Biology: Class 11 — Chapter 2: Biological Classification

• Aristotle: The earliest to attempt a scientific basis for classification. He used simple morphological characters to classify plants into trees, shrubs, and herbs, and divided animals into two groups: those with red blood and those without.
• Linnaeus: Developed the Two Kingdom system of classification, dividing organisms into kingdoms Plantae and Animalia. This system did not distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms, or photosynthetic and non-photosynthetic organisms.
• R.H. Whittaker (1969): Proposed the Five Kingdom Classification. The kingdoms are Monera, Protista, Fungi, Plantae, and Animalia. The main criteria used include cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships.
• Three-domain system: A system that divides the Kingdom Monera into two domains, leaving the remaining eukaryotic kingdoms in the third domain, resulting in a six-kingdom classification.

• Coccus (pl. cocci): Spherical shape.
• Bacillus (pl. bacilli): Rod-shaped.
• Vibrium (pl. vibrio): Comma-shaped.
• Spirillum (pl. spirilla): Spiral shape.

These bacteria live in some of the most harsh habitats due to a different cell wall structure:

• Halophiles: Live in extreme salty areas.
• Thermoacidophiles: Live in hot springs.
• Methanogens: Live in marshy areas and the gut of several ruminant animals (responsible for biogas production from dung).

Known as "true bacteria", characterized by the presence of a rigid cell wall and a flagellum if motile.

• Photosynthetic Autotrophs: Cyanobacteria (blue-green algae) contain chlorophyll a. They are unicellular, colonial, or filamentous. Some can fix atmospheric nitrogen in specialized cells called heterocysts (e.g., Nostoc and Anabaena).
• Chemosynthetic Autotrophs: Oxidise inorganic substances (nitrates, nitrites, ammonia) to release energy for ATP production. They play a major role in recycling nutrients like nitrogen, phosphorus, iron, and sulphur.
• Heterotrophic Bacteria: Most abundant in nature and act as important decomposers. They are helpful in making curd, producing antibiotics, and fixing nitrogen in legumes, but some are pathogens causing diseases like cholera, typhoid, tetanus, and citrus canker.
• Mycoplasma: Organisms that completely lack a cell wall. They are the smallest living cells known, can survive without oxygen, and are pathogenic in animals and plants.

Includes diatoms and golden algae (desmids). Found in fresh and marine environments.

• Diatoms have cell walls forming two thin overlapping shells that fit together like a soap box.
• Walls are embedded with silica, making them indestructible, leaving behind deposits known as diatomaceous earth.
• Diatoms are the chief "producers" in the oceans.

Mostly marine and photosynthetic, appearing in various colors based on key pigments.

• Cell wall has stiff cellulose plates on the outer surface.
• Most have two flagella (one longitudinal, one transverse in a furrow).
• Red dinoflagellates (e.g., Gonyaulax) undergo rapid multiplication causing red tides and releasing toxins that can kill marine life.

Freshwater organisms found in stagnant water.

• Lack a cell wall; instead, they have a protein-rich layer called pellicle which makes their body flexible.
• Have two flagella (one short, one long).
• Photosynthetic in sunlight, but behave as heterotrophs when deprived of light (e.g., Euglena).

Saprophytic protists whose bodies move along decaying twigs and leaves engulfing organic matter.

• Under suitable conditions, they form an aggregation called plasmodium.
• Under unfavorable conditions, the plasmodium differentiates to form fruiting bodies bearing spores with true walls.

Heterotrophs that live as predators or parasites. Divided into four major groups:

• Amoeboid protozoans: Move and capture prey using pseudopodia (e.g., Amoeba). Marine forms have silica shells. Some are parasites (e.g., Entamoeba).
• Flagellated protozoans: Free-living or parasitic with flagella. Parasitic forms cause diseases like sleeping sickness (e.g., Trypanosoma).
• Ciliated protozoans: Aquatic, actively moving using thousands of cilia. Have a cavity (gullet) that opens to the outside (e.g., Paramoecium).
• Sporozoans: Have an infectious spore-like stage in their life cycle (e.g., Plasmodium, the malarial parasite).

• Fungi are filamentous (except unicellular yeast). Their bodies consist of long, slender thread-like structures called hyphae.
• A network of hyphae is called mycelium. Hyphae can be continuous tubes filled with multinucleated cytoplasm (coenocytic hyphae) or have septae.
• Cell walls are composed of chitin and polysaccharides.
• Fungi can be saprophytes, parasites, or symbionts (as lichens with algae, or mycorrhiza with roots of higher plants).

Reproduction occurs vegetatively (fragmentation, fission, budding), asexually (by spores like conidia, sporangiospores, or zoospores), and sexually (by oospores, ascospores, or basidiospores).

The sexual cycle involves three steps:

1. Plasmogamy: Fusion of protoplasms between two motile or non-motile gametes.
2. Karyogamy: Fusion of two nuclei.
3. Meiosis in zygote resulting in haploid spores.

In Ascomycetes and Basidiomycetes, an intervening dikaryotic stage ($n+n$) occurs, called a dikaryon, and the phase is called dikaryophase.

• Phycomycetes: Found in aquatic habitats and on decaying wood. Mycelium is aseptate and coenocytic. Asexual reproduction by zoospores or aplanospores. Zygospores are formed by fusion of gametes (e.g., Mucor, Rhizopus, Albugo).
• Ascomycetes (Sac-fungi): Mostly multicellular (e.g., Penicillium) or rarely unicellular (e.g., yeast). Mycelium is branched and septate. Asexual spores are conidia produced exogenously. Sexual spores are ascospores produced endogenously in sac-like asci (e.g., Aspergillus, Claviceps, Neurospora).
• Basidiomycetes: Includes mushrooms, bracket fungi, or puffballs. Mycelium is branched and septate. Asexual spores are generally absent. Plasmogamy occurs by fusion of somatic cells to form a dikaryotic structure, giving rise to a basidium where karyogamy and meiosis occur to produce four exogenous basidiospores (e.g., Agaricus, Ustilago, Puccinia).
• Deuteromycetes (Imperfect Fungi): Only asexual or vegetative phases are known. They reproduce only by asexual spores called conidia. Mycelium is septate and branched. Many are decomposers of litter (e.g., Alternaria, Colletotrichum, Trichoderma).

Includes all eukaryotic chlorophyll-containing organisms with cellulosic cell walls. Some are partially heterotrophic (e.g., insectivorous plants like bladderwort and Venus fly trap, or parasites like Cuscuta).

The life cycle exhibits alternation of generations between the diploid sporophytic phase and the haploid gametophytic phase.

Characterised by heterotrophic eukaryotic multicellular organisms lacking cell walls. They ingest food and digest it in an internal cavity, storing reserves as glycogen or fat. Nutrition is holozoic.

Non-cellular organisms characterized by having an inert crystalline structure outside the living cell. They are obligate parasites.

• Discovery: Dmitri Ivanowsky (1892) recognized microbes causing mosaic disease of tobacco. M.W. Beijerinck (1898) called the infectious fluid Contagium vivum fluidum. W.M. Stanley (1935) showed that viruses could be crystallized.
• Structure: Contain a protein coat called capsid made of small subunits called capsomeres, protecting the genetic material (either RNA or DNA, never both).
• Diseases: Cause mumps, smallpox, herpes, influenza, and AIDS in humans. In plants, symptoms include mosaic formation, leaf rolling, yellowing, and stunted growth.

Discovered by T.O. Diener (1971) as a new infectious agent smaller than viruses. Causes potato spindle tuber disease. It consists of free RNA of low molecular weight and lacks a protein coat.

Infectious agents consisting of abnormally folded proteins, similar in size to viruses. They transmit neurological diseases such as bovine spongiform encephalopathy (BSE/mad cow disease) in cattle and Creutzfeldt-Jakob disease (CJD) in humans.

Symbiotic, mutually useful associations between algae (phycobiont, autotrophic) and fungi (mycobiont, heterotrophic). Algae prepare food while fungi provide shelter and absorb nutrients. They are highly sensitive to pollution and do not grow in polluted areas.