Biological Classification

Azotobacter, Oscillatoria, Anabaena, and Nostoc are diazotrophs (bacteria/cyanobacteria capable of nitrogen fixation). Volvox is a colonial green alga (eukaryote) that cannot fix atmospheric nitrogen.

In Whittaker's five-kingdom classification, all unicellular eukaryotic organisms are placed in the kingdom Protista.
$Chlamydomonas$ and $Chlorella$ are unicellular, eukaryotic green algae, so in this system they were brought together with $Paramoecium$ and $Amoeba$ under Protista (NCERT states this directly).
Monera contains only prokaryotes (bacteria and cyanobacteria), so it cannot include these eukaryotic cells.
Plantae contains multicellular, eukaryotic, chlorophyll-bearing organisms; 'Algae' is not a kingdom in the five-kingdom system.
Hence $Chlamydomonas$ and $Chlorella$ are included in Protista.

The morphology of the mycelium, mode of spore formation and fruiting bodies form the basis for dividing the kingdom Fungi into various classes. Mode of nutrition is not a criterion for the classification of fungi.

$Rhizopus$ – bread mould $Ustilago$ – smut fungus $Puccinia$ – rust fungus $Agaricus$ – mushroom Correct match: A-III, B-II, C-IV, D-I.

Reference: NCERT Class XI, pages 13, 14, 16.

Bacteria (Monera) show the most extensive metabolic diversity.
They may be photosynthetic autotrophs, chemosynthetic autotrophs or heterotrophs (saprophytes, parasites, symbionts).
Plants are mostly autotrophic, and fungi and animals are heterotrophic.

Heterotrophic bacteria are the most abundant in nature; most are important decomposers.
They are helpful in making curd from milk, producing antibiotics and fixing nitrogen in legume roots.

Nostoc is a cyanobacterium, a prokaryote; its genetic material is not enclosed by a nuclear membrane.
Penicillium and Agaricus (fungi) and Volvox (alga) are eukaryotes with a nuclear membrane.

Cyanobacteria have chlorophyll a like green plants and are photosynthetic autotrophs; they are also called blue-green algae.
Golden algae are chrysophytes, which are protists.

In Whittaker's five-kingdom system, kingdom Monera includes all prokaryotes (organisms without a true nucleus and membrane-bound organelles).
Archaebacteria (archaea), such as methanogens, halophiles and thermoacidophiles, are prokaryotes.
Nitrogen-fixing organisms such as Rhizobium, Azotobacter and cyanobacteria (Nostoc, Anabaena) are also prokaryotes.
So both are classified in kingdom Monera.

Viroids are infectious agents consisting of free RNA without a protein coat (found by T.O. Diener, causing potato spindle tuber disease).

Amanita muscaria (fly agaric) is a poisonous mushroom that contains hallucinogenic compounds.
Morchella is an edible morel, Neurospora is used in genetics and Ustilago causes smut.

Ringworm is caused by fungi of the genera $Microsporum$, $Trichophyton$ and $Epidermophyton$.
So $Microsporum$ belongs to the Kingdom Fungi.
$Rhizopus$, the bread mould, is also a fungus (a phycomycete).
$Ascaris$, $Taenia$ and $Wuchereria$ are helminths belonging to Kingdom Animalia.
Hence $Microsporum$ belongs to the same kingdom as $Rhizopus$.

Rust diseases are caused by basidiomycete fungi of the genus $Puccinia$.
$Puccinia\ graminis\ tritici$ causes black (stem) rust of wheat.
$Alternaria\ solani$ causes early blight of potato.
$Ustilago\ nuda$ causes loose smut of wheat and barley.
$Xanthomonas\ oryzae$ is a bacterium that causes bacterial blight of rice.
Hence black rust of wheat is caused by $Puccinia\ graminis$.

Ergot of rye is a plant disease caused by the fungus Claviceps purpurea, a member of Ascomycetes.
The fungus infects the ovaries of the rye flowers. The so-called ergot that replaces the grain of the rye is a dark, purplish sclerotium, from which the sexual stage of the life cycle forms after overwintering.
The sclerotia contain poisonous alkaloids; eating bread made from infected grain causes the disease ergotism. The hallucinogenic drug LSD is also derived from ergot alkaloids.
Uncinula causes powdery mildew of grapes.
Ustilago causes smut diseases of cereals.
Phytophthora infestans causes late blight of potato.
So ergot of rye is caused by a species of Claviceps.

Mucor is a saprophytic fungus (a mould of the class Phycomycetes) that grows on moist, stale bread and other decaying organic matter.
Temperature: fungi grow best at moderately warm temperatures of about 25 to $30^\circ C$. At about $5^\circ C$ (refrigerator temperature) growth is very slow, which is why food is preserved in a refrigerator. So A is required, not B.
Humidity: fungi need plenty of moisture for the germination of spores and the growth of hyphae, so a high relative humidity of about 95 percent is required. At 5 percent humidity the bread would dry up and the fungus would not grow. So D is required, not C.
Light: fungi have no chlorophyll and do not need light; bright light and the accompanying dryness are unfavourable, so they grow best in a shady or dark place. So E is required, not F.
So the essential conditions are A, D and E only.

Fungi store their reserve food as glycogen (and oil droplets), like animals.
Chitin is a structural component of their cell walls, not a food reserve. Starch is the reserve food of green plants.
So the stored food in fungi is glycogen.

Reference: NCERT Class XI, page 21.

Bovine spongiform encephalopathy (mad cow disease) in cattle and Creutzfeldt-Jakob disease in humans are caused by prions, which are abnormally folded proteins.

Prions are infectious agents made only of abnormally folded protein; they contain no nucleic acid.
They cause neurological diseases such as bovine spongiform encephalopathy (mad cow disease) in cattle, scrapie in sheep, and kuru and Creutzfeldt–Jakob disease (CJD) in humans.
Viroids are the opposite: free, infectious RNA of low molecular weight without any protein coat (e.g., potato spindle tuber disease).
Satellite viruses and gemini viruses contain nucleic acid along with protein.
Hence infectious proteins are present in prions.

In 1971 T.O. Diener discovered a new infectious agent, smaller than viruses, that caused potato spindle tuber disease.
It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid.
The RNA of the viroid is of low molecular weight.
Bacteriophages are viruses that infect bacteria (discovered by Twort and d'Herelle), and infectious proteins are prions (Prusiner).
Hence Diener discovered a free infectious RNA.

Mad-cow disease is bovine spongiform encephalopathy (BSE), a fatal disease of the nervous system of cattle.
It is caused by a prion, an infectious agent that consists only of an abnormally folded protein and contains no nucleic acid.
The abnormal prion protein makes the normal proteins of the brain cells change into the abnormal form, and their accumulation destroys the nerve cells, giving the brain a spongy appearance.
Its analogous variant in humans is Creutzfeldt-Jakob disease (CJD); scrapie of sheep is another prion disease.
It is not caused by a bacterium, a worm or a virus.
So the causative agent of mad-cow disease is a prion.

Reference: NCERT Class XI, page 21.

A lichen is a symbiotic (mutualistic) association between an alga (phycobiont) and a fungus (mycobiont).
The algal component contains chlorophyll and prepares food by photosynthesis for both partners; some cyanobacterial partners also fix nitrogen.
The fungal component forms the bulk of the thallus. It provides shelter and protection to the alga, anchors the lichen to the substratum, and absorbs water and mineral nutrients for it.
So the fungus does not provide food, fix nitrogen or release oxygen; these are done by the algal partner.
Lichens are very good indicators of air pollution, because they do not grow in polluted areas.
So the fungus provides protection, anchorage and absorption for the alga.

A lichen is a symbiotic association (mutualism) between an alga (phycobiont) and a fungus (mycobiont).
The algal partner is autotrophic and prepares food by photosynthesis for both.
The fungal partner provides shelter, absorbs water and minerals and protects the alga from drying.
Both partners benefit, and they are so closely associated that the lichen looks like a single organism.
So in a lichen the fungus has a symbiotic relationship with the alga. (Lichens are good indicators of air pollution, as they do not grow in polluted areas.)

Archaebacteria are able to survive in harsh conditions because of branched lipid chains in the cell membrane, which reduce fluidity of the cell membrane. Halophiles (a group of archaebacteria) are exclusively found in saline habitats.

Archaebacteria live in extreme habitats: halophiles in salty water, thermoacidophiles in hot springs and methanogens in marshes and the guts of ruminants.
Their cell walls and membrane lipids are different from other bacteria, which helps them survive such conditions.
They are considered the oldest living organisms, believed to be survivors of the earliest forms of life on earth.
Not all are halophiles or photosynthetic, and they are living organisms, not just fossils.
So archaebacteria are the oldest living beings.

Reference: NCERT Class XI, page 89.

Reference: NCERT Class XI, page 17.

$Saccharomyces$, i.e. yeast, is a eukaryote (unicellular fungus). $Mycobacterium$ – a bacterium; $Oscillatoria$ and $Nostoc$ are cyanobacteria.

Ciliates differ from other protozoans in having two types of nuclei, e.g. $Paramoecium$ has a macronucleus and a micronucleus.

Diatoms are the chief producers of the ocean.

Mycoplasmas are the smallest, wall-less prokaryotes, pleomorphic in nature. They are pathogenic on both plants and animals.

Mycoplasmas are the smallest living cells known, and they completely lack a cell wall.
Penicillin acts by inhibiting the synthesis of the peptidoglycan cell wall of bacteria. Since mycoplasmas have no cell wall, penicillin has no effect on them; they are resistant, not sensitive, to penicillin.
So the statement that they are sensitive to penicillin is wrong. (They are sensitive to antibiotics that act on protein synthesis, such as tetracycline.)
Because they lack a rigid wall they have no fixed shape and are pleomorphic.
They are pathogenic in animals and plants, for example causing little leaf of brinjal and witches' broom.
They are also called PPLO (pleuropneumonia-like organisms).
The other three statements are therefore correct.

Viroids are sub-viral agents: infectious RNA particles without a protein coat.

Protists are single-celled eukaryotes. Chrysophytes, dinoflagellates, euglenoids, protozoans and slime moulds are included in kingdom Protista.

In Whittaker's five-kingdom classification, all single-celled eukaryotes are placed under the kingdom Protista.
Protista includes chrysophytes (diatoms), dinoflagellates, euglenoids, slime moulds and protozoans.
Monera and Archaea contain prokaryotes, which lack a true nucleus.
Fungi are mostly multicellular, heterotrophic eukaryotes with chitinous cell walls.
Hence single-celled eukaryotes are included in Protista.

Slime moulds are saprophytic protists. Their body moves along decaying twigs and leaves, engulfing organic material.
Under suitable conditions they form an aggregation called a plasmodium, which may grow and spread over several feet.
Physarum (for example Physarum polycephalum, the 'many-headed slime') is an acellular slime mould that lives in shady, cool, moist places such as decaying leaves and logs.
During unfavourable conditions the plasmodium differentiates and forms fruiting bodies bearing spores at their tips.
Thiobacillus is a chemoautotrophic sulphur bacterium.
Anabaena is a filamentous cyanobacterium (blue-green alga).
Rhizopus is the bread mould, a fungus of the class Phycomycetes.
So the slime mould is Physarum.

Archaebacteria live in extreme conditions like salty areas, hot springs and marshy areas; they have a distinctive cell wall structure.
Methanogens are archaebacteria that live in the gut of ruminants like cows and buffaloes and produce methane gas during fermentation.

Eubacteria are the 'true bacteria'. They have a rigid cell wall and flagellum and are motile. Cyanobacteria, photosynthetic and chemosynthetic autotrophs belong to eubacteria.
So calling eubacteria 'false bacteria' is wrong.

Chitin is a polysaccharide made up of N-acetyl glucosamine. It is found mainly in the cell wall of fungi and the exoskeleton of arthropods.

Viroids are virus-like agents smaller than viruses, composed of naked strands of RNA without a capsid. Their RNA has low molecular weight.

Methanogens (archaebacteria) present in the gut of cows and buffaloes are responsible for the partial digestion of cellulose and produce methane.

Archaebacteria have a different cell wall structure and unusual branched-chain lipids in their cell membrane, which lets them survive in extreme conditions.

Archaebacteria live in the harshest habitats, such as deep-sea hydrothermal vents, hot springs and salty areas.

Methanogens are archaebacteria that are strict anaerobes and produce methane.
They live in the gut (rumen) of ruminant animals such as cows and buffaloes, where they help in the digestion of cellulose.
The dung of cattle (gobar) is therefore rich in these bacteria, and they produce methane (biogas) from it.
So they are most abundant in a cattle yard; they are also common in marshy areas and flooded paddy fields.
Hot springs are the habitat of thermoacidophiles, another group of archaebacteria.
Hence methanogens are most abundant in a cattle yard.

Tobacco mosaic virus is rod-shaped, with a single coiled RNA strand surrounded by protein subunits called capsomeres arranged helically.

A virus is a nucleoprotein: its genetic material is enclosed in a protein coat (capsid). It has either DNA or RNA, never both, and no nucleus.

Viruses are obligate parasites, are not affected by antibiotics, and direct the host machinery to make their nucleic acids and proteins.
Their capsids may be helical (e.g. TMV), polyhedral (e.g. adenovirus) or complex (e.g. bacteriophage).
So the statement that all of them have helical symmetry is wrong.

A bacteriophage (e.g., T-even phage) has a polyhedral head, a short collar, a tail with a contractile sheath, a basal plate and tail fibres.
A is the polyhedral structure at the top that encloses the nucleic acid (DNA): the head.
C is the narrow ring-like region just below the head, joining it to the tail: the collar.
B is the long cylindrical, contractile covering of the tail: the sheath.
D is the set of long thread-like leg structures arising from the base plate, with which the phage attaches to the bacterium: the tail fibres.
Hence A – head, B – sheath, C – collar, D – tail fibres.

A virus is a nucleoprotein particle: it has genetic material (DNA or RNA) surrounded by a protein coat.
The protein coat that envelops and protects the nucleic acid is called the capsid; it is made of small subunits called capsomeres arranged in helical or polyhedral forms.
The core is the central nucleic acid, the virion is the complete infectious virus particle outside the host, and nucleoprotein refers to the nucleic acid together with its protein.
Hence the covering (envelope) of the virus is known as the capsid.

A virus consists of nucleic acid (DNA or RNA, never both) enclosed in a protein coat. It has no metabolic machinery of its own.
Outside a host cell a virus is inert, like a chemical (it can even be crystallised). It can replicate only inside a living host cell, using the host's enzymes, ribosomes and energy.
Similarly, an isolated chromosome cannot replicate on its own; it needs the environment of a living cell.
Viruses do not need food molecules or oxygen for respiration, since they have no metabolism.
So viruses are no more alive than isolated chromosomes because both require the environment of a cell to replicate.

Viruses have no cellular organisation and no metabolic machinery of their own (no ribosomes or enzymes for energy production).
They can multiply only inside living host cells by using the host's machinery. So they are obligate (intracellular) parasites.
A virus contains either DNA or RNA, never both.
The protein coat of a virus is the capsid (made of capsomeres); the nucleic acid is the core.
So the correct statement is that viruses are obligate parasites.

Viruses have no cellular structure and no metabolic machinery. They consist of nucleic acid enclosed in a protein coat, and use the enzymes of the host cell to multiply.
(Some viruses carry a few enzymes inside the particle, such as reverse transcriptase or lysozyme, but they do not have metabolic enzymes.)
Fungi, algae and cyanobacteria are living cells with many enzymes.
So enzymes are not found in viruses.

Viruses show one important characteristic of living things: inside a living host cell, they reproduce (replicate) and make copies of themselves, and they have genetic material that can mutate.
They do not respire or carry out metabolism on their own, and outside the host they are inert.
So viruses are considered living because they multiply in host cells.

Yeast (Saccharomyces) is a unicellular fungus of the class Ascomycetes.
Nostoc and Anabaena are cyanobacteria (Monera); Paramecium and Plasmodium are protists while Penicillium is a fungus; a lichen is an association of an alga and a fungus.

Slime moulds are saprophytic protists that move along decaying twigs and leaves, engulfing organic material.
The vegetative body of an acellular slime mould (Myxomycetes) is a naked, multinucleate mass of protoplasm without cell walls.
This body is called a plasmodium; under suitable conditions it may grow and spread over several feet.
During unfavourable conditions the plasmodium differentiates and forms fruiting bodies, which bear spores at their tips.
Protonema is the juvenile filamentous stage of mosses.
Mycelium is the body of a fungus, made up of hyphae.
So the thalloid body of a slime mould is known as the plasmodium.

Clostridium botulinum is a rod-shaped, spore-forming bacterium.
It is an obligate anaerobe: it can grow only in the complete absence of oxygen, and oxygen is harmful to it.
That is why it multiplies in improperly sterilised canned and sealed foods, where conditions are anaerobic.
There it produces botulinum toxin, a powerful neurotoxin that causes the severe food poisoning called botulism.
A facultative anaerobe can live with or without oxygen, and an obligate aerobe can live only in the presence of oxygen.
So Clostridium botulinum is an obligate anaerobe.

Paramecium is a ciliated protozoan that shows nuclear dimorphism: it has two kinds of nuclei.
The large, polyploid macronucleus controls the vegetative (trophic) functions such as metabolism, growth and the synthesis of proteins.
One or more small, diploid micronuclei take part in reproduction, especially in conjugation (sexual process), and give rise to the new macronucleus.
Trypanosoma, Euglena and Amoeba have only one kind of nucleus.
So the organism is Paramecium.

Viruses that infect bacteria are called bacteriophages.
In the lytic cycle, the phage injects its DNA, takes over the host cell's machinery, makes many new phage particles, and finally breaks open (lyses) the bacterial cell to release them. Such phages are called lytic (virulent) phages, e.g. T4.
Lysogenic (temperate) phages integrate their DNA into the bacterial chromosome as a prophage and do not lyse the cell immediately.
Lysozyme is an enzyme that digests bacterial cell walls; lipolytic refers to fat-digesting activity.
So viruses that infect bacteria, multiply and cause their lysis are called lytic.

During unfavourable conditions such as drought, extreme temperature or lack of food, Amoeba withdraws its pseudopodia, becomes rounded and secretes a thick, protective three-layered cyst wall around itself. This is encystment.
Inside the cyst it can survive for a long time; it may also divide by multiple fission (sporulation) to form many pseudopodiospores.
When favourable conditions return, the cyst wall breaks and the young amoebae are released.
So the chief advantage of encystment is the ability to survive during adverse physical conditions.

Mycorrhiza is an association between a fungus and the roots of a higher plant (e.g. Glomus with many plants, and fungi with Pinus roots).
The fungal hyphae absorb water and minerals, especially phosphorus, from the soil and pass them to the plant; they also protect the roots from pathogens.
The plant supplies sugars and other organic compounds to the fungus.
Since both partners benefit, it is a mutualistic symbiotic relationship.
So mycorrhiza is an example of a symbiotic relationship.

Chemoautotrophs make their own food from $CO_2$ using energy obtained by oxidising reduced inorganic substances such as ammonia, nitrite, hydrogen sulphide, ferrous iron or hydrogen.
Examples: nitrifying bacteria Nitrosomonas ($NH_3 \rightarrow NO_2^-$) and Nitrobacter ($NO_2^- \rightarrow NO_3^-$), sulphur bacteria and iron bacteria.
Photoautotrophs get their energy from light; saprozoic organisms absorb dissolved organic matter.
So such organisms are called chemoautotrophs.

Black (stem) rust of wheat is caused by the basidiomycete fungus Puccinia graminis tritici, which needs two hosts (wheat and barberry) to complete its life cycle.
Ustilago causes smut, Albugo causes white rust of crucifers, and Phytophthora causes late blight of potato.
So black rust of wheat is caused by Puccinia.