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Schemes of Biological Classification

Updated on June 4, 2014
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Many organisms are composed of many cells; they are said to be multicellular. Some consist of any one cell each; they are said to be unicellular. Unicellular organisms, such as those you observed in the preceding demonstration are so small that they can be seen clearly only with the use of instruments such as microscopes. They are often referred to as microorganisms which means small.

Living things on this planet are usually classified as either plants or animals. Plants generally do not move about as much as animals do. Plants also generally possess the green pigment chlorophyll which enables them to manufacture food from the raw materials from the environment. This pigment is absent in animals. The system which all living things under either Kingdom Plantae or Kingdom Animalia is known as the Two-Kingdom Scheme of biological classification.

Two-Kingdom Scheme of Classification

Cell Nucleus
Number of Cells
Food Making
prokaryotic and eukaryotic
unicellular and multicellular
photosynthetic and nonphotosynthetic
prokaryotic and eukaryotic
unicellular and multicellular

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There are, however, some microorganisms which challenge the classification just cited. Examples ate those you observed in the demonstration. The euglena is a unicellular organism. The scientific name of euglena is Euglena gracilis. It moves its elastic body slowly and gracefully and uses a thread-like structure called flagellum at its anterior end. The flagellum is more animal-like than plant-like. The green color of Euglena is due to chlorophyll. Because of this pigment, euglena, like plants, can manufacture food in the presence of light.

The structure and characteristic of Euglena make it difficult for us to classify it as either plant or animal. Because of this, some biologist proposed to add to the plant and animal kingdom a third group which would take care of Euglena and other unicellular organisms – Kingdom Protista. This is known as the Three Kingdom Scheme of biological classification.

Another development is the Four-Kingdom Scheme of classification in which bacteria and blue-green algae are put together in separate group; Monera. The main reasons for doing so are the following:

a. Blue-green algae and bacteria are similar in cellular structure. Both are prokaryotic.

b. Some species of bacteria and blue-green algae can make use of nitrogen in the air.

Due to these similarities, blue-green algae apparently are more closely related to bacteria that to algae. Hence, blue-green algae are now called cyanobacteria. Recently, Whittaker proposed the Five-Kingdom Scheme of classification which separates the non-photosynthetic fungi from the rest of the plant kingdom. The fifth kingdom is simply called Kingdom Fungi.

Five-Kingdom Scheme of Classification

Cell Nucleus
Number of Cells
Food Making
multicellular and unicellular
unicellular and multicellular
nonphotosynthetic and photosynthetic
nonphotosynthetic and photosynthetic or chemosynthetic

Margulis and Schwartz adopted this scheme with some modifications. They introduced Kingdom Protoctista to include the heterotrophic protozoans of Kingdom Protista, all photosynthetic and eukaryotic algal phyla and the fungus-like slime molds formerly classified under Kingdom Plantae.

Just what are the characteristic of organisms around which changes in classification at the kingdom level revolve? The main characteristic are:

a. Nature of the cell nucleus

1. In some organisms, the cell nucleus lacks a nuclear membrane. The nucleus is described as incipient. The cell is described as prokaryotic. And the organisms are known as prokaryotes.

2. Other organisms have true nucleus, meaning, the nuclear materials are enclosed by nuclear membrane. The cell is described as eukaryotic. And the organisms are know as eukaryotes.

b. Number of cells

1. Some organisms are single-celled or unicellular

2. Other are many-celled or multicellular

c. Ability to synthesize food from raw materials in the environment.

1. Some of them manufacture food with the aid of sunlight. They are said to be photosynthetic.

2. Others cannot manufacture food; they are non-photosynthetic.

Biologist differs in opinion with regards to classification. And this is true with other man-made devices. Furthermore, biologists may change the classification of a particular organism as they discover more information about it. Thus, classification of organisms is not final; it is arbitrary.

Five-Kingdom Scheme of Biological Classification

Common Groups
Phyla with Subphyla, Classes or Orders
Schizophyta , Cyanobacteria
Mastigophora (Flagellates), Ciliophora (Ciliates), Sarcodina (Rhizopoda), Sporozoa
Chlorophycophyta, Phaeophycophyta, Rhodophycophyta, Chrysophycophyta
Slime molds
Zygomycota, Ascomycota, Basidiomycota, Deuteromycota
Bryophyta (Musci (Mosses), Hepaticae (Liverworts), Anthocerotae (Hornworts) )
Filicinophyta (Pterophyta)
Cycadophyta, Coniferophyta
Angiospermophyta (Anthophyta) (Dicot, Monocot)
Echinodermata, Mollusca, Cnidaria (Coelentarata), Porifera, Nematoda, Platyhelminthes, Arthropoda, Annelida
Chordata (Tunicata, Cephalochordata, Vertebrata (Mammalia, Aves, Reptalia, Amphibia, Osteichthyes, Chondrichthyes) )

References ; Science and Technology by Lilia M. Rabago Ph. D , Crescensia C. Joaquin Ph.D, Catherine B. Lagunzad , PH. D

Do you think this biological classification might change in future?

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    • KenDeanAgudo profile imageAUTHOR

      Kenneth C Agudo 

      5 years ago from Tiwi, Philippines

      @sheilamyers yes, you are right sheila. It really helps especially i have put tables on it.

    • profile image


      5 years ago

      I chose "I am interested" to answer your first poll, but after reading the hub I can tell you I learned something new. Unless I was sleeping in classes, I don't ever remember hearing any of my professors mention there being a possibility of adding kingdoms to the classification system. Is this something new or have scientists discussed it for years and my professors didn't think it was that important? With the information you present, I can see that adding kingdoms makes sense and would make classification easier.


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