AIPMT 1998 Biology Question Paper with Answer and Solution

(D) Involuntary muscles (smooth muscles) are found in the walls of internal organs such as blood vessels,glandular ducts,and the iris. The urethra,however,contains both involuntary smooth muscles (internal urethral sphincter) and voluntary skeletal muscles (external urethral sphincter) to control micturition. Therefore,the urethra does not consist solely of involuntary muscles.

(A) Microtubules are structural components of the cytoskeleton,composed of tubulin proteins.
They play a crucial role in cell division by forming the spindle apparatus,which helps in the separation of chromosomes during mitosis and meiosis.
They are also involved in maintaining cell shape and intracellular transport.
Therefore,the correct option is $A$.

(C) The condition in which the kidneys fail to produce urine is known as $Anuria$.
$Anuria$ is defined as the failure of the kidneys to produce urine,typically defined as a urine output of less than $100 \ mL$ per day.
$Deamination$ is the process of removing an amino group from an amino acid.
$Atrophy$ refers to the wasting away or decrease in size of an organ or tissue.

(B) In mammals,the lower jaw is composed of a single pair of bones known as the $Dentary$ bones.
Unlike other vertebrates where the lower jaw consists of multiple bones,in mammals,the $Dentary$ is the only bone that forms the lower jaw and articulates with the squamosal bone of the skull.
Therefore,the correct answer is $B$.

(A) Muscle contraction is primarily driven by the interaction between two key contractile proteins: $Actin$ and $Myosin$.
$Actin$ forms the thin filaments,while $Myosin$ forms the thick filaments.
These proteins slide past each other during the contraction process,known as the sliding filament theory.
$Tubulin$ is a structural protein involved in the formation of microtubules,not muscle contraction.
Therefore,both $Actin$ and $Myosin$ are contractile proteins,making the correct choice 'All of the above'.

(B) Each hind limb of a human consists of $30$ bones.
These bones are distributed as follows:
$1$ Femur (thigh bone),
$1$ Patella (knee cap),
$1$ Tibia,
$1$ Fibula,
$7$ Tarsals (ankle bones),
$5$ Metatarsals (sole bones),
$14$ Phalanges (digits).
Total = $1 + 1 + 1 + 1 + 7 + 5 + 14 = 30$ bones.

(B) The thyroid gland secretes two main hormones: thyroxine $(T_4)$ and triiodothyronine $(T_3)$.
These hormones play a crucial role in regulating the basal metabolic rate $(BMR)$ of the body.
They also support the process of red blood cell formation,control the metabolism of carbohydrates,proteins,and fats,and maintain water and electrolyte balance.
Therefore,the correct answer is the thyroid gland.

(C) Cellulose is a polysaccharide consisting of a linear,unbranched chain of several hundred to many thousands of $\beta-D$-glucose units.
These glucose units are linked together by $\beta-1, 4$-glycosidic bonds.
Unlike starch (which contains $\alpha$-linkages),the $\beta$-linkages in cellulose result in a straight,rigid,and fibrous structure that provides structural support to plant cell walls.

(D) Lactose is a disaccharide sugar that is commonly found in milk.
It is formed by the condensation reaction between two monosaccharide units.
The two monosaccharide units that make up lactose are $Glucose$ and $Galactose$.
These are linked together by a $\beta-1,4-glycosidic$ bond.
Therefore,the correct option is $D$.

(C) During the $S$-phase of the cell cycle,$DNA$ replication occurs.
$DNA$ is composed of euchromatin (transcriptionally active,loosely packed) and heterochromatin (transcriptionally inactive,tightly packed).
Both types of chromatin undergo replication during the $S$-phase to ensure that the daughter cells receive a complete set of genetic material.
Since radioactive thymine is a precursor for $DNA$ synthesis,it will be incorporated into the newly synthesized $DNA$ strands of both euchromatin and heterochromatin.
Therefore,both types of chromatids will become radioactive.

(C) The number of divisions $(n)$ is calculated by dividing the total time by the time taken for one division: $n = 175 / 35 = 5$.
After $n$ divisions,the number of cells is given by the formula $N = N_0 \times 2^n$,where $N_0$ is the initial concentration.
Given $N_0 = 10^5$ cells $/ ml$ and $n = 5$,the final concentration $N = 10^5 \times 2^5$.
Since $2^5 = 32$,the final concentration $N = 10^5 \times 32 = 32 \times 10^5$ cells $/ ml$.

(A) By definition, the water potential $(\Psi_w)$ of pure water at standard temperature and pressure is defined as zero.
Osmotic pressure is the pressure required to prevent the movement of water into a solution across a semi-permeable membrane.
Since pure water contains no solutes, its osmotic pressure is zero.
Therefore, both the water potential and the osmotic pressure of pure water are zero.

(A) Protochlorophyll is the immediate precursor to chlorophyll $a$ in the biosynthetic pathway of chlorophyll.
The structural difference between protochlorophyll and chlorophyll $a$ is the presence of two additional hydrogen atoms in the $IV$ pyrrole ring of the chlorophyll molecule.
Therefore,protochlorophyll lacks these $2$ hydrogen atoms compared to chlorophyll $a$ in one of its pyrrole rings.

(C) Phytochrome is a pigment found in plants that acts as a photoreceptor. It is primarily involved in sensing light quality and duration,which regulates various developmental processes such as flowering,seed germination,and stem elongation. Specifically,phytochrome plays a crucial role in $Photoperiodism$,which is the physiological reaction of organisms to the length of day or night. Therefore,the correct answer is $Photoperiodism$.

(B) In plant tissue culture,the balance between auxins and cytokinins is crucial for organogenesis. Cytokinins are specifically known for promoting cell division and are essential for shoot induction (caulogenesis) in callus cultures. When the ratio of cytokinin to auxin is high,it promotes shoot development,whereas a high auxin to cytokinin ratio promotes root development.

(B) The physiological response of plants to the relative lengths of light and dark periods is called $Photoperiodism$.
$Phototropism$ refers to the growth of a plant in response to a light direction.
$Phototaxis$ is the movement of an organism in response to light.
$Vernalization$ is the induction of a plant's flowering process by exposure to the prolonged cold of winter.
Therefore,the correct answer is $Photoperiodism$.

(D) Ethylene $(C_2H_4)$ is a gaseous plant hormone that promotes fruit ripening. In commercial applications,a mixture containing $80\% C_2H_4$ and $20\% CO_2$ is often used to accelerate the ripening process of fruits like bananas and mangoes. The presence of $CO_2$ helps in controlling the rate of respiration during the ripening process.

(C) Kwashiorkor is a severe form of malnutrition caused by a deficiency of protein in the diet. It is most commonly seen in children in developing countries. Symptoms include edema (swelling),a bloated abdomen,and skin changes. Eczema is a skin condition,cirrhosis is a liver disease,and night blindness is caused by a deficiency of Vitamin $A$.

(A) Lacteals are specialized lymphatic capillaries found in the villi of the small intestine,specifically in the ileum region.
They are responsible for the absorption of dietary fats and fat-soluble vitamins from the digested food in the intestinal tract.
Therefore,the correct location is the ileum.

(C) The contraction of the gallbladder and the relaxation of the sphincter of Oddi are primarily stimulated by the hormone $Cholecystokinin$ $(CCK)$.
$CCK$ is secreted by the $I$-cells in the mucosal epithelium of the small intestine in response to the presence of fats and proteins in the chyme.
Once released into the bloodstream, it travels to the gallbladder, triggering its contraction to release bile into the duodenum to aid in fat digestion.

(C) Erythrocytes (Red Blood Cells) require specific factors for their maturation in the bone marrow.
Vitamin $- B_{12}$ (Cyanocobalamin) and Folic acid are essential for $DNA$ synthesis and the maturation of red blood cells.
$A$ deficiency of Vitamin $- B_{12}$ leads to pernicious anemia,where the red blood cells are immature and large (megaloblastic).
Therefore,Vitamin $- B_{12}$ is a critical factor for the maturation of erythrocytes.

(B) The enamel of the teeth is the hardest substance in the human body. It is secreted by specialized epithelial cells known as $Ameloblasts$. These cells are derived from the enamel organ during tooth development. Once the enamel is fully formed,these cells typically degenerate. In contrast,$Odontoblasts$ are responsible for the secretion of dentin,while $Osteoblasts$ are involved in bone formation.

(A) The hormone $Gastrin$ is secreted by the $G-cells$ located in the pyloric antrum of the stomach.
It acts on the gastric glands to stimulate the secretion of gastric juice,which includes $HCl$ and pepsinogen.
$Renin$ is a proteolytic enzyme found in the gastric juice of infants.
$Enterokinase$ is an enzyme secreted by the intestinal mucosa that activates trypsinogen.
$Enterogastrone$ is a hormone that inhibits gastric secretion and motility.

(A) The correct answer is $A$.
Vitamin $K$ is essential for blood clotting, and its deficiency leads to excessive bleeding.
Beriberi is caused by the deficiency of Vitamin $B_1$ (Thiamine), not Vitamin $K$.
Vitamin $D$ deficiency causes Rickets in children.
Vitamin $C$ deficiency causes Scurvy.
Vitamin $A$ deficiency causes Xerophthalmia (dryness of the eyes).

(A) The exchange of gases ($O_2$ and $CO_2$) between the alveoli and the blood in the pulmonary capillaries occurs primarily through the process of simple diffusion.
This process is driven by the partial pressure gradients of the gases.
Oxygen moves from the alveoli (higher partial pressure) into the blood (lower partial pressure),while carbon dioxide moves from the blood (higher partial pressure) into the alveoli (lower partial pressure).
Simple diffusion is a passive process that does not require metabolic energy.

(C) Lymph nodes are secondary lymphoid organs that act as filters for the lymph fluid.
Their primary functions include:
$1$. Trapping microorganisms (like bacteria) and antigens present in the lymph.
$2$. Providing a site for the activation and proliferation of lymphocytes (white blood cells).
$3$. Production of antibodies by activated $B$-lymphocytes.
Red blood cells are produced in the bone marrow,not in the lymph nodes. Therefore,the production of red blood cells is $NOT$ a function of lymph nodes.

(D) The chemical $2, 4-D$ ($2, 4-dichlorophenoxyacetic$ acid) is a synthetic auxin that acts as a herbicide.
It is widely used to control broad-leaved weeds and is also known as a defoliant,meaning it causes leaves to fall off from plants or trees.
Therefore,it is used to remove leaves from forest trees.

(D) The response of plants to periods of day and night (light and dark) is known as $Photoperiodism$.
$Phototropism$ is the growth of an organism in response to a light stimulus.
$Phototaxis$ is the movement of an organism in response to light.
$Vernalization$ is the induction of a plant's flowering process by exposure to the prolonged cold of winter.

(D) The Assertion is incorrect because insectivorous plants grow in nitrogen-deficient soils,not oxygen-deficient environments. They trap insects to fulfill their nitrogen requirements.
The Reason is correct because these plants possess chlorophyll and perform photosynthesis (autotrophic),but they also consume insects to obtain essential nutrients like nitrogen (heterotrophic).
Therefore,the Assertion is incorrect,but the Reason is correct.

(C) gynandromorph is an organism that contains both male and female tissues. In certain insects like Drosophila,this occurs due to the loss of one $X$ chromosome during early mitotic divisions in a developing embryo. If a cell that is $XX$ (female) loses one $X$ chromosome,it becomes $XO$ (male). As the embryo continues to develop,the tissues derived from the $XX$ cells become female,while the tissues derived from the $XO$ cells become male,resulting in a gynandromorph.

(A) Pleiotropy is a genetic phenomenon where a single gene influences multiple,seemingly unrelated phenotypic traits.
For example,in humans,the gene responsible for sickle cell anemia affects the shape of red blood cells,causes anemia,and leads to various other systemic complications.
Therefore,the correct term for this phenomenon is Pleiotropy.

(C) Mendel studied $7$ pairs of contrasting traits in pea plants,which have $14$ chromosomes $(2n = 14)$.
If the plant had only $12$ chromosomes $(2n = 12)$,it would mean there are only $6$ pairs of chromosomes.
Since Mendel studied $7$ traits,at least two of these traits would have to be located on the same chromosome pair.
Genes located on the same chromosome are linked and do not assort independently.
Therefore,if Mendel had studied a plant with $12$ chromosomes,he would have observed linkage for some traits and would not have been able to formulate the Law of Independent Assortment.

(D) Spontaneous point mutations often occur due to the deamination of $5$-methylcytosine.
When $5$-methylcytosine undergoes deamination,it is converted into thymine.
Since thymine is a natural base in $DNA$,the repair mechanisms of the cell often fail to recognize this as a mutation,leading to a permanent $C \rightarrow T$ transition mutation.
Therefore,$5$-methylcytosine is considered a 'hotspot' for spontaneous mutations.

(A) Mental retardation in humans is often associated with sex chromosomal aneuploidy,such as $Klinefelter$ syndrome $(47, XXY)$ or $Turner$ syndrome $(45, X)$.
Specifically,the condition known as $Klinefelter$ syndrome,which involves the presence of an extra $X$ chromosome $(47, XXY)$,is frequently associated with varying degrees of mental retardation.
Therefore,the gain of an $X$ chromosome is a common cause of such sex-linked abnormalities.

(B) To determine the number of different types of gametes produced by an organism,we use the formula $2^n$,where $n$ is the number of heterozygous gene pairs.
In the given genotype $AABbcc$:
- $AA$ is homozygous (not heterozygous).
- $Bb$ is heterozygous.
- $cc$ is homozygous (not heterozygous).
Therefore,the number of heterozygous gene pairs $(n)$ is $1$ (only $Bb$).
Using the formula: $2^n = 2^1 = 2$.
The two types of gametes produced will be $ABc$ and $Abc$.

(D) Crossing over is a biological process that occurs during the $pachytene$ stage of $prophase-I$ of $meiosis$.
It involves the exchange of genetic material between non-sister chromatids of homologous chromosomes.
This process leads to the formation of new combinations of alleles on a chromosome,which is known as genetic recombination.
Therefore,crossing over is responsible for the recombination of linked genes,which increases genetic variation in sexually reproducing organisms.

(B) Albinism is an autosomal recessive disorder. Let the normal allele be '$A$' and the albino allele be '$a$'.
Since the first child is an albino $(aa)$,both parents must be carriers of the recessive allele $(Aa)$.
When two carrier parents $(Aa \times Aa)$ have a child,the possible genotypes are $AA, Aa, Aa, aa$.
The probability of an offspring being an albino $(aa)$ is $1/4$ or $25\%$.
Each child's birth is an independent event,so the probability for the second child remains $25\%$.

(C) Let $X^H$ be the gene for normal blood clotting and $X^h$ be the gene for hemophilia. Let $X^C$ be the gene for normal vision and $X^c$ be the gene for color blindness. The woman is a carrier for hemophilia $(X^H X^h)$ and also carries the color blindness gene on one $X$ chromosome. Assuming the genes are linked,her genotype is $X^{Hc} X^h$ (where $c$ is the color blindness allele). The normal man has the genotype $X^{HC} Y$.
When they cross:
$1$. $X^{Hc} X^h \times X^{HC} Y$
$2$. Offspring genotypes: $X^{Hc} X^{HC}$ (Carrier daughter),$X^{Hc} Y$ (Hemophilic and color-blind son),$X^h X^{HC}$ (Carrier daughter),$X^h Y$ (Hemophilic son).
Thus,the sons will be $50\%$ hemophilic and color-blind,and $50\%$ hemophilic.

(D) Regulatory genes are responsible for controlling the expression of other genes by producing proteins (repressors or activators) that bind to specific $DNA$ sequences. These proteins regulate the transcription of structural genes,effectively turning them on or off. This mechanism is a fundamental part of operon models,such as the $lac$ operon in $E. coli$.

(D) Transposons,also known as '$jumping$ $genes$',are specific $DNA$ sequences that have the ability to change their position within the genome.
They can move from one location to another on the same chromosome or to a different chromosome.
This process is known as transposition.
Exons are coding sequences,introns are non-coding sequences,and cistrons are functional units of $DNA$ encoding a polypeptide.

(D) Crossing over is a biological process that occurs during the pachytene stage of prophase-$I$ of meiosis.
It involves the exchange of genetic material between non-sister chromatids of homologous chromosomes.
This process results in the formation of new combinations of alleles on the chromosomes,which is known as genetic recombination.
Therefore,crossing over is responsible for the recombination of linked alleles,leading to genetic variation in the offspring.

(B) $Dryopithecus$ and $Ramapithecus$ were existing about $15$ million years ago. Among the given options,$25 \times 10^6$ years ago is the closest approximation for the geological period associated with the early hominoids like $Dryopithecus$ in many standard textbooks,although they are often cited as living around $15-20$ million years ago. Given the options provided,$25 \times 10^6$ years ago is the most appropriate choice.

(C) The statement describes the concept that 'ontogeny recapitulates phylogeny',which is known as the Biogenetic Law or Recapitulation Theory proposed by Ernst Haeckel.
This law suggests that the developmental stages of an embryo (ontogeny) reflect the evolutionary history of the species (phylogeny).
The development of the human heart from a two-chambered state (fish-like) to a three-chambered state (amphibian-like) and finally to a four-chambered state is a classic example used to support this theory.

(A) Genetic drift refers to the change in the frequency of an existing gene variant (allele) in a population due to random sampling of organisms.
It is a stochastic process that has a significant impact on the gene pool of small populations.
In large populations,the effects of genetic drift are negligible because the random fluctuations in allele frequencies tend to cancel each other out over time.
Therefore,genetic drift is primarily observed and has a pronounced effect in small populations.

(B) The correct statement is that $Homo$ $erectus$ is an ancestor of humans.
$Homo$ $erectus$ lived between $1.5$ to $2$ million years ago and is considered a key evolutionary link in the human lineage.
Cro-Magnon man was discovered in France,not Ethiopia.
Neanderthal man ($Homo$ $neanderthalensis$) is considered a separate species or a subspecies that coexisted with early $Homo$ $sapiens$ but is not a direct ancestor.
$Australopithecus$ is an early hominid,but it is generally considered a side branch rather than a direct ancestor of modern $Homo$ $sapiens$.

(B) $HIV$ (Human Immunodeficiency Virus) is a retrovirus.
It contains two identical strands of single-stranded $RNA$ as its genetic material.
It is enclosed within a protein coat (capsid) and an outer envelope derived from the host cell membrane.
Therefore,the correct option is $B$.

(D) $Clostridium$ $botulinum$ produces a potent neurotoxin called botulinum toxin.
This toxin specifically targets the $neuromuscular$ $junction$.
It prevents the release of the neurotransmitter $acetylcholine$ from the nerve endings.
As a result, the muscle fibers cannot receive the signal to contract, leading to flaccid paralysis.

(B) Typhoid fever is a bacterial infection caused by the bacterium $Salmonella$ $typhi$.
It is primarily transmitted through the ingestion of contaminated food or water.
Symptoms include high fever, weakness, stomach pain, headache, and loss of appetite.
Therefore, the correct option is $B$.

(D) Vegetatively propagated crops are ideal for maintaining hybrid varieties because vegetative propagation is a form of asexual reproduction.
In asexual reproduction,the offspring are genetically identical to the parent plant (clones).
Therefore,once a desirable hybrid genotype is obtained,it can be maintained indefinitely through vegetative propagation without the risk of segregation or recombination of traits that occurs during sexual reproduction (meiosis).
This ensures that the specific characteristics of the hybrid are preserved across generations.

(B) $Azotobacter$ is a free-living nitrogen-fixing bacterium that lives in the soil and fixes atmospheric nitrogen.
$VAM$ (Vesicular-Arbuscular Mycorrhiza) is a symbiotic fungus.
$Anabaena$ is a cyanobacterium that can be free-living or symbiotic (e.g.,in $Azolla$).
$Rhizobium$ is a symbiotic bacterium that forms nodules in the roots of leguminous plants.
Therefore,$Azotobacter$ is the correct example of a free-living biofertilizer.

(C) The industrial production of citric acid is primarily carried out using the fungus $Aspergillus$ $niger$.
This fungus is highly efficient at converting sugars into citric acid through fermentation processes.
$Lactobacillus$ is typically used for lactic acid production,while other species are used for different industrial products.

(B) Agent Orange is a powerful herbicide and defoliant that was used by the $U$.$S$. military during the Vietnam War. It is notorious for containing a toxic contaminant known as $2,3,7,8$-tetrachlorodibenzo-p-dioxin $(TCDD)$,which is a type of dioxin. This chemical is highly toxic and has been linked to various health issues,including cancer and birth defects.

(D) Third-generation pesticides are defined as substances that are highly specific to target pests and have minimal impact on non-target organisms and the environment.
Insect hormone analogues (such as juvenile hormone mimics) are classified as third-generation pesticides because they disrupt the growth and development of specific insects without causing widespread environmental toxicity.
Unlike first-generation (inorganic chemicals) and second-generation (synthetic organic chemicals like $DDT$) pesticides,these are more selective and safer.

(B) $Azolla$ $pinnata$ is a small aquatic fern that has a symbiotic relationship with the nitrogen-fixing cyanobacterium $Anabaena$ $azollae$.
This fern is widely used as a biofertilizer in rice fields because it fixes atmospheric nitrogen and provides it to the rice plants.
Studies have shown that the use of $Azolla$ can increase rice production by more than $50\%$ due to the enrichment of soil nitrogen content.

(B) In genetic engineering,$Escherichia \ coli$ $(E. \ coli)$ is widely used as a host organism for cloning and expression of recombinant $DNA$ due to its well-studied genome and rapid growth. $Agrobacterium \ tumefaciens$ is extensively used as a natural genetic engineer in plants because it can transfer its $T-DNA$ into the host plant genome,causing crown gall disease. By modifying this $T-DNA$,scientists use it as a vector to introduce desired genes into plants. Therefore,the correct answer is $Escherichia$ and $Agrobacterium$.

(D) Genetic engineering,or recombinant $DNA$ technology,relies on the ability to manipulate $DNA$ molecules.
Restriction endonucleases are enzymes that act as 'molecular scissors' by cutting $DNA$ at specific recognition sequences.
These enzymes were originally discovered in bacteria,where they serve as a defense mechanism against bacteriophages.
By purifying these enzymes from bacteria,scientists can use them in the laboratory to cut $DNA$ at precise locations,which is the fundamental step in creating recombinant $DNA$ molecules.

(D) Restriction endonucleases are enzymes that cut $DNA$ at specific recognition sequences.
These enzymes are essential tools in recombinant $DNA$ technology as they allow for the precise cleavage of $DNA$ molecules to create fragments that can be joined with vectors.
Primase is involved in $DNA$ replication,exonucleases remove nucleotides from the ends of $DNA$,and ligase joins $DNA$ fragments together.
Therefore,the correct enzyme for cutting $DNA$ is restriction endonuclease.

(A) Restriction endonucleases are enzymes that recognize specific palindromic nucleotide sequences in $DNA$ and cut the $DNA$ duplex at specific positions,usually within or near the recognition site. These are essential tools in recombinant $DNA$ technology,often referred to as 'molecular scissors'.

(B) The phenomenon described is $Pseudocopulation$.
In this interaction,the orchid $Ophrys$ mimics the female bee in terms of morphology,colour,and pheromones.
The male bee,deceived by this resemblance,attempts to mate with the flower,which is termed as $Pseudocopulation$.
As a result of this attempt,the bee inadvertently picks up or deposits pollen,facilitating pollination for the orchid.

(B) species that is found exclusively in a particular geographical region and nowhere else in the world is known as an $Endemic$ $species$.
$Sibling$ $species$ are morphologically similar but reproductively isolated.
$Sympatric$ $species$ are those that live in the same area but are reproductively isolated.
$Allopatric$ $species$ are those that inhabit different geographical areas.

(C) In a marine aquatic ecosystem,the upper layer is known as the photic zone or euphotic zone,where sunlight can penetrate.
This region supports primary productivity through photosynthesis.
Phytoplankton (microscopic algae) are the primary producers found in this layer.
Zooplankton (microscopic animals) feed on these phytoplankton and are also abundant in the upper layers.
Therefore,both phytoplankton and zooplankton are characteristic of the upper layer of a marine ecosystem.

(A) In an ecosystem, energy flow is strictly unidirectional.
Solar energy enters the ecosystem, is captured by producers (plants) through photosynthesis, and is then transferred to consumers (herbivores, carnivores, etc.) at different trophic levels.
At each transfer, a significant portion of energy is lost as heat (following the $10\%$ law), and it cannot be recycled back to the producers.
In contrast, nutrients like $Carbon$, $Nitrogen$, and $Potassium$ undergo biogeochemical cycles and are recycled within the ecosystem.

(C) In an ecosystem, the flow of energy always starts from producers (green plants). According to Lindeman's $10\%$ law, energy is lost at each trophic level, so the number and biomass of producers are always greater than primary consumers. Therefore, the statement that producers are more than primary consumers is true.

(D) Gross Primary Productivity $(GPP)$ of an ecosystem is the rate of production of organic matter during photosynthesis.
Plants capture solar energy and convert it into chemical energy stored in organic molecules.
Therefore,the total amount of organic matter synthesized by producers per unit area per unit time is called $GPP$.

(B) $Prosopis$ (Khejri),$Acacia$ (Babul),and $Capparis$ (Kair) are xerophytic plants adapted to survive in arid and semi-arid regions.
These plants are characteristic of tropical thorn forests,where water availability is low and temperatures are high.
Therefore,they represent the tropical thorn forest biome.

(A) In any ecosystem,energy flows from producers to consumers.
According to the $10\%$ law of energy transfer,only $10\%$ of the energy is transferred from one trophic level to the next,while the rest is lost as heat.
$T_1$ represents the producers (plants),which capture solar energy and convert it into chemical energy.
Since $T_1$ is the first level and receives the maximum energy from the sun,it contains the highest amount of energy compared to subsequent levels $(T_2, T_3, T_4)$.
Therefore,the correct option is $T_1$.

(C) The $El$ $Niño$ is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean.
It typically occurs every $5$ to $8$ years.
This phenomenon significantly impacts global weather patterns,leading to increased rainfall in some regions and droughts in others.
Therefore,the correct answer is $El$ $Niño$.

(D) The presence of $Escherichia$ $coli$ $(E. coli)$ in water is widely used as an indicator of fecal contamination. $E. coli$ is a bacterium commonly found in the lower intestine of warm-blooded organisms. Its presence in water samples indicates that the water has been contaminated with fecal matter,which may contain other pathogenic microorganisms. Therefore,it serves as a reliable indicator of water quality regarding biological safety.

(D) Supersonic jets emit nitrogen oxides $(NO_x)$ into the stratosphere.
These nitrogen oxides react with ozone $(O_3)$ and cause its depletion or thinning.
Therefore,supersonic jets contribute to the thinning of the ozone layer ($O_3$ layer).

(C) In the sewage treatment process,the secondary treatment is also known as biological treatment.
During this process,the effluent from the primary settling tank is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it.
This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures).
While growing,these microbes consume the major part of the organic matter in the effluent.
Once the $BOD$ of sewage or wastewater is reduced significantly,the effluent is then passed into a settling tank where the bacterial 'flocs' are allowed to sediment.
This sediment is called 'activated sludge'.
$A$ small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. This is known as recycling of bacteria.

(B) $CO_2$ is a major greenhouse gas that traps heat in the Earth's atmosphere,a phenomenon known as the greenhouse effect.
This effect is essential for maintaining the Earth's surface temperature within a range suitable for life.
Without $CO_2$,the greenhouse effect would be significantly reduced,causing heat to escape into space.
Consequently,the Earth's surface temperature would be much lower than it is currently.

(C) Carbon monoxide $(CO)$ is a major air pollutant.
It has a much higher affinity for hemoglobin than oxygen $(O_2)$.
When $CO$ is inhaled,it binds to hemoglobin to form carboxyhemoglobin,which is a stable compound.
This prevents hemoglobin from binding with oxygen,thereby reducing the oxygen-carrying capacity of the blood and leading to hypoxia.

(D) Agricultural fields are a significant source of nitrous oxide $(N_2O)$.
This occurs primarily due to the use of nitrogen-based fertilizers in the soil.
Microbial processes such as nitrification and denitrification in the soil convert these nitrogenous compounds into $N_2O$,which is a potent greenhouse gas.