BIOLOGY

• Since the dawn of civilization, there have been attempts to classify living organisms instinctively using non-scientific criteria for food, shelter, and clothing.
• Aristotle was the first to attempt a scientific basis for classification using morphological characters, classifying plants into trees, shrubs, and herbs, and animals based on the presence of red blood.
• In the time of Linnaeus, a Two Kingdom system of classification was developed, categorizing all organisms into Plantae and Animalia.
• This two-kingdom system did not differentiate between:
    - Eukaryotes and Prokaryotes
    - Unicellular and Multicellular organisms
    - Photosynthetic (like green algae) and Non-photosynthetic organisms (like fungi)
• Over time, it was found inadequate due to many organisms not fitting neatly into these categories.
• Classification systems underwent changes over time leading to inclusions of various characteristics:
    - Cell structure
    - Nature of the wall
    - Mode of nutrition
    - Habitat
    - Methods of reproduction
    - Evolutionary relationships

BIOLOGICAL CLASSIFICATION

• Kingdoms Defined by WHITTAKER (1969):
    - Monera
    - Protista
    - Fungi
    - Plantae
    - Animalia

• Main criteria for classification by Whittaker:
    - Cell structure
    - Body organization
    - Mode of nutrition
    - Reproduction
    - Phylogenetic relationships

• The three-domain system further divides King Monera, leading to the Six Kingdom classification, which includes a more detailed understanding of organisms.

• **Historical classifications:
    - Earlier classifications included bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms under Plants.
    - These classifications were based on organisms sharing the trait of having a cell wall.
      - This approach grouped prokaryotic bacteria with eukaryotic groups, which showed differing characteristics.

• The criteria for classification are evolving, and future changes are expected, reflecting new understandings of morphology, physiology, and evolutionary relationships.

CHARACTERISTICS OF THE FIVE KINGDOMS

KINGDOM MONERA

• Bacteria are abundant microorganisms, occurring almost everywhere (like in a handful of soil).
    - Live in extreme habitats (hot springs, deserts, snow, deep oceans).
    - Many are parasites, living in/on other organisms.

• Bacteria are categorized based on their shape:
    - Coccus (spherical)
    - Bacillus (rod-shaped)
    - Vibrio (comma-shaped)
    - Spirillum (spiral)

• Bacterial structure is simple, but the behavior is complex, showcasing extensive metabolic diversity.
    - Autotrophic bacteria synthesize food from inorganic substrates, either being:
      - Photosynthetic autotrophs or chemosynthetic autotrophs.
    - Majority are heterotrophs, relying on organic matter.

SUBDIVISIONS OF KINGDOM MONERA

Archaebacteria

• Survive in extreme habitats (halophiles, thermoacidophiles, methanogens).
• Differ in having a distinct cell wall structure.
    - Methanogens in the guts of ruminant animals produce methane from dung.

Eubacteria

• Characterized by rigid cell walls, some possess flagella.

• Cyanobacteria (blue-green algae) exhibit photosynthetic autotrophy, possessing chlorophyll a.
    - These are unicellular, colonial, or filamentous.
    - They can fix atmospheric nitrogen in specialized cells (heterocysts).
    - Crucial for nutrient recycling.

• Heterotrophic bacteria play key roles as decomposers and in human applications (e.g. fermentation).

• Reproduction occurs via fission or by producing spores under unfavourable conditions.

• Also, Mycoplasma, which lack a cell wall, represent the smallest living cells known and can be pathogenic.

KINGDOM PROTISTA

• Encompasses single-celled eukaryotes, boundaries not well-defined.
• Includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, and Protozoans.
• Primarily aquatic, forming links with plants, animals, and fungi.
• Cells possess well-defined nuclei and membrane-bound organelles,

SUBGROUPS OF KINGDOM PROTISTA

Chrysophytes

• Comprising diatoms & golden algae (desmids), exist in both freshwater and marine habitats.
• Diatoms have silica-embedded indestructible cell walls, contributing to diatomaceous earth used in filtration, polishing, etc.

Dinoflagellates

• Mainly marine, photosynthetic, exhibit varied pigmentation.
• Have stiff cell walls with cellulose plates & two flagella, often causing red tides.

Euglenoids

• Found in stagnant freshwater, possess a flexible pellicle instead of a cell wall.
• Photosynthetic in sunlight; can switch to heterotrophic behavior.

Slime Moulds

• Saprophytic life forms, converge into plasmodium under conditions.
• Resistant spores are formed when conditions are poor.

Protozoans

• All heterotrophs; believed to be primitive relatives of animals.
• Groups:
    - Amoeboid (eg. Amoeba)
    - Flagellated (eg. Trypanosoma)
    - Ciliated (eg. Paramecium)
    - Sporozoans (eg. Plasmodium)

KINGDOM FUNGI

• A unique kingdom of heterotrophic organisms differentiated by morphology and habitat.
• Examples include mushrooms, yeast, and parasites causing diseases.
• Bodies consist of hyphae, and the interconnected network is called mycelium.

SUBGROUPS OF KINGDOM FUNGI

Phycomycetes

• Found in aquatic habitats or damp areas, mycelium is aseptate and coenocytic.
• Reproduce via zoospores or by forming zygospores from gametes.

Ascomycetes

• Known as sac-fungi (eg. Penicillium, yeast), have branched mycelium.
• Reproduce asexually through conidia and sexually through ascospores in sac-like asci.

Basidiomycetes

• Comprising mushrooms and puffballs with branched mycelium.
• Reproduce vegetatively, and sex organs are absent, but plasmogamy occurs leading to dikaryotic forms.

Deuteromycetes

• Known as imperfect fungi; characterized by incomplete knowledge of their reproduction cycle.
• Only asexual reproduction is known as conidia.

KINGDOM PLANTAE

• Includes all eukaryotic chlorophyll-containing organisms; some exhibit partial heterotrophy.
• Includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
• Exhibit alternation of generations in their lifecycles.

KINGDOM ANIMALIA

• Defined by multicellular, heterotrophic organisms that lack cell walls.
• Follow a holozoic mode of nutrition via ingestion.
• A definitive growth pattern proceeds towards maturity.

VIRUSES, VIROIDS, PRIONS, AND LICHENS

• Viruses are non-cellular, characterized by taking over host cell machinery, not classified as living. Examples: tobacco mosaic virus, and influenza.
• Viroids lack protein coats and consist solely of RNA, discovered by T.O. Diener.
• Prions are infectious agents of abnormally folded proteins causing neurological diseases (e.g. mad cow disease).
• Lichens symbolize symbiosis between algae (phycobiont) and fungi (mycobiont).

SUMMARY

• Classification has evolved from Aristotle's early attempts to Linnaeus's two kingdoms and Whittaker's five kingdoms based on varied criteria such as cell structures and nutritional modes.
• The five kingdoms incorporate vast diversity in living organisms, different modes of nutrition and reproductive strategies, indicating a sophisticated understanding of biology.
• Understanding these classifications facilitates insights into evolutionary relationships and biodiversity.

EXERCISES

1. Discuss the changes in classification systems over time.
2. State two economic uses each of heterotrophic bacteria and archaebacteria.
3. Describe the nature of cell walls in diatoms.
4. Explain ‘algal bloom’ and ‘red-tides’.
5. Distinguish between viroids and viruses.
6. Briefly outline the four groups of Protozoa.
7. Identify partially heterotrophic plants.
8. Define phycobiont and mycobiont.
9. Compare the classes of Kingdom Fungi regarding mode of nutrition and reproduction modes.
10. Discuss the characteristics of Euglenoids.
11. Describe viruses concerning structural and genetic material alongside naming four viral diseases.
12. Discuss class opinions: Are viruses living or non-living?