The Living World Overview

Biological Classification

• Biological classification is the scientific process of organizing organisms into groups and subgroups based on similarities and differences, placing them in a hierarchical structure.
• Classification is essential because it's impossible to study every organism. Studying a few representative organisms within a group reveals key characteristics of the entire group.
• Classification facilitates the identification of new organisms by providing a framework for recognizing their similarities to known groups.
• Classification helps understand the relationships between different groups of organisms.
• Understanding past organisms is only possible with a comprehensive classification system.

Types of Classification Systems

• Artificial Classification: Organisms are grouped based on one or two easily observable physical traits, avoiding evolutionary relationships. Examples include grouping plants as flowering or non-flowering, or animals as vertebrates and invertebrates. (e.g., Aristotle).

• Natural Classification: This approach considers numerous characteristics to reveal natural similarities and differences, and hence natural relationships among organisms. This system aims to depict evolutionary relationships accurately. (e.g., Bentham and Hooker).

• Phylogenetic Classification: Groups organisms based on their evolutionary relationships. Organisms are classified based on how they evolved from earlier forms on Earth, from primitive to highly evolved. (e.g., Engler and Prantl, Hutchinson).

Kingdom System

• Two Kingdom System (Plantae and Animalia): Organisms categorized based on presence or absence of cell walls. Linnaeus proposed this system.

• Three Kingdom System (Plantae, Protista, and Animalia): Separates unicellular organisms (algae, fungi, and some animals) as a separate category. Haeckel developed the system.

• Five Kingdom System (Monera, Protista, Fungi, Plantae, Animalia): Organisms divided by characteristics like cell structure (prokaryotic or eukaryotic), mode of nutrition, and other features. Recognizes and distinguishes different groups. R.H. Whittaker developed the system.

Monera

• Includes all prokaryotes.
• Unicellular, prokaryotic organisms, the most ancient.
• Lack membrane-bound organelles (e.g., nucleus).
• Genetic material (DNA) is not in a nucleus.
• Reproduction typically asexual.
• Includes mycoplasma, bacteria, actinomycetes, and cyanobacteria.

Eubacteria

• True bacteria; rigid cell walls.
• May be motile if they possess flagella.
• Includes Photosynthetic bacteria; some are photosynthetic (Cyanobacteria).
• Some are heterotrophic (acquire food by consuming organic matter) and others perform chemosynthesis (oxidize inorganic substances to produce energy).

Protista

• Mostly unicellular; some colonial eukaryotes—eukaryotic life forms.
• Mode of nutrition is diverse (photosynthetic, saprophytic, parasitic, or holozoic).
• Includes diatoms, dinoflagellates, euglenoids, slime molds, and protozoa.
• Many protists are responsible for photosynthesis in aquatic environments.

Fungi

• Achlorophyllous (lacking green pigments) organisms; heterotrophic- obtain food from other organisms.
• Cell walls contain chitin or fungal cellulose; reserves are in glycogen.
• Modes of nutrition include saprophytic, parasitic, and symbiotic.
• Reproduction occurs vegetatively, asexually, or sexually. Important groups are phycomycetes, ascomycetes, basidiomycetes, and deuteromycetes.

Phycomycetes

• Found in moist, aquatic, or decaying-wood habitats; mycelium (vegetative body) is aseptate and coenocytic.
• Reproduce asexually (zoospores and aplanospores). Example: Rhizopus (bread mold).

Ascomycetes

• Saprophytes, decomposers, parasites, and coprophilous organisms.
• Mycelium is branched and septate; asexual spores are conidia; sexual spores are ascospores produced within ascocarps. Example: Neurospora, Penicillium, Saccharomyces.

Basidiomycetes

• Saprophytic, decomposers, or coprophilous; mycelium is branched and septate; asexual spores are not found; sexual reproduction via basidiospores in basidiocarps (fruiting body). Example: Agaricus (mushroom).

Deuteromycetes ("Imperfect Fungi")

• Only vegetative and asexual phases observed; mycelium is septate and branched. Example: Alternaria and Penicillium.

Plantae (Plants)

• Eukaryotic organisms; photosynthetic mode of nutrition; cell wall composed of cellulose.
• Life cycle involves both diploid (sporophytic) and haploid (gametophytic) phases.
• Includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.

Animalia (Animals)

• Multicellular, eukaryotic organisms; heterotrophic mode of nutrition; no cell wall.
• Reproduce sexually; mode of nutrition: holozoic (ingesting whole food).
• Includes several phyla (e.g., Chordata, Arthropoda, Mollusca).

Viruses, Viroids, and Lichens

• Not included in the five-kingdom system.
• Viruses are non-cellular; consist of a protein coat and nucleic acids (DNA or RNA); replicate within a host cell; cause many plant and animal diseases
• Viroids are smaller than viruses; consist of only RNA; cause plant diseases.
• Lichens are symbiotic associations between algae (phycobiont) and fungi (mycobiont); indicator species for pollution.

Additional Details

• DNA Viruses: The protein coat (capsid) made of subunits called capsomeres is used to protect nucleic acid. Capsomers arranged structurally (for example, in helical or polyhedral geometric forms).
• Viroids: Discoverd by T.O. Diener, infectious agents smaller than viruses; contain only RNA, no protein coat. Potato spindle tuber disease is an example.