Biological Classification

The One Thing

Biological classification organizes organisms according to shared characteristics and evolutionary relationships. Modern systems use cell structure, body organization, nutrition, reproduction, and molecular evidence together rather than relying on a single feature.

Reactions, Processes and Experiments

Key Terms

• Biological classification: The scientific arrangement of organisms into groups and categories according to similarities, differences, and evolutionary relationships.
• Taxonomy: The science of identification, naming, and classification of organisms.
• Five-kingdom classification: A system proposed by R. H. Whittaker in 1969 that recognizes Monera, Protista, Fungi, Plantae, and Animalia.
• Monera: The kingdom containing prokaryotic organisms, including bacteria, cyanobacteria, and archaea.
• Prokaryotic cell: A cell without a true nucleus or membrane-bound organelles.
• Protista: A kingdom of mainly unicellular eukaryotes, including unicellular algae, protozoans, and slime moulds.
• Eukaryotic cell: A cell with a distinct nucleus surrounded by a nuclear membrane and usually containing membrane-bound organelles.
• Fungi: Mostly multicellular, non-photosynthetic, absorptive heterotrophs with chitinous cell walls.
• Plantae: Multicellular, generally photosynthetic eukaryotes with cellulose cell walls and mostly autotrophic nutrition.
• Animalia: Multicellular eukaryotes that lack cell walls and obtain food by ingestion.
• Archaebacteria: Ancient prokaryotes with distinctive cell membranes and cell walls, adapted to extreme environments such as salt lakes, hot springs, and oxygen-free habitats.
• Eubacteria: True bacteria with varied shapes, nutrition, and habitats; many have a peptidoglycan cell wall.
• Cyanobacteria: Photosynthetic bacteria, often called blue-green algae, that may fix atmospheric nitrogen.
• Mycoplasma: Very small organisms without a cell wall; some species are pathogenic and can survive without oxygen.
• Diatoms: Photosynthetic protists with silica-containing cell walls called frustules; their remains contribute to diatomaceous earth.
• Dinoflagellates: Mostly marine or freshwater unicellular protists with two flagella; some cause red tides.
• Euglenoids: Freshwater protists with a flexible protein-rich pellicle; they can photosynthesise in light and obtain food heterotrophically in darkness.
• Slime moulds: Decomposing protists that form a multinucleate mass during their feeding stage and produce resistant spores.
• Protozoans: Animal-like unicellular protists that are generally heterotrophic and include amoeboid, flagellated, ciliated, and sporozoan forms.
• Phycomycetes: Fungi with aseptate, coenocytic hyphae; examples include Rhizopus and Mucor.
• Ascomycetes: Sac fungi that produce sexual spores called ascospores inside sac-like asci; examples include yeast, Aspergillus, and Penicillium.
• Basidiomycetes: Club fungi that produce basidiospores on basidia; examples include mushrooms, rusts, and smuts.
• Deuteromycetes: Fungi historically grouped as imperfect fungi because only their asexual stage was known.
• Lichen: A mutual association between a fungus and a photosynthetic alga or cyanobacterium; the photosynthetic partner provides food and the fungus provides shelter, water, and minerals.
• Virus: An acellular infectious particle containing genetic material, either DNA or RNA, surrounded by a protein coat; it reproduces only inside a living host cell.
• Viroid: A very small infectious agent made only of naked circular RNA, without a protein coat.
• Prion: An infectious, abnormally folded protein that can cause certain nervous-system diseases.
• Binomial nomenclature: The system of naming each species with two words: the genus name followed by the specific epithet, such as Homo sapiens.
• Species: The basic unit of classification, generally consisting of organisms that share fundamental features and can interbreed to produce fertile offspring.
• Nomenclature: The standardized process of assigning scientific names to organisms.

Easily Confused

• Monera and Protista: Monera contains prokaryotes, whereas Protista contains mainly unicellular eukaryotes.
• Archaebacteria and Eubacteria: Archaebacteria have distinctive membranes and cell walls and commonly inhabit extreme environments; Eubacteria are true bacteria, many with peptidoglycan cell walls.
• Cyanobacteria and algae: Cyanobacteria are photosynthetic prokaryotic bacteria, whereas algae classified in Protista are eukaryotic.
• Euglenoids and plants: Euglenoids photosynthesise in light but can obtain food heterotrophically in darkness and possess a pellicle rather than a rigid cell wall.
• Fungi and Plantae: Fungi are absorptive heterotrophs with chitinous cell walls; plants are generally photosynthetic autotrophs with cellulose cell walls.
• Hypha and mycelium: A hypha is an individual thread-like fungal filament; a mycelium is a network of hyphae.
• Plasmogamy and karyogamy: Plasmogamy is fusion of cytoplasm, whereas karyogamy is fusion of nuclei.
• Virus and viroid: A virus contains DNA or RNA enclosed in a protein capsid; a viroid consists only of naked circular RNA.
• Virus and prion: A virus is a nucleic-acid-containing infectious particle, whereas a prion is an abnormally folded infectious protein.
• Binomial nomenclature and taxonomy: Binomial nomenclature concerns the two-part naming of species; taxonomy includes identification, naming, and classification.
• Species and genus: A species is the basic classification unit; a genus is a broader group containing one or more closely related species.
• Five-kingdom and three-domain systems: Whittaker’s five-kingdom system uses Monera, Protista, Fungi, Plantae, and Animalia; the three-domain system uses Bacteria, Archaea, and Eukarya and relies especially on molecular evidence.

What Gets Asked

• Identify the criteria used by Whittaker: Questions may require cell structure, body organisation, mode of nutrition, reproduction, and evolutionary relationships. A common error is treating one characteristic as sufficient for classification.
• Compare the five kingdoms: Students may need to distinguish Monera from the four eukaryotic kingdoms. The key slip is classifying Monera as eukaryotic.
• Describe microorganisms and their examples: Questions may ask about methanogens, halophiles, thermoacidophiles, cyanobacteria, mycoplasma, diatoms, dinoflagellates, euglenoids, slime moulds, or protozoans. Marks are lost by omitting the defining feature of the named group.
• Explain fungal reproduction: Questions may distinguish fragmentation, fission, budding, asexual spores, plasmogamy, karyogamy, and meiosis. The specific stages must not be conflated: plasmogamy is cytoplasmic fusion, while karyogamy is nuclear fusion.
• Compare viruses, viroids, and prions: The required distinctions concern cellular status, genetic material, protein coat, and dependence on host cells. In particular, a virus contains either DNA or RNA, whereas a viroid contains naked circular RNA and a prion is infectious protein.
• Apply scientific naming and classification hierarchy: Questions may require the correct formatting of Homo sapiens or the sequence from species to domain. Marks are lost by failing to capitalize the genus, lowercase the specific epithet, or italicize the scientific name.