Throughout history, a number of systems have been used to classify life. In 1735, Carl Linnaeus published his system of classifying life into two main groups, or kingdoms, animalia and vegetabilia. By 1969, a five kingdom model was proposed by Robert Whitaker. The most commonly used model today includes six kingdoms of life, incoporating plants, animals, archaebacteria, eubacteria, fungi and protists.
Plants are autotrophs, which means they make their own food, often through photosynthesis. Plants are also distinguished because of their complex cells.
Unlike plants, animals are heterotrophs, meaning they eat other organisms. Like plants, animals are also made up of many complex cells.
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Archaebacteria, or just archaea, are found in extreme environments on Earth, such as in deep vents at the bottom of an ocean or in hot springs. They are single-celled organisms that are prokaryotic, meaning the cells don't have membrane-bound organelles.
Eubacteria are "true" bacteria. They include all single-celled bacteria that aren't archaea. They are also prokaryotic. Depending on the type of cell wall, eubacteria may be Gram positive or Gram negative.
Fungi include multicellular mushrooms, mildew, mold and the unicellular yeasts. Their cells are eukaryotic, meaning they have membrane-bound organelles.
The final kingdom in the six kingdom system of classification is the protists. They are eukaryotic and include slime molds and algae. Essentially, this kingdom consists of all microscopic organisms that aren't bacteria, plants, animals or fungi.