Unicellular organism

Organisms consisting of a single cell

Unicellular organisms are organisms that consist of a single cell, unlike multicellular organisms which are made up of multiple cells. Unicellular organisms can be either prokaryotic or eukaryotic.

Types of Unicellular Organisms

Prokaryotes

Prokaryotic unicellular organisms include bacteria and archaea. These organisms lack a nucleus and other membrane-bound organelles. Bacteria are found in virtually every habitat on Earth, from soil to water to the human body. Archaea, although similar in appearance to bacteria, have distinct genetic and biochemical characteristics.

Eukaryotes

Eukaryotic unicellular organisms include protists, algae, and some fungi. These organisms have a nucleus and other organelles enclosed within membranes. Examples include the Paramecium, a type of ciliate, and various species of unicellular algae such as diatoms.

Reproduction

Unicellular organisms reproduce in various ways, including binary fission, budding, and spore formation. In binary fission, the cell divides into two genetically identical cells. Budding involves the formation of a new organism from a small projection on the parent organism. Some unicellular organisms can also exchange genetic material through processes such as conjugation.

Ecological Role

Unicellular organisms play crucial roles in ecosystems. They are primary producers in many aquatic environments, forming the base of the food web. They also contribute to the nitrogen cycle and other biogeochemical cycles. Some unicellular organisms are pathogens, causing diseases in humans, animals, and plants.

Evolutionary Significance

Unicellular organisms are believed to be the earliest forms of life on Earth. Fossil evidence, such as stromatolites, suggests that unicellular life existed over 3.5 billion years ago. These organisms have evolved into a wide variety of forms, leading to the diversity of life seen today.

Gallery

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  Ventricaria ventricosa, a large unicellular alga

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  Stromatolites in Shark Bay, Western Australia

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  Bacteria with a capsule

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  Echinoderm larvae, an example of a multicellular organism

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  Paramecium tetraurelia, a ciliate

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  Diatom, a type of unicellular algae

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  Yeast, a unicellular fungus

Related pages

• Multicellular organism
• Prokaryote
• Eukaryote
• Microorganism

References

• Madigan, M.T., Martinko, J.M., Bender, K.S., Buckley, D.H., & Stahl, D.A. (2014). Brock Biology of Microorganisms. Pearson.
• Margulis, L., & Schwartz, K.V. (1998). Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth. W.H. Freeman.

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  Ventricaria ventricosa

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  Stromatolites in Shark Bay

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  Bacteria with capsule

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  Paramecia tetraurelia

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  SEM diatom

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  Unicellular organism