Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Chapter Introduction
Biological Classification is Chapter 2 of CBSE Class 11 Biology and belongs to the Diversity in the Living World unit. This chapter moves beyond the two-kingdom (Plantae and Animalia) and five-kingdom classification systems and examines each kingdom in detail. You will also learn about acellular infectious agents — viruses, viroids, prions, and the symbiotic association called lichens.
Understanding classification is essential because it determines how we study, treat, and communicate about organisms. The five-kingdom system by Whittaker is the foundation for all subsequent chapters in your Biology syllabus.
Key Definitions
| Term | Definition |
|---|---|
| Biological classification | The scientific procedure of arranging organisms into groups and sub-groups based on their similarities, differences, and evolutionary relationships. |
| Kingdom Monera | Kingdom of prokaryotic organisms — bacteria, cyanobacteria, mycoplasma. |
| Kingdom Protista | Kingdom of eukaryotic, primarily unicellular organisms — chrysophytes, dinoflagellates, euglenoids, slime moulds, protozoans. |
| Kingdom Fungi | Kingdom of heterotrophic, eukaryotic organisms with chitinous cell walls that reproduce by spores. |
| Kingdom Plantae | Kingdom of eukaryotic, multicellular, autotrophic organisms with cellulose cell walls. |
| Kingdom Animalia | Kingdom of eukaryotic, multicellular, heterotrophic organisms without cell walls. |
| Virus | An infectious agent with a protein coat (capsid) enclosing either RNA or DNA, but never both. |
| Viroid | An infectious agent consisting of only a short, free RNA without a protein coat. Discovered by T.O. Diener. |
| Prion | An infectious proteinaceous particle with no nucleic acid. Causes neurodegenerative diseases. |
| Lichen | A symbiotic association between a fungus and an alga or cyanobacterium. |
Detailed Concepts
1. Evolution of Classification Systems
| System | Proposed By | Kingdoms | Basis |
|---|---|---|---|
| Two-kingdom | Ancient / Linnaeus | Plantae, Animalia | Presence or absence of cell wall, mode of nutrition |
| Five-kingdom | R.H. Whittaker (1969) | Monera, Protista, Fungi, Plantae, Animalia | Cell structure, body organisation, mode of nutrition, reproduction, phylogenetic relationships |
Problems with the two-kingdom system:
- It placed fungi (heterotrophic) and photosynthetic plants together.
- It grouped unicellular and multicellular organisms together.
- It did not distinguish between prokaryotes and eukaryotes.
- It grouped chlamydomonas (unicellular) and chlamydobacteria (prokaryotic) together with higher plants.
2. Five Kingdoms — Comparison Table
| Feature | Monera | Protista | Fungi | Plantae | Animalia |
|---|---|---|---|---|---|
| Cell type | Prokaryotic | Eukaryotic | Eukaryotic | Eukaryotic | Eukaryotic |
| Cell wall | Present (peptidoglycan) | Present in some | Present (chitin) | Present (cellulose) | Absent |
| Body organisation | Unicellular | Primarily unicellular | Multicellular/unicellular | Multicellular | Multicellular |
| Mode of nutrition | Autotrophic or heterotrophic | Autotrophic or heterotrophic | Heterotrophic (saprophytic/parasitic) | Autotrophic (photosynthetic) | Heterotrophic (holozoic) |
| Reproduction | Asexual (fission, spores) | Asexual and sexual | Asexual (spores), sexual in some | Asexual and sexual | Primarily sexual |
| Examples | Bacteria, Cyanobacteria, Mycoplasma | Amoeba, Euglena, Paramecium | Mushroom, Rhizopus, Penicillium | Mango, Fern, Moss | Human, Fish, Insects |
3. Kingdom Monera
Monera includes prokaryotes — organisms lacking a true nucleus and membrane-bound organelles.
Archaebacteria:
- Live in the harshest habitats (halophiles in salty areas, thermophiles in hot springs, methanogens in marshy areas).
- Cell wall lacks peptidoglycan; instead has pseudomurein.
- Methanogens are found in the guts of ruminants (cows, buffaloes) and produce methane.
Eubacteria (True Bacteria):
- Cell wall has peptidoglycan.
- Cyanobacteria (Blue-green algae): Photosynthetic, have chlorophyll-a. Colonies are surrounded by a gelatinous sheath. Some fix atmospheric nitrogen in specialised cells called heterocysts (e.g., Nostoc, Anabaena).
- Chemosynthetic bacteria: Oxidise inorganic substances (nitrates, nitrites, ammonia) for energy. Important in nutrient recycling.
- Heterotrophic bacteria: Decomposers, some are pathogens (cholera, typhoid, tetanus). Helpful ones make antibiotics, fix nitrogen, or are used in curd making (Lactobacillus).
- Reproduction: primarily by fission; some by spores; sexual reproduction by DNA transfer (conjugation).
Mycoplasma:
- Smallest living cells known.
- Lack a cell wall entirely.
- Can survive without oxygen.
- Many are pathogenic in animals and plants.
4. Kingdom Protista
Protista includes all eukaryotic, primarily unicellular organisms. They form a link with plants, animals, and fungi.
| Group | Key Features | Examples |
|---|---|---|
| Chrysophytes | Diatoms and golden algae; cell walls of diatoms have silica forming two overlapping halves (epitheca and hypotheca); major producers in oceans; diatomaceous earth used in polishing, filtration | Diatoms, Desmids |
| Dinoflagellates | Marine, photosynthetic; appear yellow, green, brown, blue, or red depending on pigments; cell wall has stiff cellulose plates; two flagella (longitudinal and transverse); cause red tides (e.g., Gonyaulax — releases toxins killing fish) | Gonyaulax, Noctiluca |
| Euglenoids | Freshwater; lack cell wall but have a protein-rich pellicle; photosynthetic in light (chlorophyll present), heterotrophic in dark; flagella for movement; flexible body | Euglena |
| Slime Moulds | Saprophytic; body is a plasmodium (multinucleate mass of protoplasm); under unfavourable conditions, form spores with cellulose walls; resistant to adverse conditions | Physarum, Fuligo |
| Protozoans | Heterotrophic; all are parasites or predators; four groups: (1) Amoeboid — pseudopodia, e.g., Amoeba; (2) Flagellated — flagella, e.g., Trypanosoma; (3) Ciliated — cilia, e.g., Paramoecium; (4) Sporozoans — spore-like stage, e.g., Plasmodium (malaria) | Amoeba, Trypanosoma, Paramoecium, Plasmodium |
5. Kingdom Fungi
Fungi are heterotrophic eukaryotes with chitinous cell walls. They obtain nutrition by absorption (extracellular digestion followed by absorption). They can be:
- Saprophytes: Derive nutrition from dead and decaying matter.
- Parasites: Live on or in a host and cause disease.
- Symbionts: Live in mutually beneficial associations (e.g., lichens, mycorrhiza).
The body of a fungus is a mycelium made of thread-like hyphae. Hyphae may be aseptate (coenocytic, multinucleate) or septate (divided by cross-walls).
Classes of Fungi — Comparison Table:
| Feature | Phycomycetes | Ascomycetes | Basidiomycetes | Deuteromycetes |
|---|---|---|---|---|
| Hyphae | Aseptate, coenocytic | Septate | Septate, dolipore septa | Septate |
| Asexual spores | Zoospores or aplanospores | Conidia | Not typical | Conidia |
| Sexual spores | Zygospore (by gametangial contact) | Ascospores (in asci/sac-like structures) | Basidiospores (on basidia/club-shaped structures) | Not known (absent) |
| Fruiting body | Not conspicuous | Ascocarp | Basidiocarp | Not known |
| Examples | Mucor, Rhizopus, Albugo | Penicillium, Aspergillus, Neurospora, Claviceps, Yeast (Saccharomyces) | Agaricus (mushroom), Ustilago (smut), Puccinia (rust) | Alternaria, Colletotrichum, Trichoderma |
Note: Yeast is unicellular ascomycete. Neurospora is used extensively in biochemical and genetic research. Morels and truffles are edible ascomycetes.
6. Kingdom Plantae and Kingdom Animalia
- Plantae: Eukaryotic, chlorophyll-containing, autotrophic organisms with cellulose cell walls. Includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms. (Detailed in Chapter 3.)
- Animalia: Eukaryotic, multicellular, heterotrophic organisms without cell walls. (Detailed in Chapter 4.)
7. Viruses, Viroids, Prions, and Lichens
Viruses:
- Not considered truly living — they are obligate parasites, inert outside a host.
- Consist of a protein coat (capsid) enclosing genetic material (RNA or DNA, never both).
- Capsid is made of smaller units called capsomeres.
- RNA viruses: Retroviruses (HIV), TMV (Tobacco Mosaic Virus), Influenza virus.
- DNA viruses: Bacteriophages (T4), Herpes virus.
- Viruses infect bacteria (bacteriophages), plants (TMV, cauliflower mosaic), and animals (HIV, flu, polio).
- Discovered by Dmitri Ivanowsky (1892) — showed filterable agents cause mosaic disease in tobacco. Named "virus" by Beijerinck (contagium vivum fluidum). Crystallised by Stanley (1935).
Virus vs Viroid — Comparison Table:
| Feature | Virus | Viroid |
|---|---|---|
| Size | Larger | Smaller than viruses |
| Genetic material | RNA or DNA (never both) | Only free RNA (circular) |
| Protein coat | Present (capsid made of capsomeres) | Absent |
| Infects | Plants, animals, bacteria | Only plants |
| Discovery | Ivanowsky (1892) | T.O. Diener (1971) |
| Disease caused | AIDS, flu, polio, mosaic disease, etc. | Potato spindle tuber disease |
| Examples | TMV, HIV, Bacteriophage T4 | Potato spindle tuber viroid (PSTVd) |
Viroids:
- Discovered by T.O. Diener (1971).
- Smaller than viruses; consist only of a short, free, circular RNA — no protein coat.
- Cause potato spindle tuber disease.
- Much smaller than viruses.
Prions:
- Infectious protein particles with no genetic material (no nucleic acid).
- Cause BSE (bovine spongiform encephalopathy / mad cow disease), Creutzfeldt-Jakob disease in humans, scrapie in sheep.
- Extremely difficult to destroy — resist heat, UV, and chemicals.
Lichens:
- Symbiotic association of a fungus (mycobiont) and an alga or cyanobacterium (phycobiont).
- Fungus provides structure, minerals, and water; alga provides food through photosynthesis.
- Do not grow in polluted areas — act as bio-indicators of air quality.
- Common in tundra regions; important in initiating ecological succession on bare rocks.
- Three types of lichens: Crustose (crust-like, closely attached), Foliose (leaf-like), Fruticose (shrub-like, branching).
8. Economic Importance of Each Kingdom
| Kingdom | Beneficial Uses | Harmful Effects |
|---|---|---|
| Monera | Nitrogen fixation (Rhizobium), curd making (Lactobacillus), antibiotic production (Streptomyces), sewage treatment, biogas production (methanogens) | Diseases — cholera, typhoid, tetanus, tuberculosis; food spoilage |
| Protista | Diatoms — diatomaceous earth for polishing and filtration; plankton — base of aquatic food chains; Euglena — research on nutrition | Diseases — malaria (Plasmodium), sleeping sickness (Trypanosoma), amoebic dysentery (Entamoeba); red tides (Gonyaulax toxins) |
| Fungi | Antibiotics (Penicillium), food (mushrooms, truffles, morels), fermentation (yeast), ripening of cheese (Penicillium roqueforti), vitamin B production | Diseases — ringworm, athlete's foot, rust and smut of crops; food spoilage (bread mould); aflatoxins (Aspergillus flavus) |
| Plantae | Food, medicines, timber, fibre, oxygen, carbon fixation | Weeds, allergenic pollen |
| Animalia | Food, pollination, silk, lac, labour, companionship | Diseases, crop pests, parasites |
9. Scientist Timeline — Key Discoveries
| Scientist | Year | Contribution |
|---|---|---|
| Aristotle | 4th century BCE | Early classification of animals |
| Linnaeus | 1753 | Two-kingdom system; binomial nomenclature |
| Ernst Haeckel | 1866 | Three-kingdom system (added Protista) |
| Copeland | 1938 | Four-kingdom system (added Monera) |
| R.H. Whittaker | 1969 | Five-kingdom classification |
| Dmitri Ivanowsky | 1892 | Discovered viruses (filterable agent causing mosaic disease) |
| M.W. Beijerinck | 1898 | Named "virus" (contagium vivum fluidum) |
| W.M. Stanley | 1935 | Crystallised TMV |
| T.O. Diener | 1971 | Discovered viroids |
| Carl Woese | 1990 | Three-domain system based on rRNA |
| C.B. van Niel | 1941 | Proposed classification based on cell type |
Important Diagrams and Labelling
1. Structure of a Virus (Bacteriophage): Draw the head (protein coat enclosing DNA), collar, tail sheath, tail fibres, and base plate. Label each part.
2. TMV (Tobacco Mosaic Virus): Draw the rod-shaped structure showing the RNA helix surrounded by capsomeres arranged in a helical pattern.
3. Structure of a Viroid: Draw a simple circular RNA molecule (no protein coat) to contrast with virus structure.
4. Five-Kingdom System Diagram: Draw a branching diagram or table showing the five kingdoms branching from a common ancestor, with key features listed.
Common Mistakes to Avoid
| Mistake | Correction |
|---|---|
| Saying viruses are living organisms | Viruses are obligate parasites and inert outside a host; they are not considered truly living |
| Confusing viroids and viruses | Viroids have no protein coat (only free RNA); viruses have a protein coat (capsid) enclosing genetic material |
| Saying mycoplasma has a cell wall | Mycoplasma lacks a cell wall entirely — they are the smallest cells without a wall |
| Confusing cyanobacteria with algae | Cyanobacteria are prokaryotic (Monera); algae are eukaryotic (Protista or Plantae) |
| Listing yeast under Basidiomycetes | Yeast (Saccharomyces) is an Ascomycete |
| Saying Deuteromycetes have sexual reproduction | Deuteromycetes are called "fungi imperfecti" because sexual reproduction is not observed |
| Saying prions contain nucleic acid | Prions are purely protein with no nucleic acid |
| Forgetting that dinoflagellates cause red tides | Gonyaulax releases toxins that kill fish and cause red tides |
| Confusing archaebacteria and eubacteria | Archaebacteria lack peptidoglycan in cell walls; eubacteria have peptidoglycan |
Exam Tips
1. For 1-mark questions: Know the scientist names — Whittaker (five-kingdom), Ivanowsky (virus discovery), Beijerinck (named "virus"), Stanley (crystallised TMV), Diener (viroids). Know which kingdom is prokaryotic (Monera). 2. For 2-mark questions: Differentiate between any two kingdoms, or between virus and viroid, or between archaebacteria and eubacteria. 3. For 3-mark questions: Write about a kingdom in detail (Monera, Protista, or Fungi) with sub-groups and examples. Or explain the five-kingdom basis of classification. 4. For 5-mark questions: Prepare the entire Fungi classes comparison table with examples. Or write about viruses, viroids, and prions together. Or discuss the Protista sub-groups with features and examples. 5. Draw diagrams: Bacteriophage structure, TMV helical structure, and the five-kingdom classification tree — these carry presentation marks. 6. Use NCERT language: Write "contagium vivum fluidum," "obligate parasites," "fungi imperfecti," "coenocytic," "dolipore septa" — these keywords score marks.
NCERT Exercise Questions — How to Approach
| NCERT Question Type | Strategy |
|---|---|
| "Discuss how classification systems have changed over time" | Start with two-kingdom, describe its problems, then explain Whittaker's five-kingdom system and its basis. Mention Woese's three-domain system as a further refinement. |
| "What is the nature of cell walls in diatoms?" | State that diatom cell walls are made of silica (not cellulose). The two halves are called epitheca and hypotheca. They form diatomaceous earth. |
| "Organise a discussion on whether viruses are living or non-living" | Present both sides — living: they replicate, have genetic material, mutate. Non-living: no cellular organisation, inert outside host, can be crystallised. Conclude they are on the borderline. |
| "What are the major divisions of Kingdom Plantae?" | Algae, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms — based on differentiation of plant body, vascular tissue, and seed production. |
Practice Questions
1. Differentiate between the two-kingdom and five-kingdom systems of classification. 2. Describe the five groups of Kingdom Protista with one example each. 3. Compare the four classes of Kingdom Fungi in a tabular form. 4. How are viroids different from viruses? Who discovered viroids? 5. What are prions? Name two diseases caused by prions. 6. Explain the economic importance of Kingdom Fungi with at least four examples. 7. Why are lichens called bio-indicators? 8. Describe the structure of a bacteriophage with a labelled diagram. 9. What is the difference between archaebacteria and eubacteria? 10. Why was the two-kingdom system of classification found inadequate?
Quick Revision Notes
- Whittaker (1969): Five kingdoms — Monera, Protista, Fungi, Plantae, Animalia. Based on cell structure, body organisation, nutrition, reproduction, phylogeny.
- Monera: Prokaryotic. Archaebacteria (harsh habitats, methanogens), Eubacteria (cyanobacteria, heterotrophic bacteria), Mycoplasma (no cell wall, smallest cells).
- Protista: Eukaryotic, unicellular. Five groups — Chrysophytes (diatoms, silica walls), Dinoflagellates (red tides, Gonyaulax), Euglenoids (pellicle, photosynthetic + heterotrophic), Slime moulds (plasmodium, spores), Protozoans (Amoeba, Paramoecium, Plasmodium).
- Fungi: Heterotrophic, chitinous wall, hyphae, mycelium. Four classes — Phycomycetes (aseptate), Ascomycetes (asci, conidia), Basidiomycetes (basidia, mushrooms), Deuteromycetes (no sexual reproduction).
- Viruses: Protein coat + DNA or RNA. Ivanowsky discovered; Beijerinck named; Stanley crystallised TMV.
- Viroids: Free RNA, no protein coat. Diener discovered. Cause potato spindle tuber.
- Prions: Protein only, no nucleic acid. Cause BSE, Creutzfeldt-Jakob, scrapie.
- Lichens: Fungus + alga/cyanobacterium symbiosis. Bio-indicators of pollution. Do not grow in polluted areas.
10. Monera vs Protista vs Fungi — Quick Comparison
| Feature | Monera | Protista | Fungi |
|---|---|---|---|
| Cell type | Prokaryotic | Eukaryotic | Eukaryotic |
| Nucleus | Absent (nucleoid) | Present (true nucleus) | Present (true nucleus) |
| Cell wall | Peptidoglycan (or absent in Mycoplasma) | Present in some; absent in others | Chitin |
| Nutrition | Autotrophic or heterotrophic | Autotrophic or heterotrophic | Heterotrophic only (saprophytic/parasitic/symbiotic) |
| Organisation | Unicellular | Primarily unicellular | Unicellular (yeast) or multicellular (mycelium) |
| Reproduction | Fission, spores | Asexual and sexual | Spores (asexual + sexual in some) |
| Unique feature | Smallest cells (Mycoplasma); nitrogen fixers | Link between plants, animals, fungi; eukaryotic unicells | Hyphae and mycelium; cell wall of chitin |
11. Why Viruses Are on the Borderline of Life
This is a frequently asked exam question. Present arguments from both sides:
Features suggesting viruses are living:
- They contain genetic material (DNA or RNA).
- They replicate (inside a host cell).
- They mutate and evolve.
- They can be transmitted from one host to another.
Features suggesting viruses are non-living:
- No cellular organisation (no cytoplasm, organelles).
- Inert outside a living host — cannot carry out metabolism independently.
- Can be crystallised and stored like chemicals (Stanley, 1935).
- No energy-producing system of their own.
- Do not grow or reproduce outside a host.
Conclusion: Viruses are considered obligate intracellular parasites that exist on the borderline between living and non-living. They are not placed in any of the five kingdoms.