Biological Classification
Welcome to HSLC Guru! This page presents detailed and well-organized question answers for Class 11 Biology Chapter 2 — Biological Classification, prepared strictly as per the ASSEB (Assam State School Education Board) syllabus. Students of the English Medium will find chapter summary, short and long answer questions, MCQs, fill in the blanks, true/false statements, and a glossary table to make exam preparation easier and more effective.
Chapter Summary
The diversity of living organisms on Earth is enormous, and to study them systematically we need a logical system of classification. The earliest attempt was the two-kingdom system proposed by Carolus Linnaeus, which divided all organisms into Plantae and Animalia. However, this system could not differentiate between prokaryotes and eukaryotes, autotrophs and heterotrophs, or unicellular and multicellular forms. As more organisms were discovered with mixed characteristics, the need for a more refined classification became evident.
To overcome these limitations, R. H. Whittaker (1969) proposed the five-kingdom classification based on cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships. The five kingdoms are Monera, Protista, Fungi, Plantae and Animalia. Later, Carl Woese (1990) introduced the three-domain system based on molecular phylogeny, dividing living organisms into Archaea, Bacteria and Eukarya. This domain system places the kingdom Monera into two domains — Archaea and Bacteria — while all other kingdoms (Protista, Fungi, Plantae, Animalia) are kept under Eukarya.
Kingdom Monera includes all prokaryotic organisms, mainly bacteria. Bacteria are grouped into archaebacteria (live in extreme habitats like hot springs, salty areas, marshy places — methanogens, halophiles, thermoacidophiles), eubacteria or true bacteria (including useful and harmful bacteria), cyanobacteria (blue-green algae with chlorophyll-a, performing photosynthesis and nitrogen fixation), and mycoplasma (smallest living cells without a cell wall, can survive without oxygen, causing diseases in plants and animals). Kingdom Protista includes single-celled eukaryotes — chrysophytes (diatoms, golden algae), dinoflagellates (mostly marine, photosynthetic, can cause red tide), euglenoids (Euglena — both autotrophic and heterotrophic), slime moulds (saprophytic protists), and protozoans (Amoeba, Paramoecium, Plasmodium, Trypanosoma).
Kingdom Fungi are heterotrophic eukaryotes with cell walls of chitin. They are classified into Phycomycetes (Mucor, Rhizopus — found in moist places), Ascomycetes or sac fungi (Aspergillus, Penicillium, Neurospora, yeast), Basidiomycetes or club fungi (mushrooms, bracket fungi, puffballs, smuts, rusts), and Deuteromycetes or imperfect fungi (Alternaria, Colletotrichum, Trichoderma — sexual stage unknown). Kingdom Plantae includes all eukaryotic, multicellular, chlorophyll-bearing autotrophs — algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Kingdom Animalia includes multicellular, heterotrophic eukaryotes that lack cell walls and depend on holozoic nutrition. Apart from the five kingdoms, certain acellular entities are studied separately — viruses (non-cellular, infectious agents made of nucleic acid and protein coat), viroids (smaller than viruses, only free RNA without protein coat — discovered by T. O. Diener), prions (infectious proteinaceous particles causing diseases like mad cow disease and Creutzfeldt-Jakob disease), and lichens (symbiotic association between algae and fungi — algae provide food and fungi provide shelter and minerals).
Very Short Answer Type Questions (1 Mark)
Q1. Who proposed the five-kingdom system of classification?
Answer: R. H. Whittaker proposed the five-kingdom classification system in 1969.
Q2. Name the scientist who proposed the three-domain system.
Answer: Carl Woese proposed the three-domain system of classification in 1990.
Q3. What are the three domains of life?
Answer: The three domains are Archaea, Bacteria and Eukarya.
Q4. Name the smallest living cells without a cell wall.
Answer: Mycoplasma — they are the smallest known living cells and lack a cell wall.
Q5. What are methanogens?
Answer: Methanogens are archaebacteria found in marshy places and the gut of ruminants. They produce methane gas (biogas) from cattle dung.
Q6. Who discovered viroids?
Answer: T. O. Diener discovered viroids in 1971.
Q7. What is a lichen?
Answer: A lichen is a symbiotic association between an alga (phycobiont) and a fungus (mycobiont).
Q8. Name two examples of Ascomycetes.
Answer: Aspergillus and Penicillium are examples of Ascomycetes.
Q9. Which protistan group is known as the “chief producers in the oceans”?
Answer: Diatoms (Chrysophytes) are known as the chief producers in the oceans.
Q10. What are prions?
Answer: Prions are infectious proteinaceous particles that cause diseases like mad cow disease and Creutzfeldt-Jakob disease in humans.
Short Answer Type Questions (2-3 Marks)
Q1. What were the main drawbacks of the two-kingdom classification system?
Answer: The two-kingdom system proposed by Linnaeus had several drawbacks. It did not differentiate between prokaryotes and eukaryotes, unicellular and multicellular organisms, or photosynthetic (algae) and non-photosynthetic (fungi) organisms. Organisms with mixed characters such as Euglena could not be properly placed. As a result, a more comprehensive system was needed.
Q2. Differentiate between archaebacteria and eubacteria.
Answer: Archaebacteria live in extreme environments such as hot springs (thermoacidophiles), salty areas (halophiles) and marshy regions (methanogens). Their cell walls have a unique structure that helps them survive these conditions. Eubacteria, or true bacteria, are common bacteria with a rigid peptidoglycan cell wall and live in normal environmental conditions. They include both useful (like Lactobacillus) and harmful (disease-causing) bacteria.
Q3. Write a short note on cyanobacteria.
Answer: Cyanobacteria, also known as blue-green algae, are prokaryotic photosynthetic organisms containing chlorophyll-a. They are unicellular, colonial or filamentous and may be found in freshwater, marine or terrestrial habitats. Some cyanobacteria such as Nostoc and Anabaena fix atmospheric nitrogen in specialized cells called heterocysts, which makes them very important for soil fertility.
Q4. Write a short note on Euglenoids.
Answer: Euglenoids are mostly freshwater protists found in stagnant water. They lack a cell wall but have a protein-rich layer called pellicle. They have two flagella, one short and one long. In sunlight they behave as autotrophs (photosynthesis) and in absence of sunlight they behave as heterotrophs, feeding on smaller organisms. Euglena is a common example.
Q5. What are slime moulds? Give their characteristics.
Answer: Slime moulds are saprophytic protists. Under suitable conditions, they form an aggregation called plasmodium which may grow and spread over several feet. During unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls and are extremely resistant.
Q6. Distinguish between viruses and viroids.
Answer: Viruses are non-cellular infectious agents made of nucleic acid (DNA or RNA) surrounded by a protein coat called capsid. They infect plants, animals and bacteria. Viroids, on the other hand, are smaller than viruses and contain only free RNA without a protein coat. They were discovered by T. O. Diener and cause diseases like potato spindle tuber disease.
Long Answer Type Questions (5-7 Marks)
Q1. Describe the five-kingdom classification proposed by R. H. Whittaker.
Answer: R. H. Whittaker, in 1969, proposed the five-kingdom system of classification based on cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.
1. Kingdom Monera: Includes all prokaryotic organisms — bacteria, cyanobacteria, archaebacteria and mycoplasma. They have no true nucleus or membrane-bound organelles.
2. Kingdom Protista: Includes all single-celled eukaryotes such as chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans.
3. Kingdom Fungi: Heterotrophic eukaryotes with chitinous cell wall — Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes.
4. Kingdom Plantae: Multicellular, eukaryotic, autotrophic organisms with cellulose cell wall — algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
5. Kingdom Animalia: Multicellular, eukaryotic, heterotrophic organisms without cell wall, depending on holozoic nutrition.
Q2. Describe the four classes of Kingdom Fungi with examples.
Answer: Kingdom Fungi is divided into four major classes:
1. Phycomycetes: Found in aquatic habitats and on decaying wood in moist and damp places. Mycelium is aseptate and coenocytic. Asexual reproduction occurs by zoospores or aplanospores. Examples: Mucor, Rhizopus (bread mould), Albugo.
2. Ascomycetes: Commonly known as sac-fungi. Mostly multicellular (Penicillium) or rarely unicellular (yeast). Sexual spores are called ascospores produced inside sac-like asci. Examples: Aspergillus, Claviceps, Neurospora, Saccharomyces (yeast).
3. Basidiomycetes: Commonly known as club fungi. Includes mushrooms, bracket fungi, puffballs and rusts. Sexual reproduction occurs by basidiospores produced on basidia. Examples: Agaricus (mushroom), Ustilago (smut), Puccinia (rust).
4. Deuteromycetes: Also called imperfect fungi because only the asexual stage is known. Reproduction is by conidia. Examples: Alternaria, Colletotrichum, Trichoderma.
Q3. Write an essay on the Kingdom Protista.
Answer: Kingdom Protista includes all single-celled eukaryotes that exhibit a wide range of body forms and modes of nutrition. Members of Protista are mostly aquatic. They include the following major groups:
1. Chrysophytes: This group includes diatoms and golden algae (desmids). They are mostly photosynthetic. Diatoms have indestructible cell walls made of silica that form diatomaceous earth. They are the chief producers in oceans.
2. Dinoflagellates: Mostly marine and photosynthetic, appearing in different colours like yellow, green, brown, blue or red, depending on their pigments. The red dinoflagellates such as Gonyaulax cause red tides which kill marine animals.
3. Euglenoids: Mostly freshwater protists found in stagnant water. Example: Euglena. They are mixotrophic — autotrophic in light, heterotrophic in dark.
4. Slime Moulds: Saprophytic protists which form a plasmodium that may spread over several feet under favourable conditions and produce fruiting bodies in unfavourable times.
5. Protozoans: All are heterotrophs and live as predators or parasites. Examples: Amoeba (rhizopod), Paramoecium (ciliate), Trypanosoma (flagellate), Plasmodium (sporozoan, malarial parasite).
Q4. Write short notes on (a) Viruses (b) Viroids (c) Prions (d) Lichens.
Answer:
(a) Viruses: Viruses are non-cellular infectious agents that show characteristics of both living and non-living. Outside the host cell they are inert, but inside the host they reproduce. They consist of a nucleic acid (DNA or RNA) covered by a protein coat called capsid. Examples: TMV (Tobacco Mosaic Virus), HIV.
(b) Viroids: Discovered by T. O. Diener in 1971, viroids are smaller than viruses. They contain only free RNA without a protein coat. They cause diseases like potato spindle tuber disease.
(c) Prions: Prions are infectious proteinaceous particles having a similar size to that of viruses. They cause diseases such as Bovine Spongiform Encephalopathy (mad cow disease) in cattle and Creutzfeldt-Jakob disease (CJD) in humans.
(d) Lichens: Lichens are symbiotic associations between algae (phycobiont) and fungi (mycobiont). The alga provides food through photosynthesis, and the fungus provides shelter, water and minerals. Lichens are very good pollution indicators — they do not grow in polluted areas.
Q5. Describe the three-domain system of classification proposed by Carl Woese.
Answer: Carl Woese, in 1990, proposed the three-domain system of classification on the basis of phylogenetic relationships and ribosomal RNA (rRNA) sequencing. The three domains are:
1. Archaea: Includes archaebacteria — primitive prokaryotes living in extreme environments such as hot springs, salty water and anaerobic places. Examples: methanogens, halophiles, thermoacidophiles.
2. Bacteria: Includes all true bacteria (eubacteria) such as cyanobacteria, mycoplasma and other common bacteria.
3. Eukarya: Includes all eukaryotic organisms — protists, fungi, plants and animals.
This system shows that archaebacteria are more closely related to eukaryotes than to true bacteria, which was a major advancement over the earlier five-kingdom system.
Multiple Choice Questions (MCQs)
Q1. Who proposed the five-kingdom classification?
(a) Linnaeus (b) Carl Woese (c) R. H. Whittaker (d) Aristotle
Answer: (c) R. H. Whittaker
Q2. Mycoplasma differs from other bacteria in lacking:
(a) Nucleus (b) Cell wall (c) Cytoplasm (d) Ribosomes
Answer: (b) Cell wall
Q3. Which of the following is a sac fungus?
(a) Rhizopus (b) Agaricus (c) Penicillium (d) Alternaria
Answer: (c) Penicillium
Q4. Diatoms belong to which group of Protista?
(a) Chrysophytes (b) Dinoflagellates (c) Euglenoids (d) Slime moulds
Answer: (a) Chrysophytes
Q5. Red tide is caused by:
(a) Diatoms (b) Dinoflagellates (c) Euglena (d) Amoeba
Answer: (b) Dinoflagellates
Q6. Viroids were discovered by:
(a) Iwanowsky (b) Stanley (c) T. O. Diener (d) Beijerinck
Answer: (c) T. O. Diener
Q7. Which of the following is a club fungus?
(a) Mucor (b) Yeast (c) Agaricus (d) Aspergillus
Answer: (c) Agaricus
Q8. The three domains of life were proposed by:
(a) Whittaker (b) Carl Woese (c) Linnaeus (d) Haeckel
Answer: (b) Carl Woese
Q9. Which fungi are called imperfect fungi?
(a) Phycomycetes (b) Ascomycetes (c) Basidiomycetes (d) Deuteromycetes
Answer: (d) Deuteromycetes
Q10. Lichen is a symbiotic association between:
(a) Fungi and bacteria (b) Algae and fungi (c) Algae and bacteria (d) Two algae
Answer: (b) Algae and fungi
Fill in the Blanks
Q1. The five-kingdom classification was proposed by __________.
Answer: R. H. Whittaker
Q2. __________ are the smallest living cells without a cell wall.
Answer: Mycoplasma
Q3. Diatoms belong to the group __________.
Answer: Chrysophytes
Q4. Mad cow disease is caused by __________.
Answer: Prions
Q5. Lichen is a symbiotic association between algae and __________.
Answer: Fungi
True or False
Q1. Carl Woese proposed the three-domain system of classification.
Answer: True
Q2. Bacteria are eukaryotic organisms.
Answer: False (Bacteria are prokaryotic.)
Q3. Mucor and Rhizopus are examples of Phycomycetes.
Answer: True
Q4. Viroids contain both RNA and protein coat.
Answer: False (Viroids contain only free RNA, no protein coat.)
Q5. Lichens are good indicators of air pollution.
Answer: True
Glossary
| Term | Meaning |
|---|---|
| Classification | The arrangement of organisms into groups based on similarities and differences. |
| Monera | Kingdom of prokaryotic organisms — bacteria, cyanobacteria, mycoplasma, archaebacteria. |
| Protista | Kingdom of single-celled eukaryotic organisms. |
| Fungi | Heterotrophic eukaryotes with chitinous cell walls. |
| Archaebacteria | Primitive bacteria living in extreme environments. |
| Eubacteria | True bacteria with a peptidoglycan cell wall. |
| Cyanobacteria | Prokaryotic photosynthetic blue-green algae. |
| Mycoplasma | Smallest known living cells lacking a cell wall. |
| Chrysophytes | Diatoms and golden algae — chief producers of oceans. |
| Dinoflagellates | Marine photosynthetic protists; cause red tide. |
| Euglenoids | Mixotrophic freshwater protists with pellicle. |
| Slime moulds | Saprophytic protists forming plasmodium. |
| Protozoans | Heterotrophic single-celled eukaryotes. |
| Phycomycetes | Aquatic fungi with aseptate mycelium (Mucor, Rhizopus). |
| Ascomycetes | Sac fungi producing ascospores in asci. |
| Basidiomycetes | Club fungi producing basidiospores. |
| Deuteromycetes | Imperfect fungi with no known sexual stage. |
| Virus | Non-cellular infectious agent of nucleic acid and protein coat. |
| Viroid | Smaller than virus; only free RNA, no protein coat. |
| Prion | Infectious proteinaceous particle causing diseases. |
| Lichen | Symbiotic association of algae and fungi. |
| Three-Domain System | Classification by Carl Woese — Archaea, Bacteria, Eukarya. |