AIIMS 1992 Biology Question Paper with Answer and Solution

(A) Taxonomy based on all available information,including the study of origin,evolution,and variations to classify organisms,is known as classical taxonomy.
$A$ taxonomist who is engaged in studying the origin,evolution,variations,and classification of organisms is referred to as a classical taxonomist.

(D) $T.S. Sadasivan$ is a renowned Indian mycologist known for his work on the physiology of infection by $Fusarium$.
Other famous Indian mycologists include $K.C. Mehta$,$B.B. Mundkur$,and $C.V. Subramanian$.

(B) $Plasmogamy$ is the first stage of sexual reproduction in fungi,in which the cytoplasm of two motile or non-motile gametes fuses with each other.
In this process,the nuclei of the sex cells come close to each other but do not fuse immediately.
Thus,the resulting cell contains two nuclei per cell,which is known as a $dikaryon$ or $binucleate$ stage.

(A) If the fusion of two gametes fails to occur,the gametes can develop directly into reproductive structures known as $Azygospores$.
Under unfavorable environmental conditions,the filaments of $Spirogyra$ $varians$ placed in a sugar solution can develop these structures,which are also referred to as $Parthenospores$ or $Azygospores$.

(B) In mosses,the central part of the stem axis is known as the medulla or central strand. This region consists of a conducting strand made of slightly thick-walled,elongated,dead parenchymatous cells,which is specifically referred to as the $Hadrome$. The $Hadrome$ functions in the conduction of water and minerals,analogous to the xylem in higher plants.

(D) Selaginella is heterosporous,meaning it produces two types of spores: $Megaspores$ and $Microspores$.
In contrast,Pteris (fern) is homosporous,meaning it produces only one type of spore.
Therefore,the heterosporous condition is the key character that distinguishes Selaginella from Pteris.

(B) $Rafflesia$ is a holoparasitic plant. It lacks chlorophyll and cannot perform photosynthesis. The tip of its root forms haustorial connections with the roots of the host plant (typically $Tetrastigma$ vines) to absorb water and nutrients. Therefore,it is classified as a total root parasite.

(B) In castor,the cotyledons come above the surface of the soil into the air and light due to the rapid growth and elongation of the hypocotyl.
The cotyledons turn green and finally dry up and fall off,and the seedling becomes an independent plant.
This is a type of epigeal germination where the cotyledons function as the first pair of photosynthetic leaves.

(B) $Collenchyma$ is a type of simple permanent tissue that provides mechanical support to the growing parts of the plant, such as young stems and petioles of leaves.
These cells are living at maturity and possess characteristic thickening at the corners due to the deposition of cellulose, hemicellulose, and pectin.
In contrast, $Sclerenchyma$ cells are dead at maturity, while $Parenchyma$ and $Chlorenchyma$ are primarily involved in storage and photosynthesis, respectively, and provide minimal mechanical support.

(B) After the leaf falls,the abscission zone is exposed to the air. To prevent water loss and pathogen entry,the cells of the leaf scar develop a protective layer. This layer is formed by the deposition of $suberin$ and $lignin$ on the cell walls,which makes the scar waterproof and protective. Therefore,the correct answer is $B$.

(B) The correct answer is $B$.
In a bicollateral vascular bundle,the phloem is present on both the outer and inner sides of the xylem,with two strips of cambium located between the xylem and the two phloem layers.
This type of vascular bundle is a characteristic feature of the family $Cucurbitaceae$,which includes plants like $Cucurbita$ and $Lagenaria$.

(B) Cytochrome oxidase is an enzyme that contains iron $(Fe)$ and copper $(Cu)$ as essential cofactors. Among the given options,$Fe$ is the correct element. Iron plays a crucial role in the electron transport system $(ETS)$,photosynthesis,and respiration because it is a central component of the heme group in cytochromes.

(A) The gall bladder stores and concentrates bile juice produced by the liver.
Bile juice contains bile salts which are essential for the emulsification of fats.
Emulsification breaks down large fat globules into smaller droplets,increasing the surface area for the action of the enzyme lipase.
If the gall bladder is surgically removed (cholecystectomy),the bile is no longer stored or concentrated; instead,it flows directly into the small intestine.
While the liver continues to produce bile,the lack of concentrated bile storage leads to a reduced efficiency in the digestion and absorption of dietary fats,particularly after a high-fat meal.

(A) . Absorption is the process by which the digested food of the alimentary canal passes through its mucous membrane into the circulatory system.
$B$. Assimilation is the utilization of absorbed food by the body cells.
$C$. Hydrolysis is the chemical breakdown of a compound due to reaction with water.
$D$. Defecation is the elimination of undigested waste from the body.

(C) Enzymes,vitamins,and hormones are all essential biological chemicals that act as regulators of various physiological processes within an organism.
Enzymes act as biological catalysts to speed up biochemical reactions.
Hormones act as chemical messengers that coordinate body activities.
Vitamins act as co-factors or essential components for metabolic pathways.
Therefore,they all collectively aid in the regulating mechanism of the body's metabolic and physiological functions.

(A) Vitamin $D$ is synthesized in the skin when it is exposed to the $UV$ rays of sunlight. The precursor $7$-dehydrocholesterol present in the skin is converted into cholecalciferol (Vitamin $D_3$) upon exposure to $UV$ radiation.

(D) The respiratory rhythm centre, which is primarily responsible for the regulation of respiration, is located in the $Medulla \text{ } oblongata$ region of the brain.
This centre contains specialized chemosensitive areas and neurons that generate nerve impulses to stimulate the diaphragm and intercostal muscles, thereby facilitating the process of breathing.
Therefore, the correct option is $(d)$.

(C) The pulmonary vein carries oxygenated blood from the lungs to the left atrium of the heart.
During the process of pulmonary circulation,blood passes through the alveoli in the lungs where gas exchange occurs.
As a result,the blood becomes oxygenated,meaning it has a higher concentration of $O_2$ compared to the deoxygenated blood returning from the rest of the body.
Therefore,the blood returning via the pulmonary vein has more oxygen per $ml$ of blood.

(D) Purkinje fibres are specialized conducting fibres located in the inner ventricular walls of the heart.
They are composed of modified cardiac muscle cells that are larger than ordinary cardiomyocytes.
These fibres are responsible for the rapid conduction of electrical impulses from the $AV$ node to the ventricular myocardium,ensuring coordinated contraction of the ventricles.
Therefore,they are modified muscle cells.

(B) The process of $Erythropoiesis$ (production of red blood cells) is primarily regulated by the oxygen levels in the blood.
When the oxygen content in the arterial blood decreases (hypoxia),the kidneys detect this deficiency and release the hormone $Erythropoietin$.
This hormone stimulates the bone marrow to increase the production of red blood cells to restore oxygen-carrying capacity.
Therefore,a deficiency of oxygen is the primary stimulus for $Erythropoiesis$.

(B) In a $4$-chambered heart,the heart is divided into two atria and two ventricles.
This complete separation ensures that oxygenated (arterial) blood and deoxygenated (venous) blood do not mix.
In contrast,$2$-chambered hearts (found in fish) have a single circulation where blood passes through the heart once,and $3$-chambered hearts (found in amphibians and most reptiles) allow for the mixing of oxygenated and deoxygenated blood in the single ventricle.

(C) The urinary excretion of $Na^+$ is primarily regulated by aldosterone,which is a mineralocorticoid hormone.
Aldosterone is secreted by the zona glomerulosa of the adrenal cortex.
It acts on the distal convoluted tubules and collecting ducts of the nephron to increase the reabsorption of $Na^+$ and water,and the excretion of $K^+$ and phosphate ions.
Therefore,the correct option is $C$.

(D) Insufficient filtration in the Bowman's capsule leads to the retention of waste products and fluids in the body.
$(a)$ Accumulation of fluid occurs because the kidneys fail to excrete excess water.
$(b)$ Increase in blood pressure occurs due to fluid retention and the activation of the renin-angiotensin system.
$(c)$ Increase in blood urea level (uremia) occurs because urea is not efficiently filtered from the blood.
$(d)$ Loss of glucose through urine (glycosuria) is primarily caused by insufficient reabsorption in the proximal convoluted tubule,not by filtration failure in the Bowman's capsule. Therefore,this is the correct exception.

(D) During muscle contraction, $ATP$ is rapidly consumed. The immediate regeneration of $ATP$ is facilitated by $Creatine$ phosphate (also known as phosphocreatine).
In resting muscle, excess $ATP$ is used to synthesize $Creatine$ phosphate, which acts as an energy reservoir.
When muscle contraction begins, the enzyme creatine kinase transfers a phosphate group from $Creatine$ phosphate back to $ADP$ to quickly reform $ATP$.

(B) The perception of the type of stimulus (e.g.,light,sound,touch) depends on the specific pathway through which the nerve impulse travels to the brain.
Each sensory receptor is specialized to respond to a specific type of stimulus.
When a receptor is stimulated,it sends an impulse along a dedicated neural pathway to a specific region of the central nervous system $(CNS)$.
Therefore,the brain interprets the stimulus based on the specific connection to the receptor organ and the area of the brain receiving the signal,rather than the nature of the impulse itself,which is essentially the same (an action potential) for all stimuli.

(D) . The $Eustachian$ $tube$ connects the middle ear cavity with the nasopharynx.
Its primary function is to equalize the air pressure on both sides of the tympanic membrane (eardrum),which is essential for proper hearing and preventing damage to the membrane.

(C) The thyroid gland secretes thyroid hormones (like $T_3$ and $T_4$),which are iodinated amino acid derivatives. The adrenal gland secretes various hormones,including glucocorticoids (like cortisol) and mineralocorticoids (like aldosterone),which are steroid hormones,as well as catecholamines (like adrenaline),which are amino acid derivatives. However,both thyroid hormones and certain adrenal hormones (like glucocorticoids) play a significant role in increasing glucose metabolism in the body. Thyroid hormones increase the basal metabolic rate and glucose utilization,while glucocorticoids stimulate gluconeogenesis and increase blood glucose levels. Therefore,the common functional similarity is that they both increase glucose metabolism.

(D) The secretion of aldosterone,a mineralocorticoid,is primarily regulated by the Renin-Angiotensin-Aldosterone System $(RAAS)$ and plasma electrolyte levels.
$1$. Angiotensin-$II$: When blood pressure or blood volume decreases,the kidneys release renin,which leads to the production of angiotensin-$II$. Angiotensin-$II$ directly stimulates the adrenal cortex to secrete aldosterone.
$2$. Plasma $K^+$ concentration: An increase in plasma potassium levels $(hyperkalemia)$ directly stimulates the adrenal cortex to release aldosterone to promote potassium excretion in the kidneys.
Therefore,both plasma $K^+$ concentration and the level of blood angiotensin-$II$ are key regulators of aldosterone secretion.

(D) In many dicotyledonous plants, such as $Bean$ ($Phaseolus$ $vulgaris$) and $Castor$ ($Ricinus$ $communis$), the cotyledons emerge above the soil during germination (epigeal germination).
Once they emerge, they turn green and perform photosynthesis, effectively acting as the first pair of leaves for the seedling before the true foliage leaves develop.
In contrast, monocots like $Maize$ and $Rice$ typically exhibit hypogeal germination where the cotyledon remains below the soil.

(D) Dendrochronology is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed.
Each year,a tree adds a new layer of wood to its trunk,forming an annual ring.
By counting these rings,scientists can determine the age of the tree and study past climatic conditions.
Dendrology is the study of trees,while the other options are unrelated to tree aging.

(C) $Parthenium \, hysterophorus$ (carrot grass / congress grass) is a highly invasive weed belonging to the family $Asteraceae$.
It was introduced into India from the $USA$ in the $1950$s as a contaminant along with imported wheat.
Since its introduction,it has spread rapidly across all parts of India,causing severe ecological and health issues.

(D) Testosterone is the primary male sex hormone. It is synthesized and secreted by the Leydig cells,also known as interstitial cells. These cells are located in the interstitial spaces,which are the regions situated between the seminiferous tubules in the testes. Therefore,the correct answer is the cells that lie between the seminiferous tubules.

(A) In an orthotropous ovule,the body of the ovule is straight and not curved.
As a result,the micropyle,chalaza,and the funiculus all lie in a single straight vertical line.
This is considered the most primitive type of ovule and is found in members of the families Polygonaceae and Urticaceae.

(D) Self-pollination is prevented by several mechanisms in plants to promote cross-pollination:
$1$. $Self-sterility$ (or self-incompatibility): $A$ genetic mechanism where pollen grains fail to germinate on the stigma of the same flower.
$2$. $Herkogamy$: $A$ physical or mechanical barrier between the anther and the stigma of the same flower, preventing self-pollination.
$3$. $Dichogamy$: $A$ condition where the anthers and stigma of the same flower mature at different times (protandry or protogyny).
Therefore, all these mechanisms prevent self-pollination.

(D) The gynoecium represents the female reproductive part of a flower, consisting of one or more carpels (pistils).
Each carpel is composed of three distinct parts: the stigma, the style, and the ovary.
$1$. The $Stigma$ serves as the landing platform for pollen grains.
$2$. The $Style$ is the elongated, slender part beneath the stigma.
$3$. The $Ovary$ is the basal, swollen part that contains the ovules.
Therefore, the part that receives the pollen is the $Stigma$.

(B) $Corpus$ $luteum$ literally means yellow body.
The luteal cells secrete small amounts of estradiol hormone and significant amounts of the progesterone hormone.
$Corpus$ $luteum$ also secretes the hormone relaxin.

(D) The placental barrier refers to the layers of tissue separating maternal blood from foetal blood.
In some mammals,there are up to six layers (epitheliochorial placenta).
In humans,the placenta is of the hemochorial type,where the maternal blood comes in direct contact with the chorionic villi,reducing the number of barriers to only three.
Therefore,the placental barrier is minimum in humans.

(C) In humans,males are $XY$ and females are $XX$.
Since males have only one $X$ chromosome,any recessive gene present on the $X$ chromosome will be expressed because there is no corresponding allele on the $Y$ chromosome to mask its effect.
This condition is known as hemizygous.
Therefore,a single recessive gene on the $X$ chromosome of a male is sufficient to express the trait.

(B) Haemophilia is an $X$-linked recessive disorder.
Let $X^H$ be the normal allele and $X^h$ be the haemophilic allele.
The father is haemophilic,so his genotype is $X^hY$.
The mother is normal,so her genotype is $X^HX^H$ (assuming she is not a carrier).
When these parents produce offspring,the father contributes his $Y$ chromosome to his daughters and his $X^h$ chromosome to his daughters.
The mother contributes an $X^H$ chromosome to all her children.
Therefore,the genotype of the daughter will be $X^HX^h$.
Since the daughter has one normal allele $(X^H)$ and one haemophilic allele $(X^h)$,she will be a carrier of the disease.

(B) Haemophilia is a sex-linked recessive genetic disorder in which the blood fails to clot properly.
It is caused by a deficiency of the Anti-Haemophilic Factor $(AHF)$,also known as Factor $VIII$,which is essential for the blood coagulation cascade.
Therefore,the correct option is $B$.

(D) Albinism is an autosomal recessive disorder. Let the normal allele be $A$ and the recessive allele for albinism be $a$. An albino individual must have the genotype $aa$. For a child to be $aa$,they must receive one $a$ allele from each parent. Therefore,both parents must carry at least one $a$ allele. If both parents are phenotypically normal but carry the gene,they are heterozygous $(Aa)$. If one parent is an albino $(aa)$ and the other is a carrier $(Aa)$,the child can also be $aa$. Thus,both $(A)$ and $(C)$ are possible scenarios that result in an albino child.

(C) Sickle cell anaemia is an autosomal recessive genetic disorder caused by a mutation in the $Hb^S$ gene.
In the heterozygous condition $(Hb^A Hb^S)$,the individual is a carrier and exhibits the sickle cell trait,which provides some resistance to malaria.
In the homozygous recessive condition $(Hb^S Hb^S)$,the individual suffers from severe sickle cell anaemia.
This condition leads to the production of abnormal haemoglobin,causing red blood cells to become sickle-shaped,which results in severe anaemia and often leads to premature death due to organ failure and complications. Thus,the lethal effect is observed in the homozygous recessive state.

(A) $Ethnology$ is the branch of anthropology that compares and analyzes the characteristics of different peoples and the relationship between them. It specifically focuses on the study of human races,their origins,distribution,and cultural characteristics.

(A) Teratology is the branch of biology that deals with the study of abnormalities of physiological development,specifically the study of congenital malformations and abnormal embryonic growth.
Ontogeny refers to the developmental history of an individual organism.
Morphogenesis is the biological process that causes an organism to develop its shape.
Parthenogenesis is a form of asexual reproduction in which an embryo develops from an unfertilized egg.

(B) . The fruits of $Emblica$ $officinalis$ (Amla) are of great medicinal value and are considered one of the richest natural sources of vitamin $C$ (ascorbic acid).

(B) $Coir$ is the fibrous material found between the hard,internal shell and the outer coat of a coconut.
It is obtained from the mesocarp of the fruit of $Cocos$ $nucifera$ (coconut palm).
Therefore,the husk fibres obtained from dry coconut are known as $Coir$.

(B) The correct answer is $B$.
Crude birth rate is defined as the number of live births per $1000$ individuals in a population per year.
It is a standard demographic measure used to assess the fertility of a population.

(C) The rapid growth of the human population is primarily driven by a high birth rate. To control the population explosion,the most effective and direct approach is to implement strategies that reduce the birth rate. This is achieved through family planning programs,education,and the use of contraceptives,which directly address the root cause of overpopulation.