Biological Classification: A Comprehensive Study Guide

Historical Development and Methods of Biological Classification

• Early Instinctive Human Classification: Since the dawn of civilization, humans have attempted to classify living organisms. These early attempts were not based on scientific criteria but were borne out of the practical need to utilize organisms for food, shelter, and clothing.
• Aristotle's Scientific Basis: Aristotle was the earliest known individual to attempt a more scientific basis for classification.
  • He utilized simple morphological characters to classify plants into three distinct groups: trees, shrubs, and herbs.
  • He categorized animals into $2$ groups: those that possessed red blood and those that did not.
• Linnaeus' Two Kingdom System: During the time of Linnaeus, a Two Kingdom system was developed, consisting of Kingdom Plantae and Kingdom Animalia.
  • Kingdom Plantae included all plants, while Kingdom Animalia included all animals.
  • Inadequacies of the Two Kingdom System: The system was easy to understand but failed because a large number of organisms did not fall into either category. It did not distinguish between:
    • Eukaryotes and prokaryotes.
    • Unicellular and multicellular organisms.
    • Photosynthetic (e.g., green algae) and non-photosynthetic (e.g., fungi) organisms.
• Evolution of Classification Criteria: Over time, scientists felt the need to include characteristics beyond gross morphology, such as:
  • Cell structure.
  • Nature of the cell wall.
  • Mode of nutrition.
  • Habitat.
  • Methods of reproduction.
  • Evolutionary (phylogenetic) relationships.

R.H. Whittaker's Five Kingdom Classification ($1969$)

• The Five Kingdoms: R.H. Whittaker proposed a Five Kingdom Classification system in $1969$. The kingdoms are named Monera, Protista, Fungi, Plantae, and Animalia.
• Main Criteria for Classification: Whittaker used the following criteria for his system:
  • Cell structure.
  • Body organisation.
  • Mode of nutrition.
  • Reproduction.
  • Phylogenetic relationships.
• Comparison of Characteristics across the Five Kingdoms (Table $2.1$):
  • Kingdom Monera:
    • Cell type: Prokaryotic.
    • Cell wall: Noncellulosic (Polysaccharide $+$ amino acid).
    • Nuclear membrane: Absent.
    • Body organisation: Cellular.
    • Mode of nutrition: Autotrophic (chemosynthetic and photosynthetic) and Heterotrophic (saprophytic/parasitic).
  • Kingdom Protista:
    • Cell type: Eukaryotic.
    • Cell wall: Present in some.
    • Nuclear membrane: Present.
    • Body organisation: Cellular.
    • Mode of nutrition: Autotrophic (Photosynthetic) and Heterotrophic.
  • Kingdom Fungi:
    • Cell type: Eukaryotic.
    • Cell wall: Present with chitin.
    • Nuclear membrane: Present.
    • Body organisation: Multicellular / loose tissue.
    • Mode of nutrition: Heterotrophic (Saprophytic / Parasitic).
  • Kingdom Plantae:
    • Cell type: Eukaryotic.
    • Cell wall: Present (cellulose).
    • Nuclear membrane: Present.
    • Body organisation: Tissue / organ.
    • Mode of nutrition: Autotrophic (Photosynthetic).
  • Kingdom Animalia:
    • Cell type: Eukaryotic.
    • Cell wall: Absent.
    • Nuclear membrane: Present.
    • Body organisation: Tissue / organ / organ system.
    • Mode of nutrition: Heterotrophic (Holozoic / Saprophytic etc.).
• Three Domain System: This system divides Kingdom Monera into $2$ domains and leaves the remaining eukaryotic kingdoms in the third domain, effectively creating a six-kingdom classification.

Kingdom Monera

• General Characteristics: Bacteria are the sole members of Kingdom Monera. They are the most abundant microorganisms and can be found almost everywhere.
  • Hundreds of bacteria can exist in a handful of soil.
  • They inhabit extreme environments such as hot springs, deserts, snow, and deep oceans where other life forms struggle to survive.
  • Many act as parasites on or in other organisms.
• Classification by Shape (Figure $2.1$): Bacteria are grouped into $4$ categories based on shape:
  • Coccus (pl.: cocci): Spherical shape.
  • Bacillus (pl.: bacilli): Rod shape.
  • Vibrium (pl.: vibrio): Comma shape.
  • Spirillum (pl.: spirilla): Spiral shape.
• Bacterial Nature: Although their structure is simple, their behavior is complex. As a group, they show the most extensive metabolic diversity compared to other organisms.
  • Autotrophic: Synthesize food from inorganic substrates. Sub-types include photosynthetic autotrophic and chemosynthetic autotrophic.
  • Heterotrophic: The vast majority depend on other organisms or dead organic matter for food.
• Archaebacteria: Special bacteria that live in harsh habitats.
  • Halophiles: Live in extreme salty areas.
  • Thermoacidophiles: Live in hot springs.
  • Methanogens: Live in marshy areas and in the gut of ruminant animals (e.g., cows, buffaloes). They produce methane (biogas) from animal dung.
  • Survival in extreme conditions is due to a different cell wall structure.
• Eubacteria (True Bacteria): Characterized by a rigid cell wall and, if motile, a flagellum.
  • Cyanobacteria (Blue-green algae): Contain chlorophyll $a$ (similar to green plants) and are photosynthetic autotrophs. They can be unicellular, colonial, or filamentous, and are found in freshwater, marine, or terrestrial environments. Colonies are usually surrounded by a gelatinous sheath and can form blooms in polluted water.
  • Heterocysts: Specialised cells in organisms like Nostoc and Anabaena used to fix atmospheric nitrogen.
  • Chemosynthetic Autotrophic Bacteria: Oxidise inorganic substances like nitrates, nitrites, and ammonia to produce $ATP$. They recycle nutrients like nitrogen, phosphorous, iron, and sulphur.
  • Heterotrophic Bacteria: Most abundant in nature; majority are decomposers. Their roles include:
    • Making curd from milk.
    • Producing antibiotics.
    • Fixing nitrogen in legume roots.
    • Acting as pathogens causing diseases like cholera, typhoid, tetanus, and citrus canker.
• Reproduction and Size:
  • Mainly reproduce by fission.
  • Under unfavorable conditions, they produce spores.
  • Perform a primitive type of DNA transfer (sexual reproduction).
  • Mycoplasma: Organisms that completely lack a cell wall. They are the smallest living cells known and can survive without oxygen. Many are pathogenic.

Kingdom Protista

• Definition and Scope: Includes all single-celled eukaryotes. Boundaries are not well defined; some may be considered plants by some and protistans by others.
  • Primarily aquatic habitats.
  • Forms a link between Monera and the kingdoms of Fungi, Plantae, and Animalia.
  • Contains well-defined nuclei and membrane-bound organelles.
• Chrysophytes: Includes diatoms and golden algae (desmids).
  • Found in fresh and marine water; mostly photosynthetic.
  • Diatoms: Cell walls form $2$ thin overlapping shells like a soap box. Walls are embedded with silica and are indestructible.
  • Diatomaceous Earth: Accumulations of cell wall deposits over billions of years. Used for polishing and filtration of oils and syrups. Diatoms are the chief 'producers' in the oceans.
• Dinoflagellates: Mostly marine and photosynthetic. Various colors (yellow, green, brown, blue, red) based on pigments.
  • Cell wall has stiff cellulose plates.
  • Possess $2$ flagella; one longitudinal and one transverse in a furrow.
  • Red Tides: Rapid multiplication of red dinoflagellates like Gonyaulax. Toxins released can kill marine animals like fish.
• Euglenoids: Mostly freshwater organisms found in stagnant water.
  • Lack a cell wall; have a protein-rich layer called a pellicle which provides flexibility.
  • Possess $2$ flagella (one short, one long).
  • Mixotrophic: Photosynthetic in sunlight; heterotrophic (predatory) in darkness.
  • Pigments are identical to higher plants (e.g., Euglena).
• Slime Moulds: Saprophytic protists.
  • Body moves along decaying matter engulfing organic material.
  • Plasmodium: Large aggregation formed under suitable conditions.
  • Under unfavorable conditions, they form fruiting bodies with spores at the tips. Spores have true walls and are highly resistant, dispersed by air currents.
• Protozoans: All are heterotrophs (predators or parasites) and considered primitive relatives of animals.
  • Amoeboid protozoans: Use pseudopodia (false feet) to capture prey (e.g., Amoeba). Marine forms have silica shells. Some are parasites (e.g., Entamoeba).
  • Flagellated protozoans: Free-living or parasitic. Parasitic forms cause diseases like sleeping sickness (e.g., Trypanosoma).
  • Ciliated protozoans: Aquatic and move via thousands of cilia. Have a cavity called a gullet. (e.g., Paramoecium).
  • Sporozoans: Have an infectious spore-like stage in the life cycle. Key example: Plasmodium (malarial parasite).

Kingdom Fungi

• General Characteristics: Unique kingdom of heterotrophic organisms. Diverse morphology and habitat.
  • Include molds on bread, mushrooms, and toadstools.
  • White spots on mustard leaves are parasitic fungi.
  • Commercial use: Yeast (unicellular) for bread and beer; Penicillium for antibiotics.
  • Pathogens: Puccinia causes wheat rust.
• Structure: Composed of long, slender thread-like hyphae. A network of hyphae is a mycelium.
  • Coenocytic hyphae: Continuous tubes filled with multinucleated cytoplasm.
  • Septate hyphae: Have cross walls (septae).
  • Cell walls consist of chitin and polysaccharides.
• Nutrition: Saprophytes (absorb organic matter from dead substrates), parasites, or symbionts (lichens with algae; mycorrhiza with plant roots).
• Reproduction:
  • Vegetative: Fragmentation, fission, and budding.
  • Asexual: Spores such as conidia, sporangiospores, or zoospores.
  • Sexual: Oospores, ascospores, and basidiospores produced in fruiting bodies.
• The Sexual Cycle ($3$ steps):
  1. Plasmogamy: Fusion of protoplasms between motile or non-motile gametes.
  2. Karyogamy: Fusion of $2$ nuclei.
  3. Meiosis: Reduction division in the zygote to form haploid spores.
• Dikaryon phase: In Ascomycetes and Basidiomycetes, an intervening stage where there are $2$ nuclei per cell ($n + n$). The phase is called dikaryophase.
• Classification of Fungi:
  • Phycomycetes: Found in aquatic habitats and damp places. Mycelium is aseptate and coenocytic. Asexual reproduction via zoospores (motile) or aplanospores (non-motile). Example: Mucor, Rhizopus (bread mould), and Albugo (mustard parasite).
  • Ascomycetes (Sac-fungi): Mostly multicellular (Penicillium), rarely unicellular (Yeast). Saprophytic, decomposers, parasitic, or coprophilous (grow on dung). Mycelium is branched and septate. Conidia produced exogenously. Ascospores produced endogenously in asci. Examples: Aspergillus, Claviceps, Neurospora (used in genetics), and edible morels/truffles.
  • Basidiomycetes: Includes mushrooms, bracket fungi, and puffballs. Parasitic forms include rusts and smuts. Mycelium is branched and septate. Asexual spores are rare; vegetative reproduction is by fragmentation. Plasmogamy occurs by fusion of somatic cells, leading to a dikaryotic basidium. Karyogamy and meiosis produce exogenously formed basidiospores. Examples: Agaricus, Ustilago, Puccinia.
  • Deuteromycetes (Imperfect Fungi): Only asexual or vegetative phases are known. Mycelium is septate and branched. Once sexual stages are found, they are often moved to Ascomycetes or Basidiomycetes. Reproduce only by conidia. Many are decomposers helping in mineral cycling. Examples: Alternaria, Colletotrichum, Trichoderma.

Kingdom Plantae and Kingdom Animalia

• Kingdom Plantae:
  • Eukaryotic chlorophyll-containing organisms.
  • Partial heterotrophs include insectivorous plants (e.g., Bladderwort, Venus fly trap) and parasites (e.g., Cuscuta).
  • Cell walls are primarily cellulose.
  • Includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
  • Alternation of Generation: Life cycle has $2$ phases: diploid sporophytic and haploid gametophytic.
• Kingdom Animalia:
  • Heterotrophic, eukaryotic, multicellular organisms without cell walls.
  • Holozoic Nutrition: Ingestion of food followed by digestion in an internal cavity.
  • Store food as glycogen or fat.
  • Capable of locomotion; follow definite growth patterns; possess neuromotor mechanisms.

Viruses, Viroids, Prions, and Lichens

• General Note: These acellular organisms are not included in Whittaker's Five Kingdom Classification.
• Viruses:
  • Non-cellular with an inert crystalline structure outside the host.
  • Infect host machinery to replicate, eventually killing the host.
  • Dmitri Ivanowsky ($1892$): Identified microbes causing mosaic disease of tobacco; smaller than bacteria.
  • M.W. Beijerinek ($1898$): Named the pathogen "virus" and called the infectious fluid "Contagium vivum fluidum".
  • W.M. Stanley ($1935$): Demonstrated viruses could be crystallized; crystals consist of proteins.
  • Genetics: Contain either $DNA$ or $RNA$, never both. Genetic material is infectious.
    • Plant viruses usually have single-stranded $RNA$.
    • Animal viruses have single/double-stranded $RNA$ or double-stranded $DNA$.
    • Bacteriophages (infect bacteria) usually have double-stranded $DNA$.
  • Structure: Protein coat called a capsid made of capsomeres arranged in helical or polyhedral forms.
  • Diseases: Mumps, small pox, herpes, influenza, $AIDS$. In plants: mosaic formation, leaf curling, stunted growth.
• Viroids ($1971$):
  • Discovered by T.O. Diener.
  • Infectious agent smaller than viruses; causes potato spindle tuber disease.
  • Consists of free $RNA$ with low molecular weight and lacks a protein coat.
• Prions:
  • Consist of abnormally folded proteins; size similar to viruses.
  • Cause neurological diseases like bovine spongiform encephalopathy ($BSE$) or "mad cow disease" in cattle, and Cr–Jacob disease ($CJD$) in humans.
• Lichens:
  • Symbiotic (mutually useful) associations between algae (phycobiont) and fungi (mycobiont).
  • Phycobionts are autotrophic (prepare food); mycobionts are heterotrophic (provide shelter, absorb minerals/water).
  • Pollution Indicators: Lichens do not grow in polluted areas and are used to monitor air quality.