Friday, July 3, 2020

Biological Classification - Notes | Class 11 | Part 1: Classification systems

2. BIOLOGICAL CLASSIFICATION

Aristotle’s classification


-    Aristotle was the earliest to attempt a more scientific basis for classification of organisms.

-  He classified plants to trees, shrubs & herbs and animals into 2 groups- those with red blood & without red blood.

Linnaeus’s Two-kingdom classification


-    Linnaeus (1758) classified organisms into Two Kingdoms- Kingdom Plantae & Kingdom Animalia.


Drawbacks of 2-kingdom classification:

  • Prokaryotes (Bacteria, cyanobacteria) and eukaryotes (fungi, mosses, ferns, gymnosperms & angiosperms) were included under ‘Plants’ based on the presence of cell wall. But they are widely differed in other characteristics.
  • It included the unicellular and the multicellular organisms in same group. E.g. Chlamydomonas and Spirogyra were placed under algae.
  • It did not differentiate between the heterotrophic fungi and the autotrophic green plants. Fungi have chitinous cell wall while the green plants have cellulosic cell wall.

Five Kingdom Classification


-    It is proposed by R.H. Whittaker (1969).

-    It includes Monera, Protista, Fungi, Plantae & Animalia.

-    This is based on cell structure, thallus organization, mode of nutrition, reproduction and phylogenetic relationships.

Three-domain system: It divides Kingdom Monera into two domains. Eukaryotic kingdoms are included in third domain.  Thus it is six-kingdom classification.



Characteristics of the five kingdoms

Characters

Monera

Protista

Fungi

Plantae

Animalia

Cell type

Prokaryotic

Eukaryotic

Eukaryotic

Eukaryotic

Eukaryotic

Cell wall

Non-cellulosic (poly-saccharide + amino acid)

Present in some

Present (Chitin & polysaccharides)

Present (Cellulose)

Absent

Nuclear membrane

Absent

Present

Present

Present

Present

Body organisation

Cellular

Cellular

Multicellular, loose tissue

Tissue/organ

Tissue/organ/ organ system

Mode of nutrition

Autotrophic (photosynthetic & chemosynthetic) and heterotrophic (saprophyte/parasite)

Autotrophic (photosynthetic) and heterotrophic

Heterotrophic (saprophytic or parasitic)

Autotrophic (photosynthetic)

Heterotrophic (holozoic, saprophytic etc.)

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