.
- The cell wall of red algae is composed of cellulose,pectin,and phycocolloids like agar.
- Green algae typically possess $2-8$ equal,apical flagella.

(B) $(i)$ True: $Torpedo$ (electric ray) possesses electric organs to produce electric shocks.
$(ii)$ True: Bony fishes use various fins like pectoral,pelvic,dorsal,anal,and caudal for swimming and balancing.
$(iii)$ False: Amphibian skin is moist and lacks scales (they are scaleless).
$(iv)$ False: Birds are homeothermic (warm-blooded) animals,not poikilothermic.
$(v)$ True: The presence of mammary glands is a unique characteristic of mammals used for nourishing young ones.
Therefore,statements $(i), (ii),$ and $(v)$ are true,while $(iii)$ and $(iv)$ are false.

(C) The Assertion is correct because the duck-billed platypus and the spiny ant-eater (echidna) are prototherian mammals that lay eggs (oviparous) but possess mammary glands.
The Reason is incorrect because,while mammals generally have $7$ cervical vertebrae,the number of cranial nerves in mammals is $12$ pairs,but this specific anatomical feature is not the defining characteristic that classifies them as mammals. Furthermore,the statement implies these are the reasons for their classification as mammals,which is scientifically inaccurate; they are classified as mammals due to the presence of mammary glands,hair,and a diaphragm.

(B) Based on the structure of a typical dicot seed (like a bean or pea):
$A$ represents the Hilum,which is the scar on the seed coat through which the developing seeds were attached to the fruit.
$B$ represents the Micropyle,a small pore located above the hilum.
When the seed is opened,the internal structure reveals:
$C$ is the Plumule,which gives rise to the shoot system.
$D$ is the Cotyledon,which stores food.
$E$ is the Radicle,which gives rise to the root system.
Therefore,the correct labeling is $A -$ Hilum,$B -$ Micropyle,$C -$ Plumule,$D -$ Cotyledon,$E -$ Radicle.

(D) Based on the anatomical structure of a dicot leaf:
$A$ represents the upper epidermis.
$B$ represents the palisade parenchyma,which consists of elongated cells located below the upper epidermis.
$C$ represents the spongy parenchyma,which consists of loosely arranged cells with intercellular spaces.
$D$ represents the sub-stomatal cavity (air space).
$E$ represents the stoma (pore).
$F$ represents the phloem,which is located towards the lower side of the vascular bundle.
$G$ represents the metaxylem,which consists of larger vessels.
$H$ represents the protoxylem,which consists of smaller vessels located towards the upper side.
Therefore,option $D$ is the correct identification of the labeled parts.

(C) Sphincters are specialized circular muscles that control the passage of substances through body openings.
$(i)$ Sphincters are mesodermal in origin,as they are composed of muscle tissue.
$(ii)$ They contain stretch receptors that help in regulating their tone and contraction based on the presence of contents.
$(iii)$ Rhythmic contractions are characteristic of cardiac muscle or peristaltic movements,not typical sphincters,which generally maintain a tonic state of contraction.
$(iv)$ Sphincters do not fatigue during the life of the animal because they are designed to maintain a constant state of contraction to prevent leakage.
Therefore,the characteristics $(i), (ii),$ and $(iv)$ are associated with sphincters.

(D) Statement $(i)$ is true: Epithelial cells are compactly packed with very little intercellular matrix.
Statement $(ii)$ is true: Specialized junctions are present in animal tissues to provide structural and functional links.
Statement $(iii)$ is true: Tight junctions prevent substances from leaking across a tissue.
Statement $(iv)$ is true: Adhering junctions perform cementing to keep neighbouring cells together.
Statement $(v)$ is true: Gap junctions facilitate communication between cells by connecting the cytoplasm of adjacent cells for the rapid transfer of ions,small molecules,and sometimes big molecules.
Since all the given statements are correct,none of them are false.

(A) $i.$ Cells exhibit diverse shapes such as disc-like,polygonal,columnar,cuboid,thread-like,or even irregular,which are adapted to their specific functions.
$ii.$ The diameter of a human red blood cell $(RBC)$ is approximately $7.0 \, \mu m$.
$iii.$ The cytoplasm is a semi-fluid matrix that occupies the volume of the cell and serves as the primary site for various cellular metabolic activities.
$iv.$ The cytoplasm contains various organelles and molecules where chemical reactions take place,ensuring the cell remains in a living state.
Therefore,all the given statements are correct.

(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) By analyzing the chemical structures of the nitrogenous bases:
$A$ is Uracil,a pyrimidine base found in $RNA$.
$B$ is Adenine,a purine base.
$C$ is Thymine,a pyrimidine base with a methyl group $(-CH_3)$ at the $5^{th}$ position.
$D$ is Guanine,a purine base.
$E$ is Cytosine,a pyrimidine base.
Therefore,the correct combination is $A-$ uracil,$B-$ adenine,$C-$ thymine,$D-$ guanine,$E-$ cytosine.

(B) In a typical enzyme-catalyzed reaction,the velocity increases with substrate concentration until it reaches a maximum $(V_{max})$ and then plateaus.
However,the provided graph shows that after reaching a certain point,the reaction velocity decreases as the substrate concentration increases.
This decline in reaction rate at higher substrate concentrations is a characteristic indicator of the presence of an enzyme inhibitor in the reaction mixture,which interferes with the enzyme's activity.

(A) Minerals enter plant roots through various mechanisms such as active transport,proton $(H^+)$ pumps,and cation exchange.
Active transport involves the movement of ions against the concentration gradient using energy.
Proton pumps create an electrochemical gradient that facilitates the uptake of minerals.
Cation exchange is the process where cations adsorbed on soil particles are exchanged with $H^+$ ions released by roots.
Foliar feeding is a technique of feeding plants by applying liquid fertilizer directly to their leaves,not a mechanism for mineral uptake through roots.

(D) The $Casparian$ strip is a band of cell wall material deposited in the radial and transverse walls of the endodermis,which is chemically composed of suberin.
It acts as a selective barrier that forces water and dissolved solutes to pass through the plasma membrane of the endodermal cells rather than moving through the apoplastic pathway.
By forcing this movement through the cytoplasm (symplastic pathway),the plant can regulate the uptake of water and minerals into the vascular cylinder (xylem).
If a plant cannot produce $Casparian$ strips,it loses the ability to selectively control the entry of water and solutes into the xylem,leading to an unregulated flow.

(C) The formula for $DPD$ is $DPD = OP - TP$.
Given values for cell $A$: $DPD = 4 \, \text{bars}$.
For cell $B$: $OP = 4, TP = 4 \implies DPD = 4 - 4 = 0 \, \text{bars}$.
For cell $C$: $OP = 10, TP = 5 \implies DPD = 10 - 5 = 5 \, \text{bars}$.
For cell $D$: $OP = 7, TP = 3 \implies DPD = 7 - 3 = 4 \, \text{bars}$.
Water always moves from a region of lower $DPD$ to a region of higher $DPD$.
Comparing the values: $B(0) < A(4) = D(4) < C(5)$.
Therefore, water will flow from cell $B$ (lowest $DPD$) towards cells $A, D,$ and $C$ (higher $DPD$).

(A) The assertion is correct because sieve tubes are the primary conducting elements for the translocation of photoassimilates (sucrose) in plants.
The reason is also correct because mature sieve tube elements lack a nucleus but retain a thin layer of parietal cytoplasm and possess perforated sieve plates at their end walls.
These structural features facilitate the mass flow of sap from one sieve tube element to the next,making the reason a correct explanation for the assertion.

(A) Light is a critical environmental factor affecting the rate of transpiration.
Most plants have stomata that open in response to light to facilitate $CO_2$ intake for photosynthesis.
When stomata open,the pathway for water vapor loss increases,thereby increasing the rate of transpiration.
Conversely,in the dark,stomata close to conserve water,which significantly reduces transpiration.
Since the reason directly explains the mechanism by which light influences transpiration,the Reason is the correct explanation of the Assertion.

(A) Statement $(A)$ is correct: The synthesis of $ATP$ from $ADP$ and inorganic phosphate in the presence of light is known as photophosphorylation.
Statement $(B)$ is correct: Kranz anatomy is a specialized structure found in the leaves of $C_4$ plants,where mesophyll cells surround the bundle sheath cells.
Statement $(C)$ is incorrect: During the Calvin cycle (biosynthetic phase),$NADPH$ is oxidized to $NADP^+$ to provide electrons for the reduction of $3-PGA$ to $G3P$. The reduction of $NADP^+$ to $NADPH$ occurs during the light-dependent reactions.
Statement $(D)$ is incorrect: In a chlorophyll molecule,the magnesium ion $(Mg^{2+})$ is located at the center of the porphyrin ring,not in the phytol tail.
Therefore,statements $(A)$ and $(B)$ are correct.

(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) The mitochondrion is the site of aerobic cellular respiration. During this process,oxygen $(O_2)$ is consumed,and organic substrates are oxidized to release energy. This energy is used to synthesize $ATP$ from $ADP$ and inorganic phosphate $(Pi)$. The byproducts of this process include carbon dioxide $(CO_2)$ and water $(H_2O)$. Therefore,the correct representation is that $O_2$ and $ADP$ + Phosphate enter the mitochondrion,while $ATP$,$CO_2$,and $H_2O$ are produced and released. This corresponds to the process shown in option $A$.

(C) In the process of glycolysis, Glucose is converted into Glucose$-6-$phosphate.
Then, Glucose$-6-$phosphate is isomerized to Fructose$-6-$phosphate $(P)$.
Fructose$-6-$phosphate is then phosphorylated to form Fructose$-1,6-$bisphosphate $(Q)$.
Fructose$-1,6-$bisphosphate splits into two triose phosphates, one of which is $3-$phosphoglyceraldehyde $(R)$.
$3$-phosphoglyceraldehyde is then oxidized to form $1,3-$bisphosphoglyceric acid $(S)$.
Therefore, the correct sequence is $P = \text{Fructose-6-phosphate}$, $Q = \text{Fructose-1,6-bisphosphate}$, $R = 3\text{-phosphoglyceraldehyde}$, and $S = 1,3\text{-bisphosphoglyceric acid}$.

(A) The Assertion is correct because phytochrome is a photoreceptor pigment that regulates various developmental processes,including flowering,in response to light.
The Reason is also correct. Phytochrome exists in two interconvertible forms: $P_R$ (inactive) and $P_{FR}$ (active). When exposed to red light,$P_R$ is converted to $P_{FR}$. The $P_{FR}$ form is the physiologically active form that migrates to the nucleus or interacts with signaling pathways to induce floral development in shoot buds.
Since the active form $P_{FR}$ is the specific component of the phytochrome system that triggers the flowering response,the Reason correctly explains the Assertion.

(A) Vernalization is the process of inducing or accelerating flowering in plants by exposing them to low temperatures. This phenomenon was first described by $T.D.$ Lysenko in $1928$.
The Assertion is correct because vernalization specifically refers to the promotion of flowering through cold treatment.
The Reason is also correct because the perception of the cold stimulus occurs primarily in the apical meristem (shoot apex) of the plant,where it triggers the production of a hypothetical flowering hormone called vernalin.
Since the Reason correctly identifies the site where the process described in the Assertion occurs,it serves as a valid explanation.

(D) Parietal cells (also known as oxyntic cells) in the stomach lining are responsible for secreting hydrochloric acid $(HCl)$ and intrinsic factor.
$HCl$ is essential for converting the inactive enzyme pepsinogen into its active form,pepsin.
Pepsin is the primary enzyme responsible for the digestion of proteins into proteoses and peptones in the acidic environment of the stomach.
If parietal cells become non-functional,$HCl$ secretion decreases,leading to a higher (less acidic) $pH$ in the stomach.
Consequently,pepsinogen cannot be activated efficiently,and proteins will not be adequately hydrolyzed by pepsin.

(A) In the condition of obstructive jaundice,the entry of bile into the small intestine is prevented due to an obstruction in the bile duct.
Bile salts are essential for the emulsification of fats,which increases the surface area for lipase action.
Furthermore,bile salts are necessary for the formation of micelles,which are water-soluble aggregates that transport fatty acids,glycerides,and fat-soluble vitamins to the intestinal mucosa for absorption.
In the absence of bile,fats cannot be properly emulsified or absorbed.
Consequently,these unabsorbed fats pass through the digestive tract and are eliminated from the body in the faeces.

(C) Carbon dioxide $(CO_2)$ is transported in the blood in three forms:
$1$. As dissolved gas in plasma $(7\%)$.
$2$. As carbamino-haemoglobin bound to haemoglobin $(20-25\%)$.
$3$. As bicarbonate ions $(HCO_3^-)$ in blood plasma and $RBCs$ $(70\%)$.
Since the majority of $CO_2$ is transported as bicarbonate,the correct answer is $C$.

(C) The $P$-wave represents the electrical excitation (or depolarization) of the atria,which leads to the contraction of both the atria.
The $QRS$ complex represents the depolarization of the ventricles,which initiates the ventricular contraction. The contraction starts shortly after $Q$ and marks the beginning of the systole.
The $T$-wave represents the return of the ventricles from an excited to a normal state (repolarization). The end of the $T$-wave marks the end of systole.
Therefore,the $QRS$ complex corresponds to the depolarization of the ventricles,which initiates ventricular contraction,effectively representing one complete pulse or heartbeat cycle in terms of electrical activity.

(A) Veins are known as capacitance vessels because they contain a large volume of blood (about $60-70\%$ of the total blood volume) at any given time.
When blood pressure drops due to a decrease in blood volume (e.g.,hemorrhage),the sympathetic nervous system triggers vasoconstriction of the veins.
This constriction reduces the capacity of the venous system,forcing the stored blood to move toward the heart and into the arterial system.
This mechanism helps to maintain venous return and blood pressure,making the Reason a correct explanation for the Assertion.

(D) $Henle's$ loop plays a crucial role in the counter-current mechanism,which is responsible for the reabsorption of water and sodium chloride from the filtrate to concentrate the urine.
In the absence of $Henle's$ loop,the kidney would lose its ability to create a hypertonic medullary interstitium.
Consequently,the water cannot be reabsorbed efficiently from the collecting duct.
Therefore,the urine produced will be significantly more dilute compared to normal urine.

(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 $(ATP)$.

(A) During the resting state of a neuron,the axonal membrane is in a polarized state.
In this state,the membrane is significantly more permeable to potassium ions $(K^+)$ due to the presence of leak channels.
Conversely,the membrane is nearly impermeable to sodium ions $(Na^+)$ because the voltage-gated sodium channels are closed.
This differential permeability,along with the action of the $Na^+-K^+$ pump,maintains the resting membrane potential.

(A) Based on the standard $NCERT$ diagram for global biodiversity:
$1$. In pie diagram $A$ (Invertebrates),the largest slice is Insects,followed by Molluscs $(I)$,Crustaceans,and other animal groups.
$2$. In pie diagram $B$ (Vertebrates),the largest slice is Fishes,followed by Birds,Mammals,Reptiles,and Amphibians $(II)$.
$3$. In pie diagram $C$ (Plants),the largest slices are Angiosperms $(IV)$ and Fungi $(III)$,followed by Algae,Mosses,Ferns and allies,and Lichens.
Therefore,$I$ is Molluscs,$II$ is Amphibians,$III$ is Fungi,and $IV$ is Angiosperms. The correct option is $A$.

(D) Seeds are highly adaptive structures for plants for the following reasons:
$1.$ They maintain dormancy,which allows seeds to survive unfavorable environmental conditions until favorable conditions return.
$2.$ The seed coat provides protection to the young embryo during its most vulnerable developmental stages.
$3.$ Seeds contain stored food (in the endosperm or cotyledons) that provides nourishment to the developing seedling until it can perform photosynthesis. Additionally,seeds possess various mechanisms that facilitate their dispersal to new habitats.
Therefore,all the given statements are correct.

(B) In sexual reproduction,the events are categorized into three stages:
$1$. Pre-fertilization events: These include Gametogenesis $(A)$ and Gamete transfer $(B)$.
$2$. Fertilization $(C)$: The fusion of male and female gametes to form a diploid zygote $(2n)$.
$3$. Post-fertilization events: These include Zygote formation $(D)$ and Embryogenesis $(E)$.
Therefore,the correct sequence is $A-$ Gametogenesis,$B-$ Gamete transfer,$D-$ Zygote formation,$E-$ Embryogenesis.

(B) During pregnancy, high levels of estrogen and progesterone in the maternal circulation exert negative feedback on the pituitary gland, which inhibits the cyclic release of pituitary gonadotropins ($FSH$ and $LH$) and prevents the menstrual cycle.
$hCG$ (human chorionic gonadotropin) is produced by the placenta, which maintains the corpus luteum of pregnancy and serves as the basis for pregnancy urine tests.
Placental hormones, along with other maternal hormones, stimulate the development of mammary gland tissues (ducts and alveoli) to prepare for lactation.
The corpus luteum of pregnancy secretes progesterone to maintain the endometrium until the placenta takes over this function.

(A) In the menstrual cycle of a human female:
$1$. $A$ represents $FSH$ (Follicle Stimulating Hormone), which shows a small peak at the beginning of the cycle.
$2$. $B$ represents Oestrogen, which rises during the follicular phase.
$3$. $C$ represents $LH$ (Luteinizing Hormone), which shows a sharp, high peak ($LH$ surge) around the $14^{th}$ day, triggering ovulation.
$4$. $D$ represents Progesterone, which rises significantly during the luteal phase (after ovulation) to maintain the endometrium.
Therefore, the correct identification is $A-FSH, B-Oestrogen, C-LH, D-Progesterone$.

$(A)$. The pill: Prevents ovulation. Oral contraceptive pills contain hormones that inhibit the release of the egg from the ovary.
$B$. Condom: Prevents sperm from reaching the cervix. It acts as a physical barrier.
$C$. Vasectomy: Semen contains no sperms. It is a surgical sterilization method in males where the vas deferens is cut or tied.
$D$. Copper $T$: Prevents implantation. It is an Intrauterine Device $(IUD)$ that releases copper ions, which suppress sperm motility and fertilization capacity, and also prevent implantation of the embryo.

(B) Huntington's disease is an autosomal dominant disorder. In an autosomal dominant pedigree,an affected individual must have at least one affected parent.
In the given pedigree,individual $11$ is affected (represented by a filled circle). Since the disease is autosomal dominant,at least one of her parents ($6$ or $7$) must also be affected (represented by a filled symbol).
However,in the pedigree,both individuals $6$ and $7$ are unaffected (represented by empty symbols). Therefore,this pedigree is erroneous because it violates the rule of autosomal dominant inheritance.

(C) In humans,sex determination is based on the $XY$ type. Males produce two types of gametes: $50\%$ carrying the $X$ chromosome and $50\%$ carrying the $Y$ chromosome. Females produce only one type of gamete,all carrying the $X$ chromosome. Therefore,the sex of the child is determined by the sperm (male gamete) that fertilizes the egg. Thus,the Assertion is correct.
Sex in humans is not a polygenic trait; it is determined by the presence or absence of the $Y$ chromosome,specifically the $SRY$ gene located on it. The Reason statement is scientifically incorrect because sex determination in humans is primarily a monogenic trait controlled by the $Y$ chromosome,not a polygenic trait.

(B) According to Chargaff's rule,the amount of adenine $(A)$ is equal to the amount of thymine $(T)$,and the amount of guanine $(G)$ is equal to the amount of cytosine $(C)$.
Given that adenine is $30\%$,then thymine must also be $30\%$.
The total percentage of $A + T$ is $30\% + 30\% = 60\%$.
The remaining percentage for $G + C$ is $100\% - 60\% = 40\%$.
Since $G = C$,the percentage of cytosine is $40\% / 2 = 20\%$.

(A) The distance between genes is proportional to the percentage of crossing over. Given:
$1$. Crossing over between $a$ and $b = 20\%$
$2$. Crossing over between $b$ and $c = 28\%$
$3$. Crossing over between $a$ and $c = 8\%$
Since the distance between $b$ and $c$ $(28\%)$ is the sum of the distances between $b$ and $a$ $(20\%)$ and $a$ and $c$ $(8\%)$,gene $a$ must be located between $b$ and $c$.
Therefore,the sequence of genes on the chromosome is $b-a-c$.

(C) In a $DNA$ molecule,the nitrogenous bases are held together by hydrogen bonds.
$A$ (Adenine) pairs with $T$ (Thymine) via two hydrogen bonds,while $G$ (Guanine) pairs with $C$ (Cytosine) via three hydrogen bonds.
Because $G-C$ pairs have three hydrogen bonds,they are more stable and require more energy to break compared to $A-T$ pairs,which have only two hydrogen bonds.
Therefore,$A-T$ rich regions melt at lower temperatures than $G-C$ rich regions.
The Assertion is correct,but the Reason is incorrect because it states the number of hydrogen bonds in reverse.

(A) In Miller's experiment,the components are labeled as follows:
$A$ represents the electrodes used to create an electric discharge to simulate lightning.
$B$ represents the mixture of gases used,which were $CH_4$,$NH_3$,$H_2$,and water vapor $(H_2O)$.
$C$ represents the condenser,which uses cold water to cool the gases.
$D$ represents the vacuum pump connection.
$E$ represents the $U$-trap,where the condensed liquid is collected.
Therefore,the correct combination is $A-$ electrodes,$B-NH_3 + H_2 + H_2O + CH_4$,$C-$ cold water,$D-$ vacuum,$E-U$ trap.

(A) Comparative biochemistry provides strong evidence for the common ancestry of living beings. The presence of similar biochemical molecules such as proteins,enzymes,hormones,and blood groups across diverse species suggests a shared evolutionary origin.
Furthermore,the genetic code is universal,meaning that the same codons specify the same amino acids in almost all living organisms,from bacteria to humans. This universality is a powerful piece of evidence supporting the theory that all life on Earth evolved from a common ancestor. Therefore,the Reason correctly explains the Assertion.

(A) Molecule $(A)$ is the chemical structure of Cocaine,which is obtained from the plant Erythroxylum coca. It interferes with the transport of the neurotransmitter dopamine.
Molecule $(B)$ is the chemical structure of a Cannabinoid,which is obtained from the inflorescences of the plant Cannabis sativa. Cannabinoids are known for their effects on the cardiovascular system and can produce hallucinations.
Option $(A)$ is incorrect because while the source is correct,the structure shown is Cocaine,not Morphine.
Option $(D)$ is incorrect because the structure shown in $(A)$ is Cocaine,not Morphine.
Therefore,the correct identification and association based on the provided structures is that $(A)$ is Cocaine and $(B)$ is a Cannabinoid. However,looking at the options provided,option $(A)$ correctly identifies the source and action of Cocaine,which matches the structure $(A)$.

(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).

(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.

(D) The cloning vector $pBR322$ is a widely used plasmid in biotechnology. Based on the standard map of $pBR322$:
$1$. Label $A$ points to the restriction site for $EcoR I$.
$2$. Label $B$ points to the restriction site for $Bam HI$.
$3$. Label $C$ represents the ampicillin resistance gene $(amp^R)$.
$4$. Label $D$ represents the origin of replication $(ori)$.
Therefore,the correct identification is $A \to EcoR I, B \to Bam HI, C \to amp^R, D \to ori$.

(A) In recombinant $DNA$ technology,human genes are inserted into host organisms like bacteria or yeast to produce large quantities of desired proteins.
Bacteria and yeast are preferred as host cells because they have a very short generation time and multiply rapidly,allowing for the creation of a large population of cells in a short period.
This large population of cells expresses the inserted gene,leading to the efficient production of the recombinant protein.
Therefore,the Reason correctly explains why human genes are transferred into these organisms.

(D) Statement $(1)$ is incorrect: $IARI$ released a mustard variety rich in Vitamin $A$ (not $C$),such as Pusa Gaurav.
Statement $(2)$ is incorrect: Pusa Sawani variety of Okra (Bhindi) is resistant to shoot and fruit borer,not aphids.
Statement $(3)$ is correct: Hairiness of leaves in several plants is associated with resistance to insect pests (e.g.,resistance to jassids in cotton).
Statement $(4)$ is incorrect: While agriculture is vital,it accounts for approximately $33\%$ of India's $GDP$ and employs nearly $62\%$ of the population based on older data,but these specific figures are often cited in historical contexts; however,in the context of standard biology curriculum questions,statement $(3)$ is the only scientifically accurate statement among the choices provided.
Therefore,none of the combinations provided are entirely correct based on the specific facts given.

(D) Statement $1$ is incorrect because Rosie was the first transgenic cow,not a buffalo.
Statement $2$ is incorrect because restriction enzymes are used to cut $DNA$ at specific sites,not for the isolation of $DNA$ from other macromolecules (which is done by chemical treatment and centrifugation).
Statement $3$ is correct as downstream processing is a vital step in $R-DNA$ technology.
Statement $4$ is correct as disarmed pathogen vectors are commonly used to transfer $R-DNA$ into host cells.
Therefore,statements $1$ and $2$ contain mistakes.

(C) The reproductive rate (or per capita growth rate) is calculated by the formula: $\text{Rate} = \frac{\text{Births} - \text{Deaths}}{\text{Initial Population}}$.
Given: $\text{Births} = 55$, $\text{Deaths} = 5$, $\text{Initial Population} = 500$.
$\text{Rate} = \frac{55 - 5}{500} = \frac{50}{500} = 0.1/ \text{year}$.

(A) In the provided graph,the $x$-axis represents the external level and the $y$-axis represents the internal level of organisms.
Line $a$ shows a constant internal level regardless of changes in the external level,which represents a regulator.
Line $b$ shows that the internal level changes in direct proportion to the external level,which represents a conformer.
Line $c$ shows that the organism can regulate its internal level only up to a certain point,after which it conforms to the external environment,representing a partial regulator.
Therefore,$a$ is a regulator,$b$ is a conformer,and $c$ is a partial regulator.

(C) Tropical rain forests are located in the equatorial regions where the annual rainfall exceeds $140\, cm$.
They are also called jungles and cover one-twelfth of the Earth's surface but contain more than half of the Earth's flora and fauna,meaning they are extremely rich in biodiversity.
Therefore,the Reason provided is factually incorrect because these forests are actually very rich in biodiversity,not poor.
Nowadays,these forests are disappearing rapidly due to excessive deforestation for domestic purposes like fuel,furniture,accommodation,clothing,resin,gum,etc.

(C) The excessive use of urea and phosphate fertilizers in nearby crops leads to nutrient enrichment in the lake,a process known as eutrophication.
This causes an algal bloom,which makes the lake water turn green and stinky.
The decomposition of these algae by microorganisms consumes dissolved oxygen,leading to a severe depletion of oxygen levels in the water.
This lack of oxygen (hypoxia) is the primary cause of the heavy mortality of fishes in the lake.
Therefore,the main causes are $1$ and $3$.

(D) $DDT$ residues are rapidly passed through the food chain causing biomagnification because $DDT$ is soluble in lipids (lipo-soluble).
Biomagnification,also known as bioamplification or biological magnification,is the increase in the concentration of a substance,such as the pesticide $DDT$,that occurs in a food chain as a consequence of:
• Persistence (slow to be broken down by environmental processes).
• Food chain energetics.
• Low (or non-existent) rate of internal degradation/excretion of the substance,which is often due to its water-insolubility and high lipid solubility,allowing it to accumulate in the fatty tissues of organisms.

(A) Methane is produced by incomplete biomass combustion and anaerobic decomposition by methanogens.
Carbon dioxide contributes about $60\%$ to total global warming,while methane $(CH_4)$ and chlorofluorocarbons $(CFCs)$ contribute $20\%$ and $14\%$ respectively.
Nitrous oxide $(N_2O)$ contributes $6\%$ to total global warming.
Efficient engines,such as multi-point fuel injection engines,improve combustion efficiency,thereby reducing the emission of unburnt hydrocarbons like methane in automobile exhausts.
Therefore,both the Assertion and the Reason are correct,and the Reason explains why methane emissions from vehicles are controlled.