NCERT Class 11 Biology Biological Classification Important Notes

Artificial Classification System:

This system was proposed by Linneaus. It was based on vegetative characters and androecium structure.

Natural Classification System: This system was used by George Bentham and Joseph Dalton Hooker. This system was based on natural affinities among organisms. In this system, both external and internal features were considered.

Two Kingdom Classification; as proposed by Linneaus:

• Animal Kingdom

• Plant Kingdom 

Five Kingdom Classification; as proposed by R.H. Whittaker (1969):

This is the most accepted system of classification. The five kingdoms are as follows:

• Monera

• Protista

• Fungi

• Plantae and

• Animalia 

The main criteria for classification used by Whittaker:

• Cell Structure

• Thallus Organization

• Mode of Nutrition

• Reproduction and

• Phylogenetic Relationships 

KINGDOM MONERA

The bacteria are kept under the Kingdom Monera. They are prokaryotic and possess cell wall. The cell wall is composed of polysaccharides and amino acids. Bacteria can be autotrophic and heterotrophic. The autotrophic bacteria can be chemosynthetic or photosynthetic. The heterotrophic bacteria can be saprophytic or parasitic.

Based on their shape, bacteria are classified into four types:

a. Spherical bacteria are called Coccus (pl.: cocci),

b. Rod-shaped bacteria are called Bacillus (pl.: bacilli),

c. Comma-shaped bacteria are called Vibrium (pl.: vibrio) and

d. Spiral shaped bacteria are called Spirillum (pl.: spirilla)

Archaebacteria: The archaebacteria live in some of the harshest habitats. The different structure of their cell wall helps them in surviving in extreme conditions. Based on their habitats, the archaebacteria are classified as follows:

a. Halophiles: They live in extremely salty areas.

b. Thermoacidophiles: They live in hot spring.

c. Methanogens: They live in marshy areas. They also live in the guts of the ruminant animals. They are responsible for the production of methane from the dung of these animals. 

Eubacteria: They are also called the ‘true bacteria’. They possess a rigid cell wall, and a flagellum (in motile bacteria). The cyanobacteria are also called ‘blue-green algae’ because they contain chlorophyll. The cyanobacteria can be unicellular or filamentous. They can live solitary or in colonies. The colony of cyanobacteria is usually surrounded by a gelatinous sheath. Some of the cyanobacteria are capable of nitrogen-fixation, e.g. Nostoc and Anabaena.

Heterotrophic: These are the most abundant in nature. Most of them have economic significance for human beings. While many of them are beneficial for humans, many others are quite harmful.

Reproduction in Bacteria:

Binary fission is the usual mode of reproduction in bacteria. Under unfavourable conditions, they reproduce by spore formation. They also reproduce by adopting a primitive type of DNA transfer from one bacterium to another. This is similar to sexual reproduction.

KINGDOM PROTISTA

They are eukaryotes and unicellular. Some of them have flagella or cilia for locomotion. Reproduction is asexual and sexual. Sexual reproduction is by a process of cell fusion and zygote formation. Kingdom Protista is divided into following groups:

Chrysophytes:

The diatoms and golden algae (desmids) are included in this group. They are found in freshwater and marine habitats. They are microscopic and float passively in water currents and hence are considered as planktons. Organisms which cannot swim against the current are called planktons. Most of the chrysophytes are photosynthetic.

The cell walls in diatoms form two thin overlapping shells; which fit together as the two parts of a soapbox. The cell walls are embedded with silica and hence are indestructible. Due to this, the diatoms leave behind a large amount of cell wall deposits in their habitat. The cell wall accumulation of diatoms; over billions of years is called ‘diatomaceous earth’. This soil is gritty and hence is used in polishing, filtration of oils and syrups. Diatoms are the main ‘producers’ in the oceans.

Dianoflagellates:

They are usually marine and photosynthetic. Depending on the main pigments in their cells; they appear yellow, green, brown, blue or red. Stiff cellulose plates are present on the outer surface of cell wall in dianoflagellates. Two flagella are present in most of them. One flagellum lies longitudinally and another transversely in a furrow between the wall plates. 

Euglenoids:

Most of them live in freshwater habitat in stagnant water. Cell wall is absent in them and instead there is a protein rich layer; called pellicle. The pellicle makes their body flexible. Two flagella; one short and another long; are present. They are photosynthetic; but behave as heterotrophs in the absence of sunlight. Example: Euglena.

Slime Moulds:

These are saprophytic. They body moves along decaying twigs and leaves and feeds on organic material. Under favourable conditions, they form an aggregation called plasmodium. The plasmodium may grow and spread over several feet. Under unfavourable conditions, the plasmodium differentiates and forms fruiting bodies. The fruiting bodies bear spores at their tips. True walls are present on the spores. The spores are extremely resistant and survive for many years. The spores are dispersed by air currents.

Protozoans:

They are heterotrophs and live as predators or parasites. The protozoans are classified into following four major groups:

a. Amoeboid protozoans: The amoeboid protozoans live in freshwater, sea water or in moist soil. They produce pseudopodia for locomotion and for capturing food. The marine forms have silica shells on their surface. Some of them are parasites, e.g. Entamoeba histolytica.

b. Flagellated protozoans: They are either free-living or parasitic. Flagella is present for locomotion. Many of them are parasites, e.g. Trypanosoma.

c. Ciliated protozoans: They are aquatic. Cilia are present for locomotion. A cavity (gullet) is present which opens to the outside of the cell surface. The coordinated movement of cilia facilitates the entry of food-laden water into the gullet. Example: Paramoecium.

d. Sporozoans: The sporozoans have an infectious spore-like stage in their life cycle. Example: Plasmodium. 

KINGDOM FUNGI:

The fungi are filamentous; except yeast (unicellular). The body consists of long, slender thread-like structures; called hyphae. The network of hyphae is called mycelium. Some hyphae are continuous tubes which are filled with multinucleated cytoplasm. Such hyphae are called coenocytic hyphae. The other type of hyphae has septae or cross-walls. The cell wall of fungi is composed of chitin and polysachharides.

Most of the fungi are heterotrophic and are saprophytes. Some are parasites. Some of the fungi also live as symbionts. Some of the symbiont fungi live as lichens; in association with algae. Some of the symbiont fungi live as mycorrhiza; in association with roots of higher plants.

Reproduction in Fungi:

Reproduction by vegetative means takes place by fragmentation, fission and budding. Some fungi reproduce asexually by forming spores which are called conidia or sporangiospores or zoospores. 

Sexual reproduction is by oospores, ascospores and basidiospores. The spores are produced in distinct structures called fruiting bodies.

The sexual cycle involves three steps which are as follows:

a. Plasmogamy: This involves fusion of protoplasm between two motile or non-motile gametes.

b. Karyogamy: This involves fusion of two nuclei.

c. Formation of haploid spores: This happens because of meiosis in zygote. 

During sexual reproduction, two haploid hyphae of compatible mating types come together and fuse. In some fungi, the fusion of two haploid cells immediately results in a diploid cell (2n). In other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage occurs. In this stage, two nuclei are present in each cell. This condition is called dikayron. The parental nuclei fuse at a later stage and the cells become diploid. Reduction division in the fruiting bodies leads to the formation of haploid spores.

On the basis of morphology of mycelium, mode of spore formation and fruiting bodies; Kingdom Fungi is divided into following classes:

Phycomycetes:

The members of phycomycetes are found in aquatic habitats and on decaying wood in moist and damp places. They can also be found as obligate parasites on plants.

Mycelium: Aseptate and coenocytic.

Reproduction:

• Asexual reproduction is by zoospores (motile) or by aplanospores (non-motile). These spores are endogeneously produced in sporangium.

• Zygospores are formed by fusion of two gametes. The gametes can be similar (isogamous) or dissimilar (anisogamous). 

Examples: Mucor, Rhizopus and Albugo (the parasitic fungi on mustard).

Ascomycetes:

They are commonly known as sac-fungi. They are unicellular or multicellular. They are saprophytic, decomposers, parasitic or coprophilous. Those growing on dung are called coprophilous.

Mycelium: Branched and septate.

Reproduction:

Asexual spores are exogenously produced on the special mycelium called conidiophores. Sexual spores are called ascospores. They are produced endogenously in sac like asci. These asci are arranged in various kinds of fruiting bodies called ascocarps.

Examples: Aspergillus, Claviceps, Neurospora, yeast, penicillium, morels, baffles, etc.

Basidiomycetes:

They grow in soil, on logs and tree stumps. Some of them also grow in living plant bodies as parasites. Mushrooms, bracket fungi or puffballs are the commonly known forms.

Mycelium: Branched and septate.

Reproduction:

Asexual spores are usually absent. Vegetative reproduction by fragmentation is common. Sex organs are absent. But plasmogamy is brought about by fusion of two vegetative cells of different strains. The resultant structure is dikaryotic which finally forms the basidium. Karyogamy and meiosis are responsible for formation of four basidiospores in a basidium. The basiodiospores are exogenously produced. Basidia are arranged in fruiting bodies called basidiocarps.

Examples: Agaricus (mushroom), Ustilago (smut) and Puccinia (rust).

Deuteromycetes:

They are usually known as imperfect fungi because only the asexual or vegetative phase of them is known. A large number of deuteromycetes are decomposers, while some members are parasites.

Mycelium: Branched and septate.

Examples: Alternaria, Colletotrichum and Trichoderma.

KINGDOM PLANTAE

All eukaryotes which have chloroplast are kept under this kingdom. Most of them are autotrophic; but some are heterotrophic. Cell wall is mainly composed of cellulose.

The life cycle of plants has two distinct phases; the diploid saprophytic and the haploid gametophytic phase. These phases alternate with each other. The lengths of the haploid and diploid phases vary among different groups of plants. This phenomenon is called Alternation of Generation.

KINGDOM ANIMALIA:

All multicellular eukaryotes which lack cell wall and are heterotrophs are kept under this kingdom. The animals indirectly or directly depend for food on plants. They follow the holozoic mode of nutrition. Holozoic nutrition involves ingestion of food and use of internal cavity for digestion of food. Most of the animals are capable of locomotion. Sexual reproduction is by copulation of male and female which is followed by embryological development.

VIRUSES, VIROIDS AND LICHENS:

Virus is non-cellular organism. It is characterized by an inert crystalline structure outside the living cell. Once a virus infects a living cell, it begins to replicate and kills the host in the process. Virus is considered as a borderline case between living and non-living.

The name virus was given by Pasteur D. J. Ivanowsky (1892). M. W. Beijerinek (1898) demonstrated that the extract of infected tobacco plants could cause infection in healthy plants. He called the fluid from the infected plant as Contagium vivum fluidum (infectious living fluid). W. M. Stanley (1935) demonstrated that viruses could be crystallized and the crystals mainly consist of proteins.

Viruses also contain genetic material; which can be either RNA or DNA. Both RNA and DNA cannot be present in the same virus. Plant infecting viruses usually have single-stranded RNA and animal infecting viruses usually have double-stranded RNA or double-stranded DNA. Bacteriophages (bacterial viruses) usually have double-stranded DNA.

The protein coat on the virus is called capsid. It is made up of small subunits called capsomeres. The capsid protects the nucleic acid. The capsomeres are arranged in helical or polyhedral geometric forms.

Mumps, small pox, herpes, influenza and AIDS are examples of viral diseases in humans. In plants; viral infection leads to the symptoms; like mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

NCERT Solution for Class 11 Biology Biological Classification

Question – 1- Discuss how classification systems have undergone several changes over a period of time?

Answer: Scientific classification of living beings was first done by Aristotle. He used morphological characters as the basis of classification. He classified the living beings into plants and animals. He further classified the plant into trees, shrubs and herbs. He further classified the animals on the basis of presence of absence of red blood.

After that, Linnaeus proposed two kingdoms, i.e. Plant Kingdom and Animal Kingdom.

But there were certain organisms which could be kept in both of the kingdoms or could not be kept in either of the kingdoms. Hence, a need was felt for a better system of classification. At present, the Five Kingdom Classification is the most accepted one. This was proposed by Robert Whittaker in 1969. Whittaker used phylogenetic relationship to classify the living beings.

Question – 2 - State two economically important uses of:

(a) Heterotrophic bacteria

Answer: Curd and antibiotic are made by using heterotrophic bacteria.

(b) Archaebacteria

Answer: Methanogens are responsible for production of biogas which can be used as fuel. The archaebacteria which live in extreme conditions give us a clue about the beginning of life on earth.

Question – 3 - What is the nature of cell-walls in diatoms?

Answer: The cell walls in diatoms form two thin overlapping shells; which fit together as the two parts of a soapbox. The cell walls are embedded with silica and hence are indestructible.

Question – 4 - Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify.

Answer: A rapid increase in the population of microscopic algae in an aquatic habitat is called algal bloom. The algal bloom involving the dianoflagellates is called the ‘red tide’ because of its red hue. Red tide can be harmful for other aquatic life forms.

Question – 5 - How are viroids different from viruses?

Answer: The free RNAs without the protein coat are called viroids, while virus have a protein coat to protect the genetic material.

Question – 6 - Describe briefly the four major groups of Protozoa.

Answer: Four major groups of Protozoa is as follows:

a. Amoeboid protozoans: The amoeboid protozoans live in freshwater, sea water or in moist soil. They produce pseudopodia for locomotion and for capturing food. The marine forms have silica shells on their surface. Some of them are parasites, e.g. Entamoeba histolytica.

b. Flagellated protozoans: They are either free-living or parasitic. Flagella is present for locomotion. Many of them are parasites, e.g. Trypanosoma.

c. Ciliated protozoans: They are aquatic. Cilia are present for locomotion. A cavity (gullet) is present which opens to the outside of the cell surface. The coordinated movement of cilia facilitates the entry of food-laden water into the gullet. Example: Paramoecium.

d. Sporozoans: The sporozoans have an infectious spore-like stage in their life cycle. Example: Plasmodium. 

Question – 7 - Plants are autotrophic. Can you think of some plants that are partially heterotrophic?

Answer: Pitcher plant, Venus fly trap and bladderwort are examples of partially heterotrophic plants.

Question – 8 - What do the terms phycobiont and mycobiont signify?

Answer: Lichens are the symbiotic association of fungi and algae. The algal part of lichen is called phycobiont and the fungal part is called mycobiont. The mycobiont part provides minerals and support, while the phycobiont part provides nutrition.

Question – 9 - Give a comparative account of the classes of Kingdom Fungi under the following:

(a) Mode of nutrition

• Answer: Phycomycetes are obligate parasites or saprophytes.

• Ascomycetes are saprophytes or parasites or coprophilous.

• Basidiomycetes are saprophytes or parasites.

• Deuteromycetes are manly saprophytes, some are parasites. 

(b) Mode of reproduction

• Answer: In phycomycetes, asexual reproduction is by zoospores (motile) or by aplanospores.

• In ascomycetes, asexual spores (conidia) and sexual spores (ascospores) are produced.

• In basidiomycetes, vegetative reproduction takes place by fragmentation. Plsamogamy is also seen.

• In deuteromycetes, only vegetative reproduction is seen. 

Question – 10 - What are the characteristic features of Euglenoids?

Answer: Most of them live in freshwater habitat in stagnant water. Cell wall is absent in them and instead there is a protein rich layer; called pellicle. The pellicle makes their body flexible. Two flagella; one short and another long; are present. They are photosynthetic; but behave as heterotrophs in the absence of sunlight.

Question – 11 - Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.

Answer: Virus contains genetic material surrounded by a protein capsule. The protein coat on the virus is called capsid. It is made up of small subunits called capsomeres. The capsid protects the nucleic acid. The capsomeres are arranged in helical or polyhedral geometric forms.

The genetic material can be either RNA or DNA. Both RNA and DNA cannot be present in the same virus. Plant infecting viruses usually have single-stranded RNA and animal infecting viruses usually have double-stranded RNA or double-stranded DNA. Bacteriophages (bacterial viruses) usually have double-stranded DNA. 

Four common viral diseases are: Common cold, mumps, jaundice and influenza.

Important Questions for NCERT Class 11 Biology Biological Classification

Ques. Bacterial leaf blight of rice is caused by a species
(a) Alternaria              (b) Erwinia
(c) Xanthomonas        (d) Pseudomonas.

Solution: C

Ques. Thermococcus, Methanococcus and Methanobacterium exemplify
(a) bacteria whose DNA is relaxed or positively supercoiled but which have a cytoskeleton as well as mitochondria
(b) bacteria that contain a cytoskeleton and ribosomes
(c) archaebacteria that contain protein homologous to eukaryotic core histones
(d) archaebacteria that lack any histones resembling those found in eukaryotes but whose DNA is negatively supercoiled.

Solution: C

Ques. In the light of recent classification of living organisms into three domains of life (bacteria, archaea and eukarya), which one of the following statements is true about archaea?
(a) Archaea completely differ from both prokaryotes and eukaryotes.
(b) Archaea completely differ from prokaryotes.
(c) Archaea resemble eukarya in all respects.
(d) Archaea have some novel features that are absent in other prokaryotes and eukaryotes. 

Solution: D

Ques. Which one of the following statements about mycoplasma is wrong?
(a) They are pleomorphic.
(b) They are sensitive to penicillin.
(c) They cause diseases in plants.
(d) They are also called PPLO. 

Solution: B

Ques. Curing of tea leaves is brought about by the activity of
(a) fungi              (b) bacteria
(c) mycorrhiza     (d) viruses. 

Solution: B

Ques. All of the following statements concerning the actinomycetous filamentous soil bacterium Frankia are correct except that Frankia
(a) can induce root nodules on many plant species
(b) can fix nitrogen in the free-living state
(c) cannot fix specialized vesicles in which the nitrogenase is protected from oxygen by a chemical barrier involving triterpene hopanoids
(d) like Rhizobium, it usually infects its host plant through root hair deformation and stimulates cell proliferation in the host’s cortex. 

Solution: B

Ques. For retting of jute the fermenting microbe used is
(a) methanophilic bacteria
(b) butyric acid bacteria
(c) Helicobacter pylori
(d) Streptococcus lactin.

Solution: B

Ques. Basophilic prokaryotes
(a) grow and multiply in very deep marine sediments
(b) occur in water containing high concentrations of barium hydroxide
(c) readily grow and divide in sea water enriched in any soluble salt of barium
(d) grow slowly in highly alkaline frozen lakes at high altitudes.

Solution: A

Ques. Which statement is correct for bacterial transduction?
(a) Transfer of some genes from one bacteria to another bacteria through virus.
(b) Transfer of genes from one bacteria to another bacteria by conjugation.
(c) Bacteria obtained its DNA directly from mother cell.
(d) Bacteria obtained DNA from other external source. 

Solution: A

Ques. In bacteria, plasmid is
(a) extra chromosomal material
(b) main DNA
(c) non functional DNA
(d) repetitive gene. 

Solution: A

Ques. Organisms which obtain energy by the oxidation of reduced inorganic compounds are called
(a) photoautotrophs    (b) chemoautotrophs
(c) saprozoic              (d) coproheterotrophs.

Solution: B

Ques. What is true for archaebacteria?
(a) All halophiles       (b) All photosynthetics
(c) All fossils             (d) Oldest living beings

Solution: D

Ques. Difference in Gram positive and Gram negative bacteria is due to
(a) cell wall        (b) cell membrane
(c) ribosome      (d) cytoplasm. 

Solution: A

Ques. Transfer of genetic information from one bacterium to another in the transduction process is through
(a) bacteriophages released from the donor bacterial strain
(b) another bacterium having special organ for conjugation
(c) physical contact between donor and recipient strains
(d) conjugation between opposite strain bacterium.

Solution: A

Ques. A bacterium divides every 35 minutes. If a culture containing 105 cells per mL is grown for 175 minutes, what will be the cell concentration per mL after 175 minutes?
(a) 35 × 105 cells            (b) 32 × 105 cells
(c) 175 × 105 cells          (d) 85 × 105 cells 

Solution: B

Ques. The DNA of E.coli is
(a) double stranded and linear
(b) double stranded and circular
(c) single stranded and linear
(d) single stranded and circular. 

Solution: B

Ques. The main role of bacteria in the carbon cycle involves
(a) chemosynthesis
(b) digestion or breakdown of organic compounds
(c) photosynthesis
(d) assimilation of nitrogenous compounds. 

Solution: B

Ques. A few organisms are known to grow and multiply at temperatures of 100-105oC. They belong to
(a) thermophilic sulphur bacteria
(b) hot spring blue-green algae
(c) methanogenic archaebacteria
(d) marine archaebacteria. 

Solution: A

Ques. The hereditary material present in the bacterium E.coli is
(a) single-stranded DNA        (b) double-stranded DNA
(c) DNA                               (d) RNA.

Solution: B

Ques. Azotobacter and Bacillus polymyxa are the examples of
(a) pathogenic bacteria    (b) decomposers
(c) symbiotic N2 fixer      (d) non-symbiotic N2 fixer.

Solution: D

Ques. What are the sex organs provided in some bacteria?
(a) Sex pili            (b) Plasmid
(c) Circular DNA    (d) Gametes 

Solution: A

Ques. Which type of DNA is found in bacteria?
(a) Circular free DNA      (b) Membrane bound DNA
(c) Straight DNA            (d) Helical DNA 

Solution: A

Ques. A large number of organic compounds can be decomposed by
(a) Azotobacter         (b) Chemolithotrophs
(c) Mycoplasma         (d) Pseudomonas.

Solution: B