Chapter 2 : Biological Classification

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

Answer. 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 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. After natural system, organisms were classified on basis of evolutionary relationships called phylogenetic system. It is based on cytotaxonomy, chemotaxonomy, numerical taxonomy and cladistic taxonomy.

2. State two economically important uses of:

(a) Heterotrophic bacteria

(b) Archaebacteria


(a) Heterotrophic bacteria

   a) They act as decomposers and help in the formation of humus.

   b) They help in the production of curd from milk.

  c) Many antibiotics are obtained from some species of bacteria.

  d) Many soil bacteria help in fixation of atmospheric nitrogen.

(b) Archaebacteria

  a) Methane gas is produced from the dung of ruminants by the methanogens.

  b) Methanogens are also involved in the formation of biogas and sewage treatment

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

Answer. The cell walls of diatoms are made of silica. Their cell wall construction is known as frustule. It consists of two thin overlapping shells that fit into each other such as a soap box. When the diatoms die, the silica in their cell walls gets deposited in the form of diatomaceous earth. This diatomaceous earth is very soft and quite inert. It is used in filtration of oils, sugars, and for other industrial purposes.

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


Algal bloom

Algal bloom refers to an increase in the population of algae or blue-green algae in water, resulting in discoloration 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 are caused by red dinoflagellates (Gonyaulax) that multiply rapidly. Due to their large 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.

5. How are viroids different from viruses?

Answer. Viroids are the smallest known agent 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 infect both plants and animals.

6. Describe briefly the four major groups of protozoa.

Answer. All protozoans are heterotrophs and live as predators or. parasites. They are be-lieved 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 a 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.
(iv)Sporozoans: This includes diverse parasitic organisms that have an infectious spore¬like stage in their life cycle. Locomotory organs are absent. The most notorious N . is Plasmodium (malarial parasite) which
causes malaria which has a staggering effect on human population.

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

Answer. Some insectivorous plants like Drosera, Nepenthes, Utricularia are partially heterotrophic plant. 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.

8. What do the terms phycobiont and mycobiont signify?

Answer.  Phycobiont refers to the algal component of the lichens and mycobiont refers to the fungal component. Algae contain chlorophyll and prepare food for fungi whereas the fungus provides shelter to algae and absorbs water and nutrients from the soil. This type of relationship is referred to as symbiotic.

9. Organise a discussion in your class on the topic – ‘Are viruses living or non-living’?

Answer. 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 crystallised.
(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 contains 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 their multiplication.
(xi)Viruses are responsible for a number of infectious’ diseases like common cold, epidemic influenza, chicken pox.

10. What are the characteristic features of Euglenoids?

Answer: Some characteristic features of Euglenoids are as follows.

• Euglenoids (such as Euglena) are unicellular protists commonly found in fresh water.

• Instead of cell wall, a protein-rich cell membrane known as pellicle is present.

• They bear two flagella on the anterior end of the body.

• A small light sensitive eye spot is present.

• They contain photosynthetic pigments such as chlorophyll and can thus prepare their own food. However, in absence of light, they behave similar to heterotrophs by capturing other small aquatic organisms.

• They have both plant and animal-like features, which makes them difficult to classify.

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

Answer: Viruses are sub-microscopic infectious agents that can infect all living organisms. A virus consists of genetic material surrounded by a protein coat. The genetic material may be present in the form of DNA or RNA.  Most of the viruses, infecting plants, have single stranded RNA as genetic material. On the other hand, the viruses infecting animals have single or double stranded RNA or double stranded DNA.

Bacteriophages or viruses infecting bacteria mostly have double stranded DNA. Their protein coat called capsid is made up of capsomere subunits. These capsomeres are arranged in helical or polyhedral geometric forms.  A.I.D.S, small pox, mumps, and influenza are some common examples of viral diseases.

12. Give a comparative account of the classes of Kingdom Fungi under the following:

(i) Mode of nutrition

(ii) Mode of reproduction


(A) Phycomycetes: This group of fungi includes members such as RhizopusAlbugo, etc.

(i) Mode of nutrition

They are obligate parasites on plants or are found on decaying matter such as wood.

(ii) Mode of reproduction

Asexual reproduction takes place through motile zoospores or non-motile aplanospores that are produced endogenously in sporangium.

Sexual reproduction may be of isogamous, anisogamous, or oogamous type. It results in the formation of thick-walled zygospore.

(B) Ascomycetes: This group of fungi includes members such as PenicilliumAspergillusClaviceps, and Neurospora.

(i) Mode of nutrition

They are sporophytic, decomposers, parasitic or coprophilous (growing on dung).

(ii) Mode of reproduction

Asexual reproduction occurs through asexual spores produced exogenously, such as conidia produced on conidiophores.

Sexual reproduction takes place through ascospores produced endogenously in saclike asci and arranged inside ascocarps.

(C) Basidiomycetes: This group of fungi includes members such as UstilagoAgaricus and Puccinia.

(i) Mode of nutrition

They grow as decomposers in soil or on logs and tree stumps. They also occur as parasites in plants causing diseases such as rusts and smuts.

(ii) Mode of reproduction

Asexual reproduction takes place commonly through fragmentation. Asexual spores are absent.

Sex organs are absent but sexual reproduction takes place through plasmogamy. It involves fusion of two different strains of hyphae. The resulting dikaryon gives rise to a basidium. Four basidiospores are produced inside a basidium.

(D) Deuteromycetes: This group of fungi includes members such as Alternaria, Trichoderma, and Colletotrichum.

(i) Mode of nutrition

Some members are saprophytes while others are parasites. However, a large number act as decomposers of leaf litter.

(ii) Mode of reproduction

Asexual reproduction is the only way of reproduction in deuteromycetes. It occurs through asexual spores called conidia.

Sexual reproduction is absent in deuteromycetes.