AIIMS 2000 Biology Question Paper with Answer and Solution

(B) The correct answer is $B$.
Species occupies a key position in biological classification.
It is considered the fundamental or basic unit of classification/taxonomy because it represents a group of individual organisms with fundamental similarities.

(B) In $Rhizopus$,sexual reproduction occurs through conjugation where two gametangia fuse to form a zygospore. If the fusion or conjugation fails,the individual gametangia may develop directly into thick-walled,resting spores without fertilization. These structures are known as $Parthenospores$ (or $Azygospores$).

(C) In mosses like $Funaria$,the sex organs (antheridia and archegonia) develop at the apex of the stem. The antheridia are produced on the male branch,while the archegonia are produced on the female branch.

(A) In $Pheretima$,the dorsal blood vessel is the main collecting vessel.
It possesses valves that prevent the backflow of blood.
These valves are located in front of the septum in each segment,ensuring the unidirectional flow of blood from the posterior to the anterior end.

(A) $Antedon$ belongs to the class $Crinoidea$ of the phylum $Echinodermata$.
It is a living fossil and is commonly known as the feather star.

(B) . Birds are homoiothermal,meaning they are capable of maintaining a constant body temperature regardless of the environmental temperature.

(B) Python is a non-venomous snake. It belongs to the family $Pythonidae$. It kills its prey by constriction (suffocation) and then swallows it whole. In contrast,$Naja$ $naja$ (Cobra),$Bungarus$ (Krait),and $Hydrophis$ (Sea snake) are highly venomous snakes.

(D) The butterfly-shaped corolla,also known as papilionaceous corolla,consists of five petals: one posterior standard (vexillum),two lateral wings (alae),and two anterior petals fused to form a keel (carina).
This type of aestivation is known as descending imbricate or vexillary aestivation.
This characteristic feature is found in the family $Papilionaceae$ (a subfamily of $Fabaceae$).

(C) The vascular cambium in roots is formed during secondary growth.
It originates from the dedifferentiation of parenchyma cells (such as conjunctive tissue and pericycle cells) located between the primary xylem and primary phloem.
Since it develops from permanent tissues that have regained the ability to divide,it is classified as a secondary meristem.

(C) . Companion cells are characteristic features of angiosperms only. In gymnosperms like $Pinus$,companion cells are absent and are replaced by albuminous cells (also known as Strasburger cells) which perform similar functions.

(D) . The whole plant body is enclosed by water. Submerged plants ($e.g.$,$Hydrilla$,$Vallisneria$,$Potamogeton$) do not have stomata because they perform gas exchange directly through their thin epidermal cell walls.

(C) The endoplasmic reticulum $(ER)$ is classified into two types based on the presence or absence of ribosomes on its surface.
$1$. When ribosomes are attached to the outer surface of the cisternae,the $ER$ appears rough under an electron microscope and is therefore called Rough Endoplasmic Reticulum $(RER)$ or Granular Endoplasmic Reticulum.
$2$. In the absence of ribosomes,the $ER$ appears smooth and is called Smooth Endoplasmic Reticulum $(SER)$ or Agranular Endoplasmic Reticulum.
Therefore,the presence of ribosomes on the cisternae characterizes the Granular Endoplasmic Reticulum.

(B) Many enzymes are composed of a protein portion and a non-protein portion.
The protein portion of an enzyme is called the $Apoenzyme$.
The non-protein portion is called the $Cofactor$.
When the $Apoenzyme$ and $Cofactor$ are bound together,the complete,catalytically active enzyme is called the $Holoenzyme$.
$Isoenzymes$ are different forms of an enzyme that catalyze the same reaction.

(A) The enzyme $Nitrogenase$ is responsible for the biological fixation of atmospheric nitrogen into ammonia. This enzyme is found exclusively in certain prokaryotes,such as $Rhizobium$ and $Azotobacter$. It catalyzes the reduction of $N_2$ to $NH_3$ using $ATP$ and electrons.

(C) Guttation is the exudation of water droplets from the margins or tips of leaves through specialized structures called hydathodes.
It occurs when the rate of water absorption by roots is high and the rate of transpiration is very low, typically during the night or early morning.
This high water content in the plant creates a $Positive root pressure$, which forces the water out through the hydathodes.
Therefore, $Positive root pressure$ is the primary cause of guttation.

(B) $Potometer$ is an apparatus used to measure the rate of water uptake by a leafy shoot,which is indirectly proportional to the rate of transpiration.
$Porometer$ is used to measure the size of stomatal aperture.
$Auxanometer$ is used to measure the growth in length of a plant.
$Tensiometer$ is used to measure the soil water tension or matric potential.

(A) Tendrils are specialized structures in climbing plants that respond to touch or contact with a solid support.
When a tendril comes into contact with an object,the cells on the side touching the object grow more slowly than the cells on the opposite side.
This differential growth causes the tendril to coil or twine around the support.
This directional growth movement in response to touch or mechanical stimulus is known as $Thigmotropism$.

(C) The hormone $Cholecystokinin$ $(CCK)$ is secreted by the duodenal mucosa. Its primary function is to stimulate the contraction of the gall bladder and the relaxation of the sphincter of $Oddi$,which facilitates the release of bile into the duodenum to aid in the digestion of fats.

(C) Haemocyanin is a copper-containing respiratory pigment that gives blood a blue color when oxygenated.
It is commonly found in the hemolymph of many mollusks,such as cephalopods (e.g.,squids,octopuses) and some gastropods.
While some annelids use haemoglobin or chlorocruorin,haemocyanin is a characteristic feature of many members of the phylum $Mollusca$.

(C) Blood pressure is measured in $mm$ of mercury $(Hg)$ by an instrument known as a sphygmomanometer.
This instrument was invented by the Italian physician Scipione Riva-Rocci in $1896$.

(A) Malpighian tubules are the main excretory organs in insects like cockroaches.
They are blind-ended tubules attached to the junction of the midgut and hindgut.
These tubules absorb nitrogenous waste products and water from the haemolymph (the body fluid of insects).
These wastes are then converted into uric acid and passed into the alimentary canal to be excreted out through the anus.

(D) Reabsorption in the nephron tubules is a selective process that involves multiple mechanisms.
$1$. Water is reabsorbed passively by osmosis.
$2$. Nitrogenous wastes are reabsorbed by passive diffusion.
$3$. Essential nutrients like glucose and amino acids are reabsorbed by active transport.
Since reabsorption involves all these processes depending on the substance,the most comprehensive answer is that it involves active transport,passive diffusion,and osmosis. However,in the context of selective reabsorption of essential substances like $Na^+$,glucose,and amino acids,active transport is a primary mechanism. Given the standard options,active transport is the most significant process for the reabsorption of essential solutes.

(A) The cranium (brain box) is formed by $8$ bones.
These include $1$ frontal bone,$2$ parietal bones,$2$ temporal bones,$1$ occipital bone,$1$ sphenoid bone,and $1$ ethmoid bone.
Therefore,the total number of bones in the cranium is $8$.

(D) In a frog, the vertebral column consists of $9$ vertebrae. The $8^{th}$ vertebra is procoelous, meaning it has a concave anterior surface and a convex posterior surface. However, the $9^{th}$ vertebra is unique because it has an anterior convex surface (biconvex) to articulate with the $8^{th}$ vertebra, and it also possesses two posterior convex surfaces (condyles) to articulate with the urostyle.

(D) The $Hypothalamus$ is a part of the forebrain that contains several centers which control body temperature,urge for eating and drinking. It acts as the body's thermostat,maintaining homeostasis by regulating heat production and heat loss.

(A) Nitrification is a biological process carried out by chemoautotrophic bacteria such as $Nitrosomonas$ and $Nitrobacter$.
These bacteria are highly sensitive to environmental conditions.
The process of nitrification is significantly influenced by the $pH$ of the soil.
Nitrification occurs most efficiently in neutral to slightly alkaline soils ($pH$ $7.0$ to $8.5$).
In acidic soils,the activity of nitrifying bacteria is severely inhibited,making $pH$ the primary limiting factor for this process.

(D) The Assertion is incorrect because bacteria have four basic shapes: coccus (spherical/round),bacillus (rod-shaped),vibrio (comma-shaped),and spirillum (spiral-shaped). The statement mentions only three shapes,omitting the vibrio form.
The Reason is correct as bacteria like cocci and bacilli can indeed form clusters (e.g.,staphylococci) or chains (e.g.,streptococci) of varying lengths.
Therefore,the Assertion is incorrect,but the Reason is correct.

(D) In collateral vascular bundles,the phloem is situated towards the outer side,while the xylem is situated towards the inner side. Therefore,the Assertion is incorrect.
In monocot stems,the vascular bundles are closed,meaning cambium is absent. Therefore,the Reason is also incorrect.
Since both the Assertion and the Reason are false,the correct option is $D$.

(C) The Assertion is correct because saline water contains $NaCl$ (sodium chloride). High intake of sodium increases water retention in the blood vessels,which leads to an increase in blood volume and consequently raises blood pressure,which is harmful to patients with hypertension.
However,the Reason is incorrect. Saline water does not drop blood pressure; rather,it increases blood pressure. Therefore,the statement provided in the Reason is scientifically false.

(A) Blood pressure refers to the pressure exerted by blood against the walls of the arteries during each heartbeat.
It is typically measured using the brachial artery in the arm.
The instrument used to measure this pressure is called a sphygmomanometer.
Since blood pressure specifically refers to the pressure within the arteries,both the Assertion and the Reason are correct.
Furthermore,the Reason explains why we can define blood pressure as arterial pressure,as the standard clinical measurement is performed on arteries using this specific device.

(A) The tongue acts as a gustatoreceptor because it contains specialized sensory structures called taste buds.
Gustatory sensations (taste) are detected by chemoreceptors located within these taste buds.
Since the tongue is the organ that houses these taste buds,the assertion is correct.
The reason correctly explains that the receptors for gustatory sensations are found in the taste buds,which justifies why the tongue is classified as a gustatoreceptor.
Therefore,both statements are correct and the reason is the correct explanation of the assertion.

(A) $Ginkgo biloba$ belongs to the order $Ginkgoales$, which evolved during the $Jurassic$ period.
It is considered a living fossil because it has persisted and remained unchanged for millions of years while its close relatives have gone extinct.
Therefore, the Assertion is correct, and the Reason provides a valid explanation for why it is classified as a living fossil.

(D) The main role of bacteria in the carbon cycle is that of a decomposer.
Bacteria break down complex organic compounds into simpler substances through the process of decomposition.
These simpler substances are then released back into the environment and reused in the carbon cycle,thereby maintaining the balance of carbon in the ecosystem.

(A) Malignant tertian malaria is a severe form of malaria characterized by high fever and potential complications.
It is caused by the protozoan parasite $Plasmodium$ $falciparum$.
$P. vivax$ and $P. ovale$ typically cause benign tertian malaria,while $P. malariae$ causes quartan malaria.

(D) $Wuchereria$ $bancrofti$ is a filarial nematode that causes filariasis in humans.
The adult worms typically reside in the lymphatic vessels and lymph glands of the human host.
It is a viviparous nematode,meaning it gives birth to live young ones called microfilariae.

(B) In a mature female gametophyte (embryo sac) of a typical angiosperm, there are $3$ antipodal cells at the chalazal end, $2$ synergids and $1$ egg cell at the micropylar end, and $1$ large central cell containing $2$ polar nuclei.
Although it contains $8$ nuclei, the fusion of the $2$ polar nuclei into a single diploid secondary nucleus results in a total of $7$ cells.
Therefore, the mature embryo sac is $7$-celled and $8$-nucleated.

(C) The growth of the pollen tube towards the embryo sac is an example of chemotaxis.
This movement is induced by chemical substances,such as calcium,boron,and inositol,which are secreted by the synergids within the embryo sac.
These chemicals create a gradient that guides the pollen tube towards the micropyle,ensuring successful fertilization.

(B) Pleiotropy is a genetic phenomenon where a single gene influences two or more seemingly unrelated phenotypic traits.
For example,in humans,the gene responsible for sickle-cell anemia can affect hemoglobin structure,red blood cell shape,and resistance to malaria,demonstrating multiple phenotypic effects from a single gene mutation.

(C) Klinefelter's syndrome is a genetic disorder caused by the presence of an extra $X$ chromosome in males.
This condition results from the nondisjunction of sex chromosomes during meiosis.
Therefore,the sex chromosome complement in individuals with Klinefelter's syndrome is typically $XXY$.

(C) $Australopithecus$ $ramidus$ is considered the most ape-like hominid ancestor.
It represents a crucial transitional stage or the 'missing link' between ancestral apes and the hominid lineage leading to humans.

(B) The correct answer is $B$.
$Plasmodium$ $falciparum$ is responsible for causing malignant tertian malaria.
This type of malaria is also known as aestivo-autumnal,pernicious malaria,or black water fever.
It is considered the most dangerous form of malaria because it can lead to cerebral malaria,which is often fatal as it affects the brain.

(A) Aquatic ecosystems are broadly classified into two types based on the flow of water: $Lentic$ and $Lotic$.
$Lentic$ ecosystems refer to standing or still water bodies such as lakes,ponds,and swamps.
$Lotic$ ecosystems refer to flowing water bodies such as rivers and streams.
Therefore,the flora and fauna found in lakes or ponds are collectively referred to as $Lentic$ biota.

(D) The greenhouse effect is a natural phenomenon that warms the earth's surface.
It is primarily caused by greenhouse gases that trap heat in the atmosphere.
Among these,$CO_2$ (carbon dioxide) is the most significant contributor,accounting for approximately $50\%$ of the increase in global temperature.
Other gases like chlorofluorocarbons (CFCs) contribute about $20\%$,while methane and nitrous oxide also play roles.
Therefore,the presence of a $CO_2$ layer in the atmosphere is the primary driver of the greenhouse effect.

(C) Aflatoxins are toxic secondary metabolites produced primarily by the fungus $Aspergillus$ $flavus$.
These toxins are highly carcinogenic and harmful to humans and animals,causing liver damage and other health issues.
Therefore,the Assertion is correct,but the Reason is incorrect because these toxins are harmful,not useful to mankind.

(A) Phenylketonuria $(PKU)$ is an autosomal recessive genetic disorder.
It is caused by the deficiency of the enzyme phenylalanine hydroxylase,which is responsible for converting the amino acid phenylalanine into tyrosine.
Due to the lack of this enzyme,phenylalanine accumulates in the body and is converted into phenylpyruvic acid and other derivatives.
These derivatives are excreted in the urine,hence the name Phenylketonuria. Therefore,both the Assertion and the Reason are correct,and the Reason explains why the condition is named as such and why the substance appears in the urine.

(B) Histones are basic proteins found in eukaryotic chromosomes,which are rich in positively charged amino acids like lysine and arginine.
These proteins are essential for the packaging of negatively charged $DNA$ into chromatin.
There are five major types of histone proteins: $H_1, H_2A, H_2B, H_3$,and $H_4$.
While both the Assertion and the Reason are scientifically correct statements,the Reason describes the types of histones but does not directly explain why they are important for packaging (which involves their basic nature and charge).
Therefore,the Reason is not the correct explanation for the Assertion.

(C) Rabies (hydrophobia) is a viral disease caused by the rabies virus (a rhabdovirus).
It is transmitted to humans through the saliva of infected animals (like dogs,cats,or bats) via bites or scratches.
The virus is neurotropic,meaning it travels to the central nervous system $(CNS)$,causing encephalitis,hydrophobia (fear of water),muscle spasms,paralysis,and eventually death.
Since the causative agent is a virus and not a bacterium,the Reason provided is incorrect.