Botany Half Yearly Revision Series - Class 11 JKBOSE Science

Diversity and Classification of Living Organisms

Biodiversity: Meaning and Quantitative Overview

Biodiversity refers to the number and types of living organisms present on Earth or in a particular area. The term combines "bio" (life) and "diversity" (variety/forms), indicating the variety of life forms.

• Globally, there are about 1.7 to 1.8 million taxonomically known species.
• Of these, approximately 1.2 million are animal species and 0.5 million are plant species.
• Among animals, the largest group is Arthropoda (insects), with more than 1,025,000 species.
• In plants, angiosperms (flowering plants) are the most dominant, constituting over 80% of…

Diversity and Classification of Living Organisms

Biodiversity: Meaning and Quantitative Overview

Biodiversity refers to the number and types of living organisms present on Earth or in a particular area. The term combines "bio" (life) and "diversity" (variety/forms), indicating the variety of life forms.

• Globally, there are about 1.7 to 1.8 million taxonomically known species.
• Of these, approximately 1.2 million are animal species and 0.5 million are plant species.
• Among animals, the largest group is Arthropoda (insects), with more than 1,025,000 species.
• In plants, angiosperms (flowering plants) are the most dominant, constituting over 80% of plant species.

Need and Basis for Classification

The vast number of species makes it impossible to study each individually, necessitating classification. Classification groups organisms based on similarities and differences, making study and understanding easier.

• Organisms are divided into groups based on observable characters.
• Similar organisms are grouped together; different ones are separated.
• Classification is a multi-step, hierarchical process involving several ranks or categories.

Taxonomy and Systematics: Definitions and Steps

Taxonomy is the branch of science dealing with the classification of living organisms according to established rules and laws. The term derives from Greek: "taxis" (arrangement) and "nomos" (rules/laws).

• The term "taxonomy" was given by A.P. de Candolle in his book "Theory of Elementary Botany."
• Carolus Linnaeus is regarded as the Father of Taxonomy.
• Taxonomy involves four main steps:
  1. Characterization: Studying all available characters (morphological, anatomical, physiological, etc.) of an organism.
  2. Identification: Recognizing the organism by comparing its characters with known data.
  3. Classification: Grouping the organism into convenient categories based on observable characters.
  4. Nomenclature: Assigning a name to the organism according to established rules.
• Systematics is an advanced branch that includes taxonomy and phylogeny (evolutionary relationships). The term "systematics" was used by Linnaeus in his book "Systema Naturae."

Nomenclature: Rules and Codes

Nomenclature is the process of assigning names to organisms. There are two types of names: common (vernacular) and scientific (biological/technical).

• Common names vary by language and region, leading to confusion (ambiguity).
• Scientific names are universal and given by biologists, following specific rules.
• Binomial Nomenclature (given by Linnaeus): Each scientific name has two components—Genus (generic name) and Specific epithet.
  • Example: Solanum tuberosum (potato), where "Solanum" is the genus and "tuberosum" is the specific epithet.
  • The species name is the complete name (genus + specific epithet).
• Rules of Binomial Nomenclature:
  • Names are generally derived from Latin or are Latinized.
  • The first word (genus) starts with a capital letter; the second (specific epithet) starts with a small letter.
  • Names are printed in italics or underlined separately when handwritten.
  • The author’s name (abbreviated) may follow the species name (not underlined or italicized).
• Codes of Nomenclature:
  • ICBN: International Code of Botanical Nomenclature (for plants)
  • ICZN: International Code of Zoological Nomenclature (for animals)

Taxonomic Categories and Hierarchy

Classification is a multi-step process involving a hierarchy of categories (ranks), each called a taxon (plural: taxa).

• The main taxonomic categories (obligate/essential) are: Kingdom, Phylum/Division, Class, Order, Family, Genus, Species.
• For animals, "phylum" is used; for plants, "division" is used.
• Mnemonic: "Keep Pots Clean Or Family Gets Sick" (Kingdom, Phylum/Division, Class, Order, Family, Genus, Species).
• Hierarchy can be arranged in ascending (species to kingdom) or descending (kingdom to species) order.
• As one moves from species to kingdom:
  • Number of similar characters (specificity) decreases.
  • Number of general characters increases.
  • Species diversity and classification complexity increase.

Definitions of Key Taxonomic Ranks

Each taxonomic rank has a specific definition and role in classification.

• Species: The real basic unit of classification; a group of individuals with fundamental similarities, capable of interbreeding and producing fertile offspring, and reproductively isolated from other species (Biological Species Concept by Ernst Mayr).
• Genus: A group of related species with more characters in common compared to species of other genera.
• Family: A group of related genera, defined in plants by vegetative and reproductive characters, and in animals by the suffix "-idae."
• Order: A group of related families; plant orders often end with "-ales."
• Class: A group of related orders.
• Phylum/Division: A group of related classes; "phylum" for animals, "division" for plants.
• Kingdom: The highest category, grouping related phyla/divisions.

Taxonomic Tools: Herbarium and Reference Systems

A herbarium is a storehouse of dead, preserved plant specimens, used as a reference system for classification and identification.

• Plant specimens are collected, dried, pressed, and mounted on sheets of standard size (29 × 41.5 cm).
• Each sheet carries a label with date/place of collection, English/common/botanical name, family, and collector’s name.
• Herbarium sheets are arranged according to the latest accepted classification system and serve as a quick reference for taxonomic work.

Biological Classification: Kingdom Schemes

Biological classification organizes living organisms into kingdoms based on observable characters.

• Two-Kingdom Classification (Linnaeus): Divided all living organisms into Plantae and Animalia, based on presence/absence of cell wall, mode of nutrition, and locomotion.
  • Drawbacks: Grouped unicellular and multicellular organisms together, mixed prokaryotes and eukaryotes, and placed some organisms (e.g., Euglena) controversially.
• Five-Kingdom Classification (R.H. Whittaker, 1969): Monera, Protista, Plantae, Fungi, Animalia.
  • Criteria: Cell structure (prokaryotic/eukaryotic), body organization (unicellular/multicellular), mode of nutrition, reproduction, and evolutionary relationships (phylogeny).
  • Drawbacks: Viruses, viroids, and prions were not accommodated.
• Six-Kingdom/Three-Domain Classification (Carl Woese): Domains—Archaea, Bacteria, Eukarya; Kingdoms—Archaebacteria, Eubacteria, Plantae, Protista, Fungi, Animalia.

Kingdom Monera: Bacteria

Kingdom Monera includes only prokaryotic organisms—bacteria.

• Bacteria are cosmopolitan, found in water, soil, air, and extreme environments.
• Four main shapes: Coccus (spherical), Bacillus (rod-shaped), Spirillum (spiral), Vibrio (comma-shaped).
• Cell structure: Glycocalyx (outermost), cell wall, plasma membrane; surface structures include fimbriae, pili (sex pili), and flagella (motility).
• Nutrition:
  • Autotrophic: Photoautotrophs (use light, e.g., cyanobacteria), chemoautotrophs (use chemical energy, e.g., Nitrosomonas).
  • Heterotrophic: Saprophytic (feed on dead matter), parasitic (on living hosts, may be pathogenic), symbiotic (mutually beneficial, e.g., Rhizobium in legume roots).
• Reproduction:
  • Asexual: Binary fission (favorable conditions), endospore formation (unfavorable conditions).
  • Sexual (Genetic Recombination): Transformation (uptake of DNA from medium), transduction (via bacteriophage), conjugation (direct transfer via sex pilus).
• Classification: Archaebacteria (extremophiles—methanogens, halophiles, thermoacidophiles) and Eubacteria (true bacteria, including cyanobacteria and mycoplasma).

Kingdom Protista: Key Groups and Features

Protista includes all unicellular eukaryotes. The term was given by Ernst Haeckel.

• Boundaries are not well-defined; members share features with other kingdoms.
• Main groups:
  • Chrysophytes: Diatoms (non-motile, siliceous cell wall, chief producers in oceans), desmids.
  • Dinoflagellates: Mostly marine, two flagella, cell wall with cellulose and pectin, cause red tides (e.g., Gonyaulax).
  • Euglenoids: Freshwater, no cell wall (have pellicle), mixotrophic nutrition (photosynthetic in light, predatory in dark), e.g., Euglena.
  • Slime Molds: Saprophytic, form plasmodium under favorable conditions, produce resistant spores in fruiting bodies.
  • Protozoans: Animal-like, heterotrophic, four groups—ciliated (Paramecium), amoeboid (Amoeba), flagellated (Trypanosoma), sporozoans (Plasmodium).

Kingdom Fungi: Structure, Nutrition, and Reproduction

Fungi are achlorophyllous, heterotrophic, and produce spores. Study of fungi is mycology.

• Body is thalloid, usually multicellular and filamentous (hyphae), except yeast (unicellular).
• Hyphae may be septate or aseptate (coenocytic).
• Nutrition: Saprophytic, parasitic, or symbiotic (lichens—algae + fungi; mycorrhiza—fungi + plant roots).
• Cell wall made of chitin and polysaccharides; food stored as glycogen and oils.
• Reproduction:
  • Vegetative: Fragmentation, fission, budding.
  • Asexual: Spore formation (sporangiospores, conidia).
  • Sexual: Plasmogamy, karyogamy, meiosis; sexual spores include oospores, ascospores, basidiospores.
• Four main classes: Phycomycetes, Ascomycetes, Basidiomycetes, Deuteromycetes.

Viruses, Viroids, and Prions

Viruses are acellular infectious agents, living only inside host cells. They are exceptions to the cell theory.

• Structure: Protein coat (capsid) made of capsomeres, enclosing genetic material (DNA or RNA, never both).
• Viruses are living inside host cells, non-living outside.
• Plant viruses usually have single-stranded RNA; animal viruses may have single/double-stranded RNA or double-stranded DNA.
• Viroids: Infectious agents with only RNA, no protein coat; smaller than viruses.
• Prions: Infectious proteins without genetic material, cause neurological diseases (e.g., BSE in cattle, CJD in humans).

Plant Kingdom: Major Groups and Life Cycles

Plant kingdom includes eukaryotic, chlorophyll-containing, cellulosic-walled, mainly autotrophic organisms.

• Two sub-kingdoms:
  • Cryptogams (non-seed plants): Algae, Bryophyta, Pteridophyta.
  • Phanerogams (seed plants): Gymnosperms (naked seeds), Angiosperms (covered seeds).
• Algae: Thalloid, non-vascular, mostly aquatic, reproduce by fragmentation, spore formation, and sexual methods (isogamy, anisogamy, oogamy).
• Bryophytes: First land plants, require water for reproduction, main plant body is gametophyte, produce multicellular, jacketed sex organs (antheridium, archegonium), embryo formation occurs.
• Pteridophytes: First vascular terrestrial plants, main plant body is sporophyte, true roots/stems/leaves, reproduce via spores, exhibit alternation of generations.
• Gymnosperms: Naked seeds, main plant body is sporophyte, heterosporous, produce cones (male and female), fertilization via pollen tube (siphonogamy), polyembryony may occur.
• Angiosperms: Covered seeds (within fruit), most evolved, two classes—dicots (two cotyledons), monocots (one cotyledon).

Key Exam Takeaways and Pitfalls

• Binomial nomenclature, taxonomic hierarchy, and codes of nomenclature are high-yield topics.
• Understand the difference between taxon and category, and between homosporous and heterosporous life cycles.
• Know the unique features and economic importance of each plant group.
• Be able to distinguish between types of reproduction and the significance of alternation of generations.
• Viruses, viroids, and prions are not classified in the five-kingdom system but are important for disease and biotechnology contexts.