NCERT Solutions Class 11 Biology Chapter 2 Biological Classification

NCERT Solutions For Class 11 Biology for Chapter 2 Biological Classification

1. Discuss how classification systems have undergone several changes over a period of time?

The biological classification system has undergone tremendous change guided by scientific understanding and a need to define and categorize each organism precisely on the basis of its morphological, anatomical and genetic attributes. The step diagram below shows how the present system of classification has evolved to its form.

2. State two economically important uses of:

(a) Heterotrophic bacteria

(b) Archaebacteria


Heterotrophic Bacteria

 Many heterotrophic bacteria help in decomposing dead and decaying organisms. This leads to production of humus which increases soil fertility.

 Heterotrophic bacteria are also used in production of antibiotics. Many known antibiotics are produced by Streptomyces, which are filamentous soil bacteria.


 Methanogens are Archaebacteria that produce methane gas as a metabolic bi-product. These bacteria are naturally found in the dung of cows and buffaloes. Some examples of methanogens are Methanococcus jannaschii and Methanobacterium thermoautotrophicum Methanogens are used for commercial production of methane gas.

 Methanogens are also useful for treatment of sewage in villages and municipal sewage treatment plants. Under anaerobic conditions soluble carbon compounds of wastes and wastewater are degraded stepwise to methane and other gases.

. What is the nature of cell-walls in diatoms?

In diatoms, the cell walls form two thin overlapping shells, which fit together closely. The walls cannot be destroyed as they are embedded with silica. The cell walls show a wide diversity in form, but are usually almost bilaterally symmetrical, hence the group name – diatom.

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

Answer: Algal bloom is a phenomenon whereby contaminants like nitrates and phosphates from untreated sewage when released into water bodies over stimulate the growth of algae. This over stimulation of algal growth (algal bloom) reduces the dissolved oxygen content of the water, which in turn causes other life forms to perish. Algal bloom affects the water quality and causes fish mortality. Some algae can be toxic to humans too. Algal blooms are often green but they could also be other colours like red or green depending upon the predominant species of algae causing the bloom.

Red Tides: Red or brown coloured algal blooms are called Red tides. Red tides are caused by red dinoflagellates (example Gonyaulax) which accumulate due to rapid multiplication making the sea appear red. Toxins released by such large numbers of algae may kill other marine animals such as fish.

5. How are viroids different from viruses? Answer: The difference between viroids and viruses are as follows:




Genome size

246 to 467 nucleotide long

2 kb or more in size


Highly complementary, circular single stranded RNA

Known to be RNA, DNA double stranded or single stranded

Protein Coat




Plant Pathogen

Both plant and animals


Potato spindle tuber viroid by Theodor Otto Diener in 1971

Tobacco mosaic virus by Martinus Beijerinck in 1898

Number known

33 till date

5000 described in detail but millions more known to exist.

. Describe briefly the four major groups of Protozoa.

Answer: The four major groups of protozoans are as follows:

Amoeboid protozoans:

 Found in fresh water, sea water or moist soil

 Move and capture the prey by using pseudopodia (false feet) as in Amoeba

 Marine forms have silica shells on their surface

 Some are parasites e.g. Entamoeba

Flagellated protozoans:

 Free-living or parasitic

 Movement by flagella

 The parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma

Ciliated protozoans:

 Aquatic and very motile

 Presence of hair-like organelles called cilia on the pellicle (cell body). Cilia are shorter than flagella and occur in large numbers Cilia are used in movement and food gathering

 Presence of a cavity (gullet) that opens to the outside of the cell surface.

 Rows of cilia move in a synchronized manner for locomotion and also to steer food into the gullet. Example: Paramecium.


 Parasitic protozoans

 They lack organelles for locomotion like cilia or flagella

 All sporozoans lack contractile vacuole

 They have complex life cycles a part of which is spent in host organisms

 Some sporozoans pass from host to host via an infectious spore stage.

 Example: Plasmodium (malarial parasite) which causes malaria.


Apart from this, for more study material for biology please click here - ncert solutions for class 11 biology chapter 3.

. Plants are autotrophi Can you think of some plants that are partially heterotrophic?

Answer: Plants are mostly autotrophic which means that they possess the pigment chlorophyll with the help of which they manufacture their own food (photosynthesis). However, a few members of the plant kingdom are partially heterotrophic, for example the insectivorous plants or parasites. These plants typically grow in nitrogen and mineral deficient soils and compensate for their nutritional requirement by their partial heterotrophic nature. These insectivorous plants have evolved mechanisms to trap insects from which they derive minerals and nutrition. Bladderwort and Venus fly trap are examples of insectivorous plants.

The other partially heterotrophic plant is Cuscuta that lives like a parasite on many trees and absorbs nutrition from the host plant.

. What do the terms phycobiont and mycobiont signify?

Answer: The phycobiont and mycobiont are the algal

and fungal partners, respectively, which are in symbiotic relationship with each other in nature. These are known as lichens and this is a mutually beneficial partnership.The algal partner, phycobiont (also called photobiont) is autotrophic and provides food to the fungi. The fungal or mycobiont partner, provides shelter, nutrients and water to the algal partner.

. Give a comparative account of the classes of

Kingdom Fungi under the following:

(i) Mode of nutrition

(ii) Mode of reproduction








Mode of Nutrition

Obligate parasites on plants. Also found in aquatic habitats and on decaying wood.

Saprophytic, decomposers, parasitic or coprophilous (growing on dung).

Grow in soil, on logs and tree stumps and in living plant bodies as parasites, e.g., rusts and smuts.

Some are saprophytes or parasites. Many others are decomposers of litter and help in mineral cycling.

Mode of


(mode of spore formation and fruiting bodies form the basis for the

division of the kingdom into various classes)

Asexual reproduction

– by zoospores (motile) or by aplanospores (non-motile). These spores are endogeneousl y produced in sporangium.



- Zygospores are formed

by fusion of

two gametes. These gametes are similar in morphology (isogamous) or


(anisogamous or


Asexual Reproduction

– asexual spores/ conidia produced exogenously


Sexual Reproduction: Sexual


ascospores produced endogenously in sac like asci. These asci are arranged in

different types of fruiting bodies called ascocarps.

Asexual Reproduction - asexual spores generally not found. Vegetative reproduction by fragmentation is common.


Reproduction -

sex organs absent, but plasmogamy by fusion of two vegetative cells of different strains or genotypes. The resultant structure is dikaryotic

which ultimately gives rise to basidium. Karyogamy and meiosis take place in the

basidium producing four basidiospores. The basidiospores are exogenously produced on the basidium The basidia are

arranged in fruiting bodies called basidiocarps.



-reproduce only by asexual spores known as conidia.


Rhizopus (the bread mould)


Agaricus (mushroom)

Alternaria, Colletotrichum and Trichoderma

10. What are the characteristic features of Euglenoids?

Answer: Characteristic features of Euglenoids are:

 Euglenoids are a group of flagellates commonly found in stagnant pools of fresh water.

 Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible. Pellicle is composed of proteinaceous strips underneath the cell membrane giving them a distinctive striation.

 They have two flagella, a short and a long one which help in locomotion.

 They are photosynthetic in the presence of sunlight, however when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.

Euglena reproduces by fission.It splits lengthwise into two.

The euglenoids have chlorophylls a and b and they store their photosynthetic product in an unusual form called paramylon starch.

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

Answer: A complete virus particle is known as virion. It consists of nucleic acid surrounded by a coat of protein called capsid. These are formed from identical protein subunits called capsomeres. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. Viruses are classified into four types based on their structure:

Polyhedral / icosahedral Viruses: These are nonenveloped viruses in which the capsomeres are arranged in geometric shape around the genetic material which is either double or single stranded DNA or RNA. The icosahedron shape has 20 equilateral triangle faces and 12 corners. Example: polio causing virus

Helical viruses: These viruses are nonenveloped with capsomeres arranged as a helix around the nucleic acid giving rise to elongated rods or flexible filaments. Example: tobacco mosaic virus

Complex viruses: The complex viruses have complex combinations of structures that may or may not be completely consistent between viruses of the same species. Example tailed bacteriophage, poxviruses

Enveloped viruses: These viruses are helical and icosahedrons shapes but have an outer envelope that surrounds the virus capsomeres. The envelope is a lipid bilayer containing glycoproteins embedded in the lipid. Example: Herpes virus.

Genetic Material of Viruses:

 It can be RNA and DNA.

 No virus contains both DNA and RNA as genetic material at the same time.

 Viruses infecting plants have single stranded RNA.

 Viruses infecting animals have either single or double stranded RNA or double stranded DNA.

 Bacteriophages usually have double stranded DNA.

Four viral diseases: Rabies, Influenza, Herpes, HIV

. Organize a discussion in your class on the topic – Are viruses living or nonliving?

The debate whether viruses are living or nonliving is an old one and till today, a consensus has not been reached.

Characteristics that classify viruses as living:

 They have complex molecule assemblies that include nucleic acids, proteins, carbohydrates, lipids and genetic material.

 They also make copies of themselves to reproduce. Characteristics that classify viruses as non-living:

 They cannot replicate on their own - they need a host cell to provide them the machinery to replicate.

 They can remain in dormant state forever till they find the right host to multiply.

Though it is difficult to categorize viruses, it can be said with enough confidence that viruses are some form of life. Nature is so diverse that the classification system that humans have made for their ease of use may not fit that diversity. This debate is not going to end soon so each one of us can decide which side we would like to be in given all the facts.



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