Biology Chapter 2 of Class 11 — Biological Classification — takes the exploration of life a step further by organizing the vast diversity of living organisms into meaningful groups. This chapter explains how scientists classify life forms based on similarities and differences in structure, function, and evolution. Students learn about the five-kingdom classification system proposed by R.H. Whittaker, and gain insights into the unique features of Monera, Protista, Fungi, Plantae, and Animalia.
The NCERT Solutions for Class 11 Biology Chapter 2 – Biological Classification provided here offer clear, step-by-step explanations to all textbook questions. They help students build a strong conceptual base, understand key distinctions among different kingdoms, and prepare thoroughly for school exams as well as competitive exams like NEET.
NCERT Solutions for Class 11 Biology Chapter 2 - All Exercise Questions
Q.
Discuss how classification systems have undergone several changes over a period of time?
Q.
State two economically important uses of:
(a) heterotrophic bacteria
(b) archaebacteria
Q.
What is the nature of cell-walls in diatoms?
Q.
Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify.
Q.
How are viroids different from viruses?
Q.
Describe briefly the four major groups of Protozoa.
Q.
Plants are autotrophic. Can you think of some plants that are partially heterotrophic?
Q.
What do the terms phycobiont and mycobiont signify?
Q.
Give a comparative account of the classes of Kingdom Fungi under the following:
(i) Mode of nutrition
(ii) Mode of reproduction
Q.
What are the characteristic features of Euglenoids?
Q.
Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.
Q.
Organise a discussion in your class on the topic – Are viruses living or non-living?
Class 11 Biology Chapter 2 Questions and Answers PDF
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Biological Classification NCERT Solutions: Class 11 Biology Chapter 2 Question and Answers
Q1. 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 steps given below show how the present system of classification has evolved into its form.
- The earliest methods of classification were based out of the need to use plants or animals and didn’t have any scientific basis.
- Aristotle defined a classification method whereby he used the morphological characters to classify plants and animals.
- Linnaeus defined a two-kingdom system of plants and animals. this included all plants and animals irrespective of their varied forms and features.
- R.H.whittaker (1969) proposed a five-kingdom classification system. This mode of classification system includes cell structure, thallus organisation, mode of nutrition, reproduction and phylogenetic relationship. In this classification system, a clear differentiation between photosynthetic and non-photosynthetic organisms was made.
Q2. State two economically important uses of (a) heterotrophic bacteria (b) archaebacteria
The two economic uses of heterotrophic bacteria are:
- Many heterotrophic bacteria help in decomposing dead and decaying organisms. This leads to the production of humus which increases soil fertility.
- Heterotrophic bacteria are also used in the production of antibiotics. Many known antibiotics are produced by Streptomyces, which are filamentous soil bacteria.
The two economic uses of archaebacteria are:
- 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 the 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.
Q3. 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.
Q4. Find out what do the terms mean; algal bloom and red-tides; signify.
Algal bloom: The algal bloom is a phenomenon occurs when contaminants like nitrates and phosphates from untreated sewage are released into water bodies. the presence of an excessive amount of nutrients such as nitrates and phosphates overstimulate the growth of algae. This overstimulation 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 water quality and causes fish and other aquatic life 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.
Q5. How are viroids different from viruses?
The difference between viroids and viruses are as follows:
|
Viroid |
Virus |
| Genome size |
246 to 467 nucleotide long |
2 kb or more in size |
| RNA/ DNA |
Highly complimentary, circular single-stranded RNA |
Known to be RNA, DNA double-stranded or single-stranded |
| Protein Coat |
Absent |
Present |
| Host |
Plant pathogen |
Both plant and animals |
| Discovery |
Potato spindle tuber viroid by Theodor Otto Diener in 1971 |
Tobacco mosaic virus by Martinus Beijerinck in 1898 |
| Number known |
Around 30 |
Around 5000 described in detail but millions more known to exist. |
Q6. Describe briefly the four major groups of Protozoa.
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 for movement and food gathering.
- Presence of a cavity (gullet) that opens to the outside of the cell surface.
- Rows of cilia move in a synchronised manner for locomotion and also to steer food into the gullet. Example: Paramecium
Sporozoans:
- 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
Q7. Plants are autotrophic. Can you think of some partially heterotrophic plant?
Plants are mostly autotrophic which means that they possess the pigment chlorophyll. With the help of chlorophyll, they manufacture their own food by performing 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 flytrap 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.
Q8. What do the terms phycobiont and mycobiont signify?
The phycobiont and mycobiont are the algal and fungal partners, respectively, which are in a 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.
Q9. Give a comparative account of the classes of Kingdom Fungi under the following:
- Mode of nutrition
- Mode of reproduction
(i) Mode of nutrition
(ii) Mode of reproduction
Q10. What are the characteristic features of Euglenoids?
- Characteristic features of Euglenoids are:
- Euglenoids are a group of flagellates commonly found in stagnant pools of freshwater and ditches or damp soil.
- Instead of a cell wall, they have a protein-rich layer called pellicle which makes their body flexible. The pellicle is composed of proteinaceous strips underneath the cell membrane giving them a distinctive striation.
- They have one or two flagella that they use for swimming. Those have two flagella, one is short and another is long. Both the flagella are used for 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 chlorophyll a and chlorophyll b and they store their photosynthetic product in an unusual form called paramylon starch.
Q11. Give a brief account of viruses with respect to their structure and nature of genetic material. Also, name four common viral diseases.
A complete virus particle is known as virion. It consists of nucleic acid, either DNA or RNA, 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 non-enveloped 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 non-enveloped 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 or DNA but not both at the same time in a virus.
- 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
Q12. Organize a discussion in your class on the topic; Are viruses living or nonliving?
The debate whether viruses are living or non-living 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.
- They are sensitive to external factors such as temperature, chemicals and radiations.
Characteristics that classify viruses as non-living:
- They cannot replicate on their own – they need a host cell to provide them with the machinery to replicate.
- They can remain in the dormant state forever till they find the right host to multiply.
- They have only one type of nucleic acid, i.e., either DNA or RNA. All the living things possess both the types of nucleic acid.
Though it is challenging to categorise 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.
NCERT Solutions for Class 11 Biology Chapter 2 – FAQs
Q1. What is biological classification?
Biological classification is the process of grouping living organisms into categories (kingdoms, phyla, classes, orders, families, genera, species) based on shared characteristics, evolutionary relationships, and genetic similarities.
Q2. Who proposed the five-kingdom classification system and why was it needed?
Robert H. Whittaker proposed the five-kingdom system (Monera, Protista, Fungi, Plantae, Animalia) in 1969, to better accommodate distinctions in cell type, nutrition, reproduction, and evolutionary relationships that the earlier two-kingdom system (plants vs animals) could not account for.
Q3. What are the characteristic features of Euglenoids?
- Lack a rigid cell wall (they have a flexible pellicle).
- Possess two flagella of unequal length.
- They are mixotrophic: autotrophic in presence of light and heterotrophic in absence of light.
- Have chlorophyll and chloroplasts (for photosynthesis) when in light.
Q4. What is the difference between viruses and viroids?
- Viruses consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid), and sometimes an envelope.
- Viroids are just naked RNA molecules (no protein coat).
- Viruses can infect animals, plants, bacteria; viroids generally infect plants.
- Viroids are much smaller than viruses.
Q5. What are lichens, and what is their significance in classification?
Lichens are symbiotic associations between a fungus (mycobiont) and an alga or cyanobacterium (photobiont). They are interesting from a classification standpoint because they blur lines between separate kingdoms—they are not themselves a kingdom, but a partnership of organisms from different kingdoms.
Q6. Which class of fungi is called “imperfect fungi” and why?
Deuteromycetes (also called Fungi imperfecti) are called “imperfect fungi” because their sexual stage is not known or has not been observed. They reproduce only asexually (by conidia).
Q7. Why is it said that classification systems have changed over time?
Because earlier systems (Aristotle’s, Linnaeus’s) were based on easily observable morphological traits and lacked ability to include microorganisms, evolutionary relationships, or genetic data. Over time, as knowledge in cell biology, genetics, and phylogeny advanced, classification systems evolved (from 2 kingdoms → 5 kingdoms → domains etc.) to better reflect real relationships and diversity.