Class 11-science NCERT Solutions Biology Chapter 2 - Biological Classification

Aristotle was the first to classify organisms based on morphological characters to differentiate plants and animals. He classified plants into trees, shrubs and herbs and animals based on the presence or absence of red-coloured blood.

Later, Linnaeus proposed two kingdoms-Plant Kingdom and Animal Kingdom. This system did not distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms and photosynthetic and non-photosynthetic organisms. Plants and animals could easily be classified under this system, but a large number of organisms did not fall under either category. So, a need was felt to include more number of characteristics such as mode of reproduction, evolutionary relationship etc. to facilitate better categorisation. 

Thus, R. H. Whittaker proposed a 'Five Kingdom Classification'.

The five kingdoms of the Five Kingdom Classification system are

 i. Monera

ii. Protista

iii. Fungi

iv. Plantae

v. Animalia

 The main criteria for the classification of organisms in the Five Kingdom Classification include cell structure, thallus organisation, mode of nutrition, reproduction and phylogenetic relationships. 

(a) Uses of heterotrophic bacteria:

(i) They are used in the manufacture of industry products such as in the preparation of antibiotics, vitamins, production of curd, cheese etc.

(ii) They are used in agriculture in the formation of manure and nitrogen fixation by Rhizobium.

(b) Uses of Archaebacteria:

(i) They are used in gobar gas production.

(ii) They are used in bioleaching of mines.  

The cell wall in diatoms is impregnated with silica, which makes characteristic patterns on the wall. It consists of two thin overlapping shells which fit together and thus looks like a soap case. The cell wall is almost indestructible and forms diatomaceous earth.  

Algal bloom refers to the excessive growth of algae, especially blue-green algae, in polluted water.

Red tides refer to the red colour imparted to the sea water by the rapid multiplication of red pigmented dinoflagellates such as Gonyaulax. 

 i.  Viroids are smaller in size than viruses.

ii.  Viroids are free RNAs without the protein coat, while viruses have a protein coat

     encapsulating the RNA.

iii.  Viroids infect only plants, whereas viruses can infect all types of organisms. 

The four major groups of Protozoa are as given below:

(a) Amoeboid Protozoans: They are found in fresh water, sea water or moist soil. They move and capture their prey with the help of pseudopodia. Example: Amoeba

(b) Flagellated Protozoans: They have flagella for movement. They are either free living or parasitic. The parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma

(c) Ciliated Protozoans: They have thousands of cilia present all over the body. The cilia help in locomotion and steering of food into the gullet. Example: Paramoecium

(d) Sporozoans: Many protozoans have an infectious spore-like stage in their life cycle. The spore-like stage helps them get transferred from one host to another host. Example: Plasmodium

Insectivorous plants and carnivorous plants are partially heterotrophic autotrophs. They are green and autotrophic, but they augment their nitrogen supply by catching and digesting small animals. Examples: Utricularia, Drosera, Nepenthes etc.

In the symbiotic association of algae and fungi, called lichens, the phycobiont is the name of the part composed of algae and mycobiont is the name of the part composed of fungi. In this association, both the organisms (algae and fungi) are benefitted. Fungi provide minerals and support to the algae, while algae provide nutrition to the fungi.

Features of Euglenoids:

 i. They do not have a cell wall.

ii. They have a protein-rich layer called pellicle, which makes their body flexible.

iii. Two flagella of different lengths are present.

iv. They are autotrophs in the presence of sunlight and behave like heterotrophs in the

    absence of sunlight by predating on smaller organisms. Example: Euglena

A virus is a crystalline structure, composed of protein, when present outside a host cell. The genetic material is present inside the crystal which can be either RNA or DNA. No virus has both RNA and DNA. Viruses which infect plants have single-stranded RNA. Viruses, infecting animals, have either single- or double-stranded RNA or double-stranded DNA. The protein coat is called a capsid. The capsid is made of smaller subunits, called capsomeres, which protect nucleic acid.

Example: Tobacco Mosaic Virus

Viral Diseases: AIDS, Mumps, Influenza, Herpes

Viruses resemble living beings in some characteristics. However, many characteristics of living beings are absent in them.

Characteristics of living beings in viruses:

i. Occurrence of genetic material.

ii. Presence of mutations.

iii. Ability to multiply.

iv. Daughter viruses resemble parent viruses.

v. They are host specific.

vi. They have antigenic properties.

vii. They are obligate parasites and are killed by ultraviolet radiations,

     autoclaving and many disinfectants.

viii. They follow a particular pattern of life cycle and reproduction.

Characteristics of non-living beings in viruses:

i. Absence of a cellular structure.

ii. Absence of metabolic machinery.

iii. Absence of energy-storing and energy-liberating systems.

iv. There is no growth.

v. They do not divide.

vi. They are inert outside the host cell.

vii. They can be crystallised.

Conclusion: Viruses are obligate parasites where different cellular structures have degenerated due to overtaking of metabolic machinery of the host.