Unicellular organisms, including bacteria, archaea, and eukaryotes, are the majority of life on Earth. They vary in size and survival strategies, with some having flagella or lobopodia. Four of the six eukaryotic supergroups are exclusively single-celled. They can be observed with a light microscope or electron microscope.
A unicellular organism is any form of life that consists of only one cell. This group includes the majority of life on Earth, with bacteria making up the majority. The major groups of unicellular life are bacteria, archaea (both prokaryotes), and eukaryotes (eukaryotes). The differences between prokaryotes and eukaryotes are significant: eukaryotes possess a nucleus, while prokaryotes lack one, and eukaryotes possess a range of subcellular organs called organelles, while prokaryotes have very minimal ones.
People can observe larger single-celled organisms, such as amoebas, using the highest settings on a light microscope. The bacteria are so small that they appear as specks under this magnification. To collect them for observation, a person can place a coverslip on the surface of the pond water and leave it overnight. By the next morning, many organisms will have developed entire colonies at the bottom of the slip. They replicate quickly: colonies can double in size in 30 minutes to a few hours.
Single-celled organisms as diverse as they are ubiquitous. The oldest forms of life existed 3.8 billion years ago, if not more. They pursue a variety of survival strategies: photosynthesis (cyanobacteria), chemotrophy (many archaea), and heterotrophy (amoebae). Some have flagella, small tails they use for locomotion, or lobopodia, extensions of the cellular skeleton (cytoskeleton), which appear as blot-like arms. The flagellum from the unicellular ancestors of man is retained all the way to animals, where it appears as flagellated sperm.
Of all six eukaryotic supergroups, four are composed exclusively of single-celled organisms. The only exceptions are the opisthoconts, made up of animals, fungi and close relatives, and the archeplastids, made up of both unicellular and multicellular plants. These organisms vary in size, with the smallest bacteria measuring 300 nanometers in diameter, up to the titanic plasmodial molds, which can grow up to 8 cm in diameter. The largest may have millions of nuclei scattered throughout the cell envelope. An expensive electron microscope is required to observe some of the smaller ones, while the larger ones can be seen with the naked eye.
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