Organism Classification

• Every organism, plant, or animal is unique.
• There is wide diversity in flora (plants) and fauna (animals).
• Current diversity resulted from 3.5 billion years of organic evolution.
• Many species have vanished from Earth, with over fifty times the existing species becoming extinct.
• Studying living organism diversity can be more effective if arranged in an orderly manner.

Basis of Classification

• Classification is based on several factors:
• Whether cells are prokaryotic or eukaryotic:
  • Prokaryotic cells lack clearly demarcated nuclei and organelles.
  • Eukaryotic cells have membrane-bound organelles, including a nucleus.
• Whether cells occur singly (unicellular) or in clusters (multicellular):
  • Unicellular organisms are made of one cell (e.g., Amoeba).
  • Multicellular organisms have cells grouped to perform specialized functions (e.g., insect).
• Whether an organism is photosynthetic or takes food from outside:
  • Green plants perform photosynthesis, synthesizing their own food.
  • Animals cannot perform photosynthesis and obtain food from outside.
• Organization of different body parts:
  • Plants are grouped by stem, root, and leaves.
  • Animals are grouped by specialized organs.
  • Body design characteristics used for plant and animal classification differ.

Hierarchy of Classification

• Organisms that closely resemble each other are placed in a group.
• Groups are placed in larger groups based on similarities.
• Larger groups are placed in still larger levels or ranks known as categories.
• Each category has a specific name.
• There are seven major categories: Species, Genus, Family, Order, Class, Phylum (animals)/Division (plants), Kingdom.

Linnaeus's System of Classification

• Two major systems of classification are studied worldwide.
• Two-Kingdom System of Classification:
  • Aristotle started classification in 600 A.E. based on food-making processes, prior to proper apparatus or microscopes.
  • Carolus Linnaeus proposed the two-kingdom scheme in 1758.
  • This divides all living organisms into Plants and Animals.
  • Organisms that manufacture food but cannot move are in the Plant kingdom.
  • Organisms that cannot make food but can move are in the Animal kingdom.
  • Plants (Autotrophs) self-make food through photosynthesis or nitrogen fixation, using inorganic compounds to create glucose.
  • Plants, fungi, algae, cyanobacteria are put in the kingdom Plantae.
  • Organisms that cannot prepare their food are known as Heterotrophs.
  • Heterotrophs get food from autotrophs (leaves, grass, meat) and are placed in the kingdom Animalia.
  • These include animals, humans, major bacteria types, and fungi.

Two Kingdom System of Classification

• Kingdom Plantae includes bacteria (Prokaryotes), photosynthetic plants, and non-photosynthetic fungi.
• Characteristic features of Kingdom Plantae:
  • Branches and asymmetrical bodies with green leaves.
  • Non-motile and fixed in place.
  • More actively involved in photosynthesis during the daytime.
  • Autotrophic mode of nutrition, synthesizing their own food.
  • Growing points for unlimited growth.
  • Absent excretory and nervous systems.
  • Starch is the reserve food material.
  • Cells have a cell wall and a large vacuole.
  • Lack centrosomes but may have inorganic crystals.
  • Reproduction occurs with air, water, and insects.
  • Asexual and vegetative reproduction is not uncommon.
• Kingdom Animalia includes unicellular protozoans and multi-cellular animals (metazoans).
• Animalia are characterized by:
  • Definite body shape and absence of branches.
  • Ability to move.
  • Respiration takes place without photosynthesis.
  • Heterotrophic nutrition since chlorophylls are absent.
  • Limited growth, stopping at a particular size and age.
  • Well-developed excretory and nervous systems.
  • Glycogen is the reserve food material.
  • Lack cell walls and have small vacuoles.
  • Centrosomes are present without inorganic crystals.
  • Sexual reproduction independent of external agents.
  • Regeneration of body parts and asexual reproduction in lower organisms only.

Limitations of Two Kingdom System of Classification

• The two-kingdom system is the earliest classification, only accounting for two kingdoms.
• Many new kingdoms have been added since.
• Limitations of Linnaeus’ two-kingdom system:
  • Fungi lack chlorophyll, yet are in the plant kingdom.
  • It does not distinguish between eukaryotic and prokaryotic cells.
  • Lichens with plant-like and animal-like characteristics are in the plant kingdom.
  • Unicellular organisms are grouped with multicellular organisms.
  • It does not acknowledge diversified life forms.
  • Many organisms lack a category.

Five-Kingdom System of Classification

• R.H. Whittaker proposed a five-kingdom system of classification in 1969.
• Separation into five kingdoms based on:
  • Complexity of cell structure.
  • Complexity of body structure.
  • Mode of nutrition.
  • Reproduction.
  • Phylogenetic relationship.
• The five kingdoms are Monera, Protista, Fungi, Plantae, and Animalia.
• Kingdom Monera (Prokaryotae):
  • Lack a well-defined body.
  • Unicellular and tiny.
  • Can be autotrophic or heterotrophic.
  • Includes bacteria, blue-green algae or cyanobacteria, and mycoplasma.
• Kingdom Protista (Protocista):
  • Unicellular and eukaryotic.
  • May be autotrophic or heterotrophic.
  • Some use appendages like cilia or flagella for movement.
  • Usually aquatic, present in soil or moist areas.
  • Includes unicellular algae, diatoms, and protozoans.