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Class 11 Biology NCERT Notes- Chapter 2: Biological Classification
ByAbhinav
Detailed Study Notes – Chapter 2: Biological Classification ( Class 11 Biology, Notes, PDFs, Quizzes, MCQs)
1. The Evolution of Classification Systems
Biological classification has evolved significantly over time, moving from practical, instinctual groupings to complex systems based on evolutionary relationships.
- Early Attempts: The earliest efforts to classify organisms were not scientific but were based on the need for food, shelter, and clothing.
- Aristotle’s System: Aristotle was the first to attempt a more scientific classification. He used simple morphological characters to group plants into trees, shrubs, and herbs, and divided animals into those with red blood and those without.
- Linnaeus and the Two Kingdom System: In the time of Linnaeus, a Two Kingdom system was established, comprising Kingdom Plantae and Kingdom Animalia. This system was easy to understand but proved inadequate for several reasons:
- It did not distinguish between prokaryotes and eukaryotes.
- It failed to separate unicellular and multicellular organisms.
- It grouped photosynthetic (green algae) and non-photosynthetic (fungi) organisms together.
- A large number of organisms did not fit into either kingdom.
- The Need for a New System: The shortcomings of the Two Kingdom system highlighted the need to consider other characteristics, such as cell structure, nature of the cell wall, mode of nutrition, habitat, reproduction methods, and evolutionary relationships (phylogenetics).
2. R.H. Whittaker’s Five Kingdom Classification
In 1969, R.H. Whittaker proposed the Five Kingdom Classification, which remains a foundational concept in biology. This system addresses the limitations of earlier models.
- The Five Kingdoms:
- Monera
- Protista
- Fungi
- Plantae
- Animalia
- Main Criteria for Classification:
- Cell structure (Prokaryotic vs. Eukaryotic)
- Body organization (Cellular, Tissue, Organ)
- Mode of nutrition (Autotrophic vs. Heterotrophic)
- Reproduction
- Phylogenetic relationships
Comparative Characteristics of the Five Kingdoms
| Character | Monera | Protista | Fungi | Plantae | Animalia |
| Cell type | Prokaryotic | Eukaryotic | Eukaryotic | Eukaryotic | Eukaryotic |
| Cell wall | Noncellulosic (Polysaccharide + amino acid) | Present in some | Present with chitin | Present (cellulose) | Absent |
| Nuclear membrane | Absent | Present | Present | Present | Present |
| Body organisation | Cellular | Cellular | Multicellular/ loose tissue | Tissue/ organ | Tissue/organ/ organ system |
| Mode of nutrition | Autotrophic (chemo- & photosynthetic) & Heterotrophic | Autotrophic (Photosynthetic) & Heterotrophic | Heterotrophic (Saprophytic/ Parasitic) | Autotrophic (Photosynthetic) | Heterotrophic (Holozoic/ Saprophytic) |
- The Three-Domain System: A more recent proposal divides Kingdom Monera into two domains, with all eukaryotic kingdoms placed in a third domain, resulting in a six-kingdom classification.
3. Kingdom Monera
This kingdom comprises bacteria, the most abundant microorganisms on Earth. They are found in nearly every environment, including extreme habitats like hot springs, deserts, and deep oceans.
- Structure and Shape: Bacteria have a simple structure but exhibit complex behavior and extensive metabolic diversity. They are grouped into four categories based on shape:
- Coccus: Spherical
- Bacillus: Rod-shaped
- Vibrium: Comma-shaped
- Spirillum: Spiral
- Nutrition:
- Autotrophic: Synthesize their own food. This includes photosynthetic autotrophs and chemosynthetic autotrophs.
- Heterotrophic: The vast majority, depending on other organisms or dead organic matter for food.
- Reproduction: Primarily reproduce by fission. Under unfavorable conditions, they produce spores. They can also engage in a primitive type of DNA transfer.
Sub-groups of Monera
- Archaebacteria: These bacteria are unique for their ability to survive in harsh environments. Their distinct cell wall structure is key to their survival.
- Halophiles: Live in extreme salty areas.
- Thermoacidophiles: Live in hot springs.
- Methanogens: Live in marshy areas and the guts of ruminant animals (cows, buffaloes), where they produce methane (biogas).
- Eubacteria (“True Bacteria”): Characterized by a rigid cell wall and a flagellum (if motile).
- Cyanobacteria (Blue-green algae): Contain chlorophyll a and are photosynthetic autotrophs. They can be unicellular, colonial, or filamentous and are found in various aquatic and terrestrial environments. Some form “blooms” in polluted water and can fix atmospheric nitrogen in specialized cells called heterocysts (e.g., Nostoc, Anabaena).
- Chemosynthetic Autotrophic Bacteria: Oxidize inorganic substances like nitrates, nitrites, and ammonia to produce ATP. They play a crucial role in nutrient recycling (nitrogen, phosphorous, iron, sulphur).
- Heterotrophic Bacteria: The most abundant type. Many are important decomposers. They have significant impacts on humans, being used to make curd, produce antibiotics, and fix nitrogen in legumes. Some are pathogens that cause diseases like cholera, typhoid, tetanus, and citrus canker.
- Mycoplasma: Organisms that completely lack a cell wall. They are the smallest known living cells, can survive without oxygen, and many are pathogenic in plants and animals.
4. Kingdom Protista
This kingdom includes all single-celled eukaryotes and serves as an evolutionary link to the plant, animal, and fungi kingdoms.
- General Characteristics:
- Primarily aquatic.
- Eukaryotic cell structure with a well-defined nucleus and other membrane-bound organelles.
- Some possess cilia or flagella for movement.
- Reproduce both asexually and sexually (via cell fusion and zygote formation).
Major Groups of Protista
- Chrysophytes: Includes diatoms and desmids (golden algae). They are microscopic plankton found in freshwater and marine environments.
- Diatoms: Are major photosynthetic producers (“chief producers”) in the oceans. Their cell walls are embedded with silica, making them indestructible. These walls fit together like a soapbox and accumulate over billions of years to form diatomaceous earth, which is used in polishing and filtration.
- Dinoflagellates: Mostly marine and photosynthetic organisms. Their cell walls have stiff cellulose plates. They typically have two flagella. Rapid multiplication of some species, like Gonyaulax, can cause red tides and release toxins harmful to marine life.
- Euglenoids: Mostly freshwater organisms found in stagnant water (e.g., Euglena).
- Instead of a cell wall, they have a flexible, protein-rich layer called a pellicle.
- They are photosynthetic in sunlight but behave as heterotrophic predators when deprived of light. Their pigments are identical to those of higher plants.
- Slime Moulds: Saprophytic protists. Their body moves along decaying organic material.
- Under suitable conditions, they form an aggregation called a plasmodium, which can grow to span several feet.
- In unfavorable conditions, the plasmodium forms fruiting bodies that produce extremely resistant spores.
- Protozoans: Heterotrophic organisms that live as predators or parasites, considered primitive relatives of animals.
- Amoeboid protozoans: Move and capture prey using pseudopodia (false feet), like Amoeba. Some are parasitic (e.g., Entamoeba).
- Flagellated protozoans: Have flagella for movement. Some are parasitic and cause diseases like sleeping sickness (e.g., Trypanosoma).
- Ciliated protozoans: Are aquatic and use thousands of cilia for movement. They have a cavity (gullet) to ingest food (e.g., Paramoecium).
- Sporozoans: Have an infectious, spore-like stage in their life cycle. The most notorious is Plasmodium, the parasite that causes malaria.
5. Kingdom Fungi
Fungi constitute a unique kingdom of heterotrophic organisms, displaying great diversity in morphology and habitat. They are cosmopolitan, preferring warm and humid environments.
- General Characteristics:
- Structure: Most are filamentous, composed of long, slender threads called hyphae. The network of hyphae is the mycelium. Yeasts are an exception, being unicellular.
- Hyphae Types: Can be coenocytic (continuous tubes with multinucleated cytoplasm) or septate (with cross-walls).
- Cell Wall: Composed of chitin and polysaccharides.
- Nutrition:
- Saprophytes: Absorb nutrients from dead substrates.
- Parasites: Depend on living plants and animals.
- Symbionts: Live in association with other organisms, such as with algae to form lichens or with roots of higher plants as mycorrhiza.
- Reproduction:
- Vegetative: Fragmentation, fission, and budding.
- Asexual: By spores like conidia, sporangiospores, or zoospores.
- Sexual: By oospores, ascospores, or basidiospores. The sexual cycle involves three steps:
- Plasmogamy: Fusion of protoplasm.
- Karyogamy: Fusion of two nuclei.
- Meiosis: In the zygote, resulting in haploid spores.
- Dikaryon Stage: In ascomycetes and basidiomycetes, an intervening stage (n+n) with two nuclei per cell occurs between plasmogamy and karyogamy.
Classes of Fungi
- Phycomycetes: Found in aquatic habitats and on decaying wood. The mycelium is aseptate and coenocytic. Asexual reproduction is by zoospores or aplanospores. Examples: Mucor, Rhizopus (bread mould), Albugo.
- Ascomycetes (Sac-fungi): Mostly multicellular (Penicillium) but some are unicellular (Saccharomyces or yeast). They can be saprophytic, decomposers, parasitic, or coprophilous (dung-growing). Mycelium is branched and septate. Asexual spores are conidia. Sexual spores are ascospores, produced in sacs called asci. Examples: Aspergillus, Neurospora (used in genetic work), morels, and truffles.
- Basidiomycetes: Includes mushrooms, bracket fungi, and puffballs. Mycelium is branched and septate. Asexual spores are generally not found. Sex organs are absent; plasmogamy occurs by fusion of somatic cells, leading to a dikaryotic structure that forms a basidium. Karyogamy and meiosis in the basidium produce four basidiospores. Examples: Agaricus (mushroom), Ustilago (smut), Puccinia (rust fungus).
- Deuteromycetes (Imperfect Fungi): So-called because only their asexual or vegetative phases are known. They reproduce only by conidia. Mycelium is septate and branched. Many are decomposers that help in mineral cycling. Examples: Alternaria, Colletotrichum, Trichoderma.
6. Kingdom Plantae
This kingdom includes all eukaryotic, chlorophyll-containing organisms, commonly known as plants.
- Characteristics:
- Eukaryotic cells with prominent chloroplasts.
- Cell wall is primarily made of cellulose.
- Most are autotrophic, but some are partially heterotrophic (e.g., insectivorous plants like Venus fly trap and parasites like Cuscuta).
- Life cycle exhibits alternation of generations, with a diploid sporophytic phase and a haploid gametophytic phase.
- Major Groups: Algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
7. Kingdom Animalia
This kingdom is characterized by multicellular, heterotrophic, eukaryotic organisms.
- Characteristics:
- Cells lack cell walls.
- Depend directly or indirectly on plants for food.
- Nutrition is holozoic (ingestion of food), digested in an internal cavity.
- Food reserves are stored as glycogen or fat.
- Exhibit a definite growth pattern.
- Most are capable of locomotion and have elaborate sensory and neuromotor mechanisms.
- Sexual reproduction is by copulation followed by embryological development.
8. Acellular Organisms and Lichens
These entities are not included in the Five Kingdom classification because they are not considered truly living in the cellular sense.
- Viruses:
- Non-cellular organisms with an inert, crystalline structure outside a living cell.
- Are obligate parasites; they take over the host cell’s machinery to replicate.
- The name “virus” means venom or poisonous fluid.
- Composed of a protein coat (capsid) made of subunits (capsomeres) and a core of genetic material (either RNA or DNA, but never both).
- Plant-infecting viruses often have single-stranded RNA.
- Animal-infecting viruses have single or double-stranded RNA or double-stranded DNA.
- Bacteriophages (viruses that infect bacteria) are usually double-stranded DNA viruses.
- Diseases: Mumps, smallpox, herpes, influenza, AIDS.
- Viroids:
- Discovered by T.O. Diener in 1971.
- They are smaller than viruses and consist only of free RNA with a low molecular weight.
- They lack a protein coat.
- Cause diseases like potato spindle tuber disease.
- Prions:
- Infectious agents consisting of abnormally folded proteins.
- Similar in size to viruses.
- Cause infectious neurological diseases like bovine spongiform encephalopathy (BSE or mad cow disease) in cattle and Creutzfeldt-Jakob disease (CJD) in humans.
- Lichens:
- Symbiotic, mutually useful associations between algae and fungi.
- The algal component (phycobiont) is autotrophic and prepares food.
- The fungal component (mycobiont) is heterotrophic and provides shelter, absorbs minerals, and provides water.
- They are excellent pollution indicators, as they do not grow in polluted areas.
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Knowledge Review Quiz
Short-Answer Questions
(Answer in 2-3 sentences.)
- What were the major limitations of the Two Kingdom classification system proposed in Linnaeus’s time?
- Describe the three main types of archaebacteria and the extreme environments they inhabit.
- What is “diatomaceous earth” and how is it formed?
- Explain the unique dual mode of nutrition found in Euglenoids.
- What is a dikaryon, and in which classes of fungi does this stage occur?
- Describe the structure of a virus, including its genetic material and protein coat.
- How are viroids structurally different from viruses?
- Explain the symbiotic relationship in lichens, identifying the roles of the phycobiont and mycobiont.
- What are the four main categories of bacteria based on their shape?
- Why are members of Deuteromycetes referred to as “imperfect fungi”?
Multiple-Choice Questions (MCQs)
- Who proposed the Five Kingdom Classification system in 1969? a) Aristotle b) Linnaeus c) R.H. Whittaker d) T.O. Diener
- Which of the following is NOT a criterion used in the Five Kingdom Classification? a) Cell structure b) Mode of nutrition c) Habitat d) Phylogenetic relationships
- The cell walls of fungi are composed mainly of: a) Cellulose b) Chitin c) Polysaccharide and amino acid d) Pellicle
- Bacteria that live in extreme salty areas are called: a) Methanogens b) Thermoacidophiles c) Halophiles d) Cyanobacteria
- Specialized cells called heterocysts, used for nitrogen fixation, are found in: a) Archaebacteria b) Chrysophytes c) Cyanobacteria d) Protozoans
- Which kingdom includes all single-celled eukaryotes? a) Monera b) Protista c) Fungi d) Plantae
- The phenomenon of “red tides” is caused by the rapid multiplication of: a) Diatoms b) Euglenoids c) Dinoflagellates d) Slime moulds
- The protein-rich layer that provides flexibility to the body of a Euglena is called the: a) Cell wall b) Pellicle c) Capsid d) Shell
- Which group of protozoans uses pseudopodia for movement and capturing prey? a) Ciliated protozoans b) Flagellated protozoans c) Sporozoans d) Amoeboid protozoans
- The network of thread-like hyphae that makes up the body of a fungus is known as the: a) Mycelium b) Mycorrhiza c) Plasmodium d) Ascocarp
- Coenocytic hyphae are described as: a) Branched tubes with cross-walls b) Unicellular structures c) Continuous tubes filled with multinucleated cytoplasm d) The fruiting body of a fungus
- In the sexual cycle of fungi, the fusion of two nuclei is called: a) Plasmogamy b) Meiosis c) Karyogamy d) Dikaryon
- Yeast (Saccharomyces) is an example of a unicellular fungus belonging to which class? a) Phycomycetes b) Ascomycetes c) Basidiomycetes d) Deuteromycetes
- The Kingdom Animalia is characterized by organisms that: a) Have cell walls made of cellulose b) Are autotrophic c) Lack cell walls and are heterotrophic d) Are prokaryotic
- An infectious agent consisting only of a free RNA molecule is a: a) Virus b) Viroid c) Prion d) Bacteriophage
- Bovine spongiform encephalopathy (mad cow disease) is caused by: a) Prions b) Viruses c) Bacteria d) Viroids
- In a lichen, the fungal partner is referred to as the: a) Phycobiont b) Autotroph c) Symbiont d) Mycobiont
- Mycoplasma are unique organisms because they: a) Have a cell wall made of chitin b) Completely lack a cell wall c) Can only survive with oxygen d) Are the largest living cells known
- Which of the following is a characteristic of Kingdom Plantae? a) Holozoic nutrition b) Life cycle exhibits alternation of generations c) Food stored as glycogen d) Cells lack a cell wall
- W.M. Stanley showed in 1935 that viruses: a) Could pass through bacteria-proof filters b) Were infectious living fluids c) Could be crystallized d) Lacked a protein coat
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Answer Key
Short-Answer Questions (Model Answers)
- The Two Kingdom system was inadequate because it did not distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms, or photosynthetic and non-photosynthetic organisms. Consequently, a large number of organisms, such as fungi and bacteria, did not properly fit into either the Plantae or Animalia kingdom.
- The three main types are halophiles (live in extreme salty areas), thermoacidophiles (live in hot springs), and methanogens (live in marshy areas). Their survival in these harsh habitats is attributed to a different cell wall structure compared to other bacteria.
- Diatomaceous earth is a large accumulation of cell wall deposits from diatoms. It forms over billions of years because the diatom cell walls are embedded with indestructible silica.
- Euglenoids exhibit a mixotrophic mode of nutrition. In the presence of sunlight, they are photosynthetic autotrophs, but when deprived of sunlight, they behave like heterotrophs by predating on other smaller organisms.
- A dikaryon is an intervening stage in the fungal sexual cycle where a cell contains two nuclei (n+n) following plasmogamy but before karyogamy. This condition occurs in the Ascomycetes and Basidiomycetes classes of fungi.
- A virus is a non-cellular particle composed of a protein coat, called a capsid, which protects its genetic material. This genetic material can be either RNA or DNA (single or double-stranded), but no virus contains both.
- Viroids are simpler and smaller than viruses. The key difference is that a viroid is an infectious agent made only of a free RNA molecule and completely lacks the protein coat (capsid) that is characteristic of viruses.
- Lichens are a symbiotic association between algae (phycobiont) and fungi (mycobiont). The phycobiont is autotrophic and produces food through photosynthesis, while the mycobiont is heterotrophic and provides shelter, water, and mineral absorption for its partner.
- The four main categories of bacteria based on shape are the spherical Coccus (pl. cocci), the rod-shaped Bacillus (pl. bacilli), the comma-shaped Vibrium (pl. vibrio), and the spiral Spirillum (pl. spirilla).
- Members of Deuteromycetes are called “imperfect fungi” because only their asexual or vegetative reproductive phases are known. When the sexual stages (the “perfect” stage) of these fungi are discovered, they are correctly identified and moved into other classes like Ascomycetes or Basidiomycetes.
Multiple-Choice Questions (MCQs)
- c) R.H. Whittaker
- c) Habitat
- b) Chitin
- c) Halophiles
- c) Cyanobacteria
- b) Protista
- c) Dinoflagellates
- b) Pellicle
- d) Amoeboid protozoans
- a) Mycelium
- c) Continuous tubes filled with multinucleated cytoplasm
- c) Karyogamy
- b) Ascomycetes
- c) Lack cell walls and are heterotrophic
- b) Viroid
- a) Prions
- d) Mycobiont
- b) Completely lack a cell wall
- b) Life cycle exhibits alternation of generations
- c) Could be crystallized
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Suggested Essay Questions
- Trace the historical development of biological classification from Aristotle’s early attempts to R.H. Whittaker’s Five Kingdom system. Discuss the key criteria and scientific understanding that prompted each major shift in classification philosophy.
- Provide a comprehensive comparison of Kingdom Monera and Kingdom Protista. Your answer should detail differences and similarities in cell structure, body organization, habitat, nutrition, and representative groups within each kingdom.
- Describe the four major classes of Kingdom Fungi (Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes). Compare and contrast their mycelium structure, modes of asexual and sexual reproduction, and provide examples of organisms from each class, including their ecological or economic importance.
- Explain why viruses, viroids, and prions are not included in the Five Kingdom classification system. Detail the structure, nature of genetic material (if any), and mode of replication for each, and name a disease caused by each type of agent.
- Discuss the vast metabolic diversity within Kingdom Monera. Elaborate on the different modes of nutrition (photosynthetic, chemosynthetic, and heterotrophic) and provide specific examples of bacteria for each, explaining their role in various ecosystems.
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Glossary of Key Terms
- Alternation of Generations: A phenomenon in the life cycle of plants where a diploid sporophytic phase and a haploid gametophytic phase alternate with each other.
- Archaebacteria: A group of bacteria that live in harsh habitats and differ from other bacteria in their cell wall structure.
- Ascospores: Sexual spores produced endogenously in sac-like asci, characteristic of Ascomycetes.
- Autotrophic: Describes organisms that can synthesize their own food from inorganic substrates.
- Bacillus: A rod-shaped bacterium.
- Bacteriophage: A virus that infects bacteria.
- Basidiospores: Sexual spores produced exogenously on a club-shaped structure called a basidium, characteristic of Basidiomycetes.
- Capsid: The protein coat of a virus that protects the nucleic acid.
- Capsomeres: The small subunits that make up the capsid of a virus.
- Chemosynthetic: A type of autotrophic nutrition where organisms use energy released from oxidizing inorganic substances to produce food.
- Chitin: The primary component of the cell walls of fungi.
- Coccus: A spherical bacterium.
- Coenocytic hyphae: Fungal hyphae that are continuous tubes filled with multinucleated cytoplasm, lacking septae.
- Diatomaceous earth: An accumulation of cell wall deposits from diatoms, which are embedded with silica.
- Dikaryon: A condition in some fungi where a cell contains two nuclei (n + n) after plasmogamy but before karyogamy.
- Eukaryotic: A cell or organism possessing a true nucleus and other membrane-bound organelles.
- Fission: The main method of reproduction in bacteria, where a cell divides into two.
- Heterocysts: Specialized cells in some cyanobacteria (e.g., Nostoc) that can fix atmospheric nitrogen.
- Heterotrophic: Describes organisms that depend on other organisms or dead organic matter for food.
- Holozoic: A mode of nutrition involving the ingestion of food, characteristic of Kingdom Animalia.
- Hyphae: Long, slender, thread-like structures that form the body of a fungus.
- Karyogamy: The fusion of two nuclei, a step in the fungal sexual cycle.
- Lichen: A symbiotic association between algae (phycobiont) and fungi (mycobiont).
- Mycelium: The network of hyphae that constitutes the body of a fungus.
- Mycobiont: The fungal component of a lichen.
- Mycoplasma: Organisms that completely lack a cell wall; the smallest known living cells.
- Mycorrhiza: A symbiotic association between a fungus and the roots of higher plants.
- Pellicle: A protein-rich layer that makes the body of euglenoids flexible.
- Phycobiont: The algal component of a lichen.
- Phylogenetic: Pertaining to the evolutionary relationships between organisms.
- Plankton: Microscopic organisms that float passively in water currents.
- Plasmodium: An aggregation of slime moulds that may grow and spread over several feet.
- Plasmogamy: The fusion of protoplasm between two gametes, a step in the fungal sexual cycle.
- Prion: An infectious agent consisting of an abnormally folded protein.
- Prokaryotic: A cell or organism lacking a true nucleus and other membrane-bound organelles.
- Pseudopodia: “False feet”; temporary projections of the cell used by amoeboid protozoans for movement and feeding.
- Saprophyte: An organism that absorbs soluble organic matter from dead substrates.
- Septate: Having cross-walls (septae) in the hyphae.
- Spirillum: A spiral-shaped bacterium.
- Vibrium: A comma-shaped bacterium.
- Viroid: An infectious agent smaller than a virus, consisting of a free RNA molecule without a protein coat.
- Virus: A non-cellular, obligate parasite with an inert crystalline structure outside a living cell, composed of a protein coat and nucleic acid.