RBSE Solutions for Class 11 Biology Chapter 2 Biological Classification

Rajasthan Board RBSE Solutions for Class 11 Biology Chapter 2 Biological Classification Textbook Exercise Questions and Answers.

RBSE Class 11 Biology Solutions Chapter 2 Biological Classification

RBSE Class 11 Biology Biological Classification Textbook Questions and Answers

Question 1. 
Discuss how classification systems have undergone several changes over a period of time?
Answer:
The system of classification have undergone several changes with time. The first attempt of classification was made by Aristotle. He classified plants as herbs, shurbs and trees. Animals on the other hand were classified on the basis of presence or absence of blood cells. This system of classification failed to classify all the known organisms.

Therefore, Linnaeus gave a two kingdom system of classification. It consists of Kingdom Plantae and Kingdom Animalia. However, this system did not differentiate between unicellular and multicellular organisms and between eukaryotes and prokaryotes. Therefore, there were large number of organisms that could not be classified under the two kingdoms. To solve these problems a five kingdom system of classification was proposed by R.H. Whittaker in 1969. On the basis of characteristics such as cell structure, mode of nutrition, presence of cell wall etc. It consists of five kingdoms namely - Monera, Protista, Fungi, Plantae and Animalia.

Question 2. 
State two economically important uses of:
(a) Heterotrophic bacteria 
(b) Archaebacteria. 
Answer:
(a) Heterotrophic Bacteria: They act as natural scavangers because they decompose the dead organic matter. This activity is economically used in sewage disposal and in improving soil fertility by formation of humus.

(b) Archaebacteria: They live as symbionants in the rumen of herbivorous animals that chew their cud (cow, buffalo) and help in fermentation of cellulose. These bacteria are used in the production of biogas (methane) from the dung and therefore are called methanogens. 

Question 3. 
What is the nature of cell-walls in diatoms? 
Answer:
The cell walls of diatoms are mainly composed of cellulose impregnented with silica. They show sculpture and ornamentations. The cellwall is composed of two overlapping halves (or theca) that fit together like two pairs of soap box. The upper box is called epitheca and the lower half’is called hypotheca. The theca contains two grooves - longitudinal sulcus and transverse girdle or annulus.

Question 4. 
Find out what do the terms ‘algal bloom’ and ‘red tides’ signify.
Answer:
Algal bloom: Algal bloom refers to an increase in the population of algae or blue green algae in water, resulting in discolouration of the water body. This causes an increase in the biological oxygen demand (BOD) resulting in the death of fishes and other aquatic animals.

Red tides: Red tides are caused by red dinoflagellates (Gonyaulax) that multiple rapidly. Due to their numbers the sea appears red in colour. They release large amounts of toxins in water that can cause death of a large number of fishes.

Question 5. 
How are viroids different from viruses?
Answer:
Viroids are smaller than viruses which lack protein. On the otherhand the viruses have genetic material surrounded by protein coat.

Question 6. 
Describe briefly the four major groups of protozoa.
Answer:
Four major groups of protozoa are: 

1. Amoeboid protozoans
2. Flagellated protozoans
3. Ciliated protozoans
4. Sporozoans.

1. Amoeboid Protozoans : They live in fresh water, sea water or moist soil. They move and capture their prey by putting out pseudopodia as in Amoeba. Marine forms of amoeboid protozoans have silica shells on their outer surface. Some of amoeboid protozoans such as Entamoeba are parasitic in nature. They are uninucleate, binucleate or multinucleate. Flagella are usually absent. Nutrition is holozoic. Asexual reproduction occurs by binary fission, multiple fission, swarm spores and budding. Sexual reproduction occurs by syngamy. For example : Amoeba, Entamoeba etc.

2. Flagellated Protozoans : They are either free living or parasitic protozoans. They have flagella. Some of them are human parasites and cause diseases. For example, Trypanosoma gambiense causes sleeping sickness and Leishmania donovani causes kala-azar. They are generally uninucleate organisms bounded by a firm pellicle. Their mode of nutrition is holozoic, saprotrophic and parasitic. They store reserve food material in the form of glycogen.

Asexual reproduction occurs by binary fission and cyst formation. Sexual reproduction has been reported only in few forms. Example : Trypanosoma, Tricholympha, Giardia, Trichomonas, Leishmania etc.

3. Ciliated Protozoans : These are aquatic, actively moving organisms due to the presence of thousands of cilia. Many of them are free living individuals in fresh and marine water (e.g., Paramoecium). A few are parasitic and pathogenic (e.g., Balantidium). Some of them are colonial and sedentary. They have specialized organelles to perform various functions equivalent to mouth, digestive tract, anus, kidney, muscle, skin and sense organs of higher animals. They possess cilia for locomotion and to capture food. Nutrition is holozoic and absorptive.

Fresh water forms possess contractive vacuoles for osmoregulation. The ciliates have nuclear dimorphism. The large macronucleus controls all vegetative metabolic activities and growth and the small micronucleus takes part in sexual reproduction. They reproduce asexually by transverse binary fission or budding and sexually by conjugation. Examples : Paramoecium, Vorticella, Opalina, Podophyra, Balantidium etc.

4. Sporozoans : This include diverse organisms that have an infectious spore like stage in their life cycle and are endoparasites and pathogenic. They cause disease. The mode of nutrition is parasitic (absorptive). They do not possess locomotary organelles. Their ingetion of food is due to parasitic life. The body is covered by an elastic pellicle or cuticle. It has single nucleus. Asexual reproduction occurs by multiple fission. The sexual reproduction takes place by fusion of gametes resulting in the formation of zygotes followed by the formation of haploid spores. Life cycle may include the two different hosts.

Question 7. 
Plants are autotrophic. Can you think of some plants that are partially heterotrophic.
Answer: 
Plants have autotrophic mode of nutrition as they contain chloroplasts. Thus, they have the ability to prepare their own organic food by the process of photosynthesis. However, some insectivorous plants are partially heterotrophic in nature. They have various means of capturing.insects so as to supplement their diet with required nutrients derived from insects causing proliferation of growth. For example, Nepenthes (Pitcher plant), Utricularia (Bladder wort) and Venus fly trap and some parasitic plants such as Cuscuta.

Question 8. 
What do the terms phycobiont and mycobiont signify?
Answer:
Each lichen consists of two components : Mycobiont and phycobiont. Mycobiont is the fungal component of lichen whereas, phycobiont is the algae component of lichen.

Question 9. 
Give a comparative account of the classes of kingdom fungi under the following :
(i) Mode of Nutrition 
(ii) Mode of reproduction.
Answer:

Question 10. 
What are the characteristic features of Euglenoids?
Answer:
The euglenoids are the most interesting organisms having a mixture of animal and plant characteristics. Majority of them are fresh water organisms found in stagnant water. However, a few forms occur in damp soil, blackish water and even in salt water. The most common Euglenoid flagellate is Euglena viridis.

These protists lack a definite cell wall. Instead the cells are covered by a thin membrane called pellicle or periplast which makes their body flexible. They have two flegella, a short and a long one. A few forms are non-flagellated. Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms. Interestingly, the pigments of euglenoids are identical to those present in higher plants.

These cells at the anterior possess a mouth (cytostome) which leads into a flask shaped cavity gullet or cytopharynx. The later then opens into a large basal reservoir. The mouth is usually ecentric. Usually a large contractile vacuole occurs at the anterior end near the base of reservoir. Each cell possess a single large nucleus which exhibits a different sort of nuclear division. Asexual reproduction occurs by longitudinal binary fission starting at the flagellar end. Sexual reproduction has not been observed in euglenoids except in one genus.

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:
Virurses are ultramicroscopic, non-cellular, highly infectious agents that multiply only introcellularly inside the living host cells without involving growth and  division. Outside the living cells they behave as inert particles. They are nucleoproteins having one or more nucleic acid molecules either DNA or RNA enclosed in a coat of protein or lipoprotein.

The name virus that means venom or poisonous fluid was given by Dmitri Ivanowsky (1892) recognised certain microbes as casual organism of mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria proof filters. M.W. Beijrinck (1898) demonstrated that the extract of the injected plant of tobacco could cause infection in healthy plants and called the fluid contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist largely proteins. Viruses are considered as obligate parasites.

Viruses are highly pathogenic and cause infectious diseases in animals and plants. They cause infection in the host cells by means of nucleic acids.

• In general viruses that infect plants have single stranded RNA.
• viruses that infect animals have either single or double stranded RNA or double stranded DNA and
• Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses.

Some viruses possess an outer thin loose covering called envelop composed of proteins, lipids and carbohydrates. The smaller subunits of envelope are called peptomere. The protein coat of viruses is called capsid made of small subunits called capsomeres protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.

Viruses cause diseases like mumps, small pox, herpes and influenza. AIDS in humans is also caused by a virus Human Immunodeficiency Virus (HIV). In plants the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearning, dwarfing and stunted growth.

Question 12. 
Organise a discussion in your class on the topic. Are viruses living or non-living?
Answer:
Viruses are microscopic organisms that have characteristics of both living and non-living. A virus consists of a strand of DNA or RNA covered by protein coat. The presence of nucleic acid (DNA or RNA) suggests that viruses are alive. In addition they can also respond to their environment (inside the host cell) in a limited manner.

However, some other characters such as their inability to reproduce without using the host cell machinery and their cellular matter indicate that viruses are non living. Therefore, classifying viruses has remained a mystery for modern systematics.