Biological Classification

Learning Outcomes:

1. Understand the historical development of classification systems.
2. Identify the key characteristics used for classifying living organisms.
3. Recognize the five kingdoms and their distinguishing features.
4. Comprehend the differences between prokaryotes and eukaryotes.
5. Learn about viruses, viroids, and prions.

The classification of living organisms has evolved significantly over time, starting from the earliest efforts by Aristotle. He classified organisms based on simple morphological features, like grouping plants into trees, shrubs, and herbs and dividing animals into two groups based on whether they had red blood. However, as scientific understanding progressed, it became clear that this was an oversimplification. The introduction of the Two Kingdom system by Linnaeus (Plantae and Animalia) also proved inadequate as it did not distinguish between prokaryotes and eukaryotes, nor did it consider important characteristics such as cellular organization or modes of nutrition.

Evolution of Classification Systems

Historically, classification was largely driven by the need to categorize organisms for practical uses like food, shelter, and clothing. As science advanced, new systems emerged that better reflected the biological relationships between organisms.

1. Linnaeus’ Two Kingdom System: Included Plantae and Animalia but failed to distinguish between unicellular and multicellular organisms, eukaryotes and prokaryotes, and autotrophic and heterotrophic organisms.
2. Whittaker’s Five Kingdom Classification (1969): Introduced the kingdoms Monera, Protista, Fungi, Plantae, and Animalia. The basis for classification included cell structure, body organization, mode of nutrition, and evolutionary relationships.
3. Three-Domain System: Later classifications, such as the three-domain system, split Monera into two separate domains—Archaea and Bacteria, with eukaryotes in a third domain.

Important Note:

The two-kingdom system was found inadequate because it did not account for the diversity of life forms, particularly microorganisms, and failed to separate autotrophs from heterotrophs or unicellular organisms from multicellular ones.

Characteristics of Five Kingdoms

The five-kingdom system was proposed by R.H. Whittaker in 1969, offering a more detailed approach to biological classification. This system relied on the following key criteria: cell type (prokaryotic/eukaryotic), cell wall structure, nuclear membrane presence, body organization (unicellular/multicellular), and mode of nutrition. Below is a comparative table summarizing these characteristics:

Key Features of Each Kingdom

1. Monera: Includes prokaryotes like bacteria. They are unicellular and lack a nuclear membrane. These organisms are incredibly diverse in shape, such as cocci (spherical), bacilli (rod-shaped), vibrio (comma-shaped), and spirilla (spiral). Some bacteria are autotrophic (photosynthetic or chemosynthetic), but most are heterotrophic. Key types of bacteria:

• Archaebacteria: Known for thriving in extreme environments such as halophiles (salty areas), thermoacidophiles (hot springs), and methanogens (marshy areas). Methanogens in the guts of ruminants produce methane.
• Eubacteria: This group includes cyanobacteria (blue-green algae) that are photosynthetic autotrophs. They play a vital role in nutrient recycling by fixing atmospheric nitrogen in specialized cells called heterocysts. Reproduction: Bacteria reproduce asexually by binary fission. Some, under unfavorable conditions, form spores. A primitive form of sexual reproduction also occurs via DNA transfer.

1. Protista: Includes all single-celled eukaryotes. Members of this kingdom are primarily aquatic and form a link between plants, animals, and fungi. The group includes Chrysophytes, Dinoflagellates, Euglenoids, Slime Moulds, and Protozoans. Key groups:

• Chrysophytes: Includes diatoms and desmids. These organisms are mainly photosynthetic and their silica-embedded walls form a significant part of diatomaceous earth, used in filtration and polishing.
• Dinoflagellates: These are marine and photosynthetic organisms, often responsible for red tides due to their rapid multiplication.
• Euglenoids: Mostly found in stagnant water, they lack cell walls but have a protein-rich pellicle for flexibility. They are photosynthetic but can switch to heterotrophic behavior in the absence of light.
• Slime Moulds: These organisms are saprophytic, living on decaying organic material. They form plasmodia, which aggregate and form fruiting bodies under unfavorable conditions.
• Protozoans: Heterotrophic protists that exist as parasites or predators. They are classified into four main groups: amoeboid, flagellated, ciliated, and sporozoan protozoans.

1. Fungi: Fungi are heterotrophic organisms that exist as decomposers, parasites, or in symbiotic relationships. Fungal cells have chitin in their walls. Fungi reproduce through asexual and sexual means, forming conidia, sporangiospores, or zoospores in asexual reproduction and ascospores, basidiospores, or oospores in sexual reproduction. Fungi are classified into:
2. Phycomycetes: Found in aquatic habitats and moist environments, they reproduce via zoospores or aplanospores.
3. Ascomycetes: Also known as sac fungi, they reproduce via conidia and ascospores. Some examples include Penicillium and Neurospora.
4. Basidiomycetes: Known for their fruiting bodies, these fungi include mushrooms, rusts, and smuts.
5. Deuteromycetes: Referred to as imperfect fungi because their sexual stages are not well understood. Many are saprophytic or parasitic. Important Note:

Fungi like Penicillium are a source of antibiotics, while others like yeast are used in the fermentation process to make bread and beer.

4. Plantae: This kingdom encompasses all eukaryotic organisms with chlorophyll capable of photosynthesis. The plant body can vary in complexity from simple forms like algae to complex organisms like angiosperms. Plants exhibit alternation of generations, with distinct haploid gametophytic and diploid sporophytic phases.
5. Animalia: Members of this kingdom are multicellular, heterotrophic eukaryotes lacking cell walls. They depend on plants or other animals for food. Animals exhibit holozoic nutrition and are capable of locomotion. Reproduction in this kingdom is typically sexual, involving the copulation of male and female organisms followed by embryological development.

Viruses, Viroids, Prions, and Lichens

While viruses, viroids, prions, and lichens do not fit neatly into the five-kingdom classification, they play significant roles in biology.

1. Viruses: Non-cellular organisms with an inert crystalline structure outside living cells. Once inside a host, viruses hijack the cellular machinery to replicate, leading to the death of the host cell. Viruses contain either DNA or RNA, never both. Viral diseases in humans include mumps, smallpox, herpes, and influenza. Example: Bacteriophages are viruses that infect bacteria. Their protein coat, or capsid, is made up of capsomeres and encloses the genetic material.
2. Viroids: Dis

covered by T.O. Diener in 1971, viroids are smaller than viruses and lack a protein coat. They consist of low molecular weight RNA and are responsible for diseases such as potato spindle tuber disease.

3. Prions: Abnormal protein particles that cause neurological diseases like mad cow disease and Creutzfeldt-Jakob disease. They lack nucleic acids but are highly infectious.
4. Lichens: Symbiotic associations between algae (autotrophic) and fungi (heterotrophic). Lichens are good indicators of air pollution as they do not grow in polluted environments.

Multiple Choice Question

Question: Which of the following organisms is responsible for nitrogen fixation in cyanobacteria?
Options:

1. Nostoc
2. Rhizopus
3. Penicillium
4. Euglena
   Answer: 1. Nostoc