8.1 - Classification System and Naming of Organisms

The Necessity of Classification System and Naming of Organisms

Taxonomy is a field of biology that involves classification, identification and naming of organisms based on a systematic approach. This allows easier reference and discussions to be done among the scientific community with the systematic and well-organized information.

• Classification means the organisms are classified based on the characteristics observed in a taxonomic hierarchy system.
• Naming means that the organisms are named using a binomial nomenclature system.
• Identification means the organisms are identified using dichotomous keys.

Classification of Organisms

The six kingdoms are what classify all the organisms present in this world. The organisms are classed based on the types of cells, number of cells and types of nutrients.

1. Archaebacteria
2. Eubacteria
3. Fungi
4. Protista
5. Plantae
6. Animalia

• Prokaryotes refer to the type of cells that do not have nucleus and membrane-enclosed organelles.
• Eukaryotes are cells that have nucleus and membrane-enclosed organelles.
• Heterotrophs are organisms that cannot synthesize their own food, that is they need to obtain food by eating other organisms.
• Autotrophs refers to organisms that can synthesize their own food from non-organic raw materials.

The Main Features of Organisms in Each Kingdom

Archaebacteria

• Prokaryotes
• Unicellular organism
• Primitive bacteria
• Autotrophic or heterotrophic
• Cell walls do not have peptidoglycan
• Live in regions that are very hot, acidic, salty or anaerobic environments
• Can be classified into 3 groups based on their habitats
  • Methanogen are obligate anaerobic bacteria which can be found in swamps and the digestive tract of ruminants and humans. Methanogen produce methane as a metabolic byproduct.
  • Halophile can be found in very salty places such as the Dead Sea.
  • Thermophile are bacteria that can withstand very high temperature between 60°C to 80°C such as the Yellowstone National Park, US.
• Examples are Sulfolobus sp. (sulfur-oxidising bacteria) and Halobacterium salinarum

Eubacteria

• Prokaryotes
• Unicellular organism
• 'true' bacteria
• Autotrophic or heterotrophic
• Cell walls contain peptidoglycan, also known as murein (a type of polymer formed from sugars and amino acids)
• Cytoplasm contains ribosomes and plasmid, but does not have membrane-enclosed organelles such as mitochondria, endoplasmic reticulum and others
• Categorized based on their shapes
• Examples are Streptococcus pneumoniae and Vibrio cholerae

Protista

• Eukaryotes
• Organisms can be unicellular or multicellular
• Autotrophic or heterotrophic
• Simple cell organisation with no specialized tissues
• Nucleus is enclosed by a nuclear membrane and contain membrane-bound organelles
• Can be divided into 3 groups – protozoa, alga and slime mould
• Examples of protozoa are Euglena sp., Amoeba sp. and Paramecium sp.
• Examples of alga are Chlamydomonas sp. and Spirogyra sp.
• Example of slime mould is Physarum polycephalum

Fungi

• Eukaryotes
• Organisms can be unicellular or multicellular
• Heterotrophic (saprophyte or parasite)
• Cell walls contain chitin
• The body is made up of network of hyphae called the mycelium
• Examples are Saccharomyces cerevisiae (yeast) and Agarius sp. (mushroom)

Animalia

• Eukaryotes
• All multicellular animals
• Heterotrophic
• Most of the animals can move
• Reproduce sexually
• Examples are invertebrate (starfish) and vertebrate (elephant)

Plantae

• Eukaryotes
• Multicellular organism
• Autotrophic ( photoautotroph )
• Reproduce asexually or sexually
• Examples include seedless plants (ferns) and plants with seeds (all flowering plants)

Taxonomy Hierarchy

Linnaeus hierarchy system is the system used in taxonomy. Classification of organisms starts from species to domain. The orders of the hierarchy are as below:

Figure 1 - Taxonomy hierarchy. Domain is the highest taxonomic rank of organisms. Each kingsom is divided into smaller groups called phylum. Organisms within the same phylum have mutual characteristics. Phylum can be further divided into class. Class can be divided into order. Order can still be divided into family , and family into genus and lastly species. Organisms of the same species can reproduce among themselves.

Binomial Nomenclature System

Linnaeus binomial system is a system that gives the scientific names to the organisms. The way of writing the scientific name of organisms is as follow:

• Scientific names consist of two words. The first word is the name of the genus , followed by the name of the species.
• The first letter of the genus is capitalised whereas all the name of the species are small letters.
• Both of the names must be printed in italics. If handwritten, the two names must be underlined separately.

Example of a scienific name is Oryza sativa or in handwritten, Oryzasativa.

Dichotomous Key

Dichotomous key refers to the tool used by taxonomists to identify organisms based on their similarities and differences. Dichotomous key is built by using a series of couplets. Each couplet consists of two statements about the traits of the organism or its grouping.

The dichotomous key is specific to each types of identification. The dichotomous key is different based on the types of organisms identified.