Biology Chapter 2: Biological Classification

Study Notes

Classification of living organisms began instinctively, evolving to meet human needs for food, shelter, and clothing.
Aristotle pioneered a more scientific approach using morphology to categorize plants and animals, distinguishing groups based on blood types.
The Two Kingdom system established by Linnaeus included Plantae and Animalia but was inadequate for classifying all organisms, particularly prokaryotes and eukaryotes.

Organisms were traditionally grouped as plants or animals, but many did not fit into these categories due to diverse characteristics.
Essential criteria for classification expanded to include cell structure, mode of nutrition, reproductive methods, and evolutionary relationships.
The understanding of plant and animal kingdoms has evolved, with definitions of included groups changing over time among scientists.

R.H. Whittaker proposed a Five Kingdom Classification in 1969 consisting of Monera, Protista, Fungi, Plantae, and Animalia.
Key criteria defining these kingdoms include:
- Cell Type: Prokaryotic (Monera) vs. Eukaryotic (others).
- Cell Wall Composition:
  - Monera: Noncellulosic (polysaccharide + amino acid).
  - Fungi: Chitin.
  - Plantae: Cellulose.
  - Animalia: Absent.
- Nuclear Membrane: Absent in Monera; present in others.
- Body Organization: Monera and Protista are cellular; Fungi are multicellular/loose tissue; Plantae and Animalia are tissue/organ systems.
- Mode of Nutrition:
  - Monera: Autotrophic (chemosynthetic/photosynthetic) and heterotrophic (saprophytic/parasitic).
  - Protista: Autotrophic (photosynthetic) and heterotrophic.
  - Fungi: Heterotrophic (saprophytic/parasitic).
  - Plantae: Autotrophic (photosynthetic).
  - Animalia: Heterotrophic (various modes).

A six kingdom classification system divides Kingdom Monera into two domains, with the remaining eukaryotic kingdoms grouped into a third domain.
This system highlights the significant differences between prokaryotic and eukaryotic organisms, allowing for a more nuanced classification.

Earlier systems grouped diverse organisms such as bacteria, algae, fungi, and plants based solely on the presence of cell walls, leading to misleading classifications.
Distinctions between autotrophic plants and heterotrophic fungi were not recognized until fungi were eventually classified into a separate kingdom due to their distinct characteristics, including wall composition (chitin vs. cellulose).