NCERT Solutions for Class 11 Biology Chapter 2-Biological classification
Question 1: Discuss how classification systems have undergone several changes over a period of time?
Biological classification is the scientific procedure of arranging organisms in a hierarchical
series of groups and sub-groups on the basis of their similarities and dissimilarities. Scientists
have proposed different systems of classification which have undergone several changes from
time to time.
Earlier Aristotle proposed artificial system of classification, which divided animals and plants on the basis of habitat. E.g., Aquatic (fish, whale), terrestrial (e.g., reptiles, cattle) and aerial (e.g., bat, birds). Then natural system of classification was based on morphology, anatomy, physiology, reproduction, ontogeny, cytochemistry, etc. Post that, organisms were classified on basis of evolutionary relationships called phyloge-netic system. It is based on cytotaxonomy, chemotaxonomy, numerical taxonomy and cladistic taxonomy.
Question 2: State two economically important uses of:
(a) Heterotrophic bacteria
(a) Heterotrophic bacteria:
i). Saprotrophic bacteria act as natural scavengers as they decompose the dead bodies, organic wastes and release raw materials leading to reuse of organic matter. They also help in sewage disposal, manure production etc.
ii) Symbiotic bacteria help in atmospheric nitrogen fixation.
iii) Some bacteria are employed in the production of a number of industrial products like lactic acid, curd, cheese, butter, vinegar etc. iv. Some bacteria are used in the preparation of serum, vaccines, vitamins, enzymes, antibiotics, etc. e.g., Pseudomonas, Xanthomonas, etc.
i. Archaebacteria are employed in the production of gobar gas from dung and sewage and in ruminants, they cause fermentation of cellulose.
ii. Some Archaebacteria live in the guts of ruminants such as cows and buffaloes and help in their digestion.
Question 3: What is the nature of cell-walls in diatoms?
The cell walls of diatoms are called frustules. The cell wall is chiefly composed of cellulose impregnated with glass – like silica. It is composed of two overlapping halves (or theca) that fit together like two parts of a soap box or petri dish. The upper half (lid) is called epitheca and the lower half (case) is called hypotheca. The outer covering possesses very fine markings, pits, pores and ridges. The siliceous frustules of diatoms do not decay easily. They pile up at the bottom of water reservoirs and form big heaps called diatomite or diatomaceous earth. It may extend for several hundred meters in certain areas from where the same can be mined.
Question 4: Find out what do the terms ‘algal bloom’ and ‘red tides’ signify.
The rapid increase in populations of algae and other phytoplanktons, in particular cyanobacteria, in water bodies rich in organic matter is called algal bloom. The density of the organisms may be such that it may prevent light from passing to lower depths in the water body. Algal blooms are caused by an increase in levels of nitrate, a mineral ion essential for algal and bacterial growth.
The source of increased nitrate may be from agricultural fertilizers, which are leached – into water systems from the land, or sewage effluent.
Red tides are caused by a sudden, often toxic proliferation of marine phytoplankton, notably dinoflagellates, that colour the sea red, brown, or yellowish due to the high concentration of the photosynthetic accessory pigments. Some dinoflagellates, such as Gonyaulax, produce potent toxins, which may kill fish and invertebrates outright or accumulate in the food chain, posing a hazard to humans eating shellfish and other seafood. These phytoplanktonic blooms may be related to nutrient – rich inputs from the land, upwelling oceanic waters, and are initiated by the activation of cyst-like forms lying on the sea bed.
Question 5: How are viroid’s different from viruses?
Viroids are the smallest known agents of infectious diseases that contain small single-stranded RNA – molecule. They lack capsid and have no proteins associated with them. Viroids infect only plants. Whereas, viruses have genetic material surrounded by a protective coat of protein or lipoprotein. The genetic material of viruses are of 4 types – double – stranded DNA, double – stranded RNA, single –stranded DNA, single – stranded RNA. They can infect both plants and animals.
Question 6: Describe briefly the four major groups of protozoa.
All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. They are classified into four groups on the basis of locomotory organelles.
(i) Amoeboid protozoans:
These organisms live in fresh water, sea water or moist soil. They move and capture their prey by developing pseudopodia (false feet) as in Amoeba. Some of them such as Entamoeba are parasites.
(ii) Flagellated protozoans:
The members of this group are either free – living or parasitic. They have flagella for locomotion. The parasitic forms cause diseases such as sleeping sickness e.g., Trypanosoma.
(iii) Ciliated protozoans:
These are aquatic, actively moving organisms because of the presence of thousands of cilia. They have cavity (gullet) that opens to the outside of the cell surface. The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet e.g., paramecium.
This includes diverse parasitic organisms that have an infectious spore like stage in their life cycle. Locomotory organs are absent. The most notorious is N. Plasmodium (malarial parasite) which causes malaria has a staggering effect on human population.
Question 7: Plants are autotrophic. Can you think of some plants that are partially heterotrophic?
Some insectivorous plants like Drosera, Nepenthes, Utricularia are partially heterotrophic plants. These plants are deficient in nitrogen content but are otherwise autotrophic. They trap various insects to obtain nitrogen from them. Rest, the food i.e., carbohydrate is manufactured by themselves by the process of photosynthesis.
Question 8: What do the terms phycobiont and mycobiont signify?
A lichen is structurally organized entity consisting of the permanent association of a fungus and an alga. The fungal component of lichen is called mycobiont and the algal component is called phycobiont. Both mycobiont and phycobiont are associated in symbiotic union in which the fungus is predominant and alga is subordinate partner. Fungus provides the structural covering that protects alga from unfavourable conditions, i.e., drought, heat, etc. It also traps moisture from the atmosphere and anchors the lichen to rock, tree bark, leaves and other similar supports. The alga prepares organic food by the process of photosynthesis from carbon dioxide. If the algal component is cyanobacteria (blue-green alga), they can fix atmospheric nitrogen in addition to preparation of food.
Question 9: Give a comparative account of the classes of kingdom Fungi under the following
(i) mode of nutrition (ii) mode of reproduction
|Class of fungi||Mode of nutrition||Mode of reproduction|
|▪ Oomycetes are mostly|
parasitic (obtain their
nourishment from the
protoplasm of living plants
▪ e.g., phytophthorainfestans).
Zygomycetes are mostly
saprophytes (absorb food from
dead or decaying organic
▪ e.g., Rhizopus), parasitic
(Absidiacornealis), some are
coprophilous (fungi which
grow on dung e.g., Mucor).
▪ In oomycetes asexual
reproduction is by zoospores
(aquatic form), aplanospore
▪ Sexual reproduction may be
isogamous or oogamous, sexual
fusion is gemetangial contact
▪ Male sex organ is antheridium
and female sex organ is
▪ Plasmogamy is followed by
karyogamy and meiosis
▪ In zygomycetes asexual
reproduction occurs by thin
walled non motile
▪ Sexual reproduction takes place
by gametangial copulation (two
identical gametangia) know as
reproduction forms diploid
|▪ Most are terrestrial and|
occur as saprophytes (e.g.,
▪ Some grow in deciduous
forests on humus rich soil
|▪ Asexual reproduction by conidia|
or conidiospores (Aspergillus),
▪ Sexual reproduction by gametic
copulation e.g., yeast),
gametangial contact (e.g.,
▪ Sexual reproduction takes place
in three stages plasmogamy
(fusion of protoplast),
karyogamy (fusion of nucleus)
▪ Ascospores are formed in ascus.
Generally each ascus consists of
eight ascospores. Fructification
are known as ascocarp
(cleistothecium, e.g., penicilium,
perithecium e.g., Neurospora,
and apothecium e.g., peziza).
|Basidiomycetes||▪ Mostly saprophytes|
(Agaricus), found on
humus, bark, decaying
▪ Some are obligate
parasites (e.g., rusts,
powdery midews, which
live entirely on the
living protoplasm of
their hosts and can never
grow on dead tissue),
some are facultative
saprophytes (somesmuts, which are usually
parasitic in their mode
of life but later may pass
their mode of life as
|▪ The asexual spores are|
generally not found, but
vegetative reproduction by
fragmentation is common.
▪ The sex organs are absent,
but plasmogamy is brought
about by fusion of two
vegetative or somatic cells
of different strains or
▪ The resultant structure is
dikaryotic which ultimately
gives rise to bassidium.Karyogamy and meiosis take
place in the basidium
▪ The basidiospores are
exogenously produced on
the basidium (pl.:basidia).
▪ The basidia are arranged in
fruiting bodies called
|Deuteromycetes||Mostly parasitic.||▪ Asexual reproduction by|
conidia and some other
spores. Sexual reproduction
is either absent or yet to be
Question 10: What are the characteristic features of Euglenoids?
The euglenoid flagellates are the most interesting organisms having a mixture of animal and plant characteristics. The characteristic features are:
(i) They are unicellular flagellates.
(ii) These protists lack a definite cellulose cell wall. Instead, the cells are covered by a thin membrane known as pellicle. The pellicle is composed of protein, lipid and carbohydrates.
(iii) One or two flagella are present which help these protists in active swimming. If two flagella are present, then one is long and other is short. They are tinsel – shaped i.e., with two longitudinal rows of fine hairs. Each flagellum has its own basal granule. The two flagella join with each other at a swelling, called paraflagellar body and finally only one long flagellum emerges out through the cytostome.
(iv) Cell at the anterior end possesses an eccentric mouth or cytostom which leads into a flask shaped cavity viz. gullet or cytopharynx. Gullet opens into a large basal reservoir.
(v) At one end of the reservoir, the cytoplasm contains an orange red stigma (eye spot). The eye spot is a curved plate with orange-red granules and contains red pigment astaxanthin. Both paraflagellar body and eye spot act as photoreceptors.
(vi) Just below the reservoir is found a contractile vacuole having many feeding canals. The contractile vacuole takes part in osmoregulation. It expands and pumps its fluid contents in the reservoir.
(vii) The mode of nutrition in euglenoids is holophytic or photoautotrophic. Some euglenoids show mixotrophic nutrition (both holophytic as well as saprobic mode).
(viii) Cytoplasm is differentiated into ectoplasm and endoplasm. Nucleus is large and occurs roughly in middle. The envelope and nucleolus persist during cell division.
(ix) Each chloroplast is composed of a granular matrix traversed by 10-45 dense bands and is covered by 3-membraned envelope. They contain the photosynthetic pigments-chlorophyll –b. They store carbohydrates as paramylon bodies, scattered throughout the cytoplasm.
(x) Asexual reproduction occurs by longitudinal binary fission. The flagellum is duplicated before cell division.
(xi) Under unfavorable condition the euglenoids form cysts to perennate the dry period.
(xii) Sexual reproduction is not observed.
Question 11: Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.
Virus (L. poisonous fluid) is a group of ultramicroscopic, non-cellular, highly infectious agents that multiply only intracellularly – inside the living host cells without involving growth and division. Outside the host cells, they are inert particles. They are nucleoproteins having one or more nucleic acid molecule, either DNA or RNA, encased in a protective coat of protein or lipoprotein. A virus consist of two parts – nucleoid (genome) and capsid. An envelope and few enzymes are present in some cases,
The nucleic acid present in the virus is called nucleoid and it represents the viral chromosome. It is made up of a single molecule of nucleic acid. It may be linear or circular and nucleic acid can be DNA or RNA. It is the infective part of virus which utilizes the metabolic machinery of the host cell for synthesis and assembly of viral components.
It is a protein covering around genetic material. Capsid have protein subunits called capsomeres. Capsid protects nucleoid from damage from physical and chemical agents.,
It is the outer loose covering present in certain viruses like animal viruses (e.g., HIV) but rarely present in plant and bacterial viruses and is made of protein of viral origin and, lipid and carbohydrate of host. Outgrowths called spikes may be present. Envelope proteins have subunits called peplomers. A virus without envelope is naked virus.
Rarely, lysozymes are found in bacteriophages. Reverse transcriptase enzyme (catalyses RNA to DNA synthesis) is found in some RNA viruses like HIV. Some common viral diseases are – influenza, polio, measles, chickenpox, hepatitis, AIDS, bird flu, SARS (Severe Acute Respiratory Syndrome), etc.
Question 12: Organize a discussion in your class on the topic – ‘Are viruses living or non-living’?
Viruses are regarded as intermediate between non-living entities and living organisms. It is very difficult to ascertain whether they are living or non- living. Some characters of viruses suggest their non-living nature whereas many other characters suggest their living nature.
They resemble non-living objects in
(i) Lacking protoplast.
(ii) Ability to get crystallized.
(iii) Inability to live independent of living cell.
(iv) High specific gravity which is found only in non-living objects.
(v) Absence of respiration.
(vi) Absence of energy storing system.
(vii) Absence of growth and division. Instead, different parts are synthesized separately.
Viruses resemble living beings in
(i) Being formed of organic macromolecules which occur only in living beings.
(ii) Presence of genetic material.
(iii) Ability to multiply or reproduce although only inside living cell.
(iv) Occurrence of mutations.
(v) Occurrence of enzyme transcriptase in most viruses.
(vi) Some viruses like Pox virus contain vitamins like riboflavin and biotin.
(vii) Infectivity and host specificity.
(viii) Viruses are ‘killed’ by autoclaving and ultraviolet rays.
(ix) They breed true to their type. Even variations are inheritable.
(x) They take over biosynthetic machinery of the host cell and produce chemicals required for
(xi) Viruses are responsible for a number of infectious’ diseases like common cold, epidemic
influenza, chicken pox.