NCERT Solutions for Class 11 Biology Chapter 2: Biological Classification

Early attempts at classification were based on simple morphological characters, habits, and habitats. This was called artificial classification. The main difficulties encountered were:

1. Equal Weightage to All Characters: Early systems often gave equal importance to both vegetative (like leaf shape, number) and sexual (like stamen structure) characters. This was problematic because vegetative characters are more easily affected by the environment and are less stable.
2. Convergent Evolution: Unrelated organisms often develop similar features because they adapt to similar environments or ways of life. For example, the streamlined body shape of a dolphin (a mammal) and a shark (a fish). Classifying them together based on this single character would be incorrect.
3. Inclusion of Diverse Organisms: Early systems placed vastly different organisms in the same group. For example, the two-kingdom classification (Plantae and Animalia) put bacteria, fungi, and algae under 'Plants' just because they had a cell wall, ignoring major differences in cell structure, nutrition, and organisation.
4. Exclusion of Key Features: Important characteristics like cell structure (prokaryotic vs. eukaryotic), mode of nutrition, and evolutionary relationships (phylogeny) were not considered, leading to an unnatural and unscientific system.

To classify a new specimen, I would follow these systematic steps, moving from general to more specific characteristics:

1. Determine Cell Structure: First, I would observe the cell(s) under a microscope to determine if it is prokaryotic (lacks a defined nucleus) or eukaryotic (has a well-defined nucleus). This would place it in either Kingdom Monera or one of the other four kingdoms.
2. Determine Body Organisation: Next, I would check if the organism is unicellular (single-celled) or multicellular (made of many cells). If it's unicellular and eukaryotic, it would likely belong to Kingdom Protista.
3. Check for Cell Wall: I would test for the presence or absence of a cell wall. If a cell wall is present, I would analyze its chemical composition (e.g., chitin in fungi, cellulose in plants). If there is no cell wall and it's a multicellular eukaryote, it belongs to Kingdom Animalia.
4. Determine Mode of Nutrition: I would observe how the organism obtains its food.
   • Autotrophic (Photosynthetic): If it has chlorophyll and makes its own food, it belongs to Kingdom Plantae (or could be a photosynthetic protist).
   • Heterotrophic: If it is Saprophytic or Parasitic, it is likely Kingdom Fungi. If it is Holozoic (ingests food), it belongs to Kingdom Animalia.
5. Examine Other Features: Finally, I would look at other characteristics like method of reproduction (sexual/asexual), motility (presence of flagella), and habitat to pinpoint its exact group within the kingdom.

When choosing between two characteristics for classification, the following principles are followed:

1. Fundamental vs. Superficial Characters: We should choose fundamental characteristics over superficial ones. For example, the presence or absence of a nucleus (a fundamental feature) is a much better basis for classification than the presence or absence of wings (a superficial feature).
2. Stable vs. Variable Characters: We should prefer characters that are stable and not easily influenced by the environment. For instance, the structure of the flower (a reproductive character) is more stable than the height of the plant (a vegetative character).
3. Homologous vs. Analogous Characters: Classification should be based on homologous characters (those that have a common evolutionary origin but may perform different functions, like the forelimbs of a man, whale, and bat). Analogous characters (those with different origins but similar functions, like the wings of a bird and an insect) can be misleading and should be avoided.
4. Qualitative vs. Quantitative Characters: Qualitative characteristics (like the presence or absence of a feature) are often more reliable than quantitative ones (like the number of leaves), which can vary.

Therefore, I would choose the characteristic that is more fundamental, stable, and reflects a common evolutionary ancestry.

The five-kingdom classification was proposed by R.H. Whittaker in 1969. The main criteria for this classification are:

1. Cell Structure: Whether the cells are Prokaryotic (as in Kingdom Monera) or Eukaryotic (as in the other four kingdoms).
2. Body (Thallus) Organisation: Whether the organism is unicellular (Monera, Protista) or multicellular (Fungi, Plantae, Animalia).
3. Mode of Nutrition: How the organism obtains its energy.
   • Autotrophic: Photosynthesis (Plantae).
   • Heterotrophic: Saprophytic (most Fungi), Parasitic (some Fungi, Bacteria), or Holozoic (Animalia).
4. Reproduction: The methods used for reproduction, whether asexual or sexual.
5. Phylogenetic Relationships: The evolutionary history and relationships between different groups of organisms. This was a key addition that made Whittaker's system superior to previous ones.

Protozoans are unicellular, eukaryotic heterotrophs grouped based on their locomotory structures.

1. Amoeboid Protozoans: They move and capture prey using pseudopodia (false feet). Example: Amoeba. The parasitic form Entamoeba causes amoebic dysentery.
2. Flagellated Protozoans: They have one or more whip-like flagella for movement. Example: Trypanosoma causes sleeping sickness.
3. Ciliated Protozoans: They are actively moving due to thousands of hair-like cilia. They have a cavity called the gullet for feeding. Example: Paramecium.
4. Sporozoans: They are exclusively endoparasites and lack specific locomotory structures. Their life cycle involves an infectious, spore-like stage. Example: Plasmodium (the malarial parasite).

Yes, some plants are partially heterotrophic to supplement their nutritional requirements, especially in nitrogen-deficient soil.

1. Insectivorous Plants: These plants trap and digest insects to obtain nitrogen. They are photosynthetic.
   • Examples: Pitcher Plant (Nepenthes), Venus Flytrap (Dionaea), Bladderwort (Utricularia).
2. Parasitic Plants: These plants derive nutrition from a host plant.
   • Example: Cuscuta (Amarbel) is a total stem parasite that lacks chlorophyll and absorbs nutrients using haustoria.

The terms phycobiont and mycobiont refer to the two components of a Lichen, which is an example of symbiosis.

• Phycobiont: This is the algal component. It is autotrophic and performs photosynthesis to produce food for both partners.
• Mycobiont: This is the fungal component. It provides structure, shelter, and protection, and absorbs mineral nutrients and water for the alga.

The key features of Euglenoids (e.g., Euglena) are:

1. Habitat: Majority are found in stagnant freshwater.
2. Cell Wall: Instead of a cell wall, they have a flexible, protein-rich outer layer called the pellicle.
3. Flagella: They have two flagella (one short, one long) for locomotion.
4. Mode of Nutrition (Mixotrophic): They are photosynthetic in sunlight but act as heterotrophs in the dark.
5. Pigments: Photosynthetic pigments are chlorophyll a and b, similar to higher plants.
6. Storage: Carbohydrates are stored as paramylon granules.
7. Reproduction: Reproduce asexually by longitudinal binary fission.

Structure of Viruses:

• Genetic Material: The core contains either DNA or RNA, but never both. It can be single-stranded (ss) or double-stranded (ds).
• Capsid: The genetic material is protected by an outer protein coat called the capsid, made of subunits called capsomeres.
• Envelope: Some animal viruses have an additional outer lipid layer called an envelope.

Nature of Viruses:

Viruses are obligate intracellular parasites. They are inert outside a living host cell and can only replicate by taking over the host's cellular machinery.

Four Common Viral Diseases:

1. Common Cold
2. Influenza (Flu)
3. AIDS (Acquired Immuno Deficiency Syndrome)
4. COVID-19

Lichens are very sensitive to air pollutants, especially sulfur dioxide (SO₂). They readily absorb substances from the atmosphere. High levels of SO₂ damage their chlorophyll and inhibit photosynthesis, leading to their death. Therefore, the absence of lichens in an area is a strong indicator of high air pollution.

The five-kingdom classification was proposed by R.H. Whittaker in 1969. The one major criterion he introduced that was absent in Linnaeus's two-kingdom classification was phylogenetic relationships (evolutionary history). Other new criteria included cell structure, body organisation, and mode of nutrition.

Rusts and smuts are diseases in plants caused by parasitic fungi. They appear as rusty or sooty black spots/powders on the leaves and stems of host plants, like wheat. They belong to the class Basidiomycetes. Example: Puccinia (rust fungus).

(Guidance: Students should draw a diagram showing the key parts of a bacteriophage: a polyhedral Head containing DNA, a Collar, a hollow Sheath, a Base Plate, and Tail Fibres.)

Key Characteristics of Kingdom Monera:

• Cell Type: They are prokaryotic, lacking a true nucleus and membrane-bound organelles.
• Unicellular: They are unicellular organisms, though some may form colonies or filaments.
• Cell Wall: The cell wall is generally present and made of peptidoglycan (except in archaebacteria).
• Nutrition: They show the most extensive metabolic diversity. They can be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic or parasitic).
• Genetic Material: A single, circular chromosome (naked DNA) is present in a region called the nucleoid.
• Reproduction: Mainly reproduce asexually by binary fission. Sexual reproduction occurs through a primitive method of DNA transfer (conjugation).

Examples of economically important bacteria:

1. Lactobacillus: Used in the dairy industry to convert milk into curd and cheese.
2. Rhizobium: A nitrogen-fixing bacterium that lives in the root nodules of leguminous plants, enriching the soil with nitrogen.

The lytic cycle is one of the two main cycles of viral replication, which results in the death of the host cell. The steps are as follows:

1. Attachment (Adsorption): The virus attaches itself to a specific receptor site on the host cell's surface.
2. Penetration (Injection): The virus injects its genetic material (DNA or RNA) into the host cell's cytoplasm, leaving the capsid outside.
3. Synthesis (Replication): The viral genetic material takes control of the host cell's machinery and forces it to produce viral components—viral DNA/RNA and viral proteins.
4. Assembly (Maturation): The newly synthesized viral components are assembled into new virus particles (virions) inside the host cell.
5. Lysis (Release): The virus produces an enzyme that breaks down (lyses) the host cell wall, causing it to burst. The new viruses are released and are free to infect other nearby cells.

Based on the observation of *Rhizopus* on stale bread:

1. What are the thread-like structures called?
   The thread-like structures are called hyphae, and their network is a mycelium.
2. To which class of fungi does *Rhizopus* belong?
   Rhizopus (bread mould) belongs to the class Phycomycetes.
3. What is the mode of nutrition shown by this fungus?
   It shows a saprophytic mode of nutrition, growing on dead organic matter (bread).
4. What would the black, round bodies contain?
   The black, round bodies are sporangia, containing asexual spores (sporangiospores).