Biological Classification Summary

Biological Classification

Early Classification Attempts

  • Historically, living organisms were classified based on instinct, primarily for human use (food, shelter, clothing).
  • Aristotle attempted a more scientific classification using simple morphological characters.
    • Plants were classified into trees, shrubs, and herbs.
    • Animals were divided into those with red blood and those without.
  • Linnaeus developed a Two Kingdom system (Plantae and Animalia).
    • This system did not differentiate between eukaryotes and prokaryotes.
    • It also failed to distinguish between unicellular and multicellular organisms, and photosynthetic and non-photosynthetic organisms.
  • The two-kingdom classification proved inadequate as many organisms did not fit into either category.

Need for New Systems

  • Beyond gross morphology, there was a need to include characteristics like:
    • Cell structure
    • Nature of cell wall
    • Mode of nutrition
    • Habitat
    • Methods of reproduction
    • Evolutionary relationships
  • Classification systems have undergone several changes over time.
  • Plant and animal kingdoms have remained constant, but understanding of which organisms belong to these kingdoms has evolved.
  • The number and nature of other kingdoms have been understood differently by various scientists.

Five Kingdom Classification

  • R.H. Whittaker (1969) proposed a Five Kingdom Classification: Monera, Protista, Fungi, Plantae, and Animalia.
  • Main criteria for classification included:
    • Cell structure
    • Body organization
    • Mode of nutrition
    • Reproduction
    • Phylogenetic relationships
  • The three-domain system divides Kingdom Monera into two domains, placing eukaryotic kingdoms in the third domain, resulting in a six-kingdom classification.

Changes and Considerations

  • Earlier systems grouped bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms under ‘Plants’ because they all had cell walls.
  • This system grouped together organisms with widely differing characteristics.
  • It combined prokaryotic bacteria and blue-green algae (cyanobacteria) with eukaryotic groups.
  • It grouped unicellular (e.g., Chlamydomonas) and multicellular organisms (e.g., Spirogyra) together under algae.
  • The classification did not differentiate between heterotrophic fungi and autotrophic green plants, despite differences in cell wall composition (chitin in fungi, cellulose in plants).
  • Fungi were later placed in a separate Kingdom Fungi.
  • All prokaryotic organisms were grouped under Kingdom Monera.
  • Unicellular eukaryotic organisms were placed in Kingdom Protista.
  • Kingdom Protista brought together organisms previously classified in different kingdoms (e.g., Chlamydomonas and Chlorella from Plants, Paramoecium and Amoeba from Animalia).
  • Criteria for classification have changed over time.
  • Classification systems aim to reflect morphological, physiological, and reproductive similarities, as well as phylogenetic (evolutionary) relationships.

Kingdom Monera

  • Bacteria are the sole members.
  • They are the most abundant micro-organisms, found almost everywhere.
  • They live in extreme habitats (hot springs, deserts, snow, deep oceans) and as parasites in or on other organisms.
  • Bacteria are grouped into four categories based on shape:
    • Coccus (spherical)
    • Bacillus (rod-shaped)
    • Vibrium (comma-shaped)
    • Spirillum (spiral)
  • Bacterial structure is simple, but their behavior is complex.
  • They show the most extensive metabolic diversity.
    • Some are autotrophic (synthesize their own food from inorganic substrates).
      • Photosynthetic autotrophic
      • Chemosynthetic autotrophic
    • Most are heterotrophic (depend on other organisms or dead organic matter).

Archaebacteria

  • Live in harsh habitats (extreme salty areas, hot springs, marshy areas).
    • Halophiles (salty areas)
    • Thermoacidophiles (hot springs)
    • Methanogens (marshy areas)
  • Differ from other bacteria in cell wall structure, enabling survival in extreme conditions.
  • Methanogens in the gut of ruminant animals (cows, buffaloes) produce methane (biogas) from dung.

Eubacteria

  • Thousands of different types exist; they are the 'true bacteria'.
  • Characterized by a rigid cell wall and, if motile, a flagellum.
  • Cyanobacteria (blue-green algae) have chlorophyll similar to green plants and are photosynthetic autotrophs.
    • Can be unicellular, colonial, or filamentous.
    • Found in freshwater, marine, or terrestrial environments.
    • Colonies are often surrounded by a gelatinous sheath.
    • They often form blooms in polluted water bodies.
    • Some fix atmospheric nitrogen in specialized cells called heterocysts (e.g., Nostoc and Anabaena).
  • Chemosynthetic autotrophic bacteria oxidize inorganic substances (nitrates, nitrites, ammonia) and use the released energy for ATP production ($ ATP$ Synthesis).
    • They play a crucial role in recycling nutrients (nitrogen, phosphorus, iron, and sulfur).
  • Heterotrophic bacteria are most abundant and are important decomposers.
    • They significantly impact human affairs.
      • Helpful in making curd from milk.
      • Production of antibiotics.
      • Fixing nitrogen in legume roots.
    • Some are pathogens, causing diseases in humans, crops, farm animals, and pets.
      • Examples: Cholera, typhoid, tetanus, citrus canker.
  • Bacteria reproduce mainly by fission. Under unfavorable conditions, they produce spores.
  • They also reproduce via a primitive type of DNA transfer from one bacterium to another (sexual reproduction).
  • Mycoplasma lack a cell wall.
    • They are the smallest known living cells and can survive without oxygen.
    • Many mycoplasma are pathogenic in animals and plants.

Kingdom Protista

  • All single-celled eukaryotes are placed under Protista.
  • The boundaries of this kingdom are not well-defined.
  • Includes Chrysophytes, Dinoflagellates, Euglenoids, Slime molds, and Protozoans.
  • Members are primarily aquatic.
  • This kingdom links plants, animals, and fungi.
  • Being eukaryotes, cells contain a well-defined nucleus and membrane-bound organelles.
  • Some have flagella or cilia.
  • Protists reproduce asexually and sexually (cell fusion and zygote formation).

Chrysophytes

  • Includes diatoms and golden algae (desmids).
  • Found in freshwater and marine environments.
  • Microscopic and float passively in water currents (plankton).
  • Most are photosynthetic.
  • Diatom cell walls form two thin overlapping shells embedded with silica, making them indestructible.
  • Accumulation of these cell wall deposits over billions of years forms ‘diatomaceous earth’.
  • Diatomaceous earth is used in polishing, filtration of oils, and syrups due to its gritty nature.
  • Diatoms are the chief ‘producers’ in the oceans.

Dinoflagellates

  • Mostly marine and photosynthetic.
  • Appear yellow, green, brown, blue, or red based on pigments present.
  • Cell wall has stiff cellulose plates on the outer surface.
  • Have two flagella: one longitudinal and one transverse in a furrow between the wall plates.
  • Red dinoflagellates (e.g., Gonyaulax) can undergo rapid multiplication, causing ‘red tides’.
  • Toxins released during red tides can kill other marine animals.

Euglenoids

  • Majority are fresh water organisms found in stagnant water.
  • Instead of a cell wall, they have a protein-rich layer called a pellicle, which makes their body flexible.
  • Have two flagella (one short, one long).
  • Photosynthetic in sunlight; behave as heterotrophs by predating on smaller organisms when deprived of sunlight.
  • Pigments are identical to those in higher plants.
  • Example: Euglena.

Slime Moulds

  • Saprophytic protists.
  • The body moves along decaying twigs and leaves, engulfing organic material.
  • Under suitable conditions, they form an aggregation called plasmodium, which can grow and spread over several feet.
  • During unfavorable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips.
  • Spores possess true walls and are extremely resistant, surviving for many years even under adverse conditions.
  • Spores are dispersed by air currents.

Protozoans

  • All are heterotrophs, living as predators or parasites.
  • Believed to be primitive relatives of animals.
  • Four major groups:
    • Amoeboid protozoans:
      • Live in fresh water, sea water, or moist soil.
      • Move and capture prey using pseudopodia (false feet), e.g., Amoeba.
      • Marine forms have silica shells.
      • Some, like Entamoeba, are parasites.
    • Flagellated protozoans:
      • Free-living or parasitic.
      • Have flagella.
      • Parasitic forms cause diseases like sleeping sickness, e.g., Trypanosoma.
    • Ciliated protozoans:
      • Aquatic, actively moving organisms with thousands of cilia.
      • Have a cavity (gullet) that opens to the outside of the cell surface.
      • Coordinated movement of cilia steers water laden with food into the gullet, e.g., Paramoecium.
    • Sporozoans:
      • Diverse organisms with an infectious spore-like stage in their life cycle.
      • Plasmodium (malarial parasite) causes malaria, a disease with a staggering effect on human population.

Kingdom Fungi

  • Fungi constitute a unique kingdom of heterotrophic organisms.
  • They exhibit great diversity in morphology and habitat.
  • Examples include fungi on moist bread, rotten fruits, mushrooms, and toadstools.
  • Some unicellular fungi (e.g., yeast) are used to make bread and beer.
  • Other fungi cause diseases in plants and animals (e.g., wheat rust caused by Puccinia).
  • Some are sources of antibiotics (e.g., Penicillium).
  • Fungi are cosmopolitan, occurring in air, water, soil, and on animals and plants.
  • They prefer warm and humid places.
  • With the exception of yeasts, fungi are filamentous.
  • Bodies consist of long, slender thread-like structures called hyphae.
  • The network of hyphae is known as mycelium.
  • Some hyphae are continuous tubes filled with multinucleated cytoplasm (coenocytic hyphae).
  • Others have septae or cross walls in their hyphae.
  • Cell walls are composed of chitin and polysaccharides.
  • Most fungi are heterotrophic and absorb soluble organic matter from dead substrates (saprophytes).
  • Those that depend on living plants and animals are called parasites.
  • They can also live as symbionts:
    • In association with algae as lichens.
    • With roots of higher plants as mycorrhiza.
  • Reproduction occurs by vegetative means (fragmentation, fission, and budding).
  • Asexual reproduction is by spores (conidia, sporangiospores, or zoospores).
  • Sexual reproduction is by oospores, ascospores, and basidiospores.
  • Spores are produced in distinct structures called fruiting bodies.
  • The sexual cycle involves three steps:
    • Fusion of protoplasms between two motile or non-motile gametes (plasmogamy).
    • Fusion of two nuclei (karyogamy).
    • Meiosis in zygote, resulting in haploid spores.
  • When fungi reproduce sexually, two haploid hyphae of compatible mating types fuse.
  • In some fungi, fusion of two haploid cells immediately results in diploid cells (2n).
  • In other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage (n + n, two nuclei per cell) occurs; this is called dikaryon and the dikaryophase.
  • Later, parental nuclei fuse, and cells become diploid.
  • Fungi form fruiting bodies in which reduction division occurs, leading to formation of haploid spores.
  • The morphology of the mycelium, mode of spore formation, and fruiting bodies form the basis for the division of the kingdom into various classes.

Phycomycetes

  • Found in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants.
  • Mycelium is aseptate and coenocytic.
  • Asexual reproduction occurs by zoospores (motile) or aplanospores (non-motile).
  • Spores are endogenously produced in sporangium.
  • A zygospore is formed by fusion of two gametes.
  • Gametes can be similar in morphology (isogamous) or dissimilar (anisogamous or oogamous).
  • Examples: Mucor, Rhizopus (bread mold), and Albugo (parasitic fungi on mustard).

Ascomycetes

  • Commonly known as sac-fungi.
  • Mostly multicellular (e.g., Penicillium) or rarely unicellular (e.g., yeast - Saccharomyces).
  • Saprophytic, decomposers, parasitic, or coprophilous (growing on dung).
  • Mycelium is branched and septate.
  • Asexual spores are conidia, produced exogenously on special mycelium called conidiophores.
  • Conidia on germination produce mycelium.
  • Sexual spores are ascospores, produced endogenously in sac-like asci.
  • Asci are arranged in different types of fruiting bodies called ascocarps.
  • Examples: Aspergillus, Claviceps, and Neurospora.
  • Neurospora is used extensively in biochemical and genetic work.
  • Many members like morels and truffles are edible and considered delicacies.

Basidiomycetes

  • Commonly known forms: mushrooms, bracket fungi, or puffballs.
  • Grow in soil, on logs and tree stumps, and in living plant bodies as parasites (e.g., rusts and smuts).
  • Mycelium is branched and septate.
  • Asexual spores are generally not found, but vegetative reproduction by fragmentation is common.
  • Sex organs are absent, but plasmogamy is brought about by fusion of two vegetative or somatic cells of different strains or genotypes.
  • The resultant structure is dikaryotic, which ultimately gives rise to basidium.
  • Karyogamy and meiosis take place in the basidium, producing four basidiospores.
  • Basidiospores are exogenously produced on the basidium (pl.: basidia).
  • Basidia are arranged in fruiting bodies called basidiocarps.
  • Examples: Agaricus (mushroom), Ustilago (smut), and Puccinia (rust fungus).

Deuteromycetes

  • Commonly known as imperfect fungi because only the asexual or vegetative phases are known.
  • When sexual forms are discovered, they are moved to the classes they rightly belong to (ascomycetes and basidiomycetes).
  • Reproduce only by asexual spores known as conidia.
  • Mycelium is septate and branched.
  • Some members are saprophytes or parasites, while a large number are decomposers of litter and help in mineral cycling.
  • Examples: Alternaria, Colletotrichum, and Trichoderma.

Kingdom Plantae

  • Includes all eukaryotic chlorophyll-containing organisms (plants).
  • A few members are partially heterotrophic (insectivorous plants or parasites).
    • Bladderwort and Venus flytrap are examples of insectivorous plants.
    • Cuscuta is a parasite.
  • Plant cells have a eukaryotic structure with prominent chloroplasts and a cell wall mainly made of cellulose.
  • Plantae includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
  • Life cycle has two distinct phases:
    • Diploid sporophytic
    • Haploid gametophytic
  • These phases alternate with each other (alternation of generations).

Kingdom Animalia

  • Characterized by heterotrophic eukaryotic organisms that are multicellular and lack cell walls.
  • They directly or indirectly depend on plants for food.
  • They digest their food in an internal cavity and store food reserves as glycogen or fat.
  • Nutrition is holozoic (ingestion of food).
  • They follow a definite growth pattern and grow into adults that have a definite shape and size.
  • Higher forms show elaborate sensory and neuromotor mechanisms.
  • Most are capable of locomotion.
  • Sexual reproduction involves copulation of male and female followed by embryological development.

Viruses, Viroids, Prions and Lichens

  • Not included in Whittaker's five-kingdom classification.

Viruses

  • Non-cellular organisms characterized by an inert crystalline structure outside the living cell.
  • Once they infect a cell, they take over the host cell's machinery to replicate themselves, killing the host.
  • The term