Biological Classification — NCERT Class 11 Biology

Classification systems have evolved significantly: Two-kingdom system (Plantae and Animalia by Linnaeus) was inadequate because it grouped prokaryotes and eukaryotes together, and unicellular and multicellular organisms in the same kingdom. Five-kingdom system (Whittaker, 1969) improved this by separating Monera (prokaryotes) and Protista (unicellular eukaryotes), and further dividing multicellular eukaryotes into Fungi, Plantae, and Animalia based on cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships. Modern systems incorporate molecular evidence (rRNA sequences) and have proposed three domains: Bacteria, Archaea, and Eukarya.

(a) Heterotrophic bacteria: 1. They help in making dairy products like curd (yoghurt) from milk — Lactobacillus and Streptococcus. 2. They are used in production of antibiotics — Streptomyces produces streptomycin. 3. They act as decomposers and help in nutrient recycling. 4. Rhizobium fixes atmospheric nitrogen in root nodules of legumes.

(b) Archaebacteria: 1. Methanogens are used in biogas production — they produce methane from organic matter in biogas plants. 2. Thermoacidophiles and halophiles are studied for enzymes that function in extreme conditions (industrial applications like PCR — Taq polymerase from Thermus aquaticus).

The cell walls of diatoms are made of silica (silicon dioxide). These walls fit together like a soap box (two overlapping halves — epitheca and hypotheca). They are called frustules and are beautifully ornamented with intricate patterns. When diatoms die, their siliceous cell walls accumulate at the bottom of water bodies, forming diatomaceous earth (used in polishing, filtration, and as insulating material).

• Algal bloom: When nutrients (especially nitrogen and phosphorus) become abundant in water, phytoplankton — particularly cyanobacteria (blue-green algae) — multiply rapidly, forming a dense green layer on the water surface. This is called an algal bloom. It blocks sunlight, depletes oxygen, and can release toxins harmful to aquatic life.
• Red tides: Caused by rapid multiplication of dinoflagellates (especially Gonyaulax, Karenia brevis), which release toxins that can kill fish and other marine organisms. The water appears red due to the red pigments in these dinoflagellates.

Viroids are infectious free RNA molecules without any protein coat, while viruses have a nucleic acid core surrounded by a protein capsid. Viroids are much smaller than viruses.

(See Intext Page 13 Q1 — answered above.)

On the basis of shape, bacteria are classified into four types: 1. Coccus (spherical) — e.g., Streptococcus, Staphylococcus 2. Bacillus (rod-shaped) — e.g., Lactobacillus, Bacillus 3. Vibrio (comma-shaped) — e.g., Vibrio cholerae 4. Spirillum (spiral-shaped) — e.g., Treponema

The 'fingerprint region' of DNA refers to the variable number tandem repeats (VNTRs) — short, non-coding sequences of DNA that are repeated a variable number of times. These regions are highly variable among individuals (except identical twins) and are used in DNA fingerprinting for identification, paternity disputes, and forensic analysis.

Organisms reproduce by two main methods: 1. Asexual reproduction: Without fusion of gametes — binary fission, multiple fission, budding, fragmentation, spore formation (conidia, zoospores, sporangiospores). 2. Sexual reproduction: Involving fusion of male and female gametes — isogamy, anisogamy, oogamy.

In fungi, sexual reproduction involves three stages: plasmogamy (fusion of protoplasts), karyogamy (fusion of nuclei), and meiosis in the zygote.

1. Phycomycetes: Found in aquatic and damp environments. Hyphae are aseptate and coenocytic (multinucleate). Asexual reproduction by zoospores or aplanospores. Sexual reproduction by isogamy, anisogamy, or oogamy. Examples: Rhizopus (bread mould), Mucor, Albugo. 2. Ascomycetes: Sac fungi. Hyphae are septate. Asexual reproduction by conidia; sexual reproduction by ascospores produced in asci (sac-like structures). Examples: Neurospora, Aspergillus, Claviceps, Penicillium, yeasts (Saccharomyces). 3. Basidiomycetes: Club fungi. Hyphae are septate. Asexual reproduction by conidia, fragmentation, or budding; sexual reproduction by basidiospores produced on basidia. Examples: Agaricus (mushroom), Ustilago (smut), Puccinia (rust). 4. Deuteromycetes: Fungi Imperfecti. Only asexual or vegetative reproduction known — sexual stage not discovered. Examples: Alternaria, Trichoderma, Colletotrichum.

• They are freshwater, flagellated organisms found in stagnant water.
• They have a protein-rich cell wall called the pellicle (not cellulose), which makes them flexible.
• They have two flagella — one short and one long.
• They are mixotrophic: photosynthetic in the presence of sunlight (they have chloroplasts with photosynthetic pigments) and heterotrophic in the absence of sunlight (they ingest food).
• They have a stigma (eyespot) that helps detect light for phototactic movement.
• They reproduce by longitudinal binary fission.
• Example: Euglena.

Structure: A virus consists of: 1. Nucleoid: The genetic material — either DNA or RNA, never both. It can be single-stranded or double-stranded. 2. Capsid: A protein coat made of smaller subunits called capsomeres that surrounds and protects the genetic material. 3. Some viruses have an additional envelope of lipoprotein around the capsid (e.g., HIV, influenza virus).

Nature of genetic material: Viruses may contain:

• Double-stranded DNA (e.g., Herpes, Poxvirus)
• Single-stranded DNA (e.g., φX174)
• Double-stranded RNA (e.g., Reovirus)
• Single-stranded RNA (e.g., TMV, HIV, Influenza)

Four common viral diseases: 1. Influenza (flu) 2. AIDS (HIV) 3. Common cold (Rhinovirus) 4. Polio (Poliovirus)

A lichen is a symbiotic association between a fungus (mycobiont) and a photosynthetic partner (phycobiont — usually a green alga or cyanobacterium). The fungal partner provides structure, protection, and mineral absorption, while the algal partner performs photosynthesis and provides organic food. Lichens are excellent pollution indicators — they do not grow in polluted areas (especially areas with high SO₂). Examples: Usnea (hanging lichen), Crustose lichens on rocks.