Biological Classification: Kingdoms

Chapter 2: Biological Classification

• Early classification was instinctive, based on usage for food, shelter, and clothing, rather than scientific criteria.
• Aristotle attempted a scientific classification using simple morphological characters, dividing plants into trees, shrubs, and herbs, and animals by the presence of red blood.
• Linnaeus developed a Two Kingdom system which uses Plantae and Animalia, but this system failed to differentiate between eukaryotes and prokaryotes, unicellular and multicellular organisms, and photosynthetic and non-photosynthetic organisms
• The two-kingdom system proved inadequate as many organisms did not fit into either category.
• Modern classification considers cell structure, wall nature, nutrition mode, habitat, reproduction methods, and evolutionary relationships.
• Plant and animal kingdoms have remained constant, while understanding of groups within them has evolved.

Five Kingdom Classification

• Proposed by R.H. Whittaker in 1969, this classification includes Monera, Protista, Fungi, Plantae, and Animalia.
• The Five Kingdom classification is based on cell structure, body organization, nutrition mode, reproduction, and phylogenetic relationships.
• There has also been a suggestion made for a three-domain system, dividing Monera and placing eukaryotic kingdoms in the third domain, which would create a six kingdom classification.
• Prior systems grouped bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms under 'Plants' based on the presence of cell walls.
• Earlier systems grouped bacteria and blue-green algae (prokaryotes) with eukaryotes
• Unicellular and multicellular organisms, such as Chlamydomonas and Spirogyra, were grouped together previously as algae.
• Fungi, heterotrophic, and green plants, autotrophic, were grouped together regardless of fungi having chitin and plants having cellulose.
• Fungi were placed in their own kingdom, with all prokaryotes in Monera and unicellular eukaryotes in Protista based on the differing cell wall compositions
• The plant and animal kingdoms will be discussed in depth in subsequent chapters

Kingdom Monera

• Bacteria is the sole member and they are abundant microorganisms found everywhere.
• Bacteria thrive in extreme conditions, from hot springs to deep oceans, and can be parasitic
• Bacteria are grouped by shape: spherical coccus, rod-shaped bacillus, comma-shaped vibrium, and spiral spirillum.
• Despite simple structures, bacteria exhibit complex behaviors and metabolic diversity, with some being autotrophic (photosynthetic or chemosynthetic) and most being heterotrophic.

Archaebacteria

• These bacteria live in harsh environments such as salty areas (halophiles), hot springs (thermoacidophiles), and marshy areas (methanogens).
• Their unique cell wall structure allows survival
• Methanogens in ruminant guts produce methane from animal dung.

Eubacteria

• Known as 'true bacteria'
• Characterized by a rigid cell wall and a flagellum if motile.
• Cyanobacteria, or blue-green algae, are photosynthetic autotrophs with chlorophyll a, similar to green plants
• Cyanobacteria can be unicellular, colonial, or filamentous, in various environments.
• Some cyanobacteria, like Nostoc and Anabaena, fix atmospheric nitrogen using heterocysts.
• Chemosynthetic autotrophic bacteria recycle nutrients by oxidizing inorganic substances.
• Heterotrophic bacteria are abundant and act as decomposers
• Some heterotrophic bacteria make curd from milk, produce antibiotics, and fix nitrogen in legumes.
• Bacteria reproduce via fission but may produce spores in harsh conditions and can reproduce sexually by transferring DNA from one bacterium to another
• Mycoplasmas lack a cell wall, are the smallest living cells, can survive without oxygen, and may be pathogenic.

Kingdom Protista

• Contains single-celled eukaryotes; its boundaries are not well-defined.
• Includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, and Protozoans.
• Most protists are aquatic and serve as a link between plants, animals, and fungi.
• Protists' cells contain a well-defined nucleus and membrane-bound organelles.
• Some have flagella or cilia, and reproduce asexually and sexually.

Chrysophytes

• Includes diatoms and golden algae (desmids) in freshwater and marine environments
• Microscopic and float passively in water currents.
• Diatom cell walls form silica-embedded overlapping shells, creating "diatomaceous earth" deposits, used as a polishing and filtering agent
• Diatoms are chief producers in the ocean

Dinoflagellates

• Mostly marine and photosynthetic, appearing in various colors based on their pigments.
• Their cell wall has stiff cellulose plates and two flagella for movement.
• Red dinoflagellates, like Gonyaulax, can rapidly multiply and cause "red tides," releasing toxins that kill marine animals.

Euglenoids

• Most live in fresh stagnant water
• Their body is flexible with a protein-rich pellicle instead of a cell wall.
• They have two flagella of different lengths.
• They are photosynthetic in sunlight but can become heterotrophic when deprived of light.
• Euglenoid pigments are identical to those in higher plants.

Slime Moulds

• Saprophytic protists that engulf organic material on decaying matter.
• Form a plasmodium aggregation under suitable conditions.
• During unfavourable conditions, they differentiate and form fruiting bodies bearing resistant spores.
• Spores can survive for years and are dispersed by air.

Protozoans

• Heterotrophic and live as predators or parasites; believed to be primitive relatives of animals.
• Divided into four group
  • Amoeboid protozoans live in fresh or sea water, or moist soil, using pseudopodia to capture prey; some marine forms have silica shells and some like Entamoeba are parasitic.
  • Flagellated protozoans are free-living or parasitic, possessing flagella, parasitic forms can cause sleeping sickness, e.g. Trypanosoma.
  • Ciliated protozoans are aquatic, actively moving using cilia and have a gullet to steer food, e.g. Paramoecium.
  • Sporozoans have an infectious spore-like stage, like Plasmodium, a malarial parasite.

Kingdom Fungi

• It's a unique kingdom of heterotrophic organisms with diverse morphology and habitat; examples include mushrooms, toadstools, and parasitic forms on plants and animals.
• Some fungi are unicellular like yeast
• Some can be the source of antibiotics
• They grow in warm, humid condition
• Most fungi are filamentous except unicellular yeasts
• Composed of hyphae, creates a network called mycelium; Some hyphae are continuous tubes with multinucleated cytoplasm known as coenocytic hyphae, while others are separated by cross walls/septa
• The cell walls of fungi are composed of chitin and polysaccharides
• Most fungi are heterotrophic, absorbing soluble organic matter from dead substrates as saprophytes, or from living organisms as parasites
• Fungi act as symbionts in lichens (with algae) and mycorrhiza (with plant roots).
• Fungi reproduce vegetatively by fragmentation, fission, and budding
• Asexual reproduction occurs using spores called conidia, sporangiospores, or zoospores
• Sexual reproduction occurs through oospores, ascospores, and basidiospores, formed in fruiting bodies

Fungi Reproduction

• Sexual reproduction involves three steps: plasmogamy (fusion of protoplasms), karyogamy (fusion of nuclei), and meiosis.
• Some fungi have a dikaryotic stage (n + n) before parental nuclei fuse.
• Fungi morphology, spore formation, and fruiting bodies are use to classify into classes.

Phycomycetes

• Are commonly found in aquatic environments and on decaying wood
• They are obligate parasites of plants
• Aseptate and coenocytic mycelium
• Reproduce asexually through motile zoospores or non-motile aplanospores produced endogenously in sporangium.
• A zygospore forms from the fusion of two gametes which can be similar/isogamous or dissimilar/anisogamous Ex: Mucor, Rhizopus and Albugo.

Ascomycetes

• Commonly known as sac-fungi
• Mostly multicellular, e.g., Penicillium, or rarely unicellular, e.g., yeast
• They are saprophytic, parasitic or coprophilous
• Mycelium is branched and septate
• Asexual spores are conidia
• Sexual spores called ascospores are produced endogenously in sac like asci, which are arranged in fruiting bodies called ascocarps
• Neurospora is used extensively in biochemical and genetic work, and some are delicacies

Basidiomycetes

• Common forms are 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
• The sex organs are absent
• Plasmogamy occurs with the fusion of two vegetative or somatic cells
• Karyogamy and meiosis occur in the basidium, which produces basidiospores

Deuteromycetes

• Known as imperfect fungi because only the asexual or vegetative phases of these fungi are known
• They are moved into ascomycetes and basidiomycetes once their sexual stages are identified; otherwise, the asexual spores are called conidia
• The mycelium is septate and branched
• They act as saprophytes, parasites, or decomposers that aid in mineral cycling

Kingdom Plantae

• Includes all eukaryotic chlorophyll-containing organisms commonly called plants.
• Some are partially heterotrophic like insectivorous plants (Bladderwort and Venus fly trap) or parasites (Cuscuta).
• Plant cells have an eukaryotic structure and cell wall mainly made of cellulose.
• This group includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
• Life cycle has diploid sporophytic and the haploid gametophytic phases-alternation of generation

Kingdom Animalia

• Characterized by heterotrophic eukaryotic multicellular organisms with cells lacking cell walls
• They directly or indirectly depend on plants for food
• They ingest and digest their food in an internal cavity
• Store food as glycogen or fat
• They follow distinct growth pattern and have sensory and neuromotor mechanism and are capable of locomotion.
• Reproduce sexually and have embryological development.

Viruses, Viroids, Prions and Lichens

• Not included in Whittaker's five-kingdom classification.
• Viruses:
  • Non-cellular with an inert crystalline structure outside living cells
  • Contain either RNA or DNA, but not both
  • They causes diseases, such as mumps, small pox, herpes and influenza
  • Named 'virus' and called the fluid Contagium vivum fluidum (infectious living fluid).
  • Can be crystallized, consisting largely of proteins; inert outside their specific host cell, obligate parasites.
  • Core has nucleic acid, the protein coat called capsid made of subunits/capsomeres.
• Viroids:
  • Discovered by T.O. Diener (1971).
  • Infectious agent with free RNA, lacking a protein coat.
• Prions:
  • Infectious neurological diseases caused by abnormally folded proteins.
  • Its diseases include mad cow and Cr¬Jacob disease.
• Lichens:
  • Symbiotic associations between algae (phycobiont) and fungi (mycobiont).
  • Algae provide food, fungi provides shelter and absorbs nutrients.
  • Act as Pollution indicators; do not grow in polluted areas.